| Clark, A. A Physicalist Theory of Qualia The Monist 1985 (68)4:491-506 [html] |
| Although the capacity to discriminate between different qualia is typically admitted to have a definition in terms of functional role, the qualia thereby related are thought to elude functional definition. In this paper I argue that these views are inconsistent. Given a functional model of discrimination, one can construct from it a definition of qualia. The problem is similar in many ways to Goodman's definition of qualia in terms of 'matching', and I argue that many of his findings survive reinterpretation into a physicalistic basis which employs 'indiscriminability' as its primitive term. I show how one can identify the critical properties to which discrimination capacities are sensitive, and then identify their order. A problem arises concerning the different ways in which qualitatively distinct experiences can differ (hue, shape, and so on). Physicalist accounts have often been accused of relying in a circular fashion on some antecedent understanding of phenomenal properties in order to specify those differences. This account avoids such an accusation: ordering of critical properties is determined by the dimensionality of discriminations, and the latter is given by the structure of the discrimination pair lists. Once a topology of quality is constructed, qualia names can be defined by their relative location within the order. In the conclusion I argue that psychophysics employs physicalist techniques to define a topology of quality, and that it can provide what Thomas Nagel calls an "objective phenomenology." |
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| Kelemen, C., Szirmay-Kalos, L. A microfacet based coupled specular-matte BRDF model with importance sampling [pdf] |
| This paper presents a BRDF model based on the analysis of the photon collisions with the microfacets of the surface. The new model is not only physically plausible, i.e. symmetric and energy conserving, but provides other important features of real materials, including the off-specular peak and the mirroring limit case. Using theoretical considerations the reflected light is broken down to a specular component representing single reflections and a matte component accounting for multiple reflections and re-emissions of previously absorbed photons. Unlike most of the previous models, the proportion of the matte and specular components is not constant but varies with the viewing angle. In order to keep the resulting formulae simple, several approximations are made, which are quite accurate but allow for tabulation, fast calculation and even for accurate importance sampling. |
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| Droulez, J., Berthoz, A. A neural network model of sensoritopic maps with predictive short-term memory properties Proceedings of the National Academy of Sciences USA 1991 (88):9653-9657 [html] |
| Coordinated orienting movements can be accurately performed without direct sensory control. Ocular saccades, for instance, have been shown to be reprogrammed after target disappearance when an intervening eye movement is electrically triggered before the saccade onset. Saccadic eye movements can also be executed toward memorized targets, even when the subject has been passively moved in darkness. Two hypotheses have been proposed to account for this goal-invariance property: either (i) the goal is reconstructed and memorized in the stable frame of reference linked to the environment ("allocentric, coordinates") or (ii) the goal is selected and memorized in the sensors-related maps ("egocentric coordinates") and is continuously updated by efferent copies of the motor commands. In this paper, we shall describe a formal neural network based on this second hypothesis. The results of the simulation show that target position can be memorized and accurately updated in a topologically ordered map, using a velocity-signal feedback. Moreover, this network has been submitted to a simple learning procedure by using the intermittent visual recurring afferent signal as the teaching signal. A similar mechanism could be involved in control of limb movement. |
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| Blakemore, S.J., Wolpert, D.M., Frith, C.D. Abnormalities in the awareness of action Trends in Cognitive Sciences 2002 (6)6:237-242 [pdf] |
| Optimal motor control relies on internal representations of the actual, desired and predicted states of our limbs and the external world. Only certain components of these internal representations are available to awareness. We suggest that impairments of the components of internal representations might underlie a broad variety of neuropsychiatric symptoms, including the anarchic hand sign, phantom limbs, utilization behaviour and delusions of control. |
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| Sadato, N. & al Activation of the primary visual cortex by Braille reading in blind subjects Nature 1996 (380):526-528 [html] |
| Primary visual cortex receives visual input from the eyes through the lateral geniculate nuclei, but is not known to receive input from other sensory modalities. Its level of activity, both at rest and during auditory or tactile tasks, is higher in blind subjects than in normal controls, suggesting that it can subserve nonvisual functions; however, a direct effect of non-visual tasks on activation has not been demonstrated. To determine whether the visual cortex receives input from the somatosensory system we used positron emission tomography (PET) to measure activation during tactile discrimination tasks in normal subjects and in Braille readers blinded in early life. Blind subjects showed activation of primary and secondary visual cortical areas during tactile tasks, whereas normal controls showed deactivation. A simple tactile stimulus that did not require discrimination produced no activation of visual areas in either group. Thus in blind subjects, cortical areas normally reserved for vision may be activated by other sensory modalities. |
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| Szwed, M., Bagdasarian, K., Ahissar, E. Active encoding of vibrissal touch Neuron 2003 (40):621-630 [htm] |
| Mammals acquire much of their sensory information by actively moving their sensory organs. Yet, the principles of encoding by active sensing are not known. Here we investigated the encoding principles of active touch by rat whiskers (vibrissae). We induced artificial whisking in anesthetized rats and recorded from first-order neurons in the trigeminal ganglion. During active touch, first-order trigeminal neurons presented a rich repertoire of responses, which could not be inferred from their responses to passive deflection stimuli. Individual neurons encoded four specific events: whisking, contact with object, pressure against object, and detachment from object. Whisking-responsive neurons fired at specific deflection angles, reporting the actual whiskers' position with high precision. Touch-responsive neurons encoded the horizontal coordinate of objects' position by spike timing. These findings suggest two specific encoding-decoding schemes for horizontal object position in the vibrissal system. |
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| Sommese, A.J., Verschelde, J., Wampler, C.W. Advance in Polynomial Contination for Solving Problems in Kinematics 2002 [pdf] |
| For many mechanical systems, including nearly all robotic manipulators, the set of possible configurations that the links may assume can be described by a system of polynomial equations. Thus, solving such systems is central to many problems in analyzing the motion of a mechanism or in designing a mechanism to achieve a desired motion. This paper describes techniques, based on polynomial continuation, for numerically solving such systems. Whereas in the past, these techniques were focused on finding isolated roots, we now address the treatment of systems having higher-dimensional solution sets. Special attention is given to cases of exceptional mechanisms, which have a higher degree of freedom of motion than predicted by their mobility. In fact, such mechanisms often have several disjoint assembly modes, and the degree of freedom of motion is not necessarily the same in each mode. Our algorithms identify all such assembly modes, determine their dimension and degree, and give sample points on each. |
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| Lee, D.D., Seung, H.S. Algorithm for Non-negative Matrix Factorization 2001 (13) [pdf] |
| Non-negative matrix factorization (NMF) has previously been shown to be a useful decomposition for multivariate data. Two different multiplicative algorithms for NMF are analyzed. They differ only slightly in the multiplicative factor used in the update rules. One algorithm can be shown to minimize the conventional least squares error while the other minimizes the generalized Kullback-Leibler divergence. The monotonic convergence of both algorithms can be proven using an auxiliary function analogous to that used for proving convergence of the Expectation- Maximization algorithm. The algorithms can also be interpreted as diagonally rescaled gradient descent, where the rescaling factor is optimally chosen to ensure convergence. |
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| Meijer, P.B.L. An Experimental System for Auditory Image Representations IEEE Transactions on Biomedical Engineering 1992 (39)2:112-121 [html] |
| This paper presents an experimental system for the conversion of images into sound patterns. The system was designed to provide auditory image representations within some of the known limitations of the human hearing system, possibly as a step towards the development of a vision substitution device for the blind. The application of an invertible (1-to-1) image-to-sound mapping ensures the preservation of visual information. The system implementation involves a pipelined special purpose computer connected to a standard television camera. The time-multiplexed sound representations, resulting from a real-time image-to-sound conversion, represent images up to a resolution of 64 × 64 pixels with 16 grey-tones per pixel. A novel design and the use of standard components have made for a low-cost portable prototype conversion system having a power dissipation suitable for battery operation. Computerized sampling of the system output and subsequent calculation of the approximate inverse (sound-to-image) mapping provided the first convincing experimental evidence for the preservation of visual information in the sound representations of complicated images. However, the actual resolution obtainable with human perception of these sound representations remains to be evaluated. |
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| Woods, D., Naughton, T.J. An optical model of computation Theoretical Computer Science 2004 (x)x:xx [pdf] |
| We prove computability and complexity results for an original model of computation called the continuous space machine. Our model is inspired by the theory of Fourier optics. We prove our model can simulate analog recurrent neural networks, thus establishing a lower bound on its computational power. We also define a $\Theta(\log_{2}n)$ unordered search algorithm with our model. Keywords: continuous space machine, unconventional model of computation, analog computation, optical computing, computability, computational complexity, analog recurrent neural network, Fourier transform, binary search, unordered search |
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| Gribble, P.L., Ostry, D.J., Sanguineti, V., Laboissiere, R. Are complex control signals required for human arm movement? Journal of Neurophysiology 1998 (79):1409-1424 [pdf] |
| It has been proposed that the control signals underlying voluntary human arm movement have a "complex" nonmonotonic time-varying form, and a number of empirical findings have been offered in support of this idea. In this paper, we address three such findings using a model of two-joint arm motion based on the lambda version of the equilibrium-point hypothesis. The model includes six one- and two-joint muscles, reflexes, modeled control signals, muscle properties, and limb dynamics. First, we address the claim that "complex" equilibrium trajectories are required to account for nonmonotonic joint impedance patterns observed during multijoint movement. Using constant-rate shifts in the neurally specified equilibrium of the limb and constant cocontraction commands, we obtain patterns of predicted joint stiffness during simulated multijoint movements that match the nonmonotonic patterns reported empirically. We then use the algorithm proposed by Gomi and Kawato to compute a hypothetical equilibrium trajectory from simulated stiffness, viscosity, and limb kinematics. Like that reported by Gomi and Kawato, the resulting trajectory was nonmonotonic, first leading then lagging the position of the limb. Second, we address the claim that high levels of stiffness are required to generate rapid single-joint movements when simple equilibrium shifts are used. We compare empirical measurements of stiffness during rapid single-joint movements with the predicted stiffness of movements generated using constant-rate equilibrium shifts and constant cocontraction commands. Single-joint movements are simulated at a number of speeds, and the procedure used by Bennett to estimate stiffness is followed. We show that when the magnitude of the cocontraction command is scaled in proportion to movement speed, simulated joint stiffness varies with movement speed in a manner comparable with that reported by Bennett. Third, we address the related claim that nonmonotonic equilibrium shifts are required to generate rapid single-joint movements. Using constant-rate equilibrium shifts and constant cocontraction commands, rapid single-joint movements are simulated in the presence of external torques. We use the procedure reported by Latash and Gottlieb to compute hypothetical equilibrium trajectories from simulated torque and angle measurements during movement. As in Latash and Gottlieb, a nonmonotonic function is obtained even though the control signals used in the simulations are constant-rate changes in the equilibrium position of the limb. Differences between the "simple" equilibrium trajectory proposed in the present paper and those that are derived from the procedures used by Gomi and Kawato and Latash and Gottlieb arise from their use of simplified models of force generation. |
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| Noë, A., Thompson, E. Are there neural correlates of consciousness? Journal of Consciousness Studies 2004 (11)1:3-28 [pdf] |
| In the past decade, the notion of a neural correlate of consciousness (or NCC) has become a focal point for scientific research on consciousness (Metzinger, 2000a). A growing number of investigators believe that the first step toward a science of consciousness is to discover the neural correlates of consciousness. Indeed, Francis Crick has gone so far as to proclaim that \u2018we need to discover the neural correlates of consciousness. For this task the primate visual system seems especially attractive. No longer need one spend time attempting to endure the tedium of philosophers perpetually disagreeing with each other. Consciousness is now largely a scientific problem\u2019 (Crick, 1996, p. 486). Yet the question of what it means to be a neural correlate of consciousness is actually far from straightforward, for it involves fundamental empirical, methodological, and philosophical issues about the nature of consciousness and its relationship to the brain. Even if one assumes, as we do, that states of consciousness causally depend on states of the brain, one can nevertheless wonder in what sense there is, or could be, such a thing as a neural correlate of consciousness. |
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| Meinecke, F., Ziehe, A., Kawanabe, M., Müller, K.-R. Assessing Reliability of ICA Projections - a Resampling Approach 2001 [pdf] |
| When applying unsupervised learning techniques like ICA or temporal decorrelation for BSS, a key question is whether the discovered projections are reliable. In other words: can we give error bars or can we assess the quality of our separation? We use resampling methods to tackle these questions and show experimentally that our proposed variance estimations are strongly correlated to the separation error. We demonstrate that this reliability estimation can be used to choose an appropriate ICA-model, to enhance signifi- cantly the separation performance, and, most important, to mark the components that can really have a physical meaning. An application to data from an MEG1-experiment underlines the usefulness of our approach. |
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| Matarié, A.F.M.J., Jenkins, O.C. Automated Derivation of Primitives for Movement Classification International Conference on Humanoid Robotics 2000 : [pdf] |
| We present a new method for representing human movement compactly, in terms of a linear superimposition of simpler movements termed primitives. This method is a part of a larger research project aimed at modeling motor control and imitation using the notion of perceptuo-motor primitives, a basis set of coupled perceptual and motor routines. In our model, the perceptual system is biased by the set of motor behaviors the agent can execute, so it automatically classifies observed movements into its executable repertoire. In this paper, we describe a method for automatically deriving a set of primitives directly from human movement data. We used data from a psychophysical experiment on human imitation to derive a set of primitives, and then used those primitives as a basis for superposition and sequencing to reconstruct the original movements. We performed principal component analysis on segments from these data, resulting in a set of basis vectors. Next we clustered in the space of projections of segments onto the eigenvectors, to obtain a set of frequently used movements. To validate the approach experimentally, we used the movement obtained by expanding the cluster points in terms of the eigenvectors as a sequence of via points to control a humanoid dynamic simulation. We also developed an error metric to measure the effectiveness of the process. |
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| Minka, T.P. Automatic Choice of Dimensionality for PCA Advances in Neural Information Processing Systems 2000 :598-604 [pdf] |
| A central issue in principal component analysis PCA is choosing the number of principal components to be retained By interpreting PCA as density estimation this paper shows how to use Bayesian model selection to determine the true dimensionality of the data The resulting estimate is simple to compute yet guaranteed to pick the correct dimensionality given enough data The estimate involves an integral over the Steifel manifold of k frames which is di cult to compute exactly But after choosing an appropriate parameterization and applying Laplace s method an accurate and practical estimator is obtained In simulations it is more accurate than cross validation and other proposed algorithms plus it runs much faster |
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| Graham, G. Behaviorism 2002 [html] |
| It has sometimes been said that "behave is what organisms do." Behaviorism is built on this assumption, and its goal is to promote the scientific study of behavior. In this entry I consider different types of behaviorism. I outline reasons for and against being a behaviorist. I consider contributions of behaviorism to the study of behavior. Special attention is given to the so-called "radical behaviorism" of B. F. Skinner (1904-90). |
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| Koenderink, J.J., van Doorn, A.J., Kristin, D.J., Nayar, S. Bidirectional reflection distribution function of thoroughly pitted surfaces International Journal of Computer Vision 1999 (31)2/3:129-144 [pdf] |
| We derive the BRDF (Bidirectional Reflection Distribution Function) at the mega scale of opaque surfaces that are rough on the macro and micro scale. The roughness at the micro scale is modeled as a uniform, isotropically scattering, Lambertian surface. At the macro scale the roughness is modeled by way of a distribution of spherical concavities. These pits influence the BRDF via vignetting, cast shadow, interreflection and interposition, causing it to differ markedly from Lambertian. Pitted surfaces show strong backward scattering (so called opposition effect ). When we assume that the macro scale can be resolved, the radiance histogram and the spatial structure of the textons of the textured surface (at the mega scale) can be calculated. This is the main advantage of the model over previous ones: One can do exact (numerical) calculations for a surface geometry that is physically realizable. |
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| Churchland, P. Can Neurobiology teach us anything about consciousness ? Proceedings and Adresses of the American Philosophical Association 1994 [html] |
| Human nervous systems display an impressive roster of complex capacities, including the following: perceiving, learning and remembering, planning, deciding, performing actions, as well as the capacities to be awake, fall asleep, dream, pay attention, and be aware. Although neuroscience has advanced spectacularly in this century, we still do not understand in satisfying detail how any capacity in the list emerges from networks of neurons.[1] We do not completely understand how humans can be conscious, but neither do we understand how they can walk, run, climb trees or pole vault. Nor, when one stands back from it all, is awareness intrinsically more mysterious than motor control. Balanced against the disappointment that full understanding eludes us still, is cautious optimism, based chiefly on the nature of the progress behind us. For cognitive neuroscience has already passed well beyond what skeptical philosophers once considered possible, and continuing progress seems likely. |
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| Watts, D.J., Strogatz, S.H. Collective dynamics of "small-world" networks Nature 1998 (393):440-442 [pdf] |
| Networks of coupled dynamical systems have been used to model biological oscillators, Josephson junction arrays, excitable media, neural networks, spatial games, genetic control networks and many other self-organizing systems. Ordinarily, the connection topology is assumed to be either completely regular or completely random. But many biological, technological and social networks lie somewhere between these two extremes. Here we explore simple models of networks that can be tuned through this middle ground: regular networks 'rewired' to introduce increasing amounts of disorder. We find that these systems can be highly clustered, like regular lattices, yet have small characteristic path lengths, like random graphs. We call them 'small-world' networks, by analogy with the small-world phenomenon (popularly known as six degrees of separation. The neural network of the worm Caenorhabditis elegans, the power grid of the western United States, and the collaboration graph of film actors are shown to be small-world networks. Models of dynamical systems with small-world coupling display enhanced signal-propagation speed, computational power, and synchronizability. In particular, infectious diseases spread more easily in small-world networks than in regular lattices. |
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| Myin, E. Color and the duplication assumption Synthèse 2001 [html] |
| Susan Hurley has attacked the `Duplication Assumption', the assumption that creatures with exactly the same internal states could function exactly alike in environments that are systematically distorted. She argues that the dynamical interdependence of action and perception is highly problematic for the Duplication Assumption when it involves spatial states and capacities, whereas no such problems arise when it involves color states and capacities. I will try to establish that the Duplication Assumption makes even less sense for lightness than for some of the spatial cases. This is due not only to motor factors, but to the basic physical asymmetry between black and white. I then argue that the case can be extended from lightness perception to hue perception. Overall, the aims of this paper are: (1) to extend Susan Hurley's critique of the Duplication Assumption; (2) to argue against highly constrained versions of Inverted Spectrum arguments; (3) to propose a broader conception of the vehicle for color perception. |
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| Brown, R.O., MacLeod, D.I. Color appearance depends on the variance of surround colors Current Biology 1997 (7)11:844-9 [html] |
| BACKGROUND: The perceived color at each point in a visual scene depends on the relationship between light signals from that point, and light signals from surrounding areas of the scene. In the well known phenomenon of simultaneous color contrast, changing the overall brightness or hue of an object's surround induces a complementary shift in the perceived brightness or hue of the object's color. Color contrast is thought to contribute to color constancy with changes in illumination. RESULTS: We report a new type of simultaneous color contrast, in which changing only the variance (i.e. contrasts and saturations), but not the mean, of colors in a test spot's surround induces a complementary shift in the perceived contrast and saturation of the test spot's color. Objects appear much more vivid and richly colored against low-contrast, gray surrounds than against high-contrast, multicolored surrounds. CONCLUSIONS: Color appearance depends not just on the mean color of the surround, but also on the distribution of surround colors about the mean. This novel form of simultaneous color contrast is inconsistent with a variety of models of color appearance, including those based on sensitivity regulation at the receptor level, and those in which the effects of complex surrounds on color appearance can be reduced to adaptation to the illuminant or induction from a homogeneous 'equivalent surround'. It tends to normalize the gamut of perceived colors in each visual scene and may also contribute to color constancy under viewing conditions that affect contrast. |
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| Buchsbaum, G., Bloch, O. Color categories revealed by non-negative matrix factorization of Munsell color spectra Vision Research 2002 (42):559-563 [html] |
| Non-negative matrix factorization (NMF, Nature 401 (1999) ) is a method to derive non-negative basis functions for sets of data that are inherently non-negative, such as color spectra. We applied NMF to Munsell color spectra and investigated the color names associated with the non-negative basis functions. NMF yields basis functions compatible with established color naming categories. |
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| Bonnardel, V., Varela, F.J. Color vision in the comb frequency domain Biol. Res. 2003 (36)1 [html] |
| In 1982, Horace Barlow considered the question of human trichromacy in the context of information theory: according to the Sampling Theorem, three types of receptors covering the visible spectrum (400- -700 nm) might be sufficient to reconstruct the color signal. Although Barlow was led to reject the direct application of the Sampling Theorem to explain color dimensionality, the theoretical framework offers a fresh point of view for analyzing the color system in conjunction with the physical characteristics of natural color signals. This review aims to illustrate that if the strict mathematical reconstruction (as implied by the Sampling Theorem) is replaced by a pragmatic approximation of color signals, then trichromacy, with its subsequent opponent-color process, could be regarded as an optimization of color constancy abilities in the spectral environment of primates. Higher dimension systems (tetrachromacy) found in other species can also serve the purpose of color constancy optimization in environments where color signals exhibit a finer spectral structure. |
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| Kulikowski, J., Vaitkevicius, H. Colour constancy as a function of hue Acta Psychologica 1997 :25-35 [html] |
| The effects of colour categories on colour constancy were studied under two illuminants and two neutral grey backgrounds using Munsell chips and colour matching. It was found that the categorical colours: red, yellow, green and blue, which are processed by basic colour-opponent mechanisms, show relatively better colour constancy than intermediate colours. The dominant wavelength of these categorical colours are closely related to the typical hues obtained in experiments with narrow-band spectral hues. |
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| Riesenhuber, M., Poggio, T. Computational Models of Object Recognition in Cortex: A Review 2000 [pdf] |
| Review des principales tendances dans les théories de reconnaissance d'objet: d'une part les théories centrées sur l'extraction de données structurées indépendantes du point de vue, d'autre part les théories centrées sur l'utilisation en parallèle d'une collection de vues de différents points de vue, dans une approche soit feedforward soit top-down (avec hypothèses, tests, etc). Les études biologiques semblent indiquer que le CNS humain fonctionne avec une collection de vues et en feedforward. |
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| Lee, T.S. Computations in the early visual cortex Journal of Physiology 2003 [pdf] |
| This paper reviews some of the recent neurophysiological studies that explore the variety of visual computations in the early visual cortex in relation to geometric inference, i.e. the inference of contours, surfaces and shapes. It attempts to draw connections between ideas from computational vision and ndings from awake primate electrophysiology. In the classical feed-forward, modular view of visual processing, the early visual areas (LGN, V1 and V2) are modules that serve to extract local features, while higher extrastriate areas are responsible for shape inference and invariant object recognition. However, recent ndings in primate early visual systems reveal that the computations in the early visual cortex are rather complex and dynamic, as well as interactive and plastic, subject to in uence from global context, higher order perceptual inference, taskrequirement and behavioral experience. The evidence argues that the early visual cortex does not merely participate in the rst stage of visual processing, but is involved in many levels of visual computation. |
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| Wieber, P.-B. Constrained dynamics and parametrized control in biped walking 2000 [pdf] |
| The intermittent contact with the ground is the main speci- ficity of walking robots, allowing more versatility in their displacements, but resulting in a structural instability of these systems. A walking robot, having a free-floating base, cannot control its global movements directly and must rely on the limited interaction forces in order to move. These constraints on the movements of the robot are an obstacle to the stabilization of a trajectory. We propose then to stabilize not a single trajectory but a parametrized set qd(t, p) of all the possible trajectories, depending on the lentgth of the steps, the speed of execution... The idea is that a destabilization can be compensated by an adaptation of the walk. |
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| Mezic, i. Controllability of Hamiltonian Systems with Drift: Action-Angle variables and Ergodic Partition 2003 (3):2585- 2592 |
| Control of Hamiltonian systems with drift is investigated for the case when the drift is integrable. Transformation of the system to action-angle coordinates is used to describe the ergodic partition of the drift. This is in turn used to obtain conditions for controllability of such systems. The key idea is that control must be capable of moving the system transverse to any set in the ergodic partition of the drift Hamiltonian vector field. Using this, additional results on controllability of more general systems are obtained. |
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| Verbeek, J.J., Vlassis, N., Kr\"ose, B. Coordinating Mixtures of Probabilistic Principal Component Analyzers 2002 [html] |
| Mixtures of Probabilistic Principal Component Analyzers can be used to model data that lies on or near a low dimensional manifold in a high dimensional observation space, in effect tiling the manifold with local linear (Gaussian) patches. In order to exploit the low dimensional structure of the data manifold, the patches need to be localized and oriented in a low dimensional space, so that `local' coordinates on the patches can be mapped to `global' low dimensional coordinates. As shown by Roweis et al, this problem can be expressed as a penalized likelihood optimization problem. We show that a restricted form of the Mixtures of Probabilistic Principal Component Analyzers model allows for an efficient EM-style algorithm. The Procrustes Rotation, a technique to match point configurations, turns out to give the optimal orientation of the patches in the global space. We also show how we can initialize the mappings from the patches to the global coordinates by learning a non-penalized density model first. Some experimental results are provided to illustrate the method. |
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| Chen, T. Critical manifolds and stability in hamiltonian systems with non-holonomic constraints arXiv 2003 [html] |
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| Wallace, M.T., Stein, B. Cross-modal synthesis in the mid-brain depends on input from association cortex Journal of Neurophysiology 1994 (71):429-432 [429] |
| The synthesis of information from different sensory modalities in the superior colliculus is an important precursor of attentive and orientation behavior. 2. This integration of multisensory information is critically dependent on inputs from a small area of association cortex, the anterior ectosylvian sulcus. Removal of these corticotectal influences can have a remarkably specific effect: it can eliminate multisensory integration in superior colliculus neurons while leaving their responses to unimodal cues intact. 3. Apparently, some of the associative functions of cortex are accomplished via its target neurons in the midbrain. |
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| Barlow, H.B. Current Problems in Animal Behaviour 1960 :331-360 |
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| Damasio, A.R. Descartes' Error: Emotion, Reason and the Human Brain 1994 |
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| Olver, J.P., Sapiro, G., Tannenbaum, A. Differential Invariant Signatures and Flows in Computer Vision: A Symmetry Group Approach 1993 [html] |
| Computer vision deals with image understanding at various levels. At the low level, it addresses issues such us planar shape recognition and analysis. Some classical results on differential invariants associated to planar curves are relevant to planar object recognition under different views and partial occlusion, and recent results concerning the evolution of planar shapes under curvature controlled diffusion have found applications in geometric shape decomposition, smoothing, and analysis, as well as in other image processing applications. In this work we first give a modern approach to the theory of differential invariants, describing concepts like Lie theory, jets, and prolongations. Based on this and the theory of symmetry groups, we present a high level way of defining invariant geometric flows for a given Lie group. We then analyze in detail different subgroups of the projective group, which are of special interest for computer vision. We classify the corresponding invariant flows and show that the geometric heat flow is the simplest possible one. This uniqueness result, together with previously reported results which we review in this paper, confirms the importance of this class of flows. |
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| Wei, M., DeAngelis, G., Angelaki, D. Do visual cues contribute to the neural estimate of viewing distance used by the oculomotor system ? The Journal of Neuroscience 2003 (23)23:8340-8350 [pdf] |
| Perceived shape and depth judgments that require knowledge of viewing distance are strongly influenced by both vergence angle and the pattern of vertical disparities across large visual fields. On the basis of this established contribution of visual cues to the neural estimate of viewing distance,wehypothesized that the oculomotor system would alsomakeuse of high-level visual cues to distance. To address this hypothesis, we investigated how compensatory eye movements during whole-body translation scale with viewing distance. Monkeys viewed large-field (85 68°) random-dot stereograms that were rear projected onto a fixed screen and simulated either a textured wall or pyramid at different viewing distances. In these stereograms, we independently manipulated vergence angle, horizontal and vertical disparity gradients, relative horizontal disparities, and textural cues to viewing distance. For comparison, random-dot patterns were also projected onto a moveable screen placed at different physical distances from the animal. Several cycles of left right sinusoidal motion of the monkey at 5Hz were interleaved with several cycles of motion in darkness, and the relationship between eye movement responses and viewing distance was quantified. As expected from previous work, the amplitude of compensatory eye movements depended strongly on vergence angle. Although visual cues to distance had a statistically significant effect on eye movements, these effects were 20-fold weaker than the effect of vergence angle.Weconclude that sensory and motor systems do not share a common neural estimate of viewing distance and that the oculomotor system relies far less on visual cues than the perceptual system. |
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| Foster, D.H. Does colour constancy exist? Trends in Cognitive Sciences 2003 (7)10:439-443 [html] |
| For a stable visual world, the colours of objects should appear the same under different lights. This property of colour constancy has been assumed to be fundamental to vision, and many experimental attempts have been made to quantify it. I contend here, however, that the usual methods of measurement are either too coarse or concentrate not on colour constancy itself, but on other, complementary aspects of scene perception. Whether colour constancy exists other than in nominal terms remains unclear. |
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| Delahunt, P.B., Brainard, D.H. Does human color constancy incorporate the statistical regularity of natural daylight? Journal of Vision 2004 (4):57-81 [pdf] |
| The chromaticities of natural daylights cluster around the blackbody locus. We investigated whether the mechanisms that mediate human color constancy embody this statistical regularity of the natural environment, so that constancy is best when the illuminant change is one likely to occur. Observers viewed scenes displayed on a CRT-based stereoscope and adjusted a test patch embedded in the scene until it appeared achromatic. Scenes were rendered using physics-based graphics software (RADIANCE) coupled with custom extensions that ensured colorimetric accuracy. Across conditions, both the simulated illuminant and the simulated reflectance of scene objects were varied. Achromatic settings from paired conditions were used to compute a constancy index (CI) that characterizes the stability of object appearance across the two illuminants of the pair. Constancy indices were measured for four illuminant changes from a Neutral illuminant (CIE D65). Two of these changes (Blue and Yellow) were consistent with the statistics of daylight, whereas two (Green and Red) were not. The results indicate that constancy was least across the Red change, as one would expect for the statistics of natural daylight. Constancy for the Green direction, however, exceeded that for the Yellow illuminant change and was comparable to that for the Blue. This result is difficult to reconcile with the hypothesis that mechanisms of human constancy incorporate the statistics of daylights. Some possible reasons for the discrepancy are discussed. |
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| Richardson, D. Embodied Cognition 2000 [pdf] |
| What's the historical background to the current movement for an 'embodied' view of the mind? What is the difference between an embodied and a situated view of cognition? How does this relate to research in the fields of active vision, artificial intelligence, stimulus response compatibility and dynamical systems theory? What are the merits of the embodied view of mind, and what are its wider implications for cognitive science? |
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| Lappalainen, H., Miskin, J.W. Ensemble Learning 2000 [html] |
| Ensemble learning is a generalization of the Bayesian method in wich the prior is approximated, instead of considering the MAP alone in a parameterized model. |
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| Cardoso, J.-F. Equivariant adaptive source separation IEEE Trans. on S.P. 1996 (44)45:3017-3030 [html] |
| Source separation consists in recovering a set of independent signals when only mixtures with unknown coefficients are observed. This paper introduces a class of adaptive algorithms for source separation which implements an adaptive version of equivariant estimation and is henceforth called EASI (Equivariant Adaptive Separation via Independence) . The EASI algorithms are based on the idea of serial updating: this specific form of matrix updates systematically yields algorithms with a simple, parallelizable structure, for both real and complex mixtures. Most importantly, the performance of an EASI algorithm does not depend on the mixing matrix. In particular, convergence rates, stability conditions and interference rejection levels depend only on the (normalized) distributions of the source signals. Close form expressions of these quantities are given via an asymptotic performance analysis. This is completed by some numerical experiments illustrating the effectiveness of the proposed apprach. |
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| Robles-De-La-Torre, G., Hayward, V. Force can overcome object geometry in the perception of shape through active touch Nature 2000 (412):445-448 [pdf] |
| Haptic (touch) perception normally entails an active exploration of object surfaces over time. This is called active touch. When exploring the shape of an object, we experience both geometrical4 and force cues. For example, when sliding a Ænger across a surface with a rigid bump on it, the Ænger moves over the bump while being opposed by a force whose direction and magnitude are related to the slope of the bump5. The steeper the bump, the stronger the resistance. Geometrical and force cues are correlated, but it has been commonly assumed that shape perception relies on object geometry alone. Here we show that regardless of surface geometry, subjects identiÆed and located shape features on the basis of force cues or their correlates. Using paradoxical stimuli, for example combining the force cues of a bump with the geometry of a hole, we found that subjects perceived a bump. Conversely, when combining the force cues of a hole with the geometry of a bump, subjects typically perceived a hole. |
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| Bach-y-Rita, P., Kaczmarek, K., Tyler, M., Garcia-Lara, J. Form perception with a 49-point electrotactile stimulus array on the tongue Journal of Rehabilitation Research Development 1998 (35):427-430 [htm] |
| Form perception with the tongue was studied with a 49-point electrotactile array. Five sighted adult human subjects (3M/2F) each received 4 blocks of 12 tactile patterns, approximations of circles, squares, and vertex-up equilateral triangles, sized to 4×4, 5×5, 6×6, and 7×7 electrode arrays. Perception with electrical stimulation of the tongue is better than with fingertip electrotactile stimulation, and the tongue requires 3\% (5-15 V) of the voltage. The mean current for tongue subjects was 1.612 mA. Tongue shape recognition performance across all sizes was 79.8\%. The approximate dimensions of the electrotactile array and the dimensions of compartments built into dental retainers have been determined. The goal is to develop a practical, cosmetically acceptable, wireless system for blind persons, with a miniature TV camera, microelectronics, and FM transmitter built into a pair of glasses, and the electrotactile array in a dental orthodontic retainer. |
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| Kelly, S.D., Murray, R.M. Geometric phases and robotic locomotion 1994 [html] |
| Robotic locomotion is based in a variety of instances upon cyclic changes in the shape of a robot mechanism. Certain variations in shape exploit the constrained nature of a robot's interaction with its environment to generate net motion. This is true for legged robots, snakelike robots, and wheeled mobile robots undertaking maneuvers such as parallel parking. In this paper we explore the use of tools from differential geometry to model and analyze this class of locomotion mechanisms in a unified way. In particular, we describe locomotion in terms of the geometric phase associated with a connection on a principal bundle, and address issues such as controllability and choice of gait. We also provide an introduction to the basic mathematical concepts which we require and apply the theory to numerous example systems. |
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| Ben-Shahar, O., Zucker, S.W. Geometrical Computations Explain Projection Patterns of Long Range Horizontal Connections in Visual Cortex Neural Computation 2004 (16)3:445-476, [pdf] |
| Neurons in primary visual cortex respond selectively to oriented stimuli such as edges and lines. The long-range horizontal connections between them are thought to facilitate contour integration. While many physiological and psychophysical findings suggest that collinear or association field models of good continuation dictate particular projection patterns of horizontal connections to guide this integration process, significant evidence of interactions inconsistent with these hypotheses is accumulating. We first show that natural random variations around the collinear and association field models cannot account for these inconsistencies, a fact that motivates the search for more principled explanations.We then develop a model of long-range projection fields that formalizes good continuation based on differential geometry. The analysis implicates curvature(s) in a fundamental way, and the resulting model explains both consistent data and apparent outliers. It quantitatively predicts the (typically ignored) spread in projection distribution, its nonmonotonic variance, and the differences found among individual neurons. Surprisingly, and for the first time, this model also indicates that texture (and shading) continuation can serve as alternative and complementary functional explanations to contour integration. Because current anatomical data support both (curve and texture) integration models equally and because both are important computationally, new testable predictions are derived to allow their differentiation and identification. |
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| Tarel, J.-P. Global 3D Planar Reconstruction with Uncalibrated Cameras and Rectified Stereo Geometry Machine Graphics \& Vision 1997 (6)4:393-418 [html] |
| L'idée de la reconstruction 3D par patches, et de prendre des régions planaires comme éléments de base à associer sur les images stéréo, au lieu d'associer des points. La transformation des régions est connue, est donc en intégrant les équations sur les surfaces on obtient des estimations plus robustes. |
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| Halligan, P., Oakley, D. Greatest myth of all New Scientist 2000 (168)2265:35-39 [pdf] |
| What do you mean when you talk about "yourself"? Leading neuroscientists Peter Halligan and David Oakley are rewriting the rules on consciousness. |
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| Lutz, A., Lachaux, J.-P. , Martinerie, J., Varela, F.J. Guiding the study of brain dynamics by using first-person data: Synchrony patterns correlate with ongoing conscious states during a simple visual task Proceedings of the National Academy of Sciences USA 2002 (99)3:1586-1591 [pdf] |
| Even during well-calibrated cognitive tasks, successive brain responses to repeated identical stimulations are highly variable. The source of this variability is believed to reside mainly in fluctuations of the subject's cognitive "context" defined by his/her attentive state, spontaneous thought process, strategy to carry out the task, and so on ... As these factors are hard to manipulate precisely, they are usually not controlled, and the variability is discarded by averaging techniques. We combined first-person data and the analysis of neural processes to reduce such noise. We presented the subjects with a three-dimensional illusion and recorded their electrical brain activity and their own report about their cognitive context. Trials were clustered according to these first-person data, and separate dynamical analyses were conducted for each cluster. We found that (i) characteristic patterns of endogenous synchrony appeared in frontal electrodes before stimulation. These patterns depended on the degree of preparation and the immediacy of perception as verbally reported. (ii) These patterns were stable for several recordings. (iii) Preparatory states modulate both the behavioral performance and the evoked and induced synchronous patterns that follow. (iv) These results indicated that first-person data can be used to detect and interpret neural processes. |
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| Lie, W., Todorov, E., Pan, X. Hierarchical Optimal Control of Redundant biomechanical systems 2004 |
| Sensorimotor control occurs simultaneously onmultiple levels. We present a general approach to designing feedback control hierarchies for redundant biomechanical systems, that approximate the (non-hierarchical) optimal control law but havemuch lower computational demands. The approach is applied to the task of reaching, using a detailed model of the human arm. Our hierarchy has two levels of feedback control. The high level is designed as an optimal feedback controller operating on a simplified virtual plant. The low level is responsible for transforming the dynamics of the true plant into the desired virtual dynamics. The new method may be useful not only for modelling the neural control ofmovement, but also for designing Functional Electric Stimulation systems that have to achieve task goals by activating muscles in real time. |
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| Maloney, L.T. Illuminant cues in surface color perception: tests of three candidate cues Vision Research 2001 (41):2581-2600 [pdf] |
| Many recent computational models of surface color perception presuppose information about illumination in scenes. The models differ primarily in the physical process each makes use of as a cue to the illuminant. We evaluated whether the human visual system makes use of any of three of the following candidate illuminant cues: (1) specular highlight, (2) full surface specularity [Lee, H. C. (1986). Method for computing the scene-illuminant chromaticity from specular highlights. Journal of the Optical Society of America A, 3(10), 1694 1699; D Zmura, M., & Lennie, P. (1986). Mechanisms of color constancy. Journal of the Optical Society of America A, 3(10), 1662 1672], and (3) uniform background. Observers viewed simulated scenes binocularly in a computer-controlled Wheatstone stereoscope. All simulated scenes contained a uniform background plane perpendicular to the observer s line of sight and a small number of specular, colored spheres resting on the uniform background. Scenes were rendered under either standard illuminant D65 or standard illuminant A. Observers adjusted the color of a small, simulated test patch to appear achromatic. In a series of experiments we perturbed the illuminant color signaled by each candidate cue and looked for an influence of the changed cue on achromatic settings. We found that the specular highlight cue had a significant influence, but that the influence was asymmetric: greater when the base illuminant, CIE standard Illuminant A, was perturbed in the direction of Illuminant D65 than vice versa. Neither the full surface specularity cue nor the background cue had any observable influence. The lack of influence of the background cue is likely due to the placement of the test patch in front of the background rather than, as is typical, embedded in the background |
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| Maloney, L.T. Illuminant estimation as cue combination Journal of Vision 2002 (2):493-504 [pdf] |
| This work briefly describes a model for illuminant estimation based on combination of candidate illuminant cues. Many of the research issues concerning cue combination in depth and shape perception translate well to the study of surface color perception. I describe and illustrate a particular experimental approach (perturbation analysis) employed in the study of depth and shape that is useful in determining whether hypothetical illuminant cues are actually used in color vision. |
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| Srinivasan, R., Russell, D.P., Edelman, G.M., Tononi, G. Increased Synchronization of Neuromagnetic Responses during Conscious Perception The Journal of Neuroscience 1999 (19)13:5435-5448 [html] |
| In binocular rivalry, the observer views two incongruent images, one through each eye, but is conscious of only one image at a time. The image that is perceptually dominant alternates every few seconds. We used this phenomenon to investigate neural correlates of conscious perception. We presented a red vertical grating to one eye and a blue horizontal grating to the other eye, with each grating continuously flickering at a distinct frequency (the frequency tag for that stimulus). Steady-state magnetic fields were recorded with a 148 sensor whole-head magnetometer while the subjects reported which grating was perceived. The power of the steady-state magnetic field at the frequency associated with a grating typically increased at multiple sensors when the grating was perceived. Changes in power related to perceptual dominance, presumably reflecting local neural synchronization, reached statistical significance at several sensors, including some positioned over occipital, temporal, and frontal cortices. To identify changes in synchronization between distinct brain areas that were related to perceptual dominance, we analyzed coherence between pairs of widely separated sensors. The results showed that when the stimulus was perceived there was a marked increase in both interhemispheric and intrahemispheric coherence at the stimulus frequency. This study demonstrates a direct correlation between the conscious perception of a visual stimulus and the synchronous activity of large populations of neocortical neurons as reflected by stimulus-evoked steady-state neuromagnetic fields. |
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| Hyvärinen, A. Independent Component Analysis in the Presence of Gaussian Noise by Maximizing Joint Likelihood Neurocomputing 1998 (22):49-67 |
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| MacLeod, D.I. Influence of Scene Statistics on Colour Constancy Nature 2002 (415):637--640 [pdf] |
| The light reflected from an object depends not only on the surface properties of this object but also on the illuminant. The same is true for the excitations of the photoreceptors, which serve as the basis for the perceived colour. However, our visual system has the ability to perceive constant surface colours despite changes in illumination1. The average chromaticity of the retinal image of a scene depends on the illumination, and thus might be used by the visual system to estimate the illumination and to modulate the correction that subserves colour constancy. But this measure is not sufÆcient: a reddish scene under white light can produce the same mean stimulation as a neutral scene in red light. Higher order scene statistics for example, the correlation between redness and luminance within the image allow these cases to be distinguished. Here we report that the human visual system does exploit such a statistic when estimating the illuminant, and gives it a weight that is statistically appropriate for the natural environment. |
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| Luciani, A. Interaction as exchanged actions and their role in visual and auditory feedbacks 2004 [14] |
| Critique de la notion de force directionnelle, et en fait de causalité directionnelle, au profit de l'interaction. Cela implique la critique des notions de capteurs et actuateurs purs. |
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| Kawato, M. Internal models for motor control and trajectory planning Current Opinion in Neurobiology 1999 (9):718-727 [html] |
| A number of internal model concepts are now widespread in neuroscience and cognitive science. These concepts are supported by behavioral, neurophysiological, and imaging data; furthermore, these models have had their structures and functions revealed by such data. In particular, a specific theory on inverse dynamics model learning is directly supported by unit recordings from cerebellar Purkinje cells. Multiple paired forward inverse models describing how diverse objects and environments can be controlled and learned separately have recently been proposed. The 'minimum variance model' is another major recent advance in the computational theory of motor control. This model integrates two furiously disputed approaches on trajectory planning, strongly suggesting that both kinematic and dynamic internal models are utilized in movement planning and control. |
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| Gros, P. Introduction géométrique à la vison par ordinateur 2000 |
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| Bach-y-Rita, P. Late post-acute neurologic rehabilitation: neuroscience, engineering and clinical programs Coulter paper 2003 (): [pdf] |
| This lecture highlights my career in rehabilitation research. My principal efforts in rehabilitation have been to study: (1) mechanisms of brain plasticity related to reorganization of the brain and recovery of function; (2) late postacute rehabilitation; (3) sensory substitution; and (4) rehabilitation engineering. A principal goal has been to aid in the development of a strong scientific base in rehabilitation. |
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| Edelman, S., Intrator, N. Learning as extraction of low-dimensional representations 1997 [html] |
| Psychophysical findings accumulated over the past several decades indicate that perceptual tasks such as similarity judgment tend to be performed on a low-dimensional representation of the sensory data. Low dimensionality is especially important for learning, as the number of examples required for attaining a given level of performance grows exponentially with the dimensionality of the underlying representation space. In this chapter, we argue that, whereas many perceptual problems are tractable precisely because their intrinsic dimensionality is low, the raw dimensionality of the sensory data is normally high, and must be reduced by a nontrivial computational process, which, in itself, may involve learning. Following a survey of computational techniques for dimensionality reduction, we show that it is possible to learn a low-dimensional representation that captures the intrinsic low-dimensional nature of certain classes of visual objects, thereby facilitating further learning of tasks... |
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| Marimont, D.H., Wandell, B.A. Linear models of surface and illuminant spectra Journal of the Optical Societry of America A 1992 (9)11:1905-1913 [pdf] |
| We describe procedures for creating efficient spectral representations for color. The representations generalize conventional tristimulus representations, which are based on the peripheral encoding by the human eye. We use low-dimensional linear models to approximate the spectral properties of surfaces and illuminants with respect to a collection of sensing devices. We choose the linear-model basis functions by minimizing the error in approximating sensor responses for collections of surfaces and illuminants. These linear models offer some conceptual simplifications for applications such as printer calibration; they also perform substantially better than principal-components approximations for computer-graphics applications. |
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| Kay, P. MIT Encyclopedia of the cognitives sciences 1999 [ps] |
| Lexical color categorization consists in the division of the colorsensations into classes corresponding to the significata of the color words of a particular language. Perceptual color categorization consists in the division ofthe color sensations into classes by the perceptual processes of an organism - human or non-human, adult or neonate, possessed of knowledge of alanguage or not so possessed. Conflict among views on the relation of lexical to perceptual color categorization has prevailed for over a century.Nineteenth century classicists, anthropologists and opthalmologists were aware that all languages do not reflect identical lexical classifications of color.Some, such as the classicist (and statesman) William Gladstone, concluded that differences in color lexicons reflect differences in perceptual abilities, e.g.,"... that the organ of color and its impressions were but partially developed among the Greeks of the heroic age" (see Berlin and Kay 1969 [B&K]: 135).Others, like the opthalmologist Hugo Magnus, recognized that failure to distinguish colors lexically need not indicate inability to distinguish themperceptually (see B&K: 144f). These and other late nineteenth century scholars tended strongly to view differences in color lexicons in evolutionaryterms. |
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| MacKay, D.J.C. Maximum Likelihood and Covariant Algorithms for ICA 1996 [html] |
| Bell and Sejnowski (1995) have derived a blind signal processing algorithm for a non-linear feedforward network from an information maximization viewpoint. This paper first shows that the same algorithm can be viewed as a maximum likelihood algorithm for the optimization of a linear generative model. Second, a covariant version of the algorithm is derived. This algorithm is simpler and somewhat more biologically plausible, involving no matrix inversions; and it converges in a smaller number of iterations. Third, this paper gives a partial proof of the `folk-theorem' that any mixture of sources with high-kurtosis histograms is separable by the classic ICA algorithm. Fourth, a collection of formulae are given that may be useful for the adaptation of the non-linearity in the ICA algorithm. |
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| Schreiber, T. Measuring Information Transfer Physical review letters 2000 (85)2 [html] |
| An information theoretic measure is derived that quantifies the statistical coherence between systems evolving in time. The standard time delayed mutual information fails to distinguish information that is actually exchanged from shared information due to common history and input signals. In our new approach, these influences are excluded by appropriate conditioning of transition probabilities. The resulting transfer entropy is able to distinguish effectively driving and responding elements and to detect asymmetry in the interaction of subsystems. |
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| Feldman, j. Minimization of Boolean complexity in human concept learning Nature 2000 (407):63-633 [pdf] |
| One of the unsolved problems in the field of human concept learning concerns the factors that determine the subjective difÆculty of concepts: why are some concepts psychologically simple and easy to learn, while others seem difÆcult, complex or incoherent? This question was much studied in the 1960s1 but was never answered, and more recent characterizations of concepts as prototypes rather than logical rules2,3 leave it unsolved4±6. Here I investigate this question in the domain of Boolean concepts (categories deÆned by logical rules). A series of experiments measured the subjective difÆculty of a wide range of logical varieties of concepts (41 mathematically distinct types in six families a far wider range than has been tested previously). The data reveal a surprisingly simple empirical `law': the subjective difÆculty of a concept is directly proportional to its Boolean complexity (the length of the shortest logically equivalent propositional formula) that is, to its logical incompressibility. |
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| Macaluso, E., Frith, C., Driver, J. Modulation of human visual cortex by crossmodal spatial attention Science 2000 (289):1206-1208 [html] |
| A sudden touch on one hand can improve vision near that hand, revealing crossmodal links in spatial attention. It is often assumed that such links involve only multimodal neural structures, but unimodal brain areas may also be affected. We tested the effect of simultaneous visuo-tactile stimulation on the activity of the human visual cortex. Tactile stimulation enhanced activity in the visual cortex, but only when it was on the same side as a visual target. Analysis of effective connectivity between brain areas suggests that touch influences unimodal visual cortex via back-projections from multimodal parietal areas. This provides a neural explanation for crossmodal links in spatial attention. |
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| Brunel, N., Nadal, J.-P. Mutual information, Fisher information and population coding Neural Computation 1998 (10)7:1731-1757 [html] |
| In the context of parameter estimation and model selection, it is only quite recently that a direct linkbetween the Fisher informationandinformationtheoretic quantities has been exhibited.We give an interpretation of this link within the standard framework of information theory.We show that in the context of population coding, the mutual information between the activity of a large array of neurons and a stimulus to which the neurons are tuned is naturally related to the Fisher information. In the light of this result, we consider the optimization of the tuning curves parameters in the case of neurons responding to a stimulus represented by an angular variable. |
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| Chalmers, D.J. Neural Correlates of Consciousness: Empirical and Conceptual Questions 2000 [html] |
| The search for neural correlates of consciousness (or NCCs) is arguably the cornerstone in the recent resurgence of the science of consciousness. The search poses many difficult empirical problems, but it seems to be tractable in principle, and some ingenious studies in recent years have led to considerable progress. A number of proposals have been put forward concerning the nature and location of neural correlates of consciousness. A few of these include: |
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| MacLeod, D.I. New dimension in color perception Trends in Cognitive Sciences 2003 (7)3:97-99 [html] |
| Colors are generally ordered in three dimensions, with hue and saturation as polar coordinates of a color circle, and brightness as the third dimension. Intuitively, lines of constant hue (but variable saturation) in such a color space should converge on an achromatic point devoid of hue. However, in new experiments by Ekroll et al. using colored patches in colored surrounds, constant hue lines converge not on 'gray' but on the surround color. This paradoxical observation suggests that the standard three-dimensional conception of perceived color is inadequate. |
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| Kielau, G., Maisser, P. Nonholonomic multibody dynamics Multibody System Dynamics 2003 (9):213-236 [html] |
| The paper deals with the nonholonomic multibody system dynamics from a point of view which is caused by some actual applications in high-tec areas like high-speed train technology or biomechanics of some disciplines in high-performance sports. Obviously, looking at such problems, there are very close connections between classical analytical dynamics, differential geometry and modern control theory. But these connections cannot be used to get new composed results in solving complicated problems of multibody system dynamics because corresponding software tools are not enough in tune with each other. This paper will give some ideas for developing a unified basis for modeling, simulation and control of nonholonomic multibody systems. First, a derivative-free approach for generating Lagrangian motion equations of multibody systems with kinematical tree structure as well as for constrained multibody systems is given. This has been done by using differential-geometric concepts in a Riemannian space. Secondly, the well-known theorem of Frobenius is considered with respect to its classical interpretation by the so-called object of nonholonomy as well as by its modern interpretation in the nonlinear control theory using Lie-brackets. The ideas are illustrated by the classical rolling condition and edge condition on double-curved surfaces. Special numerical problems in simulation of multibody systems subject to additional kinematic constraints are discussed. Finally three applications are given. |
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| Cohen, J. On The Structural Properties of the Colors Australasian Journal of Philosophy 2003 (81)1 [pdf] |
| Primary quality theories of color claim that colors are intrinsic, objective, mind-independent properties of external objects | that colors, like size and shape, are examples of the sort of properties moderns such as Boyle and Locke called primary qualities of body.1 Primary quality theories have long been seen as one of the main philosophical options for understanding the nature of color. However, a recent, empirically motivated argument seems to have convinced many that primary quality theories cannot be sustained. This argument, in outline, alleges that colors bear structural relations to each other that no primary qualities bear to each other, and therefore that colors cannot be primary qualities. This argument has received considerable philosophical attention in recent years, and appears to have convinced many to abandon primary quality theories of color.2 However, I believe the argument has been misunderstood. In this paper I shall examine arguments based on the structural properties of the colors in order to discern what they do and do not show about primary quality theories of color. |
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| Stoffregen, T., Bardy, B. On specification and the senses Behavioral and Brain Sciences 2003 (24)1 [html] |
| In this target article we question the assumption that perception is divided into separate domains of vision, hearing, touch, taste, and smell. We review implications of this assumption for theories of perception, and for our understanding of ambient energy arrays (e.g., the optic and acoustic arrays) that are available to perceptual systems. We analyze three hypotheses about relations between ambient arrays and physical reality; (1) that there is an ambiguous relation between ambient energy arrays and physical reality; (2) that there is a unique relation between individual energy arrays and physical reality; (3) that there is a redundant but unambiguous relation, within or across arrays, between energy arrays and physical reality. This is followed by a review of the physics of motion, focusing on the existence and status of referents for physical motion. Our review indicates that it is not possible, in principle, for there to be a unique relation between physical motion and the structure of individual energy arrays. We argue that physical motion relative to different referents is specified only in the global array, which consists of higher-order relations across different forms of energy. The existence of specificity in the global array is consistent with the idea of direct perception, and so poses a challenge to traditional, inference-based theories of perception and cognition. However, it also presents a challenge to much work within the ecological approach to perception and action, which has accepted the assumption of separate senses. |
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| Todorov, E. Optimality principles in sensorimotor control Nature Neuroscience 2004 (7)9:907-915 [pdf] |
| The sensorimotor system is a product of evolution, development, learning and adaptation which work on different time scales to improve behavioral performance. Consequently, many theories of motor function are based on optimal performance : they quantify task goals as cost functions, and apply the sophisticated tools of optimal control theory to obtain detailed behavioral predictions. The resulting models, although not without limitations, have explained more empirical phenomena than any other class. Traditional emphasis has been on optimizing desired movement trajectories while ignoring sensory feedback. Recent work has redefined optimality in terms of feedback control laws, and focused on the mechanisms that generate behavior online. This approach has allowed researchers to fit previously unrelated concepts and observations into what may become a unified theoretical framework for interpreting motor function. At the heart of the framework is the relationship between high-level goals, and the real-time sensorimotor control strategies most suitable for accomplishing those goals. |
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| Block, N. Oxford Companion to the Mind 2003 [pdf] |
| Qualia include the ways things look, sound and smell, the way it feels to have a pain, and more generally, what it's like to have experiential mental states. ( Qualia is the plural of quale .) Qualia are experiential properties of sensations, feelings, perceptions and, more controversially, thoughts and desires as well. But, so defined, who could deny that qualia exist? |
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| Lee, B.B. Paths to colour in the retina Clinical and experimental optometry 2004 (87):239-248 [html] |
| The description of colour pathways in the primate retina has become clearer within the past decade. This review summarises current views on the pathways subserving colour vision in the primate retina, beginning in the receptors and outer retina and leading to the mechanisms in the inner retina that add and subtract the receptor signals. Although the main features of colour pathways are now well-defined, there remains uncertainty about some of the wiring details. In particular, the question of how much connectional specificity is present is unresolved. Finally, means of isolating these pathways by psychophysical tests are considered; some current tests are likely to be less specific than hoped. |
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| Philipona, D., O'Regan, J.K., Nadal, J.-P. , Coenen, O.J.-M. Perception of the structure of the physical world using unknown sensors and effectors Advances in Neural Information Processing Systems 2004 (15) [pdf] |
| Is there a way for an algorithm linked to an unknown body to infer by itself information about this body and the world it is in? Taking the case of space for example, is there a way for this algorithm to realize that its body is in a three dimensional world? Is it possible for this algorithm to discover how to move in a straight line? And more basically: do these questions make any sense at all given that the algorithm only has access to the very high-dimensional data consisting of its sensory inputs and motor outputs? We demonstrate in this article how these questions can be given a positive answer. We show that it is possible to make an algorithm that, by analyzing the law that links its motor outputs to its sensory inputs, discovers information about the structure of the world regardless of the devices constituting the body it is linked to. We present results from simulations demonstrating a way to issue motor orders resulting in fundamental movements of the body as regards the structure of the physical world. |
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| Noë, A., O'Regan, J.K. Perception, Attention and the Grand Illusion Psyche 2000 (6)15 [html] |
| This paper looks at two puzzles raised by the phenomenon of inattentional blindness. First, how can we see at all if, in order to see, we must first perceptually attend to that which we see? Second, if attention is required for perception, why does it seem to us as if we are perceptually aware of the whole detailed visual field when it is quite clear that we do not attend to all that detail? We offer a general framework for thinking about perception and perceptual consciousness that addresses these questions and we propose, in addition, an informal account of the relation between attention and consciousness. On this view, perceptual awareness is a species of attention. |
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| Chareyron, S., Wieber, P.-B. Position and Force Control of Nonsmooth Lagrangian Dynamical Systems without Friction 2004 |
| Analyses of position and force control laws in the case of perfectly rigid bodies have been made so far with strong assumptions on the state of the contacts such as supposing that they are permanent. We re interested here in having a look at what happens when no such assumptions is made: we are led therefore to propose a Lyapunov stability analysis of a position and force control law in the mathematical framework of nonsmooth Lagrangian dynamical systems. |
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| Braddick, O. & al Possible blindsight in infants lacking one cerebral hemisphere Nature 1994 (360):461-463 [html] |
| Patients with damage to the striate cortex have a subjectively blind region of the visual field, but may still be able to detect and localize targets within this region. But the relative roles in this 'blindsight' of subcortical neural systems, and of pathways to extra-striate visual areas, have been uncertain. Here we report results on two infants in whom one cerebral hemisphere, including both striate and extra-striate visual cortex, needed surgical removal in their first year. Single conspicuous targets in the half-field contralateral to the lesion could elicit fixations, implying detection and orienting by a subcortical system. In contrast, binocular optokinetic nystagmus (OKN), for which a subcortical pathway has often been thought adequate, showed a marked asymmetry. In normal neonates, fixation shifts and OKN have both been taken to reflect subcortical control; our results are consistent with subcortical control for fixation but not for OKN. |
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| Aldrovandi, R., Saeger, L. Projective Fourier Duality and Weyl Quantization Int.J.Theor.Phys. 1997 (36):573-612 [html] |
| The Weyl-Wigner correspondence prescription, which makes large use of Fourier duality, is reexamined from the point of view of Kac algebras, the most general background for noncommutative Fourier analysis allowing for that property. It is shown how the standard Kac structure has to be extended in order to accommodate the physical requirements. An Abelian and a symmetric projective Kac algebras are shown to provide, in close parallel to the standard case, a new dual framework and a well-defined notion of projective Fourier duality for the group of translations on the plane. The Weyl formula arises naturally as an irreducible component of the duality mapping between these projective algebras. |
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| Murata, N. Properties of the Empirical Characteristic Function and its Application to Testing for Independence 2001 [pdf] |
| In this article, the asymptotic properties of the empirical characteristic function are discussed. The residual of the joint and marginal empirical characteristic functions is studied and the uniform convergence of the residual in the wider sense and the weak convergence of the scaled residual to a Gaussian process are investigated. Taking into account of the result, a statistical test for independence against alternatives is considered. |
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| Kay, P., Regier, T. Resolving the question of color naming universals Proceedings of the National Academy of Sciences USA 2003 (100)15:9085-9 [pdf] |
| The existence of cross-linguistic universals in color naming is currently contested. Early empirical studies, based principally on languages of industrialized societies, suggested that all languages may draw on a universally shared repertoire of color categories. Recent work, in contrast, based on languages from nonindustrialized societies, has suggested that color categories may not be universal. No comprehensive objective tests have yet been conducted to resolve this issue. We conduct such tests on color naming data from languages of both industrialized and nonindustrialized societies and show that strong universal tendencies in color naming exist across both sorts of language. |
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| Pylyshyn, Z. Return of the mental image: are there really pictures in the brain? Trends in Cognitive Sciences 2003 (7)3:113-118 [pdf] |
| In the past decade there has been renewed interest in the study of mental imagery. Emboldened by new findings from neuroscience, many people have revived the idea that mental imagery involves a special format of thought, one that is pictorial in nature. But the evidence and the arguments that exposed deep conceptual and empirical problems in the picture theory over the past 300 years have not gone away. I argue that the new evidence from neural imaging and clinical neuropsychology does little to justify this recidivism because it does not address the format of mental images. I also discuss some reasons why the picture theory is so resistant to counterarguments and suggest ways in which non-pictorial theories might account for the apparent spatial nature of images. |
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| Glasersfled, E.(. Scheme Theory as a key to the learning paradox 2001 [PDF] |
| Carl Bereiter s article Toward a solution of the learning paradox appeared in 1985, was widely read and cited, but did not end the discussion about the learning paradox . My contribution is an attempt to show that it is in fact a spurious problem and that the paradox springs from unwarranted traditional views of knowledge and conceptualization. A constructivist orientation adopting Peirce s notion of abduction and a particular interpretation of Piaget s scheme theory opens a different and perhaps more promising approach. |
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| Bach-y-Rita, P., Tyler, M.E., Kaczmarek, K.A. Seeing with the brain Internation journal of human-computer interaction 2003 (15)2:285-295 [html] |
| We see with the brain, not the eyes (Bach-y-Rita, 1972); images that pass through our pupils go no further than the retina. From there image information travels to the rest of the brain by means of coded pulse trains, and the brain, being highly plastic, can learn to interpret them in visual terms. Perceptual levels of the brain interpret the spatially encoded neural activity, modified and augmented by nonsynaptic and other brain plasticity mechanisms (Bach-y-Rita, 1972, 1995, 1999, in press). However, the cognitive value of that information is not merely a process of image analysis. Perception of the image relies on memory, learning, contextual interpretation (e.g., we perceive intent of the driver in the slight lateral movements of a car in front of us on the highway), cultural, and other social factors that are probably exclusively human characteristics that provide qualia (Bach-y-Rita, 1996b). This is the basis for our tactile vision substitution system (TVSS) studies that, starting in 1963, have demonstrated that visual information and the subjective qualities of seeing can be obtained tactually using sensory substitution systems. |
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| Shimojo, S., Shams, L. Sensory modalities are not separate modalities: plasticity and interactions Current Opinion in Neurobiology 2001 (11):505-509 [pdf] |
| Historically, perception has been viewed as a modular function, with the different sensory modalities operating independently of each other. Recent behavioral and brain imaging studies challenge this view, by suggesting that cross-modal interactions are the rule and not the exception in perception, and that the cortical pathways previously thought to be sensory-specific are modulated by signals from other modalities. |
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| Nayar, S.K., Xi-Sheng, F., Boult, T. Separation of reflection components using color and polarization International Journal of Computer Vision 1997 (21)3:163-186 [pdf] |
| Specular reflections and interreflections produce strong highlights in brightness images. These highlights can cause vision algorithms for segmentation, shape from shading, binocular stereo, and motion estimation to produce erroneous results. A technique is developed for separating the specular and diffuse components of reflection from images. The approach is to use color and polarization information, simultaneously, to obtain constraints on the reflection components at each image point. Polarization yields local and independent estimates of the color of specular reflection. The result is a linear subspace in color space in which the local diffuse component must lie. This subspace constraint is applied to neighboring image points to determine the diffuse component. In contrast to previous separation algorithms, the proposed method can handle highlights on surfaces with substantial texture, smoothly varying diffuse reflectance, and varying material properties. The separation algorithm is applied to several complex scenes with textured objects and strong interreflections. The separation results are then used to solve three problems pertinent to visual perception; determining illumination color, estimating illumination direction, and shape recovery. |
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| Schlaer, R. Shift in binocular disparity causes compensatory change in the cortical structure of kittens Science 1971 (173):638-641 |
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| Rieke, F., Baylor, D. Single-photon detection by rod cells of the retina Reviews of Modern Physics 1998 (70)3:1027-1035 [pdf] |
| At low light levels, the visual system detects and counts photon absorptions with a reliability close to limits set by statistical fluctuations in the number of absorbed photons. Thus the rod photoreceptors that provide the input signals to the dark-adapted visual system act as nearly perfect photon counters. This elegant performance is possible because light detection in the rods satisfies four functional requirements: high quantum efficiency, sufficient amplification to produce a measurable response, low dark noise, and low trial-to-trial variability in the elementary response. The rod meets these requirements using biochemical reactions rather than the solid-state reactions of silicon detectors, yet its performance equals or exceeds that of man-made detectors in several ways. |
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| Kajimoto, H., Kawakami, N., Tachi, S., Inami, M. SmartTouch: Electric Skin to touch the untouchable IEEE Computer Graphics and Applications 2004 :36-42 [htm] |
| A haptic augmented reality system, SmartTouch, is composed of a thin electro-tactile display and a sensor mounted on the skin. The sensed information is converted to tactile sensation through electrical stimulation. Thus, the wearer not only makes physical contact with an object, but also can touch surface information of any modality, even those that are ordinarily nontouchable. The SmartTouch prototype has optical sensors. The authors endeavored to realize the tactile perception of luminance information, which was achieved by imitating the sensory nerve activity with electrical stimulation. |
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| Clark, A. Spectrum Inversion and the Color Solid Southern Journal of Philosophy 1985 (23)4:431-443 [html] |
| The possibility that what looks red to me may look green to you has traditionally been known as "spectrum inversion." This possibility is thought to create difficulties for any attempt to define mental states in terms of behavioral dispositions or functional roles. If spectrum inversion is possible, then it seems that two perceptual states may have identical functional antecedents and effects yet differ in their qualitative content. In that case the qualitative character of the states could not be functionally defined. |
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| Goebel, R. & al Sustained extrastriate cortical activation without visual awareness revealed by fMRI studies of hemianopic patients Vision Research 2001 (3):1459-74 [html] |
| Patients with lesions in the primary visual cortex (V1) may show processing of visual stimuli presented in their field of cortical blindness even when they report being unaware of the stimuli. To elucidate the neuroanatomical basis of their residual visual functions, we used functional magnetic resonance imaging in two hemianopic patients, FS and GY. In the first experiment, a rotating spiral stimulus was used to assess the responsiveness of dorsal stream areas. Although no response was detectable within denervated or destroyed early visual cortex, motion-sensitive areas (hMT+/V5) ipsilateral to the lesion showed a strong sustained hemodynamic response. In GY, this activation was at least as strong as that of his contralesional hMT+/V5 to the stimulus in the normal hemifield. In the second experiment, coloured images of natural objects were used to assess the responsiveness of ventral stream areas. Again, no activity was detectable in ipsilesional early visual areas, but extrastriate areas in the lateral occipital cortex (hMT+/V5 and LO) and within the posterior fusiform gyrus (V4/V8) showed a robust sustained hemodynamic response. In both experiments, we observed that ipsilesional areas responded to stimuli presented in either hemifield, whereas the normal hemisphere responded preferentially to stimuli in the sighted hemifield. As only one subject occasionally noticed the onset of stimulation in the impaired field, the unexpectedly strong sustained activity in ipsilesional dorsal and ventral cortical areas appears to be insufficient to generate conscious vision. |
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| Ahissar, E. Temporal-to Rate-Code Conversion by Neuronal Phase-Locked Loops Neural Computation 1998 (10):597-650 [pdf] |
| Peripheral sensory activity follows the temporal structure of input signals. Central sensory processing uses also rate coding, and motor outputs appear to be primarily encoded by rate. I propose here a simple, efficient structure, converting temporal coding to rate coding by neuronal phase-locked loops (PLL). The simplest form of a PLL includes a phase detector (that is, a neuronal-plausible version of an ideal coincidence detector) and a controllable local oscillator that are connected in a negative feedback loop. The phase detector compares the firing times of the local oscillator and the input and provides an output whose firing rate is monotonically related to the time difference. The output rate is fed back to the local oscillator and forces it to phase-lock to the input. Every temporal interval at the input is associated with a specific pair of output rate and time difference values; the higher the output rate, the further the local oscillator is driven from its intrinsic frequency. Sequences of input intervals, which by definition encode input information, are thus represented by sequences of firing rates at the PLL's output. The most plausible implementation of PLL circuits is by thalamocortical loops in which populations of thalamic "relay" neurons function as phase detectors that compare the timings of cortical oscillators and sensory signals. The output in this case is encoded by the thalamic population rate. This article presents and analyzes the algorithmic and the implementation levels of the proposed PLL model and describes the implementation of the PLL model to the primate tactile system. |
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| Bell, A.J., Sejnowski, T.J. The 'independent components' of natural scenes are edge filters Vision Research 1997 37:3327-3338 [html] |
| Field (1994) has suggested that neurons with line and edge selectivities found in primary visual cortex of cats and monkeys form a sparse, distributed representaton of natural scenes, and Barlow (1989) has reasoned that such responses should emerge from an unsupervised learning algorithm that attempts to find a factorial code of independent visual features. We show here that a new unsupervised learning algorithm that is based on information maximisation, a non-linear `infomax' network (Bell and Sejnowski, 1995) when applied to an ensemble of natural scenes, produces sets of visual filters that are localised and oriented. Some of these filters are Gabor-like and resemble those produced by the sparseness-maximisation network of Olshausen & Field (1996). In addition, the outputs of these filters are as independent as possible, since the infomax network is able to perform Independent Components Analysis (ICA). We compare the resulting ICA filters and their associated basis functions, with ... |
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| Nathans, J. The Evolution and Physiology of Human Color Vision: Insights from Molecular Genetic Studies of Visual Pigments Neuron 1999 (24):299-312 [pdf] |
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| Coppola, D., Purves, D. The extraordinarily rapid disappearance of entoptic images Proceedings of the National Academy of Sciences USA 1996 (96):8001-8004 [pdf] |
| It has been known for more than 40 years that images fade from perception when they are kept at the same position on the retina by abrogating eye movements. Although aspects of this phenomenon were described earlier, the use of close-fitting contact lenses in the 1950s made possible a series of detailed observations on eye movements and visual continuity. In the intervening decades, many investigators have studied the role of image motion on visual perception. Although several controversies remain, it is clear that images deteriorate and in some cases disappear following stabilization; eye movements are, therefore, essential to sustained exoptic vision. The time course of image degradation has generally been reported to be a few seconds to a minute or more, depending upon the conditions. Here we show that images of entoptic vascular shadows can disappear in less than 80 msec. The rapid vanishing of these images implies an active mechanism of image erasure and creation as the basis of normal visual processing. |
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| Frisoli, A. The human-robot sensorimotor coupling: an engineering perspective 2004 [8] |
| Rappels des ellipsoides de force et sensoriels: image de la métrique angulaire dans l'espace cartésien via la relation dynamique. Rappels de ce qui semble être les stratégies des organismes vivants. Notion de système passif. Simple et éclairant. |
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| Ekroll, V., Faul, F., Niederee, R., Richter, E. The natural center of chromaticity space is not always achromatic: a new look at color induction Proceedings of the National Academy of Sciences USA 2002 (99)20:13352-6 [html] |
| Although current theories of color vision differ in many respects, they all assume the existence of a uniquely defined neutral point in chromaticity space. It generally is assumed that this point satisfies several criteria simultaneously. One of these criteria is that it is perceived as achromatic. A further criterion shared by most theories is the structural assumption that lines in chromaticity space of constant hue converge on the neutral point. The basic assumption that these two criteria coincide is clearly true for isolated spots of light presented in darkness, and it usually is taken for granted that this coincidence generalizes to more complex visual stimuli. Here, we show that this is not the case. Our experiments with infields in chromatic surrounds revealed that the point in chromaticity space that appears gray is clearly different from the point on which lines of constant hue converge. A plausible interpretation of this apparently paradoxical finding in terms of color scission is proposed. |
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| Zeki, S., Aglioti, S., McKeefry, D., Berlucchi, G. The neurological basis of conscious color perception in a blind patient Proceedings of the National Academy of Sciences USA 1999 (96)24 [html] |
| We have studied patient PB, who, after an electric shock that led to vascular insufficiency, became virtually blind, although he retained a capacity to see colors consciously. For our psychophysical studies, we used a simplified version of the Land experiments [Land, E. (1974) Proc. R. Inst. G. B. 47, 23-58] to learn whether color constancy mechanisms are intact in him, which amounts to learning whether he can assign a constant color to a surface in spite of changes in the precise wavelength composition of the light reflected from that surface. We supplemented our psychophysical studies with imaging ones, using functional magnetic resonance, to learn something about the location of areas that are active in his brain when he perceives colors. The psychophysical results suggested that color constancy mechanisms are severely defective in PB and that his color vision is wavelength-based. The imaging results showed that, when he viewed and recognized colors, significant increases in activity were restricted mainly to V1-V2. We conclude that a partly defective color system operating on its own in a severely damaged brain is able to mediate a conscious experience of color in the virtually total absence of other visual abilities. |
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| Martinez-Conde, S., Macknik, S.L., Hubel, D.H. The role of fixational eye movements in visual perception Nature reviews Neuroscience 2004 (5):229-240 [pdf] |
| Our eyes continually move even while we fix our gaze on an object. Although these fixational eye movements have a magnitude that should make them visible to us, we are unaware of them. If fixational eye movements are counteracted, our visual perception fades completely as a result of neural adaptation. So, our visual system has a built-in paradox we must fix our gaze to inspect the minute details of our world, but if we were to fixate perfectly, the entire world would fade from view. Owing to their role in counteracting adaptation, fixational eye movements have been studied to elucidate how the brain makes our environment visible. Moreover, because we are not aware of these eye movements, they have been studied to understand the underpinnings of visual awareness. Recent studies of fixational eye movements have focused on determining how visible perception is encoded by neurons in various visual areas of the brain. |
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| Li, Z., Atick, J.J. Towards a theory of the striate cortex Neural Computation 1994 (6)1:127-146 [pdf] |
| We explore the hypothesis that linear cortical neurons are concerned with building a particular type of representation of the visual world --- one which not only preserves the information and the efficiency achieved by the retina, but in addition preserves spatial relationships in the input --- both in the plane of vision and in the depth dimension. Focusing on the {\it linear} cortical cells, we classify all transforms having these properties. They are given by representations of the scaling and translation group, and turn out to be labeled by rational numbers `$(p+q)/p$' ($p, q$ integers). Any given $(p,q)$ predicts a set of receptive fields which come at different spatial locations and scales (sizes) with a bandwidth of $\log_2[(p+q)/p]$ octaves, and, most interestingly, with a diversity of `$q$' cell varieties. The bandwidth affects the trade-off between preservation of planar and depth relations, and, we think, should be selected to match structures in natural scenes. For bandwidths between $1$ and $2$ octaves, which are the ones we feel provide the best matching, we find for each scale a minimum of two distinct cell types that reside next to each other and in phase quadrature, i.e., differ by $90^o$ in the phases of their receptive fields, as are found in the cortex, they resemble the ``even-symmetric'' and ``odd-symmetric'' simple cells in special cases. An interesting consequence of the representations presented here is that the pattern of activation in the cells in response to a translation or scaling of an object remains the same but merely shifts its locus from one group of cells to another. This work also provides a new understanding of color coding changes from the retina to the cortex. |
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| Chichilisky, E., Wandell, B.A. Trichromatic opponent color classification Vision Research 1999 [pdf] |
| Stimuli varying in intensity and chromaticity, presented on numerous backgrounds, were classified into red/green, blue/yellow and white/black opponent color categories. These measurements revealed the shapes of the boundaries that separate opponent colors in three-dimensional color space. Opponent color classification boundaries were generally not planar, but their shapes could be summarized by a piecewise linear model in which increment and decrement color signals are combined with different weights at two stages to produce opponent color sensations. The effect of background light on classification was largely explained by separate gain changes in increment and decrement cone signals. |
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| Kitamoto, J. & al Two visual pigments in a single photoreceptor cell: identification and histological localization of three mRNAs encoding visual pigment opsins in the retina of the butterfly Papilio xuthus Journal of Experimental Biology 1998 (201)9:1255-1261 [pdf] |
| This paper describes the localization of newly identified visual pigment opsins in the tiered retina of the Japanese yellow swallowtail Papilio xuthus. We first cloned three cDNAs encoding visual pigment opsins, PxRh1, PxRh2 and PxRh3, and then carried out histological in situ hybridization to localize their mRNAs in the retina. By combining the present data with our previous electrophysiological results, we concluded that both PxRh1 and PxRh2 correspond to visual pigments expressed in photoreceptor cells sensitive in the green wavelength region (green receptors), whereas PxRh3 corresponds to a pigment in red receptors. The in situ hybridization studies showed that some photoreceptor cells express two opsin mRNAs. In the ventral half of the eye, all green receptors in the distal tier were labelled by both PxRh1 and PxRh2 probes. The labelling by the PxRh2 and PxRh3 probes was detected throughout the eye in the proximal tier; in 18 \% of ommatidia, the probes labelled the same photoreceptor cell. These results suggest that the possible co-localization of two different visual pigments will broaden the sensitivity spectrum of the photoreceptor cells. |
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| Valberg, A. Unique hues: an old problem for a new generation Vision Research 2001 (41):1645-1657 [html] |
| The practical success of the classical theories of colour vision, such as that of Young-Helmholtz when applied to the measurement and reproduction of colour stimuli, and that of Hering's in art and architecture, has overshadowed the fact that neither theory achieved its main goal, namely to explain colour qualities. Neither the three types of cone, nor the first opponent stages of neural processing in the retina and the lateral geniculate nucleus can serve as direct correlates to the perception of elementary, or unique colours, such as red, green, yellow and blue. While our subjective experiences of these qualities do not submit to measurement, physiological conditions that are required to perceive colours of a constant hue can be identified. For instance, a constant ratio of responses of different types of opponent cells in the retina and the lateral geniculate nucleus of primates may serve as a neurophysiological correlate of a constant hue. This is, however, not the correlate for seeing a particular hue quality, say unique red. This latter correlate, if it exists as a separable entity, must be associated with yet unidentified, higher-level neural activities. The fundamental problems encountered in relating colour qualities to neural activities are discussed and references are made to the current debate about phenomenal consciousness. |
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| Kuehni, R.G. Variability in Unique Hue Selection: A Surprising Phenomenon Color Research & Application 2004 (29)2:158-162 [html] |
| Data from ten different experiments involving nearly 600 observers of determination of unique hues are compared. Six experiments involve determination using spectral lights; two use desaturated monitor colors, and the remaining two use color chip sets. Except for unique green, color chips result in narrower ranges of results than spectral lights. Unique green has a surprisingly large range of variation in both spectral light and color chip experiments, followed by red. Comparison of spectral light data indicates that one observer's unique blue can be another's unique green and vice versa, and the same for yellow and green. This finding raises significant questions for color appearance and color space/difference models, as well as philosophy of color. |
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| Webster, M., Miyahara, E., Malkoc, G., Raker, V. Variations in normal color vision. I. Cone-opponent axes Journal of the Optical Society of America A 2000 (17)9:1535-1544 |
| Early postreceptoral color vision is thought to be organized in terms of two principal axes corresponding to opposing L- and M-cone signals (LvsM) or to S-cone signals opposed by a combination of L- and M-cone signals (SvsLM). These cone-opponent axes are now widely used in studies of color vision, but in most cases the corresponding stimulus variations are defined only theoretically, based on a standard observer. We examined the range and implications of interobserver variations in the cone-opponent axes. We used chromatic adaptation to empirically define the LvsM and SvsLM axes and used both thresholds and color contrast adaptation to determine sensitivity to the axes. We also examined the axis variations implied by individual differences in the color matching data of Stiles and Burch [Opt. Acta 6, 1 (1959)]. The axes estimated for individuals can differ measurably from the nominal standard-observer axes and can influence the interpretation of postreceptoral color organization (e.g., regarding interactions between the two axes). Thus, like luminance sensitivity, individual differences in chromatic sensitivity may be important to consider in studies of the cone-opponent axes. |
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| Tye, M. Visual Qualia and Visual Content Revisited 2003 [pdf] |
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| Hirsch, H., Spinelli, D. Visual experience modifies distribution of horizontally and vertically oriented receptive fields in cats Science 1970 (168):869-871 |
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| Palmeri, T.J., Gauthier, I. Visual object understanding Nature reviews Neuroscience 2004 (5):291-304 [pdf] |
| Visual object understanding includes processes at the nexus of visual perception and visual cognition. A traditional approach separates questions that are more associated with perception how are objects represented by high-level vision from questions that are more associated with cognition how are objects identified, categorized and remembered. However, to understand the bridge between perception and cognition, it is fruitful to abandon any sharp distinction between perceptual and cognitive aspects of visual object understanding. We provide a selective review of research from both the Object Recognition and Perceptual Categorization literatures, highlighting relevant behavioural, neuropsychological, neurophysiological and theoretical research into the representations and processes that underlie visual object understanding in humans and primates. |
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| Yamauchi, Y. & al What determines unique yellow, L/M cone ratio or visual experience ? 9th Congress of the International Colour Association 2002 (4421):275-278 [pdf] |
| Unique yellow is considered to represent the equilibrium point of the red-green opponent chromatic mechanism. There are several hypotheses that attempt to explain how this equilibrium point is established. The determinant for unique yellow, however, has not yet been clarified. Here we explored whether the L/M cone ratio or visual information determines unique yellow. If the former is the case, we expect that subjects with large differences in their L/M cone ratio would set different spectral lights to appear as unique yellow. The results of such an experiment, however, did not show a substantial difference in the value of unique yellow for two subjects with very different cone ratios. On the other hand, if the latter is the case, unique yellow should change when altering the chromaticity of the surrounding visual environment. To test this hypothesis, we conducted long-term adaptation experiments, in which subjects spent 8 to 12 hours in a chromatically altered environment. A significant shift of unique yellow was observed after spending time in such an environment for several days. These results indicate that the red-green opponent channel includes a plastic normalization mechanism that adjusts its balance point based on visual experience. |
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| Block, N. What is Functionalism? The Encyclopedia of Philosophy Supplement 1996 [pdf] |
| Functionalism is one of the major proposals that have been offered as solutions to the mind/body problem. Solutions to the mind/body problem usually try to answer questions such as: What is the ultimate nature of the mental? At the most general level, what makes a mental state mental? Or more specifically, What do thoughts have in common in virtue of which they are thoughts? That is, what makes a thought a thought? What makes a pain a pain? Cartesian Dualism said the ultimate nature of the mental was to be found in a special mental substance. Behaviorism identified mental states with behavioral dispositions; physicalism in its most influential version identifies mental states with brain states. Functionalism says that mental states are constituted by their causal relations to one another and to sensory inputs and behavioral outputs. Functionalism is one of the major theoretical developments of Twentieth Century analytic philosophy, and provides the conceptual underpinnings of much work in cognitive science. |
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| Shams, L., Kamitani, Y., Shimojo, S. What you see is what you hear Nature 2000 (408):788 [pdf] |
| Traditionally, vision has been considered the dominant modality in our multi-sensory perception of the world. Here we present findings that overturn this established view, by showing that auditory information can change the percept of an unambiguous visual stimulus qualitatively (causing a strong visual illusion). These results indicate that, as with other modalities, our visual percepts are malleable by other modalities. We have discovered a visual illusion which is induced by sound: when a single visual flash is accompanied by multiple auditory beeps, the single flash is incorrectly perceived as multiple flashes. These results were obtained by flashing a uniform white disk (subtending 2° at 5° eccentricity) a variable number of times (spaced 50ms apart) on a black background. |
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| Ziemke, T. What's that thing called embodiment? 2003 [pdf] |
| Embodiment has become an important concept in many areas of cognitive science. There are, however, very different notions of exactly what embodiment is and what kind of body is required for what type of embodied cognition. Hence, while many nowadays would agree that humans are embodied cognizers, there is much less agreement on what kind of artifact could be considered embodied. This paper identifies and contrasts six different notions of embodiment which can roughly be characterized as (1) structural coupling between agent and environment, (2) historical embodiment as the result of a history of struct ural coupling, (3) physical embodiment, (4) organismoid embodiment, i.e. organismlike bodily form (e.g., humanoid robots), (5) organismic embodiment of autopoietic, living systems, and (6) social embodiment. |
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| Critchley, H.D. & al neural systems supporting interoceptive awareness Nature Neuroscience 189-195 [pdf] |
| Influential theories of human emotion argue that subjective feeling states involve representation of bodily responses elicited by emotional events. Within this framework, individual differences in intensity of emotional experience reflect variation in sensitivity to internal bodily responses. We measured regional brain activity by functional magnetic resonance imaging (fMRI) during an interoceptive task wherein subjects judged the timing of their own heartbeats. We observed enhanced activity in insula, somatomotor and cingulate cortices. In right anterior insular/opercular cortex, neural activity predicted subjects' accuracy in the heartbeat detection task. Furthermore, local gray matter volume in the same region correlated with both interoceptive accuracy and subjective ratings of visceral awareness. Indices of negative emotional experience correlated with interoceptive accuracy across subjects. These findings indicate that right anterior insula supports a representation of visceral responses accessible to awareness, providing a substrate for subjective feeling states. |
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| Clark, A. A Physicalist Theory of Qualia The Monist 1985 (68)4:491-506 |
| Haines, R. A breakdown in simultaneous information processing Presbyopia research. From molecular biology to visual adaptation 1991 (): |
| Kelemen, C., Szirmay-Kalos, L. A microfacet based coupled specular-matte BRDF model with importance sampling |
| Droulez, J., Berthoz, A. A neural network model of sensoritopic maps with predictive short-term memory properties Proceedings of the National Academy of Sciences USA 1991 (88):9653-9657 |
| Lobry, C. A propos du sens des textes mathématiques, un exemple: la théorie des "bifurcations dynamiques" Annales de l'Institut Fourier 1992 |
| Blakemore, S.J., Wolpert, D.M., Frith, C.D. Abnormalities in the awareness of action Trends in Cognitive Sciences 2002 (6)6:237-242 |
| Sadato, N. & al Activation of the primary visual cortex by Braille reading in blind subjects Nature 1996 (380):526-528 |
| Szwed, M., Bagdasarian, K., Ahissar, E. Active encoding of vibrissal touch Neuron 2003 (40):621-630 |
| Harman, K.L., Humphrey, G.K., Goodale, M.A. Active manual control of object views facilitates visual recognition Current Biology 1999 (9):1315-1318 |
| Sommese, A.J., Verschelde, J., Wampler, C.W. Advance in Polynomial Contination for Solving Problems in Kinematics 2002 |
| Lee, D.D., Seung, H.S. Algorithm for Non-negative Matrix Factorization 2001 (13) |
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