| 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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| O'Regan, J.K., Rensink, R.A., Clark, J.J. Blindness to scene changes caused by mudsplashes Nature 1999 (34):398 [html] |
| Change-blindness, occurs when large changes are missed under natural viewing conditions because they occur simultaneously with a brief visual disruption, perhaps caused by an eye movement,, a flicker, a blink, or a camera cut in a film sequence. We have found that this can occur even when the disruption does not cover or obscure the changes. When a few small, high-contrast shapes are briefly spattered over a picture, like mudsplashes on a car windscreen, large changes can be made simultaneously in the scene without being noticed. This phenomenon is potentially important in driving, surveillance or navigation, as dangerous events occurring in full view can go unnoticed if they coincide with even very small, apparently innocuous, disturbances. It is also important for understanding how the brain represents the world. |
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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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| Olshausen, B., Field, D. Emergence of simple-cell receptive field properties by learning a sparse code for natural images Nature 1996 (381):607-609 [html] |
| The receptive fields of simple cells in mammalian primary visual cortex can be characterized as being spatially localized, oriented and bandpass (selective to structure at different spatial scales), comparable to the basis functions of wavelet transforms. One approach to understanding such response properties of visual neurons has been to consider their relationship to the statistical structure of natural images in terms of efficient coding. Along these lines, a number of studies have attempted to train unsupervised learning algorithms on natural images in the hope of developing receptive fields with similar properties, but none has succeeded in producing a full set that spans the image space and contains all three of the above properties. Here we investigate the proposal that a coding strategy that maximizes sparseness is sufficient to account for these properties. We show that a learning algorithm that attempts to find sparse linear codes for natural scenes will develop a complete family of localized, oriented, bandpass receptive fields, similar to those found in the primary visual cortex. The resulting sparse image code provides a more efficient representation for later stages of processing because it possesses a higher degree of statistical independence among its outputs. |
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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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| Sharma, J., Angelucci, A., Sur, M. Induction of visual orientation modules in auditory cortex Nature 2000 (404):841-847 [pdf] |
| Modules of neurons sharing a common property are a basic organizational feature of mammalian sensory cortex. Primary visual cortex (V1) is characterized by orientation modules groups of cells that share a preferred stimulus orientation which are organized into a highly ordered orientation map. Here we show that in ferrets in which retinal projections are routed into the auditory pathway, visually responsive neurons in `rewired' primary auditory cortex are also organized into orientation modules. The orientation tuning of neurons within these modules is comparable to the tuning of cells in V1 but the orientation map is less orderly. Horizontal connections in rewired cortex are more patchy and periodic than connections in normal auditory cortex, but less so than connections in V1. These data show that afferent activity has a profound inØuence on diverse components of cortical circuitry, including thalamocortical and local intracortical connections, which are involved in the generation of orientation tuning, and long-range horizontal connections, which are important in creating an orientation map. |
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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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| Lee, D.D., Seung, H.S. Learning the Parts of Objects by Non-negative Matrix Factorization Nature 1999 (401):788 [pdf] |
| Is perception of the whole based on perception of its parts? There is psychological and physiological evidence for parts-based representations in the brain, and certain computational theories of object recognition rely on such representations. But little is known about how brains or computers might learn the parts of objects. Here we demonstrate an algorithm for non-negative matrix factorization that is able to learn parts of faces and semantic features of text. This is in contrast to other methods, such as principal components analysis and vector quantization, that learn holistic, not parts-based, representations. Non-negative matrix factorization is distinguished from the other methods by its use of non-negativity constraints. These constraints lead to a parts-based representation because they allow only additive, not subtractive, combinations. When non-negative matrix factorization is implemented as a neural network, parts-based representations emerge by virtue of two properties: the firing rates of neurons are never negative and synaptic strengths do not change sign. |
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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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| Mitchell, J.F., Stoner, G.R., Reynolds, J.H. Object-based attention determines dominance in binocular rivalry Nature 2004 (429):410-413 [pdf] |
| A question of long-standing interest to philosophers, psychologists and neuroscientists is how the brain selects which signals enter consciousness1,2. Binocular rivalry and attention both involve selection of visual stimuli, but affect perception quite differently. During binocular rivalry, awareness alternates between two different stimuli presented to the two eyes. In contrast, attending to one of two different stimuli impairs discrimination of the ignored stimulus, but without causing it to disappear from consciousness. Here we show that despite this difference, attention and rivalry rely on shared object-based selection mechanisms. We cued attention to one of two superimposed transparent surfaces and then deleted the image of one surface from each eye, resulting in rivalry. Observers usually reported seeing only the cued surface. They were also less accurate in judging unpredictable changes in the features of the uncued surface. Our design ensured that selection of the cued surface could not have resulted from spatial, ocular or feature-based mechanisms. Rather, attention was drawn to one surface, and this caused the other surface to be perceptually suppressed during rivalry. These results raise the question of how object representations compete during these two forms of perceptual selection, even as the features of those objects change unpredictably over time. |
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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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| Harris, C.M., Wolpert, D.M. Signal-dependent noise determines motor planning Nature 1998 (394):780-784 [pdf] |
| When we make saccadic eye movements or goal-directed arm movements, there is an infinite number of possible trajectories that the eye or arm could take to reach the target. However, humans show highly stereotyped trajectories in which velocity profiles of both the eye and hand are smooth and symmetric for brief movements. Here we present a unifying theory of eye and arm movements based on the single physiological assumption that the neural control signals are corrupted by noise whose variance increases with the size of the control signal. We propose that in the presence of such signal-dependent noise, the shape of a trajectory is selected to minimize the variance of the final eye or arm position. This minimum-variance theory accurately predicts the trajectories of both saccades and arm movements and the speed accuracy trade-off described by Fitt s law. These profiles are robust to changes in the dynamics of the eye or arm, as found empirically. Moreover, the relation between path curvature and hand velocity during drawing movements reproduces the empirical two-thirds power law. This theory provides a simple and powerful unifying perspective for both eye and arm movement control. |
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| von Melschner, L., Pallas, S.L., Sur, M. Visual behaviour mediated by retinal projections directed to the auditory pathway Nature 2000 (404):871-876 [html] |
| An unresolved issue in cortical development concerns the relative contributions of intrinsic and extrinsic factors to the functional specification of different cortical areas1-4. Ferrets in which retinal projections are redirected neonatally to the auditory thalamus5 have visually responsive cells in auditory thalamus and cortex, form a retinotopic map in auditory cortex and have visual receptive field properties in auditory cortex that are typical of cells in visual cortex5-8. Here we report that this cross-modal projection and its representation in auditory cortex can mediate visual behaviour. When light stimuli are presented in the portion of the visual field that is 'seen' only by this projection, 'rewired' ferrets respond as though they perceive the stimuli to be visual rather than auditory. Thus the perceptual modality of a neocortical region is instructed to a significant extent by its extrinsic inputs. In addition, gratings of different spatial frequencies can be discriminated by the rewired pathway, although the grating acuity is lower than that of the normal visual pathway. |
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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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| Sadato, N. & al Activation of the primary visual cortex by Braille reading in blind subjects Nature 1996 (380):526-528 |
| O'Regan, J.K., Rensink, R.A., Clark, J.J. Blindness to scene changes caused by mudsplashes Nature 1999 (34):398 |
| Watts, D.J., Strogatz, S.H. Collective dynamics of "small-world" networks Nature 1998 (393):440-442 |
| Davidoff, J., Davies, I., Roberson, D. Colour categories in a stone-age tribe Nature 1999 (398):203-204 |
| Blakemore, C., Cooper, G. Development of the brain depends on the visual environment Nature 1970 (228):471-478 |
| Olshausen, B., Field, D. Emergence of simple-cell receptive field properties by learning a sparse code for natural images Nature 1996 (381):607-609 |
| Blakemore, C., Mitchell, D.E. Environmental modification of the visual cortex and the neural basis of learning and memory Nature 1973 (241):467-468 |
| 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 |
| Sharma, J., Angelucci, A., Sur, M. Induction of visual orientation modules in auditory cortex Nature 2000 (404):841-847 |
| MacLeod, D.I. Influence of Scene Statistics on Colour Constancy Nature 2002 (415):637--640 |
| Humphrey, N.K., Keeble, G.R. Interactive effects of unpleasant light and unpleasant sound Nature 1975 (253)5490:346-347 |
| Lee, D.D., Seung, H.S. Learning the Parts of Objects by Non-negative Matrix Factorization Nature 1999 (401):788 |
| Feldman, j. Minimization of Boolean complexity in human concept learning Nature 2000 (407):63-633 |
| Mitchell, J.F., Stoner, G.R., Reynolds, J.H. Object-based attention determines dominance in binocular rivalry Nature 2004 (429):410-413 |
| Friedrich, R.W. Odorant receptor make scents Nature 2004 (430) |
| Braddick, O. & al Possible blindsight in infants lacking one cerebral hemisphere Nature 1994 (360):461-463 |
| Wexler, M., Panerai, F., Lamouret, I., Droulez, J. Self-motion and the perception of stationary objects Nature 2001 (409):85-88 |
| Harris, C.M., Wolpert, D.M. Signal-dependent noise determines motor planning Nature 1998 (394):780-784 |
| Bach-y-Rita, P. & al Vision substitution by tactile image projection Nature 1969 (221):963-964 |
| von Melschner, L., Pallas, S.L., Sur, M. Visual behaviour mediated by retinal projections directed to the auditory pathway Nature 2000 (404):871-876 |
| Shams, L., Kamitani, Y., Shimojo, S. What you see is what you hear Nature 2000 (408):788 |