| Romney, A.K., Indow, T. A model for the simultaneous analysis of reflectance spectra and basis factors of Munsell color samples under D65 illumination in three-dimensional Euclidean space Proceedings of the National Academy of Sciences USA 2002 (99)17:11543-11546 |
| In this paper we present the results of an analysis of the physically measured surface reflectance spectra of 360 matte Munsell chromatic color chips plus 10 flat achromatic vectors corresponding to Munsell value levels 10 (white) to 1 (near black) for a total sample size of 370. Each of the 370 spectra was multiplied by the spectral radiant power distribution of D65 light so that the final results represent the spectra of reflected light from Munsell color chips under D65 illumination. We simultaneously model the structure of the color chips and the spectra in a common three-dimensional Euclidean space, oriented to yield the most interpretable structure with respect of the Munsell color structure. In this orientation, axis 1 roughly corresponds to the mean power of the spectral reflectance (approximate Munsell value), axis 2 goes from Munsell red to blue-green, and axis 3 goes from Munsell green-yellow to purple. Basis factors for the spectra are also plotted against wavelength and Munsell hue. These plots have implications for theories of opponent processes. By plotting the chips and spectra in the same space we obtain virtually exact correspondences between the various Munsell hues and spectral values in nanometers for comparison to those obtained by previous researchers. Mathematical derivations are provided to validate the common Euclidean model. |
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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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| 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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| 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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| 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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| 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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| Romney, A.K., Indow, T. A model for the simultaneous analysis of reflectance spectra and basis factors of Munsell color samples under D65 illumination in three-dimensional Euclidean space Proceedings of the National Academy of Sciences USA 2002 (99)17:11543-11546 |
| 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 |
| 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 |
| Kay, P., Regier, T. Resolving the question of color naming universals Proceedings of the National Academy of Sciences USA 2003 (100)15:9085-9 |
| Coppola, D., Purves, D. The extraordinarily rapid disappearance of entoptic images Proceedings of the National Academy of Sciences USA 1996 (96):8001-8004 |
| 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 |
| 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 |
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