JN Makoto Kusunoki , Konstantinos Moutoussis and Semir Zeki Color - Selective Cells in Monkey Area V 4 Effect of

Effect of background colors on the tuning of color-selective cells in monkey area V4. When objects are viewed in different illuminants, their color does not change or changes little in spite of significant changes in the wavelength composition of the light reflected from them. In previous studies, we have addressed the physiology underlying this color constancy by recording from cells in areas V1, V2, and V4 of the anesthetized monkey. Truly color-coded cells, ones that respond to a patch of a given color irrespective of the wavelength composition of the light reflected from it, were only found in area V4. In the present study, we have used a different approach to test the responses of V4 cells in both anesthetized and awake behaving monkeys. Stimuli of different colors, embedded within a Mondrian-type multicolored background, were used to identify the chromatic selectivity of neurons. The illumination of the background was then varied, and the tuning of V4 neurons was tested again for each background illumination. With anesthetized monkeys, the psycho-physical effect of changing background illumination was inferred from our own experience, whereas in the awake behaving animal, it was directly reported by the monkey. We found that the majority of V4 neurons shifted their color-tuning profile with each change in the background illumination: each time the color of the background on the computer screen was changed so as to simulate a change in illumination , cells shifted their color-tuning function in the direction of the chromaticity component that had been increased. A similar shift was also observed in colored match-to-sample psychometric functions of both human and monkey. The shift in monkey psychometric functions was quantitatively equivalent to the shift in the responses of the corresponding population of cells. We conclude that neurons in area V4 exhibit the property of color constancy and that their response properties are thus able to reflect color perception. Perhaps the most striking characteristic of the organization of the visual cortex is its functional specialization, by which we mean that different attributes of the visual scene, among them color, are processed by anatomically separate and functionally specialized systems (Livingstone and Hubel 1988; Zeki 1978; Zeki et al. 1991). Functional specialization is also reflected in the temporal perceptual dimension because different attributes of the visual scene are perceived asynchronously, color being perceived before motion and form (Arnold et al. 2001; Moutoussis and Zeki 1997a,b). The notion of a …