Colour coding in the cerebral cortex: The reaction of cells in monkey visual cortex to wavelengths and colours

Abstract The experiments described here were designed to study the extent to which the responses of single wavelength-selective cells in monkey visual cortex correlate with colours as perceived by normal human observers. Consequently, the perceived colour of a surface and the reaction of a cell to it were simultaneously determined. Part I describes the reaction of wavelength-selective cells in areas V1 and the V4 complex to surfaces of perceived natural and void colours. The natural colour of an area is its perceived colour when viewed as part of a complex scene. It bears no simple relationship to the wavelength composition of the light reflected from it. 14,15 The void colour of an area is its perceived colour when viewed in isolation, by excluding everything else from the field of view. Void colours correlate with wavelength composition. The results show that (a) some wavelength-selective cells respond to an area of any natural colour provided it reflects a sufficient amount of light of their preferred wavelength. These cells are referred to as wavelength-selective or “WL” cells. (b) Others react only to surfaces whose natural colour we associate with that of their preferred wavelength. For these, unlike the WL cells, the precise wavelength composition of the light reflected from the area in their receptive fields is not critical. They are referred to as colour-coded or ‘CO’ cells. (c) The responses of WL and of CO cells do not necessarily correlate with void colours. (d) Some cells respond to areas of one perceived colour only but it is not always possible to plot a wavelength sensitivity curve for such cells. It follows that a wavelength sensitivity curve cannot, by itself, indicate whether a cell is of the WL or the CO type and, moreover, that one cannot predict from such a curve whether the responses of a cell will correlate with particular colours. Part II describes perceptual experiments undertaken to determine whether the colour of the after-image produced by viewing a surface depends upon the wavelength composition of the light reflected from that surface. The results of experiments similar to those of Land show that, just as the perceived natural colour of a surface does not bear a simple relationship to the wavelength composition of the light reflected from it alone, neither does the colour of its after-image. The colour of a void after-image, however, correlates with wavelength composition. Part III describes the reaction to natural and void colours of cells in the VI area of the visual cortex that give an ON response to some wavelengths and an OFF response to others. The results show that such cells can be made to give an ON or an OFF response to a surface of any perceived natural colour and that their responses do not necessarily correlate with void colours either. They are therefore a sub-division of the WL category (WLO cells). The implications of these results for understanding colour vision are discussed.