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HUMAN COLOUR VISION: 2. COLOUR APPEARANCE AND CORTICAL TRANSFORMATIONS

Four terms (red, green, yellow, and blue) are necessary and sufficient to describe the appearance of hue. Parvo cells of the LGN respond maximally to modulation along either an S-(L+M) or an L-M axis, but few cells are tuned to intermediate directions. These axes do not correspond to the colour appearance axes. Colour appearance requires at least one more stage of processing, a stage that transforms the second stage axes in the cortex. Receptive fields in area V1 vary much more in their colour tuning than in LGN, and they have significantly more S-cone input. Thus, a major accomplishment of striate cortex may be rotation of the colour axes by integrating S-cone input. Major differences in chromatic properties of receptive fields between area V1 and areas V2 and V3 have not been reported. V4 cells, however, have large receptive field surrounds that may support colour constancy and promote segregation of figure and ground by colour. It seems unlikely that V4 is a “colour centre” in the monkey brain, and it is equally unlikely to be homologous with those regions of human cortex associated with achromatopsia. 1. Perception of Hue The previous chapter provided an introductory account of colour matching and discrimination based on the organisation of neural signals in the retina and LGN. An observer could, of course, say much more about these stimuli if asked to describe what they look like. While our understanding of the neural mechanisms mediating colour appearance is incomplete, it is clear from what is known that hue depends on cortical transformations of the retino-geniculate signals. Receptor/colourmatching space is sometimes considered a first (functional) stage of colour processing, while a second stage is described by the S-(L+M) and L-M axes that characterise ganglion and LGN cell receptive fields. Colour appearance requires at least one more stage of processing, a third stage that is the subject of this chapter. Consideration of evidence for third stage mechanisms of colour appearance in visual cortex leads to a discussion of the interaction of colour processing with form and motion processing, colour constancy, cerebral achromatopsia and the role of consciousness in behaviour dependent on colour vision. Consider again the stimulus described in the first figure of the previous chapter. Once the two half-fields are matched metamerically, the match will be maintained with increases in intensity (scalar invariance), addition of another light (additivity) and changes in sensitivity due to moderate chromatic adaptation (persistence law). Note, however, that even though the two fields match under these stimulus manipulations, colour appearance is likely to change. If the observer is asked to describe these changes, a variety of words might be selected due to somewhat arbitrary cultural and linguistic conventions. If, however, we limit the number of terms that an observer is offered, only four hue names (red, green, yellow, and blue) are found necessary and sufficient to describe all aspects of colour appearance (Hering, 1878, 1920). Thus, as shown experimentally, what we call orange can be described just as well with the terms red and yellow, while purple can be described just as well with the terms red and blue. Because these perceptual qualities are derived from the way our nervous system codes colour, they are largely independent of culture (Abramov & Gordon, 1994). Figure 1 shows colour-naming data for 1° monochromatic fields presented as foveal 1-sec flashes. Observers scaled the hue in terms of percentages of red, green, blue or yellow. For example, a light at about 610 nm that appears orange, might be described as 60% red and 40% yellow. After a little practice, observers scale hues in this way with high reliability (Boynton & Gordon, 1965). Squares show red or green responses from 0 to 100% on the left ordinate, while blue or yellow responses are plotted from 100 to 0% on the right ordinate. The data could be plotted in this way because observers, despite being free to use the terms red and green simultaneously or the terms blue and yellow simultaneously, almost never did so. The arrows in Figure 1 denote the wavelengths of unique blue, green or yellow, i.e., unitary hues that cannot be described by combinations of any other hue terms. Hering (1920) attached special significance to the observation that while we can experience red in combination with yellow or blue, we do not experience it in combination with green at the same time and place. Similarly, blue and yellow are not experienced cotemporally and cospatially. The experiences of red and green cancel each other, as do the sensations of blue and yellow. Hering, therefore, called these paired colours, opponent hues. He further proposed that opponent-hue sensations were due to corresponding opponent-physiological processes with two modes of response, corresponding to the modern terms, excitation and inhibition. Thus, according to Hering, red-green and blue-yellow opponent physiological processes account for all the hues experienced by human trichromats. A complete description of colour appearance must also include the achromatic or black-white components of a colour. For example, pink and maroon look quite different even though both have red as the dominant hue component. The difference arises because pink also contains a prominent whiteness component while maroon has a prominent blackness component. To account for the achromatic dimensions, 1 Unique red is extraspectral, meaning that it must be produced by a mixture of two monochromatic lights such as a short-wave (reddish-blue) and a long-wave light (yellow-red). The yellow and blue components cancel each other, leaving only the red. 0 25 50 75 100 400 500 600 700 R ed o r G re en ( % ) 0