COLOR-VISION - ISOLATING MECHANISMS IN OVERLAPPING STREAMS

This chapter discusses electrophysiologically the basis of color vision, and show that cortical visual evoked potentials (VEPs) correlate with the known spatio-temporal limits of color vision. First, the extent of overlap of the spatio-temporal properties of achromatic and chromatic-opponent cells is discussed. Second, an assessment of the relationship between local and global electrophysiological measures of color vision is made, and field potential recordings are compared with single-unit and multi-unit recordings made in the same cortical site. Third, it is shown that the use of low-contrast, low-spatial frequency gratings presented at a low temporal rate dichotomize the global responses measured as VEPs: achromatic gratings elicit transient on-of responses, consistent with the responses of most transient M-cells, which are dependent on a step change in contrast, whereas chromatic gratings generate sustained responses that are different for onset and offset presentations, and thus similar to the responses of chromatic opponent cells. Fourth, chromatic VEPs are shown to be selectively diminished by administration of N-Methyl-D-aspartate (NMDA) antagonists, consistent with the notion that NMDA receptors facilitate sustained responses. Finally, chromatic VEPs recorded from subjects with lesions of cortical visual area V4 are seen to be unchanged in shape, and little different in magnitude, relative to normal. This suggests that color detection and categorization are mediated by mechanisms operating not later than V2 in the chromatic processing stream. © 1993, Academic Press Inc.