Abstract
In receptoral and early post-receptoral color vision, behavioral and physiological data have been well reconciled, however at the cortical stage these links frequently remain obscure. For example, a wealth of psychophysical studies has demonstrated that color vision has orientation-tuned mechanisms and there is little direct evidence for non-oriented mechanisms. Yet multiple neurophysiological studies have revealed a distinct subgroup of highly color sensitive, isotropic neurons in V1. To measure orientation tuning in color vision and differentiate between non-oriented and orientation-tuned responses to color contrast, we have adapted the classic method of subthreshold summation. The method uses a linking model to tie subthreshold summation data to the underlying detector bandwidths. This method also has the advantage of using very low contrast stimuli, ensuring the color pathway is well isolated from the modulatory effects of cross-orientation masking that contaminate orientation tuning measurements obtained at higher contrasts. At mid spatial frequencies, our results show evidence for orientation-tuned detectors with similar bandwidths for chromatic and achromatic contrast. At low spatial frequencies, however, orientation tuning in color vision becomes extremely broad, and is compatible with detection by non-oriented color mechanisms. These isotropic chromatic mechanisms only appear under monocular conditions. Isotropic detectors, which could be called “blob” detectors, are well equipped for the representation of surface color, whereas orientation-tuned responses are best equipped for edge and contour detection. Such links remain only speculative, however. We are also using the subthreshold summation method to determine the orientation tuning of binocular summation, discussed in a related presentation.