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Bevil Conway; Specialized color cells in V1 and beyond. Journal of Vision 2007;7(15):27. doi: 10.1167/7.15.27.
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Single-unit recording and cone-isolating stimuli were used to show that primary visual cortex (V1) contains specialized cone-opponent cells, many of them double-opponent. Cone-opponent cells in V1 are distinguished from complex-equiluminance cells, a more numerous population of neurons that are responsive to equiluminant color boundaries irrespective of the configuration of the hues forming the boundary. Cone-opponent cells and not complex-equiluminance cells, could contribute to hue perception by encoding local color contrast and contributing to color constancy; complex equiluminance cells, on the other hand, likely assist in defeating camouflage. Testing neural responses at equiluminance has been a popular assay of the color properties of neurons, but these results show that while spatial frequency tuning to equiluminant stimuli may be necessary, it is not sufficient to evaluate a neuron s contribution to color contrast. A detailed analysis of the spatial and temporal organization of the cone signals of cone-opponent cells suggests that cone increments and decrements are processed independently, resulting in unbalanced cone inputs to some cells (often S-cone dominated cells). As a population, V1 cone-opponent cells show hue tuning to a limited region of color space, roughly consistent with the red-cyan and blue-yellow chromatic axes. To identify candidate brain regions within extrastriate cortex that may be involved in encoding specific hues (red, orange, yellow, brown, green, blue, purple), we performed functional magnetic resonance imaging of alert macaques. Specialized color modules were found within posterior inferior temporal cortex (a region encompassing V4); using targeted single-unit recordings guided by fMRI within the same subjects, neurons located within these color globs were found to be strongly tuned to specific hues. These results argue that color perception is mediated at an intermediate stage by specialized cone-opponent cells in V1 and subsequently by specialized hue-selective neurons that are clustered within the extrastriate brain.
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