Columnar organization of mid-spectral and end-spectral hue preferences in human visual cortex

Multiple color-selective areas have been described in visual cortex, in both humans and non-human primates. In macaques, hue-selective columns have been reported in several areas. In V2, it has been proposed that such hue-selective columns are mapped so as to mirror the order of wavelength through the visible spectrum, within thin-type stripes. Other studies have suggested a neural segregation of mid-spectral vs. end-spectral hue preferences (e.g. red and blue vs. green and yellow), within thin- and thick-type stripes, respectively. This latter segregation could reduce the spatial 'blur' due to chromatic aberration in the encoding of fine spatial details in the thick-type stripes. To distinguish between these and related models, we tested the organization of hue preferences in human visual cortex using fMRI at high spatial resolution. We used a high field (7T) scanner in humans (n = 7), measuring responses to four independent hues, including end-spectral (i.e. red-gray and blue-gray) and mid-spectral (i.e. green-gray and yellow-gray) isoluminant gratings, and also relative to achromatic luminance-varying (control) stimuli. In each subject, thin- and thick-type columns in V2 and V3 were localized using an independent set of stimuli and scans. We found distinct hue-selective differences along the dimension of mid-vs. end-spectral hues, in striate and early extrastriate visual cortex. First, as reported previously in macaques, V1 responded more strongly to end-spectral hues, compared to mid-spectral hues. Second, the color-selective thin-type stripes in V2 and V3 showed a greater response to end- and mid-spectral hues, relative to luminance-varying gratings. Third, thick-type stripes in V2/V3 showed a significantly stronger response to mid-spectral (compared to end-spectral) hues. Fourth, in the higher-tier color-selective area in occipital temporal cortex (n = 4), responses to all four hues were statistically equivalent to each other. These results suggest that early visual cortex segregates the processing of mid-vs. end-spectral hues, perhaps to counter the challenging optical constraint of chromatic aberration.