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Soumya Chatterjee; Micromaps and blobs: fine structure of color representation in primary visual cortex. Journal of Vision 2011;11(15):23. doi: https://doi.org/10.1167/11.15.23.
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Electrophysiological and optical imaging studies have made a strong case for the existence of a functional architecture related to color selectivity in V1. The known architecture comprises small cortical patches, roughly centered on cytochrome oxidase blobs, that respond preferentially to chromatic visual stimulation. There are hints of richer substructure within these color patches, but the resolution limits of intrinsic optical imaging, as well as the difficulty of inferring functional geometry from single-unit recordings, have precluded a more detailed description. To bridge the gap, we used two-photon calcium imaging to create maps of color selectivity at single-cell resolution in V1 of the adult macaque. With spatially uniform cone-isolating stimuli, we found that there are unambiguous ensembles of segregated color cells. The ensembles can be further subdivided into small clusters of cells with distinct chromatic preferences, in effect creating micromaps of color within larger color clusters. These micromaps form functional columns that are in register with blobs. Further, we compared responses to both spatially uniform and spatially structured cone-isolating stimuli. Uniform stimuli invariably produced tightly segregated color responses centered on blobs, while significantly suppressing activity in interblobs. Spatially structured (drifting bar) stimuli roughly maintained the color patches seen with uniform stimuli, but responses spread far into interblobs as well. We speculate that cortex acts as a ‘switch’ between two systems based on the spatial frequency content of the visual input. Low spatial frequency (or uniform) color stimuli are represented by only a subset of V1 color cells that tend to lie in blobs, consistent with the lowpass nature of chromatic processing, while structured stimuli seem to engage both blobs and interblobs in a larger representation.
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