High-resolution functional MRI responses to chromatic and achromatic stimuli in V1 and V2

The non-human primate (NHP) visual system processes color and form in separate (but interacting) pathways. While post-mortem studies have suggested that these pathways in human V1 and V2 are similar to NHP V1 and V2, in that they have superficial cytochrome oxidase blobs and thin stripes associated with color processing, this has been challenging to validate in vivo. Recent advances in high-resolution neuroimaging have facilitated the mesoscopic-scale exploration of the human visual cortex. Here we used 7T fMRI to investigate differences in V1 and V2 activation to chromatic vs achromatic stimuli in five subjects across two scanning sessions. Achromatic checkerboards with low spatial frequency and high temporal frequency targeted the color-insensitive magnocellular pathway and chromatic (red-green) checkerboards with higher spatial frequency and low temporal frequency targeted the color-selective parvocellular pathway. This work resulted in three main findings: First, responses driven by chromatic stimuli had a laminar profile biased towards superficial layers of V1, compared to responses driven by achromatic stimuli. This is consistent with the finding of strong color selective responses in the cytochrome oxidase blobs in the superficial cortex of NHPs. Second, parafoveal V1 displayed a stronger preference for chromatic stimuli compared to peripheral V1. This is consistent with behavioral studies that have demonstrated decreasing sensitivity to red-green color contrast with increasing eccentricity. Finally, we found alternating, stimulus-selective bands stemming from the V1 border into V2 and V3 across both sessions. Similar alternating patterns, termed "stripes", have been previously found in both NHP and human extrastriate cortex. Our data show similar band sizing in V2 as in previous human histology studies. Together, our findings provide a strong in vivo validation that the organization of color and form processing in V1 and V2 is as predicted from previous NHP and post-mortem studies.