Preferred spatial frequency covaries with cortical magnification in human primary visual cortex

Primary visual cortex (V1) has played a key role in understanding the organization of cerebral cortex. Both structural and functional properties vary sharply throughout the human V1 map. Despite large variation, underlying constancies computed from the covariation pattern of V1 properties have been proposed. Such constancies would imply that V1 is composed of multiple identical units whose visual properties differ only due to differences in their inputs. To test this, we used fMRI to investigate how V1 cortical magnification and preferred spatial frequency covary across eccentricity and polar angle, and across individual observers (n=40). The two properties correlated across individuals, such that those with higher overall cortical magnification (i.e., larger V1 maps) had higher preferred spatial frequency (integrated across the map). Although correlated, the two properties were not proportional, and hence their ratio (mm of cortex per stimulus cycle) was not constant. Cortical magnification and preferred spatial frequency were strongly correlated across eccentricity and across polar angle, however their relation differed between these dimensions: they were proportional across eccentricity but not polar angle. The constant ratio of cortical magnification to preferred spatial frequency across eccentricity suggests a shared underlying cause of variation in the two properties, e.g., the gradient of retinal ganglion cell density across eccentricity. In contrast, the deviation from proportionality around polar angle implies that cortical variation differs from that in retina along this dimension. Thus, a constancy hypothesis is supported for one of the two spatial dimensions of V1, highlighting the importance of examining the full 2D-map, in multiple individuals, to understand how V1 is organized.