Unpacking the V1 map: Differential covariation of preferred spatial frequency and cortical magnification across spatial dimensions.

Primary visual cortex (V1) has long served as a model system for understanding cortical organization. Although its structural and functional properties vary markedly across its surface, patterns of covariation suggest possible underlying constancies. Such constancies would imply that V1 is composed of multiple identical units whose visual properties diff er only due to diff erences in their inputs. To test this, we used fMRI to investigate how V1 cortical magnifi cation and preferred spatial frequency covary with eccentricity and polar angle, measured in 40 observers. V1 cortical magnifi cation and preferred spatial frequency were strongly correlated across eccentricity and around polar angle, however their relation diff ered between these dimensions: they were proportional across eccentricity but not polar angle. The constant ratio of cortical magnifi cation to preferred spatial frequency when measured as a function of 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 diff ers 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 to understand how V1 is organized.