Abstract
Introduction: Variations in population receptive field (pRF) size are crucially related to the sensitivity to spatial frequency (SF) across visual cortex, across both cortical areas, and across the representation of retinotopic eccentricity in each area. For instance, cortical sites that represent peripheral (compared to central) visual fields have larger pRF sizes and respond more strongly to stimuli with lower SFs. However, within a given retinotopic representation, it remains unknown whether pRF size differs with variations in columnar sensitivity (e.g. color, disparity, motion, etc.). Here we tested this hypothesis, partly by leveraging a prior finding: we found a stronger response to low SF stimuli in V2/V3 thick stripes, compared to thin stripes (Tootell and Nasr, 2017). Methods: Using high-resolution fMRI (7T; voxel size: 1 mm isotropic), in 4 human subjects, we measured the pRF size by presenting moving bars at multiple orientations and motion directions. In two individuals, we also measured the reproducibility of the results by scanning them on two different days. In all subjects, we also independently localized V2/V3 thin vs. thick stripes (Tootell and Nasr, 2021). Results: Consistent with previous studies, in all individuals, we found a larger pRF in peripheral vs. central representations, and a progressively larger pRF size in V1<V2<V3<V3A. The pRF sizes were reproducible across sessions, in both deep and superficial cortical layers (r>0.39). Here, we found that average pRF size in V2/V3 was significantly larger in thick compared to thin stripes, even when the measurements were confined to iso-eccentric (3˚<r<10˚) representations. In all subjects, this effect was observed within both deep and superficial layers, consistent with a columnar organization. Conclusion: Our results show primary evidence for a heterogeneous, column-based distribution of pRF sizes at iso-eccentric sites. This supports the hypothesis that pRF size co-varies with column-scale variations in sensitivity to SF.