Cortical depth-dependent population receptive field size variation in human V1, V2 and V3

Abstract

The early visual cortex is organized in accordance with well-established principles of retinotopy and cortical magnification. Receptive field (RF) size increases systematically with eccentricity from fovea to periphery and along the visual hierarchy. However, the organization of RFs across cortical depths is not yet well understood. While non-human primate neurophysiology shows RF size variation across cortical laminae, evidence in humans is lacking. Here, we used submillimeter fMRI to map RF properties in vivo at the scale of cortical laminae. We measured gradient-echo blood oxygenation level-dependent (BOLD) responses to a drifting bar stimulus using 7 Tesla fMRI and population receptive field (pRF) mapping in four human participants. We projected the fMRI data to eight equivolumetric cortical surfaces based on white matter and pial surface reconstructions. Fitting a pRF model to the BOLD time series of each vertex of each surface, we estimated the location in visual space and pRF size that best explain visual field selectivity. We computed the pRF size at 2 degrees of eccentricity in three early visual regions of interest (ROIs) V1, V2 and V3. For each participant and ROI, we characterized the cortical depth profile of pRF size, as well as the profile of the surround suppression to center excitation ratio (suppression index). We replicate previous findings of a U-shaped relation between pRF size and cortical depth in V1. Moreover, we extend these findings by demonstrating depth-dependent patterns in V2 and V3. Similarly to V1, pRF sizes in V2 are largest in deep layers, followed by superficial and middle layers, with the reverse pattern in V3. The suppression index remains flat across depths, consistent with previous reports. Our findings demonstrate that pRF size variation across cortical depth is robustly quantifiable in humans in vivo, lending support to future examinations of feedforward and feedback mechanisms of spatial vision.