Far-periphery retinotopy reveals a nonlinear relationship between pRF size and eccentricity

The population receptive field (pRF) size is known to increase with the visual field eccentricity in the human visual cortex, and this relationship is generally assumed to be linear. However, conventional functional magnetic resonance imaging (fMRI) visual stimulation methods limit measurements largely to the central visual field, leaving the far periphery less explored. Here, we utilized wide-view stimulation to examine whether the linear pRF size-eccentricity relationship extends to the far periphery. The retinotopic stimuli consisted of rotating wedges and contracting concentric rings with checkerboard textures moving across the visual field. Observers viewed the stimuli through a wide-view binocular visual stimulation system in an MRI scanner, which allowed the visual presentation to cover approximately 90° in the visual field. We applied a 2D Gaussian pRF model to the blood oxygenation level-dependent (BOLD) activations of voxels in areas V1 to V3, hV4, and V3A/B. The relationship between the pRF size and eccentricity from the fovea to the far periphery is better captured by a nonlinear regression function than by the linear functions commonly used in previous central vision studies. Further analysis of the relationship between the pRF estimates and cortical surface location revealed that in areas V1 to V3, the pRF eccentricity and size increased with increasing cortical distance up to certain values before either saturating or decreasing. Notably, these transitions occurred at different cortical distances, revealing distinct cortical-distance dependencies of pRF eccentricity and size that characterize their nonlinear relationship in the far periphery.