Aberrant Population Receptive Fields in Human Albinism

Retinotopic organization is a fundamental organizational feature of visual cortex thought to play a vital role in encoding spatial location. One important aspect of normal retinotopic organization is the contralateral representation of the right and left hemifields in visual cortex. In human albinism, the temporal retinal afferents decussate pathologically at the optic chiasm resulting in partially superimposed right and left hemifield representations in each hemisphere of visual cortex. Thus, albinism provides a natural model of aberrant retinotopy in which to study the development and function of retinotopic maps. Previous fMRI studies in human albinism suggest that the right and left hemifields are superimposed in a mirror-symmetric manner such that each imaging voxel responds to two regions in visual space mirrored across the vertical meridian. However, to date no group has explicitly attempted to model the receptive field properties of single voxels in albinism. Here we used traditional fMRI retinotopic mapping techniques in conjunction with population receptive field (pRF) modeling in subjects with albinism to fit each voxel9s rotating wedge timecourse in V1-V3 with both single and dual pRF models. We find that most subjects with albinism (but not controls) have sizable clusters of voxels with convincing dual pRFs often corresponding to regions of R-L hemifield overlap. However, not all dual pRFs were mirror symmetrical across the vertical meridian. These asymmetric dual pRFs are not predicted by currently accepted models and suggest that retinocortical miswiring in albinism may be more complicated and heterogeneous than previously appreciated.