Aberrant Population Receptive Fields in Human Albinism
Retinotopic organization is a fundamental feature of visual cortex thought to play a vital role in encoding spatial information. One important aspect of normal retinotopy is the representation of the right and left hemifields in contralateral visual cortex. However, in human albinism, many temporal retinal afferents decussate pathologically at the optic chiasm resulting in partially superimposed representations of opposite hemifields in each hemisphere of visual cortex. Previous fMRI studies in human albinism suggest that the right and left hemifield representations are superimposed in a mirror-symmetric manner. This should produce imaging voxels which respond to two separate regions in visual space mirrored across the vertical meridian. However, it is not yet clear how retino-cortical miswiring in albinism manifests at the level of single voxel population receptive fields. Here we used fMRI retinotopic mapping in conjunction with population receptive field (pRF) modeling to fit both single and dual pRF models to the visual responses of voxels in visual areas V1-V3 of five subjects with albinism. We found that subjects with albinism (but not controls) have sizable clusters of voxels with dual pRFs consistently corresponding to, but not fully coextensive with regions of hemifield overlap. These dual pRFs were typically positioned at roughly mirror image locations across the vertical meridian but were uniquely clustered within the visual field for each subject. We also found that single pRFs are larger in albinism than controls, and that single pRF sizes in the central visual field were anti-correlated with subjects foveal cone densities. Finally, dual pRF and aberrant hemifield representation characteristics varied significantly across subjects with albinism suggesting more central heterogeneity than previously appreciated.