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 aberrantly at the optic chiasm resulting in partially superimposed representations of opposite hemifields in each hemisphere of visual cortex. Previous functional magnetic resonance imaging (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 locations 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 (pRFs). Here, we used pRF modeling to fit both single and dual pRF models to the visual responses of voxels in visual areas V1 to V3 of five subjects with albinism. We found that subjects with albinism (but not controls) have sizable clusters of voxels with unequivocal dual pRFs consistently corresponding to, but not fully coextensive with, regions of hemifield overlap. These dual pRFs were typically positioned at locations roughly mirrored across the vertical meridian and were uniquely clustered within a portion of the visual field for each subject.
This study shows that retinocortical miswiring in albinism results in single imaging voxels with bilateral dual pRFs. Voxels with dual pRFs are numerous in subjects with albinism but not control subjects and occur selectively in cortical regions where the right and left hemifield representations are superimposed. Our results agree with previous studies which predict that dual pRFs in albinism are positioned at approximately mirror image locations across the VM but suggest that this symmetry is not always precise. Thus, in order to accurately fit responses from all dual pRF voxels, models cannot be rigidly mirror symmetrical, but must instead incorporate some flexibility. Finally, dual pRFs in albinism were not evenly distributed across the visual field, but were instead uniquely clustered within a portion of the visual field for each subject. In the future, mapping subjects’ unique dual pRF distributions could guide spatially focused psychophysical tests aimed at revealing previously undetected perceptual consequences of retinocortical miswiring in albinism.