Evidence of foveal splitting in a commissurotomy patient.

A bilateral projection of the central fovea, produced by naso-temporal overlap at the retinal vertical meridian, has been proposed as the anatomical basis of macular sparing. To evaluate this claim, a commissurotomy patient was required to compare target figures presented 1 degrees or less from the retinal midline with reference figures presented 2.5 degrees from the midline in the same or opposite visual field. The subject judged whether the target and reference were the same or different. It was found that the subject's fovea was effectively split with respect to the cerebral hemispheres; targets in the same visual field as the reference were readily matched with the reference, but accuracy dropped to chance for targets in the opposite field. Ways of reconciling this data with anatomical evidence of nasotemporal overlap and reports of macular sparing are discussed.