Abnormal visual pathways in human albinos studied with visually evoked potentials

Abstract

Numerous investigations in albino mammals using neurophysiological and neuroanatomical techniques report an abnormal crossing projection from the temporal retina. Recent studies in human albinos suggest a similar defect. Averaged visually evoked potentials to flashed checkerboard patterns were recorded bilaterally from the scalp overlying the occipital regions in normal and albino human subjects. Square stimulus patterns (10°) were presented to the nasal-temporal retina and hemipatterns to the nasal or temporal retina of each eye. Observations of visual fields, monocular acuities, and stereopsis were recorded. Normal left and right hemispheric responses were symmetrical to monocular nasal-temporal stimulation; in albinos the contralateral early latency responses were larger than the ipsilateral. In monocular nasal stimulation in albinos and in normals, the amplitude of contralateral evoked responses was larger than the ipsilateral; in normals the largest responses to temporal stimulation were ipsilateral. By contrast, temporal retina stimulation of albinos usually produced larger early latency components contralaterally. Temporal stimulation at 5° from the vertical meridian in albinos produced larger contralateral responses. Stimulation beyond 15° gave smaller components contralaterally; new larger amplitude components appeared ipsilaterally. At 30 and 45° eccentricity, ipsilateral responses were reduced. Albino subjects had normal monocular visual fields but failed to fuse stimuli from random dot stereograms. Results of this study provide further evidence for abnormal visual pathways and functional anomalies in human albinos.