Pattern evoked potentials in human albinism. Evidence of two different topographical asymmetries reflecting abnormal retino-cortical projections

Abstract

In fifteen subjects with oculo-cutaneous or ocular albinism pattern-reversal visual evoked potentials (VEP) were recorded to monocular whole-field (16 degrees radius) and right and left vertical half-field (16 degrees radius) stimulation from an array of occipital electrodes. In all 15 subjects the VEP was of low amplitude and the monocular response to whole-field stimulation showed abnormal asymmetry in scalp topography. This asymmetry was similar to that produced by stimulation of the temporal half-field in the same eye. Two distinct types of VEP asymmetry were readily identified in different subjects. In 5 the major positivity of the response (P100) was distributed in the channels ipsilateral to the stimulated eye or temporal half-field, being similar in distribution to the P100 from the temporal hemifield or normal individuals. In 9 other subjects the converse occurred; the P100 was distributed contralaterally. The remaining subject was unique in that the responses from each eye had a distribution that resembled one of the two main groups. The nasal half-field responses also differentiated the two groups. On stimulation of the nasal field in the first group the P100 was usually absent or attenuated and when present it had the same topography as the temporal half-field P100. In contrast the nasal half-field P100 from the second group was always present and, although almost invariably smaller than the temporal half-field P100, it had an identical distribution. There was no correlation between these two patterns of VEP asymmetry and clinical or known genetic features. The findings confirm that in human albinos each hemisphere receives a predominantly monocular input from the contralateral eye; i.e. in addition to the temporal, half-field, approximately 20 degrees of the nasal half-field is projected to the hemisphere contralateral to each eye and not, as in normal subjects, to the ipsilateral hemisphere. Furthermore, the findings of two distinct and in some respects opposite types of VEP topographical asymmetry raises the possibility of two variants of the geniculo-cortical projection patterns existing in human albinos similar to those described in the Siamese cat.