A study of short latency photically evoked potentials in man

Abstract

The VEP and ERG are reviewed and a hypothesis is proposed that short latency early components of the VEP may originate in structures of the visual pathway intermediate between retina and visual cortex. Under conventional conditions for generating and recording the cortical VEP (VECP) these short latency potentials are of inconsistent presentation and a technique has been developed to accentuate a number of early components. A triphasic complex of early components was detected in 95% of normal subjects and scalp topographical studies revealed this to be of maximal amplitude around the upper mastoid process. The mean latencies (in m.secs.) of the peaks of the complex were P21.0, N26.2, P33.6. Monocularstimulation produced bilateral reduction in amplitude of the complex indicating a post-chiasmal origin and this evidence in conjunction with the scalp site of its maximum amplitude led to its title the "Visually Evoked Subcortical Potential" (VESP). A topographical study of the ERG showed wide distribution over the face and scalp, but at posterior temporal sites only a vestigial 'b' wave remnant existed in some subjects, the VESP therefore not being a volume-conducted ERG signal. A subcortical origin for the VESP was confirmed by findings in patients with lesions at selected sites along the visual pathway which showed a) in unilateral retinal detachment,abolition of ERG from the affected eye but bilateral preservation of VESP; b) in unilateral non-demyelinating optic nerve lesions preservation of ERGs but reduction or abolition of VESP and VECP; c) in chiasmal compression, abolition of the VESP contralateral to the stimulated eye; d) in hemianopia of subcortical origin, reduction or abolition of the VESP over the affected hemisphere; e) in hemianopia of cortical origin bilateral preservation of VESP with reduced or abolished VECP over the affected hemisphere. The VESP may, with further work, supplement the ERG and VECP in ophthalmic electrodiagnosis.