The visually evoked subcortical potential: is related to the electroretinogram?

The visually evoked subcortical potential (VESP) of mean latencies P21-N28-P36 has previously been recorded at an electrode site around the mastoid process. An initial topographical study of the potential indicated that it was independent of the electroretinogram (ERG), and monocular stimulation showed bilateral reduction, which suggests that the VESP is of postchiasmal origin. A more detailed topographical study of the scalp and facial distribution of the ERG and its oscillatory potentials has been carried out, with simultaneous recording of the VESP. Two monocular fields of ERG activity have been demonstrated, each having a wide area of distribution and interacting medially. Remnants of the ERG b-wave have been detected at many electrode sites, but they were of different amplitude and morphology from the VESP. Results are also presented from two atypical control subjects and two patients, providing further evidence of the separate genesis of the ERG and VESP.