Scalp distribution of pattern visual evoked potentials in normal and hemianopic monkeys.

We examined the hemispheric distribution of the pattern visual evoked potential in 3 Cynomolgus monkeys, before and after right optic tractotomy or left occipital lobectomy. The stimuli were vertical gratings of 4 spatial frequencies, presented using counterphase and on-off modulation at 1 and 8 Hz. In the normal monkey, the amplitude and latency of the PVEPs elicited by 1 Hz stimulation were similar across all electrode sites. While no differences were found in the phase of the PVEP elicited by 8 Hz stimulation across the different derivations, PVEP amplitude generally decreased over the lateral electrodes. Once experimental hemianopia was created, PVEPs recorded over the midline and the intact hemisphere were normal. However, partially polarity-inverted PVEPs of smaller amplitude were recorded over the deafferented hemisphere. In addition, the interhemispheric phase difference became more prominent in the PVEP recorded over the deafferented hemisphere as the spatial frequency was increased.