Hemisphere contributions to the composition of the pattern-evoked potential waveform.

The transverse distribution of scalp-recorded potentials evoked by pattern reversal stimulation was studied in 50 healthy subjects. In most individuals the full-field responses were symmetrical over the occipital scalp, but important variations in distribution, symmetry and waveform were recorded in some cases. Asymmetrical responses were similar for each eye (i.e., they were "uncrossed" or homonymous asymmetries). Full-field peak latencies and amplitudes in the lateral channels were more variable than those at midline electrodes. Half-field responses were markedly asymmetric with well-lateralised components widespread over occipital-parietal scalp. In contrast to the full-field responses, component values measured near the midline were less consistent than those from lateral channels due to waveform distortions in this area ("transitional zone"). Upper field stimulation is particularly likely to produce such midline waveform distortions. Activity recorded from the scalp contralateral to the half-field stimulated shows more inter-individual and inter-hemispheric variation than that recorded from ipsilateral electrodes. Variants in the full-field waveform can be accounted for by relative differences in amplitude and distribution of the ipsilateral and contralateral components from each half field. The algebraic sum of these half-field components does not differ significantly from the components of the separately recorded full-field response. Furthermore, responses from the surviving half-field in patients after total hemispherectomy contain all the ipsilateral and contralateral half-field components seen in healthy subjects.