Scalp distribution of visual evoked potentials to foveal pattern and luminance stimuli

Scalp potentials elicited by pattern reversal, pattern onset, and flash stimulation have been studied in normal subjects by means of a 16-channel brain mapping system. In two groups of experiments the topography of the major positive component in each response was compared with 4 degrees full- and lateral half-field stimuli and, for more detailed exploration of the pattern components by M-scaled stimuli selected to activate 1 cm2-patches of striate cortex and associated extrastriate re-projections. The 4 degrees stimuli were found to elicit scalp distributions for the pattern reversal P100 and the pattern onset C1 consistent with striate and extrastriate visual cortical origins respectively. Data on the flash P2 component suggest that stimulation was not localized accurately; this may have been due to lateral spread of neural activity in the retina. Predictably, the M-scaled stimuli showed extensive inter- and intraindividual variations depending on stimulus type and location relative to fixation. The manner in which, in some subjects, the region of maximal activity on the map appears to migrate with change of stimulus position possibly reflects local retinotopic order in visual evoked potential generator areas. However, data are neither extensive nor detailed enough to be conclusive.