Average multichannel EEG potential fields evoked from upper and lower hemi-retina: latency differences.

Checkerboard reversals shown to the upper hemiretina evoke EEG potentials which in anterior-posterior derivations are approximately inverted in polarity compared with potentials evoked by lower hemiretinal stimulation. Using a 48-channel system, EEG scalp field distributions of such responses were mapped. Examination of the map sequences shows that occipital positive maximal field values start to develop at about the same time in the two stimulus conditions, but peak much earlier and somewhat more anteriorly for upper than for lower hemiretinal stimulation. Thus, there is a difference of response latency in the two conditions, which accounts for the approximate inversion of polarity. Possible correlations with reports of higher receptor cell density, higher visual acuity, and shorter motor reaction time of the upper hemiretinal system are noted.