VISUAL-EVOKED POTENTIALS (VEPS) TO ALTITUDINAL STIMULI - EFFECTS OF STIMULUS MANIPULATIONS ON VEP SCALP TOPOGRAPHY

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimu li were recorded from 19 scalp derivations, two zygomatic and three in ion leads in 20 controls. Patterns of 1 and 3 c/deg were presented as full field (FF) stimuli, on upper and lower hemifields (UHF, LHF), upp er and lower quadrants and with the occlusion of central and periphera l altitudinal hemifields. VEPs to octant stimuli were also recorded wi th 3 c/deg patterns. VEPs to different pattern reversal stimuli invari ably consist of a sequence of three components, N2 (81 +/- 4 ms, 1 c/d eg; 91 +/- 4 ms, 3 c/deg), P1 (108 +/- 6 ms, 1 c/deg; 116 +/- 5 ms, 3 c/deg), N2 (150 +/- 8 ms, 1 c/deg; 157 +/- 9 ms, 3 c/deg) with differe nt latency ranges and differently sensitive to the stimulus spatial fr equency. The scalp distribution of the sequence depends on the positio n of the stimulus in the visual field: FF stimuli elicit N1, P1, N2 on posterior derivations and inion leads, and polarity inverted activiti es of similar latency on anterior leads. With LHF stimuli N1, P1, N2 a re recorded from posterior derivations, but the polarity inverted sequ ence is recorded also from inion. With UHF stimuli the N1, P1, N2 sequ ence is recorded only from inion leads, while scalp and zygomatic deri vations record a polarity inverted sequence. Quadrants and octants eli cit N1, P1, N2 on derivations ipsilateral to stimuli, with contralater al polarity inversion. We suggest that VEP distributions can be justif ied by the position of occipital polar and medial calcarine cortex, wh ere N1, P1, N2 sequences take origin as surface potentials, with polar ity inverted sequences appearing on inner cortical sides.