DYNAMIC PROPERTIES OF HUMAN VISUAL-EVOKED AND OMITTED STIMULUS POTENTIALS

Visual evoked potentials (VEPs) and omitted stimulus potentials (OSPs) are re-examined in scalp recordings from 19 healthy subjects. The pri ncipal finding is a distinction in form, latency and properties betwee n OSPs in the conditioning stimulus range < 2 Hz, used in previous hum an studies, and those in the range > 5 Hz, used in previous studies of selected elasmobranchs, teleost fish and reptiles. We cannot find OSP s between 2 and 5 Hz. The high frequency (''fast,'' ca. 6- > 40 Hz) an d the low frequency (''slow,'' ca. 0.3-1.6 Hz) OSPs have different for ms and latencies but both tend to a constant latency after the omissio n, over their frequency ranges, suggesting a temporally specific expec tation. Fast OSPs (typically N120, P170-230 and later components inclu ding induced rhythms at 10-13 Hz) resemble an OFF effect, and require fixation but not attention to the interstimulus interval. Slow OSPs (u sually P500-1100) require attention but not fixation; they are multimo dal, unlike the fast OSPs. Based on cited data from fish and reptiles, fast OSPs probably arise in the retina, to be modified at each subseq uent level. We have no evidence on the origin of slow OSPs. In both ra nges not only large, diffuse flashes, but weak, virtual point sources (colored LEDs) meters away suffice. They are difficult to habituate. B oth require very short conditioning periods. The transition from the s ingle, rested VEP to the steady state response (SSR) at different freq uencies is described. Around 8-15 Hz in most subjects larger SSRs sugg est a resonance. Alternation between large and small SSR amplitude occ urs around 4 Hz in some subjects and conditions of attention, and corr elates with an illusion that the flash frequency is 2 Hz or is irregul ar. Jitter of the conditioning intervals greatly reduces the slow OSP but only slightly affects the fast OSP. Differences between scalp loci are described.