EVENT-RELATED POTENTIALS TO OMITTED VISUAL-STIMULI IN A REPTILE

Visual omitted stimulus potentials (OSPs) were recorded from awake pon d turtles with arrays of 3-20 electrodes in the dorsal cortex (DC), do rsal ventricular ridge (DVR) and optic tectum. Since they are generall y longer in duration than the interstimulus interval (ISI), the standa rd experiment is a short conditioning train of regular light or dark f lashes (1-20 Hz) whose termination elicits the OSP. Tectal surface OSP s after trains > 7 Hz have 2 major positive peaks, P120-140 and P220-2 50 after the due-time of the first omission; after < 7 Hz down to the minimum of 1.5 Hz only the slower peak appears. Some deep tectal loci also have one to three 100 msec wide negative waves peaking at variabl e times from 200 to 1300 msec. Forebrain OSPs in DC and DVR are approx imately 30 msec later and often include induced 17-25 Hz oscillations, not phase-locked and attenuated in averages. Both tectal and forebrai n OSP main waves tend toward a constant latency after the due-time, ov er a wide range of ISIs, as though the system expects a stimulus on sc hedule. Jitter of ISI around the mean does not greatly reduce the OSP. At all loci higher conditioning rates cause the amplitudes of the ste ady state response (SSR) VEPs to decline and of the OSPs to increase. Some similarities and correlations of regional amplitude fluctuations between OSPs and VEPs are noted. The OSP dynamics are consistent with the hypothesis of a postinhibitory rebound of temporally specific VEP components increasingly inhibited with higher stimulation rates; much of this response is retinal but each higher brain level further modula tes. OSPs in this reptile are similar to those known in fish and to th e ''high frequency'' type in humans, quite distinct in properties from the ''low frequency'' OSPs. It will be important to look at the high frequency type in laboratory mammals to determine whether they are pre sent in the midbrain and retina, as in fish and reptiles.