ANALYSIS OF THE MIDDLE LATENCY EVOKED-POTENTIALS TO ANGULAR-ACCELERATION IMPULSES IN MAN

The middle latency vestibular evoked potential (ML=VsEP) recorded with scalp electrodes in man in response to impulses of angular accelerati on is dominated by a forehead positive peak at about 15 ms and a negat ive peak at about 20 ms: the peak to peak amplitude of this component is about 30 mu V. This is followed by slower, smaller amplitude activi ty. The latency of this initial peak is similar to the latency of the vestibule-ocular reflex (VOR) in monkeys. The present study was undert aken to elucidate the possible relation between the ML-VsEPs and VOR. This included recordings from forehead-mastoid electrodes (sites used to record VsEP) and other scalp electrodes and the recording of potent ials due to eye movement: the electro-oculogram. Direct recording of e ye movements was also conducted using an infra-red reflection device i n those experiments in which the head was not moved, The recordings we re conducted in man during vestibular stimulation eliciting VsEPs, dur ing voluntary eye movements and during caloric and optokinetic stimula tion. These experiments indicated that the 15-20 ms component of the M L-VsEP was not due to movements of the eye (corneoretinal dipole). The large amplitude 15-20 ms component of the ML-VsEP was similar in gene ral magnitude, waveform, polarity, duration and rise time to the highl y synchronous pre-saccadic spike (neural and/or myogenic) which preced es nystagmus and voluntary saccades. it therefore probably represents vestibular-initiated electrical activity in motor units of the extra-o cular muscles which then produce anti-compensatory saccades.