Brainstem and cochlea potentials evoked by rarefaction and condensation single-slope stimuli. A preliminary report

Influences of stimulus polarity on Jewett wave V are rather small when using clicks which produce two or more polarity changes within a short time interval. In order to separate pressure changes towards rarefaction (R) and towards condensation (C) we applied steep single-slope stimuli returning very slowly to baseline. Brainstem responses recorded from 8 human subjects differed markedly for R and C onset. Amplitudes were much higher for R than for C onset. The main C response was double-peaked with the first peak appearing about 0.5 ms earlier and the second, higher one, 1 ms later than the predominant R wave. The transition from single-slope to click stimulation was investigated by combining R and C slopes. For large time intervals, independent responses to either slope were observed. Down to an on/off interval of 1 ms, the on-response predominated. For still shorter intervals an equalization of R and C responses and a graduation towards click responses was found. Using the same stimuli, cochlear microphonics (CM) and compound action potentials (CAP) were recorded from 7 guinea pigs. The CM did not replicate the slow off-motion of the single-slope stimulus, but returned back to baseline after 0.6 ms. The C compared with R latency of the CAP (peak N1) was also delayed by 0.6 ms. This delay, and that of human peak V, may be explained by CAP initiation only by one direction of basilar membrane motion.