Auditory brainstem responses with optimized chirp signals compensating basilar-membrane dispersion

This study examines auditory brainstem responses (ABR) elicited by rising f requency chirps. The time course of frequency change for the chirp theoreti cally produces simultaneous displacement maxima by compensating for travel- time differences along the cochlear partition. This broadband chirp was der ived on the basis of a linear cochlea model [de Beer, "Auditory physics. Ph ysical principles in hearing theory I,'' Phys. Rep. 62, 87-174 (1980)]. Res ponses elicited by the broadband chirp show a larger wave-V amplitude than do click-evoked responses for most stimulation levels tested. This result i s in contrast to the general hypothesis that the ABR is an electrophysiolog ical event most effectively evoked by the onset or offset of an acoustic st imulus, and unaffected by further stimulation. The use of this rising frequ ency chirp enables the inclusion of activity from lower frequency regions, whereas with a click, synchrony is decreased in accordance with decreasing traveling velocity in the apical region. The use of a temporally reversed ( falling) chirp leads to a further decrease in synchrony as reflected in ABR responses that are smaller than those from a click. These results are comp atible with earlier experimental results from recordings of compound action potentials (CAP) [Shore and Nuttall, "High synchrony compound action poten tials evoked by rising frequency-swept tonebursts," j. Acoust. Sec. Am..78, 1286-1295 (1985)] reflecting activity at the level of the auditory nerve. Since the ABR components considered here presumably reflect neural response from the brainstem, the effect of an optimized synchronization at the peri pheral level can also be observed at the brainstem level. The rising chirp may therefore be of clinical use in assessing the integrity of the entire p eripheral organ and not just its basal end. (C) 2000 Acoustical Society of America. [S0001-4966(00)02603-5].