Searching for the optimal stimulus eliciting auditory brainstem responses in humans.

This study examines auditory brainstem responses (ABR) elicited by rising frequency chirps. Two chirp stimuli were developed and designed such as to compensate for cochlear travel-time differences across frequency, in order to maximize neural synchrony. One chirp, referred to as the O-chirp, was based on estimates of human basilar membrane (BM) group delays derived from stimulus-frequency otoacoustic emissions (SFOAE) at a sound pressure level of 40 dB [Shera and Guinan, in Recent Developments in Auditory Mechanics (2000)]. The other chirp, referred to as the A-chirp, was derived from latency functions fitted to tone-burst-evoked ABR wave-V data over a wide range of stimulus levels and frequencies [Neely et al., J. Acoust. Soc. Am. 83(2), 652-656 (1988)]. In this case, a set of level-dependent chirps was generated. The chirp-evoked responses, particularly wave-V amplitude and latency, were compared to click responses and to responses obtained with the original chirp as defined in Dau et al. [J. Acoust. Soc. Am. 107(3), 1530-1540 (2000)], referred to here as the M-chirp since it is based on a (linear) cochlea model. The main hypothesis was that, at low and medium stimulation levels, the O- and A-chirps might produce a larger response than the original M-chirp whose parameters were essentially derived from high-level BM data. The main results of the present study are as follows: (i) All chirps evoked a larger wave-V amplitude than the click stimulus indicating that for the chirps a broader range of spectral components contributes effectively to the ABR. (ii) Only small differences were found between the O-chirp and M-chirp responses at low and medium levels. This indicates that SFOAE may not provide a robust estimate of BM group delay, particularly at low frequencies, or that frequency-dependent neural delays exist which are not reflected in the design of these chirps. (iii) The A-chirp produced the largest responses, particularly at low stimulation levels. This chirp might therefore be valuable for clinical applications, particularly in tests where the click stimulus has been used so far.