Marine mammal auditory evoked potential measurements using swept amplitude stimuli.

Abstract

Auditory evoked potentials are routinely used to characterize hearing in marine mammals for whom behavioral testing is not practical. For frequency-specific audiometry, the most popular evoked potential method has been the measurement of the auditory steady state response (ASSR). These tests normally entail measuring the ASSR to a sequence of sinusoidally amplitude modulated tones so that the ASSR input-output function can be defined and the auditory threshold estimated. In this study, an alternative method was employed, where thresholds were estimated in response to a single amplitude modulated stimulus whose sound pressure level varied linearly with time. The tone sound pressure was therefore swept over a range of levels believed to bracket the threshold. The input-output function was obtained by analyzing the resulting grand average evoked potential using a short-time Fourier transform. The swept amplitude technique was performed with bottlenose dolphins and California sea lions, with the resulting thresholds similar to those obtained with constant amplitude tones. The tradeoffs between the swept amplitude technique and the traditional, constant amplitude approach will be discussed. [Work supported by ONR.]