Auditory steady-state response measurement of the temporal dynamics of hearing sensitivity in an echolocating bottlenose dolphin (Tursiops truncatus)

Abstract

Studies with some echolocating odontocetes demonstrate that receiver-based automatic gain control (AGC) compensates for reductions in echo strength resulting from acoustic spreading loss. This study examined AGC in an echolocating bottlenose dolphin by measuring changes in hearing sensitivity over time courses corresponding to single click-echo pairs. The electrophysiological auditory steady-state response (ASSR) elicited by a 113-kHz sinusoidally amplitude-modulated tone was recorded while the dolphin performed a target discrimination task. Auditory electrophysiological responses were extracted from the instantaneous electroencephalogram and coherently averaged using the modulation rate of the 113-kHz tone as a reference. A Fourier transform was then performed with a 10-ms sliding window to obtain the ASSR amplitude as a function of time relative to the dolphin’s outgoing click and received echo. The ASSR amplitude initially decreased at the time of click emission and then recovered over a course of 25 to 70 ms, depending on target range. This relatively long time course of recovery appears to be consistent with forward-masking, as opposed to an AGC mechanism based on the contraction and gradual release of middle ear muscles coincident with click emission. [Work funded by SSC Pacific Naval Innovative Science and Engineering (NISE) program.]