Place specificity of the click-evoked auditory brainstem response in the bottlenose dolphin (Tursiops truncatus)

Cochlear place specificity of the auditory brainstem response (ABR) was investigated in five bottlenose dolphins by measuring ABRs to broadband clicks presented simultaneously with masking noise having various high-pass cutoff frequencies. Click and noise stimuli were digitally compensated to account for the transmitting response of the piezoelectric transducers and any multipath propagation effects to achieve "white" or "pink" spectral characteristics. Narrowband evoked responses were derived by sequentially subtracting responses obtained with noise at lower high-pass cutoff frequencies from those obtained with noise having higher cutoff frequencies. The results revealed little contribution to the click-evoked brainstem response from frequency bands below 10 kHz and, in dolphins with full hearing bandwidth, the largest amplitude derived band evoked responses were obtained from the highest frequency bands. Narrowband latencies decreased with increasing frequency and were adequately fit with a power function exhibiting relatively large change in latency with frequency below ∼30 kHz and little change above ∼30 kHz. These data demonstrate that frequency bands below ∼10 kHz do not substantively contribute to the farfield ABR in the bottlenose dolphin when using place-specific approaches such as high-pass subtractive-masking techniques.