Stimulus bandwidth impact on auditory evoked potential thresholds and estimated upper-frequency limits of hearing in dolphins

The frequency range of hearing is important for assessing the potential impact of anthropogenic noise on marine mammals. Auditory evoked potentials (AEPs) are commonly used to assess toothed whale hearing, but measurement methods vary across researchers and laboratories. In particular, estimates of the upper-frequency limit of hearing (UFL) can vary due to interactions between the unintended spread of spectral energy to frequencies below the desired test frequency and a sharp decline in hearing sensitivity at frequencies near the UFL. To assess the impact of stimulus bandwidth on UFL measurement, AEP hearing tests were conducted in four bottlenose dolphins (Tursiops truncatus) with normal and impaired hearing ranges. Dolphins were tested at frequencies near the UFL and at a frequency 1/2-octave below the UFL, where hearing sensitivity was better (i.e., threshold was lower). Thresholds were measured using sinusoidal amplitude modulated (SAM) tones and tone-bursts of varying bandwidth. Measured thresholds varied inversely as a function of stimulus bandwidth near the UFL with narrow-band tone-bursts approximating thresholds measured using SAM tones. Bandwidth did not impact measured thresholds where hearing was more sensitive, highlighting how stimulus bandwidth and the rate of decline of hearing sensitivity interact to affect measured threshold near the UFL.