Role of the temporal window in dolphin auditory brainstem response onset

Input compensation of dolphin and sea lion auditory brainstem responses using frequency-modulated up-chirps

Frequency-modulated "chirp" stimuli that offset cochlear dispersion (i.e., input compensation) have shown promise for increasing auditory brainstem response (ABR) amplitudes relative to traditional sound stimuli. To enhance ABR methods with marine mammal species known or suspected to have low ABR signal-to-noise ratios, the present study examined the effects of broadband chirp sweep rate and level on ABR amplitude in bottlenose dolphins and California sea lions. "Optimal" chirps were designed based on previous estimates of cochlear traveling wave speeds (using high-pass subtractive masking methods) in these species. Optimal chirps increased ABR peak amplitudes by compensating for cochlear dispersion; however, chirps with similar (or higher) frequency-modulation rates produced comparable results. The optimal chirps generally increased ABR amplitudes relative to noisebursts as threshold was approached, although this was more obvious when sound pressure level was used to equate stimulus levels (as opposed to total energy). Chirps provided progressively less ABR amplitude gain (relative to noisebursts) as stimulus level increased and produced smaller ABRs at the highest levels tested in dolphins. Although it was previously hypothesized that chirps would provide larger gains in sea lions than dolphins-due to the lower traveling wave speed in the former-no such pattern was observed.

Output compensation of auditory brainstem responses in dolphins and sea lions

Cochlear dispersion causes increasing delays between neural responses from high-frequency regions in the cochlear base and lower-frequency regions toward the apex. For broadband stimuli, this can lead to neural responses that are out-of-phase, decreasing the amplitude of farfield neural response measurements. In the present study, cochlear traveling-wave speed and effects of dispersion on farfield auditory brainstem responses (ABRs) were investigated by first deriving narrowband ABRs in bottlenose dolphins and California sea lions using the high-pass subtractive masking technique. Derived-band ABRs were then temporally aligned and summed to obtain the "stacked ABR" as a means of compensating for the effects of cochlear dispersion. For derived-band responses between 8 and 32 kHz, cochlear traveling-wave speeds were similar for sea lions and dolphins [∼2-8 octaves (oct)/ms for dolphins; ∼3.5-11 oct/ms for sea lions]; above 32 kHz, traveling-wave speed for dolphins increased up to ∼30 oct/ms. Stacked ABRs were larger than unmasked, broadband ABRs in both species. The amplitude enhancement was smaller in dolphins than in sea lions, and enhancement in both species appears to be less than reported in humans. Results suggest that compensating for cochlear dispersion will provide greater benefit for ABR measurements in species with better low-frequency hearing.

Directional biosonar beams allow echolocating harbour porpoises to actively discriminate and intercept closely spaced targets

Echolocating toothed whales face the problem that high sound speeds in water mean that echoes from closely spaced targets will arrive at time delays within their reported auditory integration time of some 264 µs. Here, we test the hypothesis that echolocating harbour porpoises cannot resolve and discriminate targets within a clutter interference zone given by their integration time. To do this, we trained two harbour porpoises (Phocoena phocoena) to actively approach and choose between two spherical targets at four varying inter-target distances (13.5, 27, 56 and 108 cm) in a two-alternative forced-choice task. The free-swimming, blindfolded porpoises were tagged with a sound and movement tag (DTAG4) to record their echoic scene and acoustic outputs. The known ranges between targets and the porpoise, combined with the sound levels received on target-mounted hydrophones revealed how the porpoises controlled their acoustic gaze. When targets were close together, the discrimination task was more difficult because of smaller echo time delays and lower echo level ratios between the targets. Under these conditions, buzzes were longer and started from farther away, source levels were reduced at short ranges, and the porpoises clicked faster, scanned across the targets more, and delayed making their discrimination decision until closer to the target. We conclude that harbour porpoises can resolve and discriminate closely spaced targets, suggesting a clutter rejection zone much shorter than their auditory integration time, and that such clutter rejection is greatly aided by spatial filtering with their directional biosonar beam.

The offset auditory brainstem response in bottlenose dolphins (Tursiops truncatus): Evidence for multiple underlying processes

The auditory brainstem response (ABR) to stimulus onset has been extensively used to investigate dolphin hearing. The mechanisms underlying this onset response have been thoroughly studied in mammals. In contrast, the ABR evoked by sound offset has received relatively little attention. To build upon previous observations of the dolphin offset ABR, a series of experiments was conducted to (1) determine the cochlear places responsible for response generation and (2) examine differences in response morphologies when using toneburst versus noiseburst stimuli. Measurements were conducted with seven bottlenose dolphins (Tursiops truncatus) using tonebursts and spectrally "pink" broadband noisebursts, with highpass noise used to limit the cochlear regions involved in response generation. Results for normal-hearing and hearing-impaired dolphins suggest that the offset ABR contains contributions from at least two distinct responses. One type of response (across place) might arise from the activation of neural units that are shifted basally relative to stimulus frequency and shares commonalities with the onset ABR. A second type of response (within place) appears to represent a "true" offset response from afferent centers further up the ascending auditory pathway from the auditory nerve, and likely results from synchronous activity beginning at or above the cochlear nucleus.