02066nas a2200193 4500008004100000022001400041245010700055210006900162260001600231300001600247490000800263520143700271100002301708700002401731700001801755700002401773700002401797856005101821 2018 eng d a0001-496600aImpact of stimulus bandwidth on the estimate of the upper-frequency limit of hearing in toothed whales0 aImpact of stimulus bandwidth on the estimate of the upperfrequen cJan-09-2018 a1740 - 17400 v1443 a

No consensus on the stimuli used in toothed whale hearing tests using evoked potential methods currently exists. However, stimulus bandwidth should directly affect determination of the upper-frequency limit (UFL) of hearing because of the steep reduction in hearing sensitivity at frequencies immediately below the UFL—the broader the stimulus bandwidth, the greater the amount of spectral “splatter” into lower frequencies where hearing is more sensitive. To test this hypothesis, variability in auditory evoked potential thresholds to tone-pips (with variable bandwidth) and amplitude-modulated tones was determined in dolphins near their UFL of hearing and ½ to 1 octave below where hearing is more sensitive. Subjects included both normal and hearing-impaired dolphins. At frequencies where hearing was sensitive and the audiogram was relatively flat, negligible changes in the hearing threshold with changing stimulus bandwidth were observed. Conversely, thresholds near the UFL of hearing declined with increasing stimulus bandwidth, regardless of whether dolphins had normal hearing or hearing impairment, and resulted in broadened hearing bandwidth estimates. Thus, it is recommended that stimulus bandwidth be reported in hearing tests where the UFL is tested since stimulus bandwidth affects thresholds through spectral splatter into lower frequency bands where hearing is more sensitive.

1 aHouser, Dorian, S.1 aCoffinger, Sean, P.1 aMulsow, Jason1 aFinneran, James, J.1 aBurkard, Robert, F. uhttp://asa.scitation.org/doi/10.1121/1.506771701996nas a2200181 4500008004100000022001400041245016200055210006900217260001600286300001600302490000800318520134700326100001801673700002401691700002401715700002401739856005101763 2018 eng d a0001-496600aForward masking recovery in bottlenose dolphins ( Tursiops truncatus): Auditory brainstem responses to paired-click stimuli in high-pass masking noise0 aForward masking recovery in bottlenose dolphins iTursiops trunca cJan-09-2018 a1740 - 17400 v1443 a

Adaptation is a fundamental process in the auditory system and underlies the automatic gain control system of echolocating bottlenose dolphins. As pulse-echo delay increases, auditory brainstem response (ABR) amplitudes to the echoes progressively increase. This study examined adaptation across cochlear frequency regions using a paired-click (i.e., forward masking) paradigm and high-pass masking noise. Bottlenose dolphins passively listened to paired click stimuli (20–160 kHz “pink” spectra). A “conditioning” click was followed by a “probe” click with equal amplitude and a time delay ranging from 125 to 750 μs. ABRs to click pairs were obtained with and without high-pass masking noise that precluded the basal turn of the cochlea from responding to the click stimuli. The ABR evoked by a single click (temporally aligned with the first click of the paired-click condition) was subtracted from the click-pair ABR to visualize the response evoked by the probe click. Probe ABR amplitudes recovered linearly with increasing delay relative to the conditioning click, and were approximately 70% of full response amplitude at 750-μs delay. Paired-click interval ABR amplitude recovery functions were similar for the unmasked and high-pass masked conditions. [Work sponsored by ONR.]

1 aMulsow, Jason1 aCoffinger, Sean, P.1 aFinneran, James, J.1 aBurkard, Robert, F. uhttp://asa.scitation.org/doi/10.1121/1.506771801960nas a2200181 4500008004100000022001400041245015000055210006900205260001600274300001600290490000800306520132700314100002001641700002401661700001801685700002401703856005101727 2018 eng d a0001-496600aDependence of bottlenose dolphin (Tursiops truncatus) auditory brainstem responses on noise burst rise time, amplitude, and envelope shape0 aDependence of bottlenose dolphin iTursiops truncatusi auditory b cJan-09-2018 a1741 - 17410 v1443 a

A series of experiments examined the dependence of dolphin auditory brainstem response (ABR) peak amplitudes and latencies by stimulus level, rise times, and envelope shape. Stimuli were spectrally “pink” noise bursts with frequency content from 10 to 160 kHz, rise times that varied from 32 μs to 4 ms, and plateau sound pressure levels from 102 to 138 dB re 1 μPa. Cosine and cosine-cubed rise (and fall) envelopes were tested, and results were compared to those obtained for dolphins using noise bursts with linear rise envelopes [Finneran et al., J. Acoust. Soc. Am. 143, 2914–2921 (2018)]. Results for the cosine and cosine-cubed envelopes showed that ABR peak amplitudes were a function of envelope sound pressure at the end of a fixed integration window of approximately 260 μs. This is congruent with the previous findings for linear envelopes. The ABR peak latencies for the cosine and cosine-cubed envelopes were dependent on the second derivative of the envelope pressure function, as opposed to the first derivative of the pressure function for linear envelopes. These data are consistent with single-unit and nearfield response data for terrestrial mammals. [Work supported by Navy Living Marine Resources and SSC Pacific Naval Innovative Science and Engineering Programs.]

1 aJones, Ryan, A.1 aFinneran, James, J.1 aMulsow, Jason1 aBurkard, Robert, F. uhttp://asa.scitation.org/doi/10.1121/1.5067722