Spatial release from masking in a bottlenose dolphin Tursiops truncatus.

The impact of maskers on the receiving beam of a bottlenose dolphin, Tursiops truncatus, was investigated using the auditory evoked potential (AEP) method. The test signal was a train of tone pips with a 64 kHz carrier frequency. The stimulus produced AEPs as a sequence of waves replicating the pip rate: the rate following response (RFR). The masker was band-limited noise, with a passband of 45 to 90 kHz and a level of 105 dB re 1 µPa. Masker azimuths were at 0°, ±30°, ±60°, and ±90° relative to the head midline. The receiving beam was evaluated in terms of the RFR threshold dependence on the signal azimuth. The masked thresholds were higher than the baseline thresholds, which appeared mostly as a shift rather than a deformation in the receiving beam. The largest threshold shift appeared when the masker source was located in the most sensitive direction (zero azimuth); at lateral masker source positions, the threshold shift decreased. When the masker source was not at the head midline, the masked thresholds were higher at signal positions ipsilateral to the masker source than at positions contralateral to the masker source. The largest asymmetry was observed at the 30° masker azimuth in conjunction with the ±30° and ±120° signal azimuths; the asymmetries were 5.6 and 8.1 dB, respectively. This masking asymmetry was lower than expected from the previously found interaural intensity difference, which may be explained by the conflict between the test signal and the masker when it appeared at a binaural level of the auditory system.

Influence of background noise on auditory evoked responses to rippled-spectrum signals.

The resolution of spectral patterns in adaptation background noise was investigated in a beluga whale, Delphinapterus leucas, using the evoked-potential technique. The resolution of spectral patterns was investigated using rippled-spectrum test stimuli of various levels and ripple densities and recording the rhythmic evoked responses (the rate following response, RFR) to ripple phase reversals. In baseline (no adaptation background noise) experiments, the highest RFR magnitude was observed at signal sound pressure levels (SPLs) of 100-110 dB re 1 µPa; at SPLs both below the optimum (down to 80 dB re 1 µPa) and above the optimum (up to 140 dB re 1 µPa), the RFR magnitude decreased. For high signal levels (above 110 dB re 1 µPa), low-level adaptation background noise (from -10 to -20 dB re signal level) increased RFR magnitude compared to baseline. This effect is considered to be a result of the optimization of the sensation level of the high-SPL signals due to decreasing hearing sensitivity caused by the adaptation background noise.

Hearing threshold shifts and recovery after noise exposure in beluga whales, Delphinapterus leucas.

Temporary threshold shift (TTS) after loud noise exposure was investigated in a male and a female beluga whale (Delphinapterus leucas). The thresholds were evaluated using the evoked-potential technique, which allowed for threshold tracing with a resolution of ~1 min. The fatiguing noise had a 0.5 octave bandwidth, with center frequencies ranging from 11.2 to 90 kHz, a level of 165 dB re. 1 µPa and exposure durations from 1 to 30 min. The effects of the noise were tested at probe frequencies ranging from -0.5 to +1.5 octaves relative to the noise center frequency. The effect was estimated in terms of both immediate (1.5 min) post-exposure TTS and recovery duration. The highest TTS with the longest recovery duration was produced by noises of lower frequencies (11.2 and 22.5 kHz) and appeared at a test frequency of +0.5 octave. At higher noise frequencies (45 and 90 kHz), the TTS decreased. The TTS effect gradually increased with prolonged exposures ranging from 1 to 30 min. There was a considerable TTS difference between the two subjects.

Masking of rippled-spectrum-pattern resolution in diotic and dichotic presentations.

Using rippled noise probes, spectrum-pattern resolution was measured with and without a narrow-band noise masker. Diotic presentation of both the probe and masker (S(0)N(0) mode) resulted in decreased spectrum resolution as compared to the control (no masker) conditions. The effects of the low- and on-frequency maskers differed quantitatively, however in both cases the ability to discriminate the probe spectrum pattern was suppressed completely when the masker/probe level ratio exceeded 10dB (on-frequency masker) or 10-25dB, depending on the probe level (low-frequency masker). The effect of the high-frequency masker was negligible. Slight but noticeable releasing of the spectrum-pattern resolution was found when the probe was presented to both ears in-phase and the masker counter-phase (S(0)N(pi) mode). In conditions of the probe delivered to one ear and the masker to the other ear (S(L)N(R) mode), the effect on the spectrum-pattern resolution was slight or negligible within a wide range of the noise/probe ratio.

Evidence for double acoustic windows in the dolphin, Tursiops truncatus.

In a bottlenose dolphin positions of sound receiving areas on the head surface were determined by comparing the acoustic delays from different sound-source positions. For this investigation, auditory brainstem responses (ABRs) to short tone pips were recorded and their latencies were measured at different sound source positions. After correction for the latency dependence on response amplitude, the difference in ABR latencies was adopted as being the difference of the acoustic delays. These delay differences were used to calculate the position of the sound-receiving point. Measurements were conducted at sound frequencies from 16 to 128 kHz, in half-octave steps. At probe frequencies of 16 and 22.5 kHz, the receiving area was located 21.7-26 cm caudal of the melon tip, which is near the bulla and auditory meatus. At higher probe frequencies, from 32 to 128 kHz, the receiving area was located from 9.3 to 13.1 cm caudal of the melon tip, which corresponds to a proximal part of the lower jaw. Thus, at least two sound-receiving areas (acoustic windows) with different frequency sensitivity were identified.

Rippled-spectrum resolution dependence on masker-to-probe ratio.

Resolution of rippled sound spectrum (probe) in the presence of additional noise band (masker) was studied as a function of masker-to-probe ratio and sound level in normal listeners. The probe bands were 0.5-oct wide (ERB) centered at 2 kHz; the masker band either coincided with the probe (on-frequency masker), or was 3/4 octaves below (low-frequency masker), or 3/4 octaves above the probe (high-frequency masker). Ripple-density resolution in the probe band was measured by finding the highest ripple density at which an interchange of ripple peaks and valleys was detectable (the phase-reversal test). (i) The effect of the low-frequency masker increased (resolution decreased) when masker-to-probe ratio changed from -25 dB to +20 dB; the effect increased (resolution decreased) with sound level increase. (ii) The effect of the on-frequency masker steeply increased (resolution abruptly decreased) when masker-to-probe ratio exceeded 0 dB; the effect was little dependent on sound level. (iii) The high-frequency masker was little effective unless the masker-to-probe ratio reached 30-40 dB; the effect increased (resolution decreased) with sound level decrease. Thus, different position of the masker band relative to the probe resulted in qualitatively different kinds of spectrum-pattern resolution dependence on both the masker-to-probe ratio and sound level.

Rippled-spectrum resolution dependence on level.

Rippled-density resolution of a rippled sound spectrum (probe band) in both the presence and absence of another band (masker) was studied as a function of sound level in normal listeners. The resolvable ripple density in the probe band was measured by finding the highest ripple density at which an interchange of ripple peak and valley positions was detectable (the phase-reversal test). Probe bands were 0.5 oct wide with center frequencies of 1, 2, and 4 kHz. In the control condition (no masker), the ripple-density resolution was almost independent of sound level within a range of 40-90 dB SPL. When an on-frequency masker coincided with the probe band (that resulted in reduced ripple depth), resolution decreased slightly relative to the control condition but remained little dependent on level. With an off-frequency low-side masker, the ripple-density resolution was a little less than in the control but almost independent of level within a range of 40-60 dB SPL and progressively decreased with level increase from 70 to 90 dB SPL. The dependence on level was qualitatively similar at all probe frequencies and at various widths and positions of the low-side off-frequency masker band.