Monaural and interaural temporal modulation transfer functions measured with 5-kHz carriers

Amplitude modulation detection and modulation masking in school-age children and adults

Two experiments were performed to better understand on- and off-frequency modulation masking in normal-hearing school-age children and adults. Experiment 1 estimated thresholds for detecting 16-, 64- or 256-Hz sinusoidal amplitude modulation (AM) imposed on a 4300-Hz pure tone. Thresholds tended to improve with age, with larger developmental effects for 64- and 256-Hz AM than 16-Hz AM. Detection of 16-Hz AM was also measured with a 1000-Hz off-frequency masker tone carrying 16-Hz AM. Off-frequency modulation masking was larger for younger than older children and adults when the masker was gated with the target, but not when the masker was continuous. Experiment 2 measured detection of 16- or 64-Hz sinusoidal AM carried on a bandpass noise with and without additional on-frequency masker AM. Children and adults demonstrated modulation masking with similar tuning to modulation rate. Rate-dependent age effects for AM detection on a pure-tone carrier are consistent with maturation of temporal resolution, an effect that may be obscured by modulation masking for noise carriers. Children were more susceptible than adults to off-frequency modulation masking for gated stimuli, consistent with maturation in the ability to listen selectively in frequency, but the children were not more susceptible to on-frequency modulation masking than adults.

Optimizing pulse-spreading harmonic complexes to minimize intrinsic modulations after auditory filtering

All signals, except sine waves, exhibit intrinsic modulations that affect perceptual masking. Reducing the physical intrinsic modulations of a broadband signal does not necessarily have a perceptual impact: auditory filtering can reintroduce modulations. Broadband signals with low intrinsic modulations after auditory filtering have proved difficult to design. To that end, this paper introduces a class of signals termed pulse-spreading harmonic complexes (PSHCs). PSHCs are generated by summing harmonically related components with such a phase that the resulting waveform exhibits pulses equally-spaced within a repetition period. The order of a PSHC determines its pulse rate. Simulations with a gamma-tone filterbank suggest an optimal pulse rate at which, after auditory filtering, the PSHC's intrinsic modulations are lowest. These intrinsic modulations appear to be less than those for broadband pseudo-random (PR) or low-noise (LN) noise. This hypothesis was tested in a modulation-detection experiment involving five modulation rates ranging from 8 to 128 Hz and both broadband and narrowband carriers using PSHCs, PR, and LN noise. PSHC showed the lowest thresholds of all broadband signals. Results imply that optimized PSHCs exhibit less intrinsic modulations after auditory filtering than any other broadband signal previously considered.

Temporal modulation transfer function for efficient assessment of auditory temporal resolution

Two common measures of auditory temporal resolution are the temporal modulation transfer function (TMTF) and the gap detection threshold (GDT). The current study addresses the lack of efficient psychophysical procedures for collecting TMTFs and the lack of literature on the comparisons of TMTF and GDT. Two procedures for efficient measurements of the TMTF are proposed: (1) A Bayesian procedure that adaptively chooses the stimulus modulation rate and depth to maximize the information gain from each trial and (2) a procedure that reduces the data collection to two adaptive staircase tracks. Results from experiments I and II showed that, for broadband carriers, these approaches provided similar results compared to TMTFs measured using traditional methods despite taking less than 10 min for data collection. Using these efficient procedures, TMTFs were measured from a large number of naive listeners and were compared to the gap detection thresholds collected from the same ears in experiment III. Results showed that the sensitivity parameter estimated from the TMTF measurements correlated well with the GDTs, whereas the cutoff rate is either uncorrelated or positively correlated with the gap detection threshold. These results suggest caution in interpreting a lower GDT as evidence for less sluggish temporal processing.

Advantages from bilateral hearing in speech perception in noise with simulated cochlear implants and residual acoustic hearing

Acoustic simulations were used to study the contributions of spatial hearing that may arise from combining a cochlear implant with either a second implant or contralateral residual low-frequency acoustic hearing. Speech reception thresholds (SRTs) were measured in twenty-talker babble. Spatial separation of speech and noise was simulated using a spherical head model. While low-frequency acoustic information contralateral to the implant simulation produced substantially better SRTs there was no effect of spatial cues on SRT, even when interaural differences were artificially enhanced. Simulated bilateral implants showed a significant head shadow effect, but no binaural unmasking based on interaural time differences, and weak, inconsistent overall spatial release from masking. There was also a small but significant non-spatial summation effect. It appears that typical cochlear implant speech processing strategies may substantially reduce the utility of spatial cues, even in the absence of degraded neural processing arising from auditory deprivation.

Masked detection and discrimination of tone sequences under conditions of monaural and binaural masking release

Experiment 1 examined detection and discrimination of monaural four-tone sequences composed of 400-, 500-, and 625-Hz sinusoids. In the baseline conditions, the masker was monaural composed of 25-Hz-wide bands of random noise centered on 320, 400, 500, 625, and 781 Hz. In the binaural masking release conditions, the noise was presented diotically. In the monaural masking release conditions, the noise was presented to the same ear as the signal, but it was comodulated. Tones had half-amplitude durations of 30, 60, or 150 ms. There was no delay between successive tones, so the rate of frequency change depended on tone duration. Listeners discriminated between sequences composed of 500-400-625-500 Hz and 500-625-400-500 Hz. Discrimination results were poor for rapid sequences in both monaural and binaural masking release conditions relative to baseline conditions. Results from experiment 2 indicated that poor discrimination for rapid sequences could also occur in the baseline conditions, provided that the frequency separation among tonal components was small. Sluggish processing in the present paradigm was not restricted to conditions relying on binaural cues. It is argued that sluggishness may reflect a long temporal window in monaural and binaural masking release conditions or an interaction between poor cue quality and task difficulty.

Interaural correlation fails to account for detection in a classic binaural task: dynamic ITDs dominate N0Spi detection

Binaural signal detection in an NoSπ task relies on interaural disparities introduced by adding an antiphasic signal to diotic noise. What metric of interaural disparity best predicts performance? Some models use interaural correlation; others differentiate between dynamic interaural time differences (ITDs) and interaural level differences (ILDs) of the effective stimulus. To examine the relative contributions of ITDs and ILDs in binaural detection, we developed a novel signal processing technique that selectively degrades different aspects (potential cues) of binaural stimuli (e.g., only ITDs are scrambled). Degrading a particular cue will affect performance only if that cue is relevant to the binaural processing underlying detection. This selective scrambling technique was applied to the stimuli of a classic N0Sπ task in which the listener had to detect an antiphasic 500-Hz signal in the presence of a diotic wideband noise masker. Data obtained from five listeners showed that (1) selective scrambling of ILDs had little effect on binaural detection, (2) selective scrambling of ITDs significantly degraded detection, and (3) combined scrambling of ILDs and ITDs had the same effect as exclusive scrambling of ITDs. Regarding the question which stimulus properties determine detection, we conclude that for this binaural task (1) dynamic ITDs dominate detection performance, (2) ILDs are largely irrelevant, and (3) interaural correlation of the stimulus is a poor predictor of detection. Two simple stimulus-based models that each reproduce all binaural aspects of the data quite well are described: (1) a single-parameter detection model using ITD variance as detection criterion and (2) a compressive transformation followed by a crosscorrelation analysis. The success of both of these contrasting models shows that our data alone cannot reveal the mechanisms underlying the dominance of ITD cues. The physiological implications of our findings are discussed.

Binaural sluggishness precludes temporal pitch processing based on envelope cues in conditions of binaural unmasking

Binaural sluggishness refers to the binaural system's inability to follow fast changes in the interaural configuration of the incoming sound stream. Several studies have measured binaural sluggishness by measuring signal detection in conditions of binaural unmasking when the interaural configuration of the masker is changed over time. However, it has been shown that, in conditions of binaural unmasking, binaural sluggishness also affects the perception of temporal changes in the properties of the signal (i.e., its frequency or level) and not just in the interaural configuration of the masker. By measuring the temporal modulation transfer function for sinusoidally modulated noise presented in conditions of binaural unmasking, the first experiment of the current study showed that, due to binaural sluggishness, the internal representation of binaurally unmasked sounds conveys little or no information about envelope fluctuations with rates within the pitch range (i.e., above 30 Hz). The second experiment measured the masked detection threshold for musical interval recognition in binaurally unmasked harmonic tones and showed that, in conditions of binaural unmasking, pitch wanes when the harmonics become unresolved by the cochlear filters. These results suggest that binaural sluggishness precludes temporal pitch processing based on envelope cues in binaurally unmasked sounds.

Measurement of the binaural temporal window using a lateralisation task

Binaural temporal resolution was measured using the discrimination of brief interaural time delays (ITDs). In experiment 1, three listeners performed a 2I-2AFC, ITD-discrimination procedure. ITD changes of 8 to 1024micros were applied to brief probe noises. These probes, with durations of 16 to 362ms, were placed symmetrically in time within a 500-ms burst of otherwise interaurally uncorrelated noise. Psychometric functions were measured to obtain thresholds and temporal windows fitted to those thresholds. The best-fitting window was a symmetric roex shape (equivalent rectangular duration=197ms), an order of magnitude longer than monaural temporal windows and differed substantially from windows reported by Bernstein et al. [Bernstein, L.R., Trahiotis, C., Akeroyd, M.A., Hartung, K., 2001. Sensitivity to brief changes of interaural time and interaural intensity. J. Acoust. Soc. Am. 109, 1604-1615]. Experiment 2, replicated their main experiment, comparing their ITD-detection task with a similar discrimination procedure. Thresholds in the detection conditions were significantly better than those in the discrimination condition, particularly for short probe durations, indicating the use of an additional cue at these durations for the detection task and thus undermining the assumptions made in their window fit.

Influences of modulation and spatial separation on detection of a masked broadband target

Experiments explored the influence of amplitude modulation and spatial separation on detectability of a broadband noise target masked by an independent broadband noise. Thresholds were measured for all combinations of six spatial configurations of target and masker and five modulation conditions. Masker level was either fixed (Experiment 1) or roved between intervals within a trial to reduce the utility of overall intensity as a cue (Experiment 2). After accounting for acoustic changes, thresholds depended on whether a target and a masker were colocated or spatially separated, but not on the exact spatial configuration. Moreover, spatial unmasking exceeded that predicted by better-ear acoustics only when modulation cues for detection were weak. Roving increased the colocated but not the spatially separated thresholds, resulting in an increase in spatial release from masking. Differences in both how performance changed over time and the influence of spatial separation support the idea that the cues underlying performance depend on the modulation characteristics of the target and masker. Analysis suggests that detection is based on overall intensity when target and masker modulation and spatial cues are the same, on spatial attributes when sources are separated and modulation provides no target glimpses, and on modulation discrimination in the remaining conditions.

Binaural processing of modulated interaural level differences

Two experiments are presented that measure the acuity of binaural processing of modulated interaural level differences (ILDs) using psychoacoustic methods. In both experiments, dynamic ILDs were created by imposing an interaurally antiphasic sinusoidal amplitude modulation (AM) signal on high-frequency carriers, which were presented over headphones. In the first experiment, the sensitivity to dynamic ILDs was measured as a function of the modulation frequency using puretone, and interaurally correlated and uncorrelated narrow-band noise carriers. The intrinsic interaural level fluctuations of the uncorrelated noise carriers raised the ILD modulation detection thresholds with respect to the pure-tone carriers. The diotic fluctuations of the correlated noise carriers also caused a small increase in the thresholds over the pure-tone carriers, particularly with low ILD modulation frequencies. The second experiment investigated the modulation frequency selectivity in dynamic ILD processing by imposing an interaurally uncorrelated bandpass noise AM masker in series with the interaurally antiphasic AM signal on a pure-tone carrier. By varying the masker center frequencies relative to the signal modulation frequency, broadly tuned, bandpass-shaped patterns were obtained. Simulations with an existing binaural model show that a low-pass filter to limit the binaural temporal resolution is not sufficient to predict the results of the experiments.

Interaural fluctuations and the detection of interaural incoherence. III. Narrowband experiments and binaural models

In the first two articles of this series, reproducible noises with a fixed value of interaural coherence (0.992) were used to study the human ability to detect interaural incoherence. It was found that incoherence detection is strongly correlated with fluctuations in interaural differences, especially for narrow noise bandwidths, but it remained unclear what function of the fluctuations best agrees with detection data. In the present article, ten different binaural models were tested against detection data for 14- and 108-Hz bandwidths. These models included different types of binaural processing: independent-interaural-phase-difference/interaural-level-difference, lateral-position, and short-term cross-correlation. Several preprocessing transformations of the interaural differences were incorporated: compression of binaural cues, temporal averaging, and envelope weighting. For the 14-Hz bandwidth data, the most successful model postulated that incoherence is detected via fluctuations of interaural phase and interaural level processed by independent centers. That model correlated with detectability at r=0.87. That model proved to be more successful than short-term cross-correlation models incorporating standard physiologically-based model features (r=0.78). For the 108-Hz bandwidth data, detection performance varied much less among different waveforms, and the data were less able to distinguish between models.