Binaural frequency selectivity in humans.

Several behavioural studies in humans have shown that listening to sounds with two ears that is binaural hearing, provides the human auditory system with extra information on the sound source that is not available when sounds are only perceived through one ear that is monaurally. Binaural processing involves the analysis of phase and level differences between the two ear signals. As monaural cochlea processing (in each ear) precedes the neural stages responsible for binaural processing properties it is reasonable to assume that properties of the cochlea may also be observed in binaural processing. A main characteristic of cochlea processing is its frequency selectivity. In psychoacoustics, there is an ongoing discussion on the frequency selectivity of the binaural auditory system. While some psychoacoustic experiments seem to indicate poorer frequency selectivity of the binaural system than that of the monaural processing others seem to indicate the same frequency selectivity for monaural and binaural processing. This study provides an overview of these seemingly controversial results and the different explanations that were provided to account for the different results.

Comodulation masking release with random variations of flanking-band center frequencies.

Comodulation masking release (CMR) is an effect that is associated with auditory sensitivity to coherent amplitude modulations in different frequency regions. The present study investigated if this comodulation is detected by a direct comparison of auditory filter outputs, or if common masker fluctuations are first extracted by a broadly tuned stage that integrates information across a large spectral range. To this end, a modified flanking-band experiment with a narrowband noise masker at the signal frequency (on-frequency masker), and two flanking bands (FBs), one centered below and one above the signal frequency, were used. The center frequencies (CFs) of FBs changed whenever the masker had a local envelope minimum. The center frequencies were randomly chosen from a range of frequencies around the average CF of each FB. A CMR was measured even for large CF variations of FBs, where the envelopes at the off-frequency auditory filters were no longer the same as the masker envelope at the on-frequency auditory filter. This supports the hypothesis of a broadly tuned stage to determine masker comodulation. For two experimental settings, CMR deteriorated for very large variations of CFs of FBs, suggesting a spectral weighting of the off-frequency auditory filters in this broadly tuned stage.

Spectral integration of infrasound at threshold.

The present study investigated the effect of the number of spectral components on the threshold in quiet of infrasound stimuli. Stimuli with one, two, or three sinusoidal components were presented monaurally to the ear with a low-distortion infrasound reproduction system. All components of the complex tones had the same level relative to their respective pure-tone threshold. The data are consistent with the assumption of a (perceptually weighted) intensity integration at threshold: The level at threshold for each component is 3 dB lower when two components were presented simultaneously. Thresholds decrease further, when a three-tone complex is used.

Comodulation detection difference and binaural unmasking.

The present study investigated the combined effect of binaural cues and comodulation for a narrowband target noise masked by a narrowband noise. The threshold difference between a diotic condition (same stimuli in both ears) and a dichotic condition (target interaural phase difference of π and diotic masker) decreased with spectral distance between masker and target, irrespective of across-frequency envelope correlation. The threshold difference between a condition with comodulated target and masker and a corresponding uncorrelated condition, i.e., the comodulation detection difference, did not depend on target frequency and interaural correlation, indicating that these two stimulus properties are processed independently.

Simultaneous acquisition of 40- and 80-Hz auditory steady-state responses for a direct comparison of response amplitude, residual noise and signal-to-noise ratio.

PURPOSE: The purpose of this study was to establish a paradigm that allows for the simultaneous recording of auditory steady-state responses (ASSRs) to two largely different modulation rates. METHODS: In 21 normal-hearing adults, ASSRs for 40- and 80-Hz modulation rates were recorded in (1) a classical monotic single-stimulus condition, (2) a monotic simultaneous condition, where 40 Hz was paired with a 1-kHz carrier and 80 Hz with a 2-kHz carrier, and (3) a dichotic simultaneous condition with the same modulation rate/carrier pairing. Response amplitudes, residual noises, and signal-to-noise ratios were compared across conditions. RESULTS: Whereas the multiple-stimulus paradigms reduced the 40-Hz ASSR amplitude compared to the single condition, there was hardly any change for the 80-Hz ASSR. In all conditions, the 40-Hz ASSR was considerably larger than the 80-Hz ASSR. The residual noise was only 1.4 times larger for 40 Hz than for the 80 Hz. CONCLUSION: The proposed procedure using multiple stimuli with largely different modulation rates can be used to study differences in their responses and residual noise under identical states of vigilance. The amplitude reduction caused by the interaction between multiple stimuli has to be taken into account when interpreting the results.

Temporal weights in the perception of sound intensity: Effects of sound duration and number of temporal segments.

Loudness is a fundamental aspect of auditory perception that is closely related to the physical level of the sound. However, it has been demonstrated that, in contrast to a sound level meter, human listeners do not weight all temporal segments of a sound equally. Instead, the beginning of a sound is more important for loudness estimation than later temporal portions. The present study investigates the mechanism underlying this primacy effect by varying the number of equal-duration temporal segments (5 and 20) and the total duration of the sound (1.0 to 10.0 s) in a factorial design. Pronounced primacy effects were observed for all 20-segment sounds. The temporal weights for the five-segment sounds are similar to those for the 20-segment sounds when the weights of the segments covering the same temporal range as a segment of the five-segment sounds are averaged. The primacy effect can be described by an exponential decay function with a time constant of about 200 ms. Thus, the temporal weight assigned to a specific temporal portion of a sound is determined by the time delay between sound onset and segment onset rather than by the number of segments or the total duration of the sound.

Evaluation of a model of temporal weights in loudness judgments.

The onset of a sound receives a higher weight than later portions in time when its loudness is assessed, an effect commonly referred to as primacy effect. It is investigated if this effect can be predicted on the basis of an exponentially decaying function where the weight assigned to a temporal portion of a sound is the integral of this function over the segment duration. To test this model, temporal loudness weights were measured for sounds with different segment durations and total durations. The model successfully predicted essential aspects of the data.

Characteristics of spectro-temporal modulation frequency selectivity in humans.

There is increasing evidence that the auditory system shows frequency selectivity for spectro-temporal modulations. A recent study of the authors has shown spectro-temporal modulation masking patterns that were in agreement with the hypothesis of spectro-temporal modulation filters in the human auditory system [Oetjen and Verhey (2015). J. Acoust. Soc. Am. 137(2), 714-723]. In the present study, that experimental data and additional data were used to model this spectro-temporal frequency selectivity. The additional data were collected to investigate to what extent the spectro-temporal modulation-frequency selectivity results from a combination of a purely temporal amplitude-modulation filter and a purely spectral amplitude-modulation filter. In contrast to the previous study, thresholds were measured for masker and target modulations with opposite directions, i.e., an upward pointing target modulation and a downward pointing masker modulation. The comparison of this data set with previous corresponding data with the same direction from target and masker modulations indicate that a specific spectro-temporal modulation filter is required to simulate all aspects of spectro-temporal modulation frequency selectivity. A model using a modified Gabor filter with a purely temporal and a purely spectral filter predicts the spectro-temporal modulation masking data.

Effect of duration and gating of the signal on the binaural masking level difference for narrowband and broadband maskers.

Thresholds were measured for a 250-Hz signal with an interaural phase difference of 0° (diotic) or 180° (dichotic), with signal durations of 12 and 60 ms (including 6-ms ramps) and 300 ms (including 6- or 50-ms ramps). The signal-centered diotic noise masker had a bandwidth of 20 or 200 Hz. For the 20-Hz wide masker, the binaural masking level difference (BMLD), i.e., threshold difference between diotic and dichotic signal, increased with signal duration and, for the 300-ms signal, the BMLD was larger with 50-ms rather than 6-ms ramps. These signal parameters hardly affected the BMLD for the 200-Hz wide masker.

Modeling off-frequency binaural masking for short- and long-duration signals.

Experimental binaural masking-pattern data are presented together with model simulations for 12- and 600-ms signals. The masker was a diotic 11-Hz wide noise centered on 500 Hz. The tonal signal was presented either diotically or dichotically (180° interaural phase difference) with frequencies ranging from 400 to 600 Hz. The results and the modeling agree with previous data and hypotheses; simulations with a binaural model sensitive to monaural modulation cues show that the effect of duration on off-frequency binaural masking-level differences is mainly a result of modulation cues which are only available in the monaural detection of long signals.

Absence of direction-specific cross-modal visual-auditory adaptation in motion-onset event-related potentials.

Adaptation to visual or auditory motion affects within-modality motion processing as reflected by visual or auditory free-field motion-onset evoked potentials (VEPs, AEPs). Here, a visual-auditory motion adaptation paradigm was used to investigate the effect of visual motion adaptation on VEPs and AEPs to leftward motion-onset test stimuli. Effects of visual adaptation to (i) scattered light flashes, and motion in the (ii) same or in the (iii) opposite direction of the test stimulus were compared. For the motion-onset VEPs, i.e. the intra-modal adaptation conditions, direction-specific adaptation was observed--the change-N2 (cN2) and change-P2 (cP2) amplitudes were significantly smaller after motion adaptation in the same than in the opposite direction. For the motion-onset AEPs, i.e. the cross-modal adaptation condition, there was an effect of motion history only in the change-P1 (cP1), and this effect was not direction-specific--cP1 was smaller after scatter than after motion adaptation to either direction. No effects were found for later components of motion-onset AEPs. While the VEP results provided clear evidence for the existence of a direction-specific effect of motion adaptation within the visual modality, the AEP findings suggested merely a motion-related, but not a direction-specific effect. In conclusion, the adaptation of veridical auditory motion detectors by visual motion is not reflected by the AEPs of the present study.

Interaction of Object Binding Cues in Binaural Masking Pattern Experiments.

Object binding cues such as binaural and across-frequency modulation cues are likely to be used by the auditory system to separate sounds from different sources in complex auditory scenes. The present study investigates the interaction of these cues in a binaural masking pattern paradigm where a sinusoidal target is masked by a narrowband noise. It was hypothesised that beating between signal and masker may contribute to signal detection when signal and masker do not spectrally overlap but that this cue could not be used in combination with interaural cues. To test this hypothesis an additional sinusoidal interferer was added to the noise masker with a lower frequency than the noise whereas the target had a higher frequency than the noise. Thresholds increase when the interferer is added. This effect is largest when the spectral interferer-masker and masker-target distances are equal. The result supports the hypothesis that modulation cues contribute to signal detection in the classical masking paradigm and that these are analysed with modulation bandpass filters. A monaural model including an across-frequency modulation process is presented that account for this effect. Interestingly, the interferer also affects dichotic thresholds indicating that modulation cues also play a role in binaural processing.

Mid-bandwidth loudness depression in hearing-impaired listeners.

The loudness of a bandpass-filtered noise depends on its bandwidth. For bandwidths larger than a critical bandwidth, loudness increases as the bandwidth increases, an effect commonly referred to as spectral loudness summation. For bandwidths smaller than the critical bandwidth, it was shown recently for normal-hearing listeners that loudness decreases as the bandwidth increases. This study investigated if listeners with a hearing impairment of primarily cochlear origin also showed this effect. Levels at equal loudness between a 1500-Hz pure-tone reference and noise-band targets centered at 1500 Hz were measured for bandwidths in the range from 15 to 1620 Hz. The reference level was adjusted individually on the basis of the audiogram. The average level difference at equal loudness increased from 0 dB at 15 Hz up to a maximum of about 4 dB at 810 Hz. Thus, the mid-bandwidth loudness depression is also observed for hearing-impaired listeners.

Categorical scaling of partial loudness in a condition of masking release.

Categorical loudness scaling was used to measure suprathreshold release from masking. The signal was a 986-Hz sinusoid that was embedded in a bandpass-filtered masking noise. This noise was either unmodulated or was amplitude modulated with a square-wave modulator. The unmodulated noise had either the same level as the modulated noise or had a level that was reduced by the difference in thresholds for the 986-Hz signal obtained with the modulated and unmodulated noise masker presented at the same level (i.e., the masking release). A comparison with loudness matching data of the same set of subjects showed that the data obtained with loudness scaling capture main aspects of the change in suprathreshold perception of the sinusoid when the masker was modulated. The scaling data for the signal masked by the unmodulated noise with the reduced masker level were similar to that for the signal embedded in the modulated noise. This similarity supports the hypothesis that the mechanism eliciting the masking release is effectively reducing the masker level.

Spectro-temporal modulation masking patterns reveal frequency selectivity.

The present study investigated the possibility that the human auditory system demonstrates frequency selectivity to spectro-temporal amplitude modulations. Threshold modulation depth for detecting sinusoidal spectro-temporal modulations was measured using a generalized masked threshold pattern paradigm with narrowband masker modulations. Four target spectro-temporal modulations were examined, differing in their temporal and spectral modulation frequencies: a temporal modulation of -8, 8, or 16 Hz combined with a spectral modulation of 1 cycle/octave and a temporal modulation of 4 Hz combined with a spectral modulation of 0.5 cycles/octave. The temporal center frequencies of the masker modulation ranged from 0.25 to 4 times the target temporal modulation. The spectral masker-modulation center-frequencies were 0, 0.5, 1, 1.5, and 2 times the target spectral modulation. For all target modulations, the pattern of average thresholds for the eight normal-hearing listeners was consistent with the hypothesis of a spectro-temporal modulation filter. Such a pattern of modulation-frequency sensitivity was predicted on the basis of psychoacoustical data for purely temporal amplitude modulations and purely spectral amplitude modulations. An analysis of separability indicates that, for the present data set, selectivity in the spectro-temporal modulation domain can be described by a combination of a purely spectral and a purely temporal modulation filter function.

Comodulation masking release in an off-frequency masking paradigm.

Detection threshold of a sinusoidal signal masked by a broadband masker is lower when on- and off-frequency masker components have a correlated envelope, compared to a condition in which these masker components have different envelopes. This effect is commonly referred to as comodulation masking release (CMR). The present study investigated if there is a CMR in the absence of a masker component at the signal frequency, i.e., in an off-frequency masking paradigm. Thresholds were measured for a 500-Hz signal in the presence of a broadband masker with a spectral notch at the signal frequency. Thresholds were significantly lower for a (co-)modulated than for an unmodulated masker for all notch widths up to 400 Hz. An additional experiment showed that the particularly large CMR for the no-notch condition was due to the way the modulated masker was generated. No CMR was measured when the notched-noise masker was replaced by a pair of narrowband noises. The addition of more remote masker bands resulted in a CMR of about 3-4 dB. The notched-noise data were predicted on the basis of a modulation-filterbank model. The predictions of the narrowband noise conditions indicated that all mechanisms underlying CMR might still not be fully understood.

Perceptual space, pleasantness and periodicity of multi-tone sounds.

Technical sounds often contain several tonal components, forming a multi-tone sound. The present study investigates the perception of multi-tone sounds consisting of two harmonic complexes with different fundamental frequencies and combination tones with frequencies that are equal to the sum of multiple integers of the two fundamentals. The experimental parameter is the ratio between the two fundamental frequencies ρ. A total of 15 synthetic multi-tone sounds are rated by 37 participants. In the first experiment, the perceptual space is assessed based on 16 adjective scales using categorical scaling. The resulting perceptual space has the four dimensions (i) pleasant, (ii) power, (iii) temporal structure, and (iv) spectral content of the sounds. In the second experiment, the pleasantness is measured with a paired comparison test. The data consistently show that sounds based on ratios of small integers (e.g., ρ=4:3) are significantly less pleasant than sounds with ratios based on large integers which were constructed by a slight detuning from a ratio of small integers. The repetition rate derived from an autocorrelation analysis of the stimuli turns out to be a good predictor of the (un-)pleasantness sensation.

Binaural spectral resolution as a function of interaural masker correlation.

Thresholds for an antiphasic 500-Hz sinusoid (Sπ) were measured in the presence of a notched-noise masker for three different interaural masker correlations: -1 (Nπ), 1 (N0), and 0.87. The difference between thresholds for the three masker correlations was largest for a notch width of zero and decreased continuously with increasing notch width. The bandwidth of a gammatone filter fitted to the data was 85 Hz for the interaural masker correlation of -1 (NπSπ) and 182 Hz for the interaural masker correlation of 1 (N0Sπ). For the intermediate correlation (0.87) the effective filter width was 134 Hz. This result is at odds with corresponding literature results of a bandwidening experiment where an effectively larger binaural bandwidth is only found with extreme interaural disparities of signal and masker, such as N0Sπ. The notched-noise thresholds were predicted if a detrimental across-channel process was included in the model. This approach failed to predict the effect of masker correlation in the bandwidening experiments. A beneficial across-channel process successfully used in the literature to simulate bandwidening data with extreme binaural parameters also failed to predict this effect. The effect may be due to interaural parameter fluctuations currently not used in the model framework.

Interaural-phase discrimination in notched noise.

Discrimination thresholds for the interaural-phase difference (IPD) of a 500-Hz sinusoid were measured in the presence of a diotic notched-noise masker as a function of notch width. The reference sinusoid had zero IPD. The level was set to 3 dB above the individual masked threshold for the diotic sinusoid at the respective notch width. Just-noticeable IPDs increased from about 20° for notch widths up to 100 Hz to 100° at the largest notch width of 800 Hz. Model simulations show that the data are consistent with the equalization-cancellation theory, if effectively wider binaural filters are assumed.

Direction-specific adaptation of motion-onset auditory evoked potentials.

Auditory evoked potentials (AEPs) to motion onset in humans are dominated by a fronto-central complex, with a change-negative deflection 1 (cN1) and a change-positive deflection 2 (cP2) component. Here the contribution of veridical motion detectors to motion-onset AEPs was investigated with the hypothesis that direction-specific adaptation effects would indicate the contribution of such motion detectors. AEPs were recorded from 33 electroencephalographic channels to the test stimulus, i.e. motion onset of horizontal virtual auditory motion (60° per s) from straight ahead to the left. AEPs were compared in two experiments for three conditions, which differed in their history prior to the motion-onset test stimulus: (i) without motion history (Baseline), (ii) with motion history in the same direction as the test stimulus (Adaptation Same), and (iii) a reference condition with auditory history. For Experiment 1, condition (iii) comprised motion in the opposite direction (Adaptation Opposite). For Experiment 2, a noise in the absence of coherent motion (Matched Noise) was used as the reference condition. In Experiment 1, the amplitude difference cP2 - cN1 obtained for Adaptation Same was significantly smaller than for Baseline and Adaptation Opposite. In Experiment 2, it was significantly smaller than for Matched Noise. Adaptation effects were absent for cN1 and cP2 latencies. These findings demonstrate direction-specific adaptation of the motion-onset AEP. This suggests that veridical auditory motion detectors contribute to the motion-onset AEP.