Bastian Epp

Superposition of masking releases.

We are constantly exposed to a mixture of sounds of which only few are important to consider. In order to improve detectability and to segregate important sounds from less important sounds, the auditory system uses different aspects of natural sound sources. Among these are (a) its specific location and (b) synchronous envelope fluctuations in different frequency regions. Such a comodulation of different frequency bands facilitates the detection of tones in noise, a phenomenon known as comodulation masking release (CMR). Physiological as well as psychoacoustical studies usually investigate only one of these strategies to segregate sounds. Here we present psychoacoustical data on CMR for various virtual locations of the signal by varying its interaural phase difference (IPD). The results indicate that the masking release in conditions with binaural (interaural phase differences) and across-frequency (synchronous envelope fluctuations, i.e. comodulation) cues present is equal to the sum of the masking releases for each of the cues separately. Data and model predictions with a simplified model of the auditory system indicate an independent and serial processing of binaural cues and monaural across-frequency cues, maximizing the benefits from the envelope comparison across frequency and the comparison of fine structure across ears.

Combination of masking releases for different center frequencies and masker amplitude statistics

Several masking experiments have shown that the auditory system is able to use coherent envelope fluctuations of the masker across frequency within one ear as well as differences in interaural disparity between signal and masker to enhance signal detection. The two effects associated with these abilities are comodulation masking release (CMR) and binaural masking level difference (BMLD). The aim of the present study was to investigate the combination of CMR and BMLD. Thresholds for detecting a sinusoidal signal were measured in a flanking-band paradigm at three different signal frequencies. The masker was presented diotically, and various interaural phase differences (IPDs) of the signal were used. The masker components were either multiplied or Gaussian narrowband noises. In addition, a transposed stimulus was used to increase the BMLD at a high signal frequency. For all frequencies and masker conditions, thresholds decreased as the signal IPD increased and were lower when the masker components were comodulated. The data show an addition of the monaural and binaural masking releases in decibels when masker conditions with and without comodulation and the same spectrum were compared.

Objective measures of binaural masking level differences and comodulation masking release based on late auditory evoked potentials.

The audibility of important sounds is often hampered due to the presence of other masking sounds. The present study investigates if a correlate of the audibility of a tone masked by noise is found in late auditory evoked potentials measured from human listeners. The audibility of the target sound at a fixed physical intensity is varied by introducing auditory cues of (i) interaural target signal phase disparity and (ii) coherent masker level fluctuations in different frequency regions. In agreement with previous studies, psychoacoustical experiments showed that both stimulus manipulations result in a masking release (i: binaural masking level difference; ii: comodulation masking release) compared to a condition where those cues are not present. Late auditory evoked potentials (N1, P2) were recorded for the stimuli at a constant masker level, but different signal levels within the same set of listeners who participated in the psychoacoustical experiment. The data indicate differences in N1 and P2 between stimuli with and without interaural phase disparities. However, differences for stimuli with and without coherent masker modulation were only found for P2, i.e., only P2 is sensitive to the increase in audibility, irrespective of the cue that caused the masking release. The amplitude of P2 is consistent with the psychoacoustical finding of an addition of the masking releases when both cues are present. Even though it cannot be concluded where along the auditory pathway the audibility is represented, the P2 component of auditory evoked potentials is a candidate for an objective measure of audibility in the human auditory system.

Clustering of cochlear oscillations in frequency plateaus as a tool to investigate SOAE generation

Spontonaeous otoacoustic emissions (SOAE) reflect the net effect of self-sustained activity in the cochlea, but do not directly provide information about the underlying mechanism and place of origin within the cochlea. The present study investigates if frequency plateaus as found in a linear array of coupled oscillators (OAM) [7] are also found in a transmission line model (TLM) which is able to generate realistic SOAEs [2] and if these frequency plateaus can be used to explain the formation of SOAEs. The simulations showed a clustering of oscillators along the simulated basilar membrane Both, the OAM and the TLM show traveling-wave like behavior along the oscillators coupled into one frequency plateau. While in the TLM roughness is required in order to produce SOAEs, no roughness is required to trigger frequency plateaus in the linear array of oscillators. The formation of frequency plateaus as a consequence of coupling between neighbored active oscillators might be the mechanism underlying SOAEs.

Masking Release for Sweeping Masker Components with Correlated Envelopes

To separate sounds from different sound sources, common properties of natural sounds are used by the auditory system, such as coherent temporal envelope fluctuations and correlated changes of frequency in different frequency regions. The present study investigates how the auditory system processes a combination of these cues using a generalized comodulation masking release (CMR) paradigm. CMR is the effect of a better signal detectability in the presence of comodulated maskers than in the presence of maskers with uncorrelated envelope fluctuations across frequencies. Using a flanking-band paradigm, the results of the first experiment of the present study show that CMR is still observed for the masker and the signal coherently sweeping up or down in frequency over time, up to a sweep rate of six octaves per second. Motivated by the successful modeling of CMR using filters sensitive to temporal modulations and recent physiological evidence of spectro-temporal modulation filters, the second experiment investigates whether CMR is also observed for spectro-temporal masker modulations generated using time-shifted versions of the masker envelope for each component. The thresholds increase as soon as the temporally coherent masker modulation is changed to a spectro-temporal masker modulation, indicating that spectro-temporal modulation filters are presumably not required in CMR models.

Comparing Longitudinal Coupling and Temporal Delay in a Transmission‐Line Model of the Cochlea

In this paper we compare and contrast the effects of longitudinal coupling and temporal delay on a fluid‐structure transmission‐line model of the mammalian cochlea. This work is based on recent reports that, in order to qualitatively explain experimental data, models of the basilar membrane impedance must include an exponential term that represents a time‐delayed feedback. There are also models that include, e.g., a spatial feed‐forward mechanism, whose solution is often approximated by replacing the feed‐forward coupling by an exponential term. We show that there is no direct equivalence between the time‐delay and the longitudinal coupling mechanisms, although qualitatively similar results can be achieved, albeit in very different regions of parameter space. An investigation of the steady‐state outputs shows that both models can display sharp tuning, but that the time‐delay model requires negative damping for such an effect to occur. Conversely, the longitudinal coupling model provides the most promising results with small positive damping. These results are extended by a careful stability analysis. We find that, whereas a small time delay can stabilize an unstable transmission‐line model (with negative damping), that the longitudinal coupling model is stable when the damping is positive. The techniques developed in the paper are directed towards a more comprehensive analysis of nonlinear models.In this paper we compare and contrast the effects of longitudinal coupling and temporal delay on a fluid‐structure transmission‐line model of the mammalian cochlea. This work is based on recent reports that, in order to qualitatively explain experimental data, models of the basilar membrane impedance must include an exponential term that represents a time‐delayed feedback. There are also models that include, e.g., a spatial feed‐forward mechanism, whose solution is often approximated by replacing the feed‐forward coupling by an exponential term. We show that there is no direct equivalence between the time‐delay and the longitudinal coupling mechanisms, although qualitatively similar results can be achieved, albeit in very different regions of parameter space. An investigation of the steady‐state outputs shows that both models can display sharp tuning, but that the time‐delay model requires negative damping for such an effect to occur. Conversely, the longitudinal coupling model provides the most promising...

Processing strategies of the auditory system for improving the detection of masked signals

The processing of the auditory system allows the separation of complex acoustical scenes into so-called auditory objects. The goal of this thesis was to shed some light on the ability of the auditory system to use combined across-frequency and binaural cues for the analysis of complex auditory environments. The first and second part of the thesis addresses the investigation of the processing of combined across-frequency and binaural signal properties in normally hearing listeners using psychoacoustical techniques and signal processing strategies. The third part concentrates on the influence of cochlear processing on psychoacoustic effects using a physical nonlinear and active model of the cochlea. The results of this thesis point out that the auditory system is able to process comodulation across frequency and interaural phase differences independently and over a broad frequency range. Hence, improvements of masked thresholds can be used in an optimal manner. The ability of the developed cochlea model to ...

Spectral weighting of interaural time- and level differences for broadband signals

An important ability of the auditory system is to localize sound sources in complex acoustical environments. Two important cues for localization are interaural time- and level differences (ITD, ILD). The sensitivity to these cues differs across frequency and has previously been estimated through frequency-specific detection thresholds. Detection thresholds of ITDs/ILDs are, however, affected by stimulus energy in remote spectral regions, referred to as binaural interference. In this study, the spectral weights of ITD- and ILD cues in the lateralization of a broadband signal was investigated using regression analysis. The stimuli consisted of eleven 1-ERB-wide noise bands (442 Hz-5544 Hz) containing ITD or ILD cues. In experiment 1, ITDs or ILDs were applied to the noise bands and roved independently on every trial. In experiment 2, the noise bands centred at 442 Hz and 5544 Hz were removed to investigate the effect of stimulus bandwidth. In experiment 3, the same two noise bands were present, but containe...

Modelling the effect of pulse-rate on coding of interaural time differences in listeners with cochlear implants

CIs stimulate the auditory nerve fibers (ANF) with a train of amplitude modulated current pulses. Depending on polarity, the pulses can generate spikes at different sites along the ANF. The latency difference between spikes generated at the central and the peripheral axons was found to be up to 200 μs in cats and up to 450 μs in humans. These timing differences could be the reason underlying the poor performance of CI listeners in ITD perception. A model of ANF responses to electrical stimulation (Joshi et al., 2016), which includes two sites of spike generation along the ANF was used to simulate the ANF responses to constant-amplitude and modulated pulse trains for different pulse-rates. The fidelity of the temporal coding was quantified by calculating the phase-locking value. The results show that an increase in pulse-rate leads to higher uncertainty in the site of spike generation, reduction in phase-locking, and increase in variance of its distribution. This may account for impaired ITD thresholds obs...

Individual differences in auditory brainstem response wave-V latency in forward masking: A measure of auditory neuropathy?

A recent animal study suggests that noise exposure causes a preferential loss of low-spontaneous rate (low-SR) auditory nerve fibers (ANFs). This loss may leave detection thresholds normal yet degrade temporal encoding of supra-threshold sounds. Differences in the rate of recovery from forward masking in ANFs with different spontaneous rates may allow one to assess the state of different ANF populations. To test this, we measured auditory brainstem response (ABR) in a forward masking paradigm and evaluated wave-V latency changes with increasing masker-to-probe intervals (MPI). We expected that (1) loss of ANFs increases wave-V latency and forward masking thresholds, and (2) a preferential loss of low-SR fibers results in faster recovery time of wave-V latency. To test our hypotheses, we presented listeners with a broadband noise masker at two levels followed by a chirp probe at various MPIs. Initial results show that normal hearing threshold (NHT) listeners with delayed wave-V latency exhibit higher behav...