Darrin K. Reed

Lateralization of stimuli with alternating interaural time differences: The role of monaural envelope cues

A temporally acute binaural system can help to resolve inherent fluctuations in binaural information that are often present in complex auditory scenes. Using a broadband noise stimulus that rapidly alternates between two different values of interaural time difference (ITD), the ability of the binaural system to hear the lateral position resulting from one of the ITD values was investigated. Results show that listeners are able to accurately lateralize brief noise tokens of only 3-7 ms in duration. In two subsequent experiments, the role of an amplitude modulation (AM) imposed on the ITD-switching stimulus used in the first experiment was tested. For wideband stimuli, the temporal position of the ITD target relative to the phase of the AM did not influence absolute lateralization or detection performance. When the stimuli were narrowband, however, detection of the ITD target was best when temporally positioned in the rising portion of the AM. These experiments illustrate that the auditory system is capable of making accurate lateral estimates of very brief moments of ITD information. Furthermore, for these instantaneous changes in ITD information, the stimulus bandwidth can influence the role of envelope cues for the readout of binaural information.

Lateralization of noise bursts in interaurally correlated or uncorrelated background noise using interaural level differences

The interaural level difference (ILD) of a lateralized target source may be effectively reduced when the target is presented together with background noise containing zero ILD. It is not certain whether listeners perceive a position congruent with the reduced ILD or the actual target ILD in a lateralization task. Two sets of behavioral experiments revealed that many listeners perceived a position at or even larger than that corresponding to the presented target ILD when a temporal onset/offset asynchrony between the broadband target and the broadband background noise was present. When no temporal asynchrony was present, however, the perceived lateral position indicated a dependency on the coherence of the background noise for several listeners. With interaurally correlated background noise, listeners reported a reduced ILD resulting from the combined target and background noise stimulus. In contrast, several of the listeners made a reasonable estimate of the position corresponding to the target ILD for interaurally uncorrelated, broadband, background noise. No obvious difference in performance was seen between low- or high-frequency stimuli. Extension of a weighting template to the output of a standard equalization-cancellation model was shown to remove a lateral bias on the predicted target ILD resulting from the presence of background noise. Provided that an appropriate weighting template is applied based on knowledge of the background noise coherence, good prediction of the behavioral data is possible.

Figure/ground segregation enhanced by spatially directed attention

We recently reported that detecting a “figure” of repeated chords amidst “background” tones was worse when the figure was spatially separated from the background, a counterintuitive result given the published studies of spatial unmasking. Because of the way we blocked trials and provided instructions/feedback, listeners may have directed attention to the front where most figures appeared. Here, we investigated whether spatial cues would improve figure detection when listeners were instructed that spatial cues could aid performance. Figure detection was recorded for a condition where both the background and figure were presented diotically and for two binaural conditions. In one binaural condition, the figure occurred as a brief token at a lateral position separated from the background. In the other binaural condition, the background was composed of both a diotic stream and a lateralized stream, which contained the target (if present). Figure detection accuracy was better for both binaural conditions than ...

Differences in cortical responses to spatial changes in foreground versus background auditory objects

Identifying neural markers related to the integration of auditory features—such as spectrum, temporal coherence, and lateral position—can help to reveal the mechanisms underlying how the auditory system processes simultaneous sound sources. For a stimulus that is composed of randomly chosen successive inharmonic tone complexes, a subset of tones can perceptually segregate from the other simultaneous tones if the subset repeats for a sufficient number of tokens. Listeners can reliably detect this repeating pattern as a figure sound object amidst a randomly changing background. An electroencephalogram was recorded while listeners performed the detection of changes in interaural time difference (ITD) cues that were attributed to either the figure or the background. Detection of ITD changes for either auditory object elicited a fronto-central early negativity (100–200 ms) and P300 event-related potentials (ERP) associated with identification of a behaviorally relevant event. However, the ERP amplitudes were l...

Characterizing perceptual properties of a binaurally modulated stimulus.

The majority of everyday listening situations involve a complex mixture of multiple sound sources. Assuming a spectro-temporally sparse target signal, e.g., speech, the binaural cues in these complex listening environments do not simply result from a single sound source, but are often rapidly switching between the most dominant sources at any given moment. To investigate the perception of rapidly switching interaural time difference (ITD) cues, a noise stimulus that periodically alternates between two different values of ITD was created. This stimulus appears to evoke a purely binaural percept of modulation, which is the focus of the studies presented here. Results indicate that listeners can reliably discriminate this ITD-switching stimulus from a stimulus composed of stationary ITD cues. Frequency discrimination of this ITD-switching stimulus was tested in a separate experiment, which showed that listeners are generally able to discriminate a 50% change in modulation frequency for reference rates below 16 Hz. The final study investigated if modulation masking exists between the monaural and binaural auditory pathways. Although a statistically significant increase in modulation detection thresholds was observed when both types of modulations were presented together, the increase was relatively small, indicating only a weak interaction.

The temporal acuity for processing interaural cues

It was during a three month visit of the first author to his Lab in 1998 that Tino pointed out an elegant study on detecting changes in Interaural Cross Correlation of stimuli that were roved in baseline Interaural Time Delay and Interaural Level Difference (Bernstein and Trahiotis, J. Acoust. Soc. Am. 102, 1113). This study showed that, even though ICC can only change due to the presence of dynamically varying ITDs and ILDs, the ICC can be perceived as a separate cue in certain conditions. This articulate notion of three perceptually independent binaural cues, ITD and ILD, relating to perceived laterality, and ICC, related to perceived width, is applied in low bit-rate audio coding where these cues are used to encode the spatial image of stereo sound recordings. The entanglement of these cues on a signal basis raises the question of the time scale at which these three cues are perceptually evaluated. Some recent findings will be discussed that show that the ITD and ICC can be evaluated with a high tempor...

Rapid binaural processing for source segregation and lateralization

For realistic listening conditions, interaural cues will fluctuate due to the presence of multiple active sources. If it is assumed that the binaural system is sluggish, then the perceived location of the sound input would be an average of the varying interaural cues. If, however, the binaural system is fast enough to assess the rapidly changing interaural differences, then it could be possible for the binaural system to properly identify the spatial position of a target source. Using a continuous, broadband noise stimulus that contained periodically alternating interaural time differences (ITD) and, notably, no monaural cues, we investigated the binaural systems ability to lateralize brief durations of the target ITD. Results show that listeners can lateralize targets for durations of 3–6 ms indicating that the binaural system allows for a segregation and lateralization of the target and interfering noise streams. Furthermore, results indicate that the binaural system mediates the buildup of a modulated...

Lateralization of noise targets with interaural level differences presented within a noise interferer

The interaural level difference (ILD) of a lateralized target source is reduced when the target is presented together with background noise containing no ILD. It is unknown whether listeners simply use this reduced aggregate ILD or are still able to utilize the target ILD in a lateralization task. Behavioral experiments revealed that the temporal asynchrony between the onsets/offsets of the target and the background noise resulted in the population of listeners actually perceiving a larger ILD than the target ILD. For synchronous onsets/offsets, however, the perceived ILD depended on the coherence of the background noise. With coherent background noise, the population of subjects perceived a reduced ILD near the aggregate ILD. In contrast, the population of subjects made a reasonable estimate of the target ILD when the background noise was diffuse. Implementation of an Equalization Cancellation model and taking the compensatory level equalization that yields the lowest output as an estimate for the ILD re...