Many hearing aids introduce compressive gain to accommodate the reduced dynamic range that often accompanies hearing loss. However, natural sounds produce complicated temporal dynamics in hearing aid compression, as gain is driven by whichever source dominates at a given moment. Moreover, independent compression at the two ears can introduce fluctuations in interaural level differences (ILDs) important for spatial perception. While independent compression can interfere with spatial perception of sound, it does not always interfere with localization accuracy or speech identification. Here, normal-hearing listeners reported a target message played simultaneously with two spatially separated masker messages. We measured the amount of spatial separation required between the target and maskers for subjects to perform at threshold in this task. Fast, syllabic compression that was independent at the two ears increased the required spatial separation, but linking the compressors to provide identical gain to both ears (preserving ILDs) restored much of the deficit caused by fast, independent compression. Effects were less clear for slower compression. Percent-correct performance was lower with independent compression, but only for small spatial separations. These results may help explain differences in previous reports of the effect of compression on spatial perception of sound.

To clarify the role of spatial cues in sound segregation, this study explored whether interaural time differences (ITDs) are sufficient to allow listeners to identify a novel sound source from a mixture of sources. Listeners heard mixtures of two synthetic sounds, a target and distractor, each of which possessed naturalistic spectrotemporal correlations but otherwise lacked strong grouping cues, and which contained either the same or different ITDs. When the task was to judge whether a probe sound matched a source in the preceding mixture, performance improved greatly when the same target was presented repeatedly across distinct distractors, consistent with previous results. In contrast, performance improved only slightly with ITD separation of target and distractor, even when spectrotemporal overlap between target and distractor was reduced. However, when subjects localized, rather than identified, the sources in the mixture, sources with different ITDs were reported as two sources at distinct and accurately identified locations. ITDs alone thus enable listeners to perceptually segregate mixtures of sources, but the perceived content of these sources is inaccurate when other segregation cues, such as harmonicity and common onsets and offsets, do not also promote proper source separation.

Whenever an acoustic scene contains a mixture of sources, listeners must segregate the mixture in order to compute source content and/or location. Some past studies have explored whether perceived location depends on which sound elements are perceived within a source. However, no direct comparisons have been made of "what" and "where" judgments for the same sound mixtures using the same listeners. The current study tested if the sound elements making up an auditory object predict that object's perceived location. Listeners were presented with an auditory scene containing competing "target" and "captor" sources, each of which could logically contain a "promiscuous" tone complex. In separate blocks, the same listeners matched the perceived spectro-temporal content ("what") and location ("where") of the target. Generally, as the captor intensity decreased, the promiscuous complex contributed more to both what and where judgments of the target. However judgments did not agree either quantitatively or qualitatively. For some subjects, the promiscuous complex consistently contributed more to the spectro-temporal content of the target than to its location while for some it consistently contributed more to target location. These results show a dissociation between the perceived spectro-temporal content of an auditory object and where that object is perceived.

, S C (2009). Periodicity detection and localization using spike timing from the AER EAR. In: IEEE International Symposium on Circuits and Systems, 2009 (ISCAS 2009), Taipei, Taiwan, 24 May 2009 -27 May 2009 Periodicity detection and localization using spike timing from the AER EAR AbstractWe present a system consisting of a spiking cochlea chip and real-time event-based processing software that is able to discriminate between two sets of sounds based on their periodicity content. The periodicity measurements are computed from the spike timing information of asynchronous output spikes from the binaural spiking-cochlea chip. The chip consists of a matched pair of silicon cochlea with an address event interface for the output. Each section of the cochlea is modeled by a second-order low-pass filter followed by a simplified inner hair cell circuit and a spiking neuron circuit. We show discrimination results using the periodicity measure for 2 classes of sound and preliminary localization results based on a discriminated sound.Periodicity detection and localization using spike timing from the AER EAR Abstract-We present a system consisting of a spiking cochlea chip and real-time event-based processing software that is able to discriminate between two sets of sounds based on their periodicity content. The periodicity measurements are computed from the spike timing information of asynchronous output spikes from the binaural spiking-cochlea chip. The chip consists of a matched pair of silicon cochlea with an address event interface for the output. Each section of the cochlea is modeled by a second-order low-pass filter followed by a simplified Inner Hair Cell circuit and a Spiking Neuron circuit. We show discrimination results using the periodicity measure for 2 classes of sound and preliminary localization results based on a discriminated sound.

In Advances in Auditory Research: Physiology, Psychophysics and Models, edited by E. Lopez-Poveda and R. Meddis, Springer INTRODUCTION Indoors and in nature alike, the auditory scenes that we perceive unfold in reverberant environments. In a reverberant sound field, reflected acoustic waves reach the listener from all directions, interfering with the direct sound and distorting the binaural cues for sound localization such as interaural time and level differences (ITD and ILD). In previous work (Devore et al., 2009), we showed that reverberation degrades the directional sensitivity of low frequency, ITD-sensitive neurons in the inferior colliculus (IC) of anesthetized cats, although not as much as predicted by an interaural crosscorrelation model. Here, we extend this work by characterizing directional sensitivity in neurons across a wide range of the tonotopic axis in an awake rabbit preparation, while maintaining our focus on neurons that are sensitive to ITD.Low frequency IC neurons are typically sensitive to ITD in the waveform fine structure while high frequency IC neurons are sensitive to ITD in the amplitude envelopes (Batra et al., 1993;Joris, 2003; Griffin et al., 2005; Yin et al., 1984). At all characteristic frequencies (CF), the rate responses of IC neurons can be altered by imposing ILDs (Batra et al., 1993;Palmer et al., 2007); however, for stimuli with naturally co-occurring binaural cues, ILDs may be a more potent directional cue than envelope ITDs in high frequency neurons (Delgutte et al., 1995).We investigated the effects of reverberation on directional sensitivity of ITDsensitive neurons in the IC of awake rabbits using virtual auditory space (VAS) stimuli containing different binaural cues (ITD-only and ITD+ILD). We find that reverberation degrades the directional sensitivity of single neurons, although the amount of degradation depends on both the CF and the type of binaural cues available. We also compared results from IC neurons with measures of directional information extracted from coincidence analysis of spike trains recorded from auditory nerve (AN) fibers in anesthetized cats. Together our results suggest that the frequency-dependent degradation in ITD-based directional sensitivity partly originates in the auditory periphery and can be attributed to differential degradation of interaural envelopes and fine-time structure in reverberation.

Erratum: Effects of dynamic range compression on spatial selective auditory attention in normal-hearing listeners [PACS number(s): 43.66.Ts, 43.66.Pn, 43.66.Dc [ADP]This article contains some errors that originated during the copy editing process. The description of moderate compression speed on p. 2330 (Sec. I) contains an error; the statement should begin with "Moderate compression speeds on the order of tens of milliseconds could increase interactions."The description of the adaptive procedure on p. 2332 (Sec. II D) contains an error; the statement should read: "For three of these four digits, each pair of symmetric masker digits came from one of three fixed spatial separations: 615 , 630 , and 690 ."