Abstract
The ability of normal-hearing (NH) listeners to exploit interaural time difference (ITD) cues conveyed in the modulated envelopes of high-frequency sounds is poor compared to ITD cues transmitted in the temporal fine structure at low frequencies. Sensitivity to envelope ITDs is further degraded when envelopes become less steep, when modulation depth is reduced, and when envelopes become less similar between the ears, common factors when listening in reverberant environments. The vulnerability of envelope ITDs is particularly problematic for cochlear implant (CI) users, as they rely on information conveyed by slowly varying amplitude envelopes. Here, an approach to improve access to envelope ITDs for CIs is described in which, rather than attempting to reduce reverberation, the perceptual saliency of cues relating to the source is increased by selectively sharpening peaks in the amplitude envelope judged to contain reliable ITDs. Performance of the algorithm with room reverberation was assessed through simulating listening with bilateral CIs in headphone experiments with NH listeners. Relative to simulated standard CI processing, stimuli processed with the algorithm generated lower ITD discrimination thresholds and increased extents of laterality. Depending on parameterization, intelligibility was unchanged or somewhat reduced. The algorithm has the potential to improve spatial listening with CIs.
Highlights
Cochlear implants (CIs) restore hearing function in the severely and profoundly deaf, providing many users with high levels of speech understanding, when background noise is low
Localization experiments conducted under free-field conditions demonstrate that while some bilateral CI users show relatively high performance in localization tasks, they predominantly exploit ILDs in their judgments, rather than interaural time difference (ITD), even under anechoic listening conditions where ITD information is available in the absence of potentially confounding echoes or interfering sources
ITD discrimination performance was significantly improved by the application of either enhancement method, but this improvement was greatest for sourcereceiver distances of 1 m and below, which corresponded to direct-to-reverberant ratio (DRR) of À4.0 dB and higher
Summary
Cochlear implants (CIs) restore hearing function in the severely and profoundly deaf, providing many users with high levels of speech understanding, when background noise is low. Employed by most current CI devices convey temporal information (including ITD information) only in the modulated envelope of sounds; electrical pulses used to stimulate the electrode array are presented at a fixed, high rate completely unrelated to the sound’s TFS (see Fig. 1). At these high rates, ITD information carried in the timing of individual stimulation pulses is inaccessible to CI users (Laback et al, 2007). In five of seven CI users, the increase was larger than 10, and in two CI users it
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