Effect of mismatched place-of-stimulation on the salience of binaural cues in conditions that simulate bilateral cochlear-implant listening

Abstract

Although bilateral cochlear implantation has the potential to improve sound localization and speech understanding in noise, obstacles exist in presenting maximally useful binaural information to bilateral cochlear-implant (CI) users. One obstacle is that electrode arrays may differ in cochlear position by several millimeters, thereby stimulating different neural populations. Effects of interaural frequency mismatch on binaural processing were studied in normal-hearing (NH) listeners using band-limited pulse trains, thereby avoiding confounding factors that may occur in CI users. In experiment 1, binaural image fusion was measured to capture perceptual number, location, and compactness. Subjects heard a single, compact image on 73% of the trials. In experiment 2, intracranial image location was measured for different interaural time differences (ITDs) and interaural level differences (ILDs). For larger mismatch, locations perceptually shifted towards the ear with the higher carrier frequency. In experiment 3, ITD and ILD just-noticeable differences (JNDs) were measured. JNDs increased with decreasing bandwidth and increasing mismatch, but were always measurable up to 3 mm of mismatch. If binaural-hearing mechanisms are similar between NH and CI subjects, these results may explain reduced sensitivity of ITDs and ILDs in CI users. Large mismatches may lead to distorted spatial maps and reduced binaural image fusion.

Evidence for a neural source of the precedence effect in sound localization

“…By varying stimulus duration with carrier frequency, a constant "cochlear bandwidth" of ϳ3 mm (Ϯ1.5 mm), simulating the estimated average spread of excitation along the cochlea for a single electrode, can be maintained (cf. Goupell et al 2013;Kan et al 2013). While Gabor clicks provide a reasonable acoustic simulation of the CI pulsatile stimuli in terms of bandwidth, their comparability to CI stimuli is limited by their duration: At 4-kHz center frequency, the duration of a single pulse is nearly two orders of magnitude greater than the duration of a CI electrical pulse.…”

“…1). Such "Gabor clicks" have previously been used to simulate the brief yet effectively narrowband nature of CI pulsatile stimuli (e.g., Goupell et al 2013). By varying stimulus duration with carrier frequency, a constant "cochlear bandwidth" of ϳ3 mm (Ϯ1.5 mm), simulating the estimated average spread of excitation along the cochlea for a single electrode, can be maintained (cf.…”

Using speech sounds to test functional spectral resolution in listeners with cochlear implants

“…Understandably, place of articulation perception is particularly difficult for people with hearing impairment (Bilger and Wang, 1976), including people with CIs (Munson et al, 2003), and is a particularly difficult feature to recover when spectral quality is experimentally degraded for listeners with NH (Friesen et al, 2001;Xu et al, 2005). When conducting studies with NH individuals, resolution in the spectral domain can be explicitly controlled using noise vocoders (Shannon et al, 1995;Friesen et al, 2001;Xu et al, 2005), sine vocoders (Dorman and Loizou, 1997), Gaussian envelope tone vocoders (Goupell et al, 2013), and also with other methods used to achieve spectral smearing (ter Keurs et al, 1993). In these studies, place of articulation consistently emerges as a difficult feature to recover.…”

Effect of extreme adaptive frequency compression in bimodal listeners on sound localization and speech perception

“…The study of Goupell and coworkers (Goupell et al, 2013) in normal-hearing listeners showed robust lateralization responses, by using stimuli with an interaural mismatch in center frequency up to 4 kHz in one ear, and 14 kHz in the other. Francart and Wouters (2007) demonstrated ILD sensitivity in normal-hearing listeners for signals with an interaural frequency mismatch of up to one octave.…”