Sensitivity to interaural time difference with bilateral cochlear implants: Development over time and effect of interaural electrode spacing

Abstract

Sensitivity to interaural time difference (ITD) in constant-amplitude pulse trains was measured in four sequentially implanted bilateral cochlear implant (CI) subjects. The sensitivity measurements were made as a function of time beginning directly after the second ear was implanted, continued for periods of months before subjects began wearing bilateral sound processors, and extended for months while the subjects used bilateral sound processors in day-to-day listening. Measurements were also made as a function of the relative position of the left/right electrodes. The two subjects with the shortest duration of binaural deprivation before implantation demonstrated ITD sensitivity soon after second-ear implantation (before receiving the second sound processor), while the other two did not demonstrate sensitivity until after months of daily experience using bilateral processors. The interaural mismatch in electrode position required to decrease ITD sensitivity by a factor of 2 (half-width) for CI subjects was five times greater than the half-width for interaural carrier-frequency disparity in normal-hearing subjects listening to sinusoidally amplitude-modulated high-frequency tones. This large half-width is likely to contribute to poor binaural performance in CI users, especially in environments with multiple broadband sound sources.

Modeling Binaural Responses in the Auditory Brainstem to Electric Stimulation of the Auditory Nerve

“…For example, the model shows a strong dependence of ITD sensitivity on total synaptic strength, with higher frequency stimulation requiring a greater total synaptic conductance. If deafness is associated with a loss of convergent inputs to the binaural neurons, the expected reduction in synaptic strength could cause a degradation in ITD sensitivity of MSO neurons at high pulse rates, consistent with the decreased perceptual ITD acuity observed above about 300 pps in bilateral CI users (van Hoesel 2007;Poon et al 2009). In addition, a decrease in inhibitory gain observed in central auditory neurons with both long-term and shortterm deafness (Vale and Sanes 2000;Kotak et al 2008) could degrade ITD sensitivity by allowing more firings in the antiphasic condition.…”

“…Using pulsatile stimulation bypassing the clinical processor, bilateral CI users with postlingual deafness can achieve relatively good ITD sensitivity at low pulse rates, but performance rapidly degrades for pulse rates above 200-300 pps in most subjects (van Hoesel 2007;Poon et al 2009). Similarly, single neurons in the inferior colliculus (IC) of acutely deafened cats can exhibit good ITD sensitivity to electric pulse trains at low pulse rates (G100 pps) for moderate stimulus levels, but their ITD sensitivity degrades at higher pulse rates where many neurons show only an onset response (Smith andDelgutte 2007, 2008;Hancock et al 2012).…”

Congenital and Prolonged Adult-Onset Deafness Cause Distinct Degradations in Neural ITD Coding with Bilateral Cochlear Implants

“…Among four subjects tested for ITD discrimination by Laback et al (2007), the best performer had been deaf for only 2 months prior to implantation, while the other listeners experienced several years of bilateral or unilateral auditory deprivation. Among four subjects tested by Poon et al (2009), two who had experienced relatively short periods (G 6 years) of auditory deprivation showed ITD sensitivity as soon as their bilateral processor was activated, while the other two subjects who experienced over 10 years of bilateral deprivation did not regain ITD sensitivity until after several months of bilateral CI usage. While very limited, these psychophysical data are consistent with the moderate effects of auditory deprivation in our long-term deafened cats.…”

Sensitivity to interaural envelope correlation changes in bilateral cochlear-implant users

“…Pitch-matched pairs of electrodes were tested in these experiments, which often maximize sensitivity to binaural cues in CI listeners (Long et al, 2003;Poon et al, 2009;Kan et al, 2013). Thus interaural place-of-stimulation mismatch may have also reduced binaural sensitivity in the CI listeners if the pitch matches did not fully compensate for interaural frequency mismatch.…”