Sensitivity of bilateral cochlear implant users to fine-structure and envelope interaural time differences

Abstract

Bilateral cochlear implant users have poor sensitivity to interaural time differences (ITDs) of highrate pulse trains, which precludes use of these stimuli to convey fine-structure ITD cues. However, previous reports of single-neuron recordings in cats demonstrated good ITD sensitivity to 1000 pulses-per-second (pps) pulses when the pulses were sinusoidally amplitude modulated. The ability of modulation to restore ITD sensitivity to high-rate pulses in humans was tested by measuring ITD thresholds for three conditions: ITD encoded in the modulated carrier pulses alone, in the envelope alone, and in the whole waveform. Five of six subjects were not sensitive to ITD in the 1000-pps carrier, even with modulation. One subject's 1000-pps carrier ITD sensitivity did significantly improve due to modulation. Sensitivity to ITD encoded in the envelope was also measured as a function of modulation frequency, including at frequencies from 4 to 16 Hz where much of the speech envelope's energy and information resides. Sensitivity was best at the modulation frequency of 100 Hz and degraded rapidly outside of a narrow range. These results provide little evidence to support encoding ITD in the carrier of current bilateral processors, and suggest envelope ITD sensitivity is poor for an important segment of the speech modulation spectrum.

Effect of Channel Envelope Synchrony on Interaural Time Difference Sensitivity in Bilateral Cochlear Implant Listeners

“…The results show large differences in overall sensitivity level of the subjects, which is consistent with the literature (e.g., Figure 5, including data from van Hoesel et al 2009;Best et al 2011;Laback et al 2011;Kerber & Seeber 2013;Noel & Eddington 2013). Also note that in some studies listeners are preselected based on ITD sensitivity.…”

Interaural Time-Difference Discrimination as a Measure of Place of Stimulation for Cochlear-Implant Users With Single-Sided Deafness

“…However, reducing the pulse rate would not address the issue that a long pulse duration will nevertheless tend to relay less precise ITD information, regardless of resolvability or duty cycle. Furthermore, ITD sensitivity decreases with decreasing rate below about 60 to 100 pps (e.g., Hancock, Noel, Ryugo, & Delgutte, 2010 ; Noel & Eddington, 2013 ).…”

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

“…The fact that the high-rate pulse trains produced lower JNDs than the low-rate pulse trains is also in the opposite direction of the rate limitations shown in CI users for ITD discrimination of constantamplitude pulse trains (Laback and Majdak, 2008;van Hoesel et al, 2009). Note that these rate limitations are reduced or eliminated with envelope modulations (van Hoesel et al, 2009;Noel and Eddington, 2013). Therefore it seems that the ITD-based rate limitation observed in bilateral CI listeners is solely a temporal processing phenomenon and does not affect the ILD-based encoding of interaural decorrelation.…”