Interaural Time and Level Difference Thresholds for Acoustically Presented Signals in Post-Lingually Deafened Adults Fitted with Bilateral Cochlear Implants Using CIS+ Processing

Abstract

The higher ILD and ITD thresholds obtained in this study with acoustically presented signals (when compared with prior data with direct electrical stimulation) can be attributed-at least partially-to the signal processing carried out by the CI in the former case. The processing strategy effectively leaves only envelope information as a basis for ITD discrimination, which, for the acoustically presented noise stimuli, is mainly coded in the onset information. The operation of the compression circuit reduces the ILDs in the signal, leading to elevated ILD thresholds for the acoustically presented signals in this condition. The large magnitude of the ITD thresholds indicates that ITDs could not have contributed to the performance in the horizontal-plane localization task. Overall, the results suggest that for subjects using bilateral implants, localization of noise signals is mediated entirely by ILD cues, with little or no contribution from ITD information.

Lateralization of Interimplant Timing and Level Differences in Children Who Use Bilateral Cochlear Implants

“…Poorer abilities using ITDs than ILDs observed in the CI group is consistent with previous literature involving adult bilateral cochlear implant users (Grantham et al 2008;Grantham et al 2007;Laback et al 2004;Seeber and Fastl 2008;van Hoesel and Tyler 2003).…”

“…This improvement has been widely attributed to detection of inter-implant level differences (Grantham et al 2008;Grantham et al 2007;Laback et al 2004;Seeber and Fastl 2008;Senn et al 2005;Van Hoesel et al 2002).…”

“…Van Hoesel and Tyler (2003) have shown that adult bilateral CI users can have ILD sensitivities of as little as 0.17dB, as defined in terms of stimulation current, when instructions to the bilateral CI electrodes are delivered directly through a research interface rather than through the two speech processors. Studies which used acoustic stimuli presented to participants' speech processors have reported ILD sensitivities of as low as 1.4-5.2 dB (Laback et al 2004) and 0.9-3.3dB (Grantham et al 2008), in terms of acoustic input levels.…”

Modulation Enhancement in the Electrical Signal Improves Perception of Interaural Time Differences with Bimodal Stimulation

“…We are not aware of any studies that investigated bilateral CI ITD thresholds with multi-channel stimuli under direct computer control. With clinical processors, however, performance is generally poorer than in the single-channel studies (Grantham et al 2008;Laback et al 2004;Senn et al 2005). For NH listeners, Akeroyd (2003) measured JNDs in ITD for full-bandwidth vowel sounds (thus containing temporal fine structure ITD cues), and obtained values in the order of 40 μs.…”

Restoring hearing symmetry with two cochlear implants or one cochlear implant and a contralateral hearing aid

“…ITD sensitivity was moderate, generally about 100 µs and reduced quickly when the stimulation rate for unmodulated pulse trains was increased beyond a few hundred hertz. In another study, stimuli were 200 ms Gaussian noise bursts presented acoustically and delivered via headphones that fit over the implant devices [98]. ITD and ILD discrimination tasks consisted of a two-interval forced-choice adaptive procedure that determined whether the sounds were perceived as moving from right-to-left or left-to-right.…”