Sensitivity to Interaural Time Differences with Combined Cochlear Implant and Acoustic Stimulation

Abstract

The interaural time difference (ITD) is an important cue to localize sound sources. Sensitivity to ITD was measured in eight users of a cochlear implant (CI) in the one ear and a hearing aid (HA) in the other severely impaired ear. The stimulus consisted of an electric pulse train of 100 pps and an acoustic filtered click train. Justnoticeable differences (JNDs) in ITD were measured using a lateralization paradigm. Four subjects exhibited median JNDs in ITD of 156, 341, 254, and 91 μs; the other subjects could not lateralize the stimuli consistently. Only the subjects who could lateralize had average acoustic hearing thresholds at 1,000 and 2,000 Hz better than 100-dB SPL. The electric signal had to be delayed by 1.5 ms to achieve synchronous stimulation at the auditory nerves.

Electrically Evoked Auditory Steady State Responses in Cochlear Implant Users

“…Additionally, response latencies for EASSRs should be smaller than for comparable acoustic stimuli because of the direct excitation of the nerve. A comparison between wave V latencies of 4.20 ms for EABRs to basal stimulation (Shallop et al 1990) and 5.64 ms for ABRs (Nikiforidis et al 1993) shows a difference of about 1.4 ms, while the necessary delay of the electric pathway for psychoacoustically synchronous bimodal stimulation is about 1.5 ms (Francart et al 2009).…”

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

“…They were selected to have residual hearing thresholds better than 100-dB sound pressure level (SPL) at 1,000 and 2,000 Hz, since we found previously that this was related to good ITD sensitivity (Francart et al 2009). They were volunteers and signed an informed consent form.…”

“…In our previous studies assessing interaural time difference (ITD) perception with bimodal hearing (Francart et al 2009 we have found ITD sensitivity in the envelope of simple synthetic singleand three-channel stimuli. For multiple-channel stimuli, ITD sensitivity was found when the electrical signal contains deep modulations that are consistent across different channels (electrodes).…”

“…While this could be counteracted using a fast compression system, such systems can have other undesirable effects. Another problem with clinically used bimodal systems is the synchronization between devices: the CI speech processor and hearing aid (HA) have a different processing delay and the acoustically stimulated signal is delayed while traveling through the middle and inner ear (Francart et al 2009). Previously, we have demonstrated that bimodal listeners can be sensitive to ITDs in the envelope of simple synthetic single-and three-channel stimuli (Francart et al 2009.…”

“…Another problem with clinically used bimodal systems is the synchronization between devices: the CI speech processor and hearing aid (HA) have a different processing delay and the acoustically stimulated signal is delayed while traveling through the middle and inner ear (Francart et al 2009). Previously, we have demonstrated that bimodal listeners can be sensitive to ITDs in the envelope of simple synthetic single-and three-channel stimuli (Francart et al 2009. Therefore, it is reasonable to hypothesize that their inability to use interaural timing cues with clinical devices is due to the way the devices work rather than a fundamental inability to perceive ITDs.…”