  The objective was to determine the effect of pulse rate on the sensitivity to
use interaural-time-difference (ITD) cues and to explore the mechanisms behind
rate-dependent degradation in ITD perception in bilateral cochlear implant (CI)
listeners using CI simulations and electroencephalogram (EEG) measures. To
eliminate the impact of CI stimulation artifacts and to develop protocols for
the ongoing bilateral CI studies, upper-frequency limits for both behavior and
EEG responses were obtained from normal hearing (NH) listeners using
sinusoidal-amplitude-modulated (SAM) tones and filtered clicks with changes in
either fine structure ITD or envelope ITD. Multiple EEG responses were
recorded, including the subcortical auditory steady-state responses (ASSRs) and
cortical auditory evoked potentials (CAEPs) elicited by stimuli onset, offset,
and changes. Results indicated that acoustic change complex (ACC) responses
elicited by envelope ITD changes were significantly smaller or absent compared
to those elicited by fine structure ITD changes. The ACC morphologies evoked by
fine structure ITD changes were similar to onset and offset CAEPs, although
smaller than onset CAEPs, with the longest peak latencies for ACC responses and
shortest for offset CAEPs. The study found that high-frequency stimuli clearly
elicited subcortical ASSRs, but smaller than those evoked by lower carrier
frequency SAM tones. The 40-Hz ASSRs decreased with increasing carrier
frequencies. Filtered clicks elicited larger ASSRs compared to high-frequency
SAM tones, with the order being 40-Hz-ASSR>160-Hz-ASSR>80-Hz-ASSR>320-Hz-ASSR
for both stimulus types. Wavelet analysis revealed a clear interaction between
detectable transient CAEPs and 40-Hz-ASSRs in the time-frequency domain for SAM
tones with a low carrier frequency.
