Rate limitation and onset dominance in the discrimination of high-frequency click trains with dynamic envelope ITDs

Abstract

Several studies have shown that binaural processing at high carrier frequency is limited to relatively slow modulations, suggesting profound consequences for binaural processing of high-rate electrical stimulation by cochlear implants (CIs). In this study, we measured normal-hearing-envelope ITD thresholds for Gaussian impulses (clicks) with 4000 Hz carrier frequency and 2 ms total duration (BW approximately 900 Hz) and for 16-click trains with rates of 100, 200, or 500 pps. For click trains, right-leading ITDs were static (condition RR), increased linearly from diotic (condition 0R), or decreased linearly toward diotic (condition R0). Thresholds were calculated from the largest ITD in each stimulus (i.e., the onset of R0 or offset of 0R trains). Consistent with previous results for normal hearing and bilaterally implanted subjects, thresholds were strongly dependent on ICI. Low-rate (100 pps) thresholds improved for 16-click trains relative to single clicks in all conditions, while high-rate (500 pps) thresholds were similar to single clicks for RR and R0 stimuli. In condition 0R, 500 pps ITD thresholds were elevated and comparable to monaural rate discrimination. The results suggest an inability to lateralize high-rate stimuli in the absence of onset ITDs and have important consequences for binaural information processing with cochlear implants.