Perceptual organization of sequential stimuli in cochlear implant listeners: A temporal processing approach

Cochlear Implant (CI) users often suffer difficulties in perceiving speech in noisy environments that could be attributed to reduced Auditory Stream Segregation (ASS) ability. ASS is the process used to separate a complex sound into different perceptual streams. The evidence that CI listeners routinely experience stream segregation skill is limited and equivocal. The present study was aimed to investigate the effects of temporal cues on ASS performance in postlingually deaf listeners with CI.

Nineteen (age range: 28-64 years old) monaurally cochlear implanted listener participated in this study. They were presented with 30-s sequences of alternating stimuli in a repeating A-B-A-A-B-A…sequence, where "tone A" corresponds to a stimulus applied to electrode 11, and "tone B" to a stimulus on one of the other electrode. To investigate the effect of temporal cues on ASS, four different tone repetition times (TRTs) were utilized: 50, 100, 150, and 200 ms. Speech discrimination scores in noise were also recorded for every CI recipients.

Only 6 (32%) CI users demonstrated ASS pattern similar to the normal hearing subjects, while the majority of the users (n=13) possessed poorer ASS skills. An analysis of variance showed a significant effect of electrode separation (p<0.001) and TRT (p=0.041), but there was no significant interaction between electrode separation and TRT variables. The best ASS performance was obtained when TRT was 200 ms, and there was no significant effect for other TRT conditions. Moderate, significant correlations between streaming and speech discrimination measurement in noise was also observed (r=0.62), with better stream segregation associated with better understanding of speech in noise.

ASS is a contributing factor in the ability to perceive speech in background noise. The inability of some CI recipients to perform stream segregation may therefore contribute to their difficulties in noisy backgrounds. Furthermore, stream segregation ability is related to the tone repetition time between the sounds.