PULSE TIMING INTERVAL SENSITIVITY IN THE INFERIOR COLLICULUS OF COCHLEAR IMPLANTED RATS EXCEEDS THRESHOLDS OBSERVED IN HUMAN COCHLEAR IMPLANT USERS

Temporal coding is fundamental to pitch perception in normal hearing, but cochlear implant (CI) users often perform poorly in temporal discrimination tasks. We hypothesize that sustained exposure to fixed pulse-rate stimulation may desensitize CI users to fine temporal variations. Perceptual thresholds reported in psychoacoustic studies on CI users may therefore not reflect the true limits of the auditory pathway. Here we revisited a study by Fielden et al., (2014) which tested the ability of CI users to discriminate stimulus pulse pairs. Trains of pulse pairs were presented at a rate of 100 pairs/second, and the within-pair interval was increased from a minimum of 1 ms. They reported that CI users could only detect changes in within-pair intervals > 3 ms.

We suspected that the mammalian auditory pathway should be able to resolve smaller time intervals, and we fitted anesthetized, acutely deafened rats with CIs, and recorded neural responses in the inferior colliculus (IC) using a multichannel electrode array while presenting identical paired-pulse stimuli to those used in the Fielden (2014) study. Single units were isolated using spike sorting. Analyses of these responses revealed that individual IC units could discriminate pulse-pair intervals as short as 1.32 ms. Our results contribute to the growing evidence that the CI stimulated auditory pathway may be capable of significantly better temporal discrimination than experiments on volunteers habituated to standard clinical processors typically suggest. Consequently, enhanced temporal fine-structure sensitivity in CI users may be achievable through improved signal processing strategies.