Adaptation by normal listeners to upward spectral shifts of speech: Implications for cochlear implants

TLDR

Multi‑channel cochlear implants often deliver spectral information to incorrect cochlear locations, and although such shifts immediately degrade performance, it is unclear whether listeners can adapt to them. The study simulated a four‑channel implant in normal listeners, comparing unshifted spectral input with a 6.5‑mm basalward shift (1.3–2.9 octaves) and assessed performance across both conditions. While the unshifted simulation yielded ~64 % correct word identification, the shifted condition initially produced only ~1 % accuracy, but after nine 20‑minute training sessions accuracy rose to ~30 % and listeners could track shifted speech at up to 40 wpm, indicating that short‑term adaptation may underestimate long‑term effects.

Abstract

Multi-channel cochlear implants typically present spectral information to the wrong “place” in the auditory nerve array, because electrodes can only be inserted partway into the cochlea. Although such spectral shifts are known to cause large immediate decrements in performance in simulations, the extent to which listeners can adapt to such shifts has yet to be investigated. Here, the effects of a four-channel implant in normal listeners have been simulated, and performance tested with unshifted spectral information and with the equivalent of a 6.5-mm basalward shift on the basilar membrane (1.3–2.9 octaves, depending on frequency). As expected, the unshifted simulation led to relatively high levels of mean performance (e.g., 64% of words in sentences correctly identified) whereas the shifted simulation led to very poor results (e.g., 1% of words). However, after just nine 20-min sessions of connected discourse tracking with the shifted simulation, performance improved significantly for the identification of intervocalic consonants, medial vowels in monosyllables, and words in sentences (30% of words). Also, listeners were able to track connected discourse of shifted signals without lipreading at rates up to 40 words per minute. Although we do not know if complete adaptation to the shifted signals is possible, it is clear that short-term experiments seriously exaggerate the long-term consequences of such spectral shifts.