Perceptual learning following changes in the frequency-to-electrode assignment with the Nucleus-22 cochlear implant

Abstract

The goal of the present study was to investigate the time course of adaptation by experienced cochlear implant users to a shifted frequency-to-electrode assignment in their speech processors. Speech recognition performance of three Nucleus-22 cochlear implant users was measured over a 3-month period, during which the implant listeners continuously wore "experimental" speech processors that were purposely shifted by 2-4 mm in terms of the frequency-to-electrode assignment relative to their normal processor. Baseline speech performance was measured with each subject's clinically assigned speech processor just prior to implementation of the experimental processor. Baseline speech performance was measured again after the 3-month test period, immediately following the reinstallation of the clinically assigned processor settings. Speech performance with the experimental processor was measured four times during the first week, and weekly thereafter over the 3-month period. Results showed that the experimental processor produced significantly lower performance on all measures of speech recognition immediately following implementation. Over the 3-month test period, consonant and HINT sentence recognition with the experimental processors gradually approached a performance level comparable to but still significantly below the baseline and postexperiment measures made with the clinically assigned processor. However, vowel and TIMIT sentence recognition with the experimental processors remained far below the level of the baseline measures even at the end of the 3-month experimental period. There was no significant change in performance with the clinically assigned processor before or after fitting with the experimental processor. The results suggest that a long-time exposure to a new pattern of stimulation may not be able to compensate for the deficit in performance caused by a 2-4-mm shift in the tonotopic location of stimulation, at least within a 3-month period.

Adaptation by a Cochlear-Implant Patient to Upward Shifts in the Frequency Representation of Speech

“…The scores for sentences dropped 14 points. These outcomes are somewhat better than expected given previous reports of octave shifts with N22 implant patients (Fu & Shannon, 1999;Fu et al, 2002). After 1 wk of adaptation, the scores for vowels, consonant place and sentences remained slightly below the baseline scores.…”

“…The manner score was unaffected by the shift and the voicing score, which was affected, returned to the baseline value after 1 wk of experience. A similar outcome for envelope cues, i.e., either no change in scores immediately after shifting or a complete recovery of scores after experience, has been found for N22 patients after a 3-mo period of adaptation to an octave shift (Fu et al, 2002).…”

“…This project differed from a recent project of similar nature (Fu et al, 2002) in that frequency shifts were implemented by assigning a fixed set of frequency bands to electrodes located in successively more basal locations rather than by assigning different frequency bands to a fixed set of electrodes.…”

“…There are relatively few reports of adaptation to large frequency shifts (Fu, Shannon, & Galvin, 2002;Kileny, Zimmerman-Phillips, Zwolan, & Kemink, 1992;McKay & Henshall, 2002) because the experiments require patients to use, for extended periods, processors that provide distorted representations of speech.…”

Combined acoustic and electric hearing: Preserving residual acoustic hearing

“…Longer electrodes would theoretically offer a better opportunity to present low-and mid-frequency information to the "proper" place along the cochlea. However, the ability of implant listeners to adapt to new frequency maps (Fu et al, 2002), and the changes in pitch perception over time observed in short-electrode patients (Reiss et al, 2007) suggests that this may not be a crucial issue. The Hybrid patients in the Iowa studies continue to improve their performance over periods as long as 2 years; this is longer than typically observed in traditional long-electrode patients, and suggests that: 1) the stimulation pattern currently provided to these patients is not optimal to begin with, and 2) over time, the auditory system does adapt itself at least to some extent over time.…”