Miller, S., Zhang, Y., & Nelson, P. (2016). Neural correlates of phonetic learning in postlingually deafened cochlear implant listeners. Ear and Hearing.
Objective: The present training study aimed to examine the fine scale behavioral and
neural correlates of phonetic learning in adult postlingually deafened cochlear implant
(CI) listeners. The study investigated whether high variability identification training
improved phonetic categorization of the /ba/-/da/ and /wa/-/ja/ speech contrasts and
whether any training-related improvements in phonetic perception were correlated with
neural markers associated with phonetic learning. It was hypothesized that training
would sharpen phonetic boundaries for the speech contrasts and that changes in
behavioral sensitivity would be associated with enhanced mismatch negativity (MMN)
responses to stimuli that cross a phonetic boundary relative to MMN responses evoked
using stimuli from the same phonetic category.
Design: A computer-based training program was developed that featured multi-talker
variability and adaptive listening. The program was designed to help cochlear implant
listeners attend to the important second formant transition cue that categorizes the
/ba/-/da/ and /wa/-/ja/ contrasts. Nine adult cochlear implant listeners completed the
training and four additional cochlear implant listeners that did not undergo training were
included to assess effects of procedural learning. Behavioral pre-post tests consisted
of identification and discrimination of the synthetic /ba/-/da/ and /wa/-/ja/ speech
continua. The electrophysiologic MMN response elicited by an across phoneme
category pair and a within phoneme category pair that differed by an acoustically
equivalent amount was derived at pre-posttest intervals for each speech contrast as
well.
Results: Training significantly enhanced behavioral sensitivity across the phonetic
boundary and significantly altered labeling of the stimuli along the /ba/-/da/ continuum.
While training only slightly altered identification and discrimination of the /wa/-/ja/
continuum, trained CI listeners categorized the /wa/-/ja/ contrast more efficiently than
the /ba/-/da/ contrast across pre-post test sessions. Consistent with behavioral results,
pre-post EEG measures showed the MMN amplitude to the across phoneme category
pair significantly increased with training for both the /ba/-/da/ and /wa/-/ja/ contrasts,
but the MMN was unchanged with training for the corresponding within phoneme
category pairs. Significant brain-behavior correlations were observed between changes
in the MMN amplitude evoked by across category phoneme stimuli and changes in the
slope of identification functions for the trained listeners for both speech contrasts.
Conclusions: The brain and behavior data of the present study provide evidence that
substantial neural plasticity for phonetic learning in adult postlingually deafened CI
listeners can be induced by high variability identification training. These findings have
potential clinical implications related to the aural rehabilitation process following receipt
of a cochlear implant device.