Neural correlates of phonetic learning in postlingually deafened cochlear implant listeners

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 multitalker variability and adaptive listening. The program was designed to help CI listeners attend to the important second formant transition cue that categorizes the /ba/-/da/ and /wa/-/ja/ contrasts. Nine adult CI listeners completed the training and 4 additional CI 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-post test 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 CI device.