Auditory cortical activity to different voice onset times in cochlear implant users

Objective: Voice onset time (VOT) is a critical temporal cue for perception of speech in cochlear implant (CI) users. We assessed the cortical auditory evoked potentials (CAEPs) to consonant vowels (CVs) with varying VOTs and related these potentials to various speech perception measures. Methods: CAEPs were recorded from 64 scalp electrodes during passive listening in CI and normal-hearing (NH) groups. Speech stimuli were synthesized CVs from a 6-step VOT /ba/-/pa/ continuum ranging from 0 to 50 ms VOT in 10-ms steps. Behavioral measures included the 50% boundary point for categorical perception (bato pa) from an active condition task. Results: Behavioral measures: CI users with poor speech perception performance had prolonged 50% VOT boundary points compared to NH subjects. The 50% boundary point was also significantly correlated to the ability to discriminate consonants in quiet and noise masking. Electrophysiology: The most striking difference between the NH and CI subjects was that the P2 response was significantly reduced in amplitude in the CI group compared to NH. N1 amplitude did not differ between NH and CI groups. P2 latency increased with increases in VOT for both NH and CI groups. P2 was delayed more in CI users with poor speech perception compared to NH subjects. N1 amplitude was significantly related to consonant perception in noise while P2 latency was significantly related to vowel perception in noise. When dipole source modelling in auditory cortex was used to characterize N1/P2, more significant relationships were observed with speech perception measures compared to the same N1/P2 activity when measured at the scalp. N1 dipole amplitude measures were significantly correlated with consonants in noise discrimination. Like N1, the P2 dipole amplitude was correlated with consonant discrimination, but additional significant relationships were observed such as sentence and word identification. Conclusions: P2 responses to a VOT continuum stimulus were different between NH subjects and CI users. P2 responses show more significant relationships with speech perception than N1 responses. Significance: The current findings indicate that N1/P2 measures during a passive listening task relate to speech perception outcomes after cochlear implantation. (C) 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.