Central auditory plasticity: Changes in the N1-P2 complex after speech-sound training

Objective: To determine whether the N1-P2 complex reflects training-induced changes in neural activity associated with improved voice-onset-time (VOT) perception, Design: Auditory cortical evoked potentials N1 and P2 were obtained from 10 normal-hearing young adults in response to two synthetic speech variants o f the syllable \ba\, Using a repeated measures design, subjects were tested before and after training both behaviorally and neurophysiologically to de termine whether there were training-related changes. In between pre- and po st-testing sessions, subjects were trained to distinguish the -20 and -10 m sec VOT \ba\ syllables as being different from each other. Two stimulus pre sentation rates were used during electrophysiologic testing (390 msec and 9 10 msec interstimulus interval). Results: Before training, subjects perceived both the -20 msec and -10 msec VOT stimuli as /ba\, Through training, subjects learned to identify the -2 0 msec VOT stimulus as "mba" and -10 msec VOT stimulus as "ba." As subjects learned to correctly identify the difference between the -20 msec and -10 msec VOT syllabi, an increase in N1-P2 peak-to-peak amplitude was observed. The effects of training were most obvious at the slower stimulus presentat ion rate. Conclusions: As perception improved, N1-P2 amplitude increased. These chang es in waveform morphology are thought to reflect increases in neural synchr ony as well as strengthened neural connections associated with improved spe ech perception. These findings suggest that the N1-P2 complex may have clin ical applications as an objective physiologic correlate of speech-sound rep resentation associated with speech-sound training.