Central Auditory Plasticity: Changes in the N1-P2 Complex after Speech-Sound Training

TLDR

The study aimed to determine whether training‑induced changes in the N1‑P2 complex reflect improved voice‑onset‑time perception. The authors recorded N1 and P2 evoked potentials from 10 normal‑hearing adults before and after training to discriminate −20 and −10 ms VOT /ba/ syllables, using two stimulus rates (390 ms and 910 ms ISI) in a repeated‑measures design. Training improved VOT discrimination and was accompanied by a larger N1‑P2 amplitude, especially at the slower stimulus rate, indicating increased neural synchrony and suggesting the complex could serve as an objective marker of speech‑sound training.

Abstract

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 of the syllable /ba./ Using a repeated measures design, subjects were tested before and after training both behaviorally and neurophysiologically to determine whether there were training-related changes. In between pre- and post-testing sessions, subjects were trained to distinguish the −20 and −10 msec VOT /ba/ syllables as being different from each other. Two stimulus presentation rates were used during electrophysiologic testing (390 msec and 910 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 −20 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 presentation rate. Conclusions As perception improved, N1-P2 amplitude increased. These changes in waveform morphology are thought to reflect increases in neural synchrony as well as strengthened neural connections associated with improved speech perception. These findings suggest that the N1-P2 complex may have clinical applications as an objective physiologic correlate of speech-sound representation associated with speech-sound training.