Plasticity in neuromagnetic cortical responses suggests enhanced auditory object representation

Abstract

BackgroundAuditory perceptual learning persistently modifies neural networks in the central nervous system. Central auditory processing comprises a hierarchy of sound analysis and integration, which transforms an acoustical signal into a meaningful object for perception. Based on latencies and source locations of auditory evoked responses, we investigated which stage of central processing undergoes neuroplastic changes when gaining auditory experience during passive listening and active perceptual training. Young healthy volunteers participated in a five-day training program to identify two pre-voiced versions of the stop-consonant syllable ‘ba’, which is an unusual speech sound to English listeners. Magnetoencephalographic (MEG) brain responses were recorded during two pre-training and one post-training sessions. Underlying cortical sources were localized, and the temporal dynamics of auditory evoked responses were analyzed.ResultsAfter both passive listening and active training, the amplitude of the P2m wave with latency of 200 ms increased considerably. By this latency, the integration of stimulus features into an auditory object for further conscious perception is considered to be complete. Therefore the P2m changes were discussed in the light of auditory object representation. Moreover, P2m sources were localized in anterior auditory association cortex, which is part of the antero-ventral pathway for object identification. The amplitude of the earlier N1m wave, which is related to processing of sensory information, did not change over the time course of the study.ConclusionThe P2m amplitude increase and its persistence over time constitute a neuroplastic change. The P2m gain likely reflects enhanced object representation after stimulus experience and training, which enables listeners to improve their ability for scrutinizing fine differences in pre-voicing time. Different trajectories of brain and behaviour changes suggest that the preceding effect of a P2m increase relates to brain processes, which are necessary precursors of perceptual learning. Cautious discussion is required when interpreting the finding of a P2 amplitude increase between recordings before and after training and learning.

Neural Correlates of Speech Segregation Based on Formant Frequencies of Adjacent Vowels

“…The latency of this modulation was comparable to that of prior studies using pure tones2327, and may reflect a modulation of the P2 wave. The P2 wave has been associated with speech discrimination3536, and may index categorical speech perception37 and sound object identification3839. We also found a second left-lateralized modulation that peaked at about 175 ms after the onset of the second vowel within the ABA- triplet (i.e., ~325 after triplet onset) as well as another modulation peaking at 250 ms (i.e., ~445 ms after triplet onset) over the fronto-central scalp region.…”

Perceptual Learning of Acoustic Noise Generates Memory-Evoked Potentials

“…The N1-P2 complex has been associated to perceptual changes within constant-amplitude stimuli [31], and it can be modulated by repeated exposure [32,33]. Here, we demonstrated not only a modulation of N1-P2 on a much-faster timescale but also the appearance of such an ERP where there was none before learning.…”

Speech training alters consonant and vowel responses in multiple auditory cortex fields

“…Typically, speech training results in strengthened auditory cortical evoked potentials. For example, individuals trained to categorize the prevoiced consonant /ba/ exhibit an increased N1-P2 amplitude [24] and an increased P2m amplitude [25] following training. The increased P2m amplitude was localized to anterior auditory cortex, which is where we observed a stronger response strength to consonants following training as well as an increase in the response similarity between closely tuned neurons.…”