The effect of increasing acoustic and linguistic complexity on auditory processing: an EEG study

This study uses electroencephalography (EEG) to explore the pre-attentive processing of auditory stimuli with a systematic (stepwise) increase in linguistic complexity. Participants heard repetitive similar stimuli interspersed with rare deviants that differed in pitch. Responses to the deviants were compared among non-speech complex harmonic waves and speech syllables. Syllables were generated using VocalTractLab [1], a software that allows for the synthesis of natural-sounding, highly controllable artificial speech. Complex waves were generated using Praat [2], with their intensity envelopes and harmonic structure matched to those of the syllables. Thus, the two stimulus types were acoustically very comparable, differing only in linguistic characteristics like presence/absence of modelled vocal tract and laryngeal influences. We hypothesized that both deviants would evoke a Mismatch Negativity (MMN) brain response. We further explored how MMN size varied with the nature of the stimulus. Results supported our hypothesis. An MMN was observed for both pitch deviants. The MMN was larger in amplitude for the complex waves than the syllables, suggesting differences between how linguistic syllable stimuli are processed compared to non-linguistic complex waves. This study demonstrates how an increase in acoustic and linguistic complexity reflects in the MMN response and provides support for domain-specific theories of auditory processing.