Auditory cortical responses to speech are shaped by statistical learning of short-term speech input regularities

Abstract

Speech perception presents an exemplary model of how neurobiological systems flexibly adjust when input departs from the norm. Dialects, accents, and even head colds can negatively impact comprehension by shifting speech from listeners expectations. Comprehension improves with exposure to shifted speech regularities, but there is no neurobiological model of this rapid learning. We used electroencephalography to examine human auditory cortical responses to utterances that varied only in fundamental frequency (F0, perceived as voice pitch) as we manipulated the statistical distributions of speech acoustics across listening contexts. Participants overtly categorized speech sampled across two acoustic dimensions that signal /b/ from /p/ (voice onset time [VOT] and F0) to model typical English speech regularities or an expectation-violating accent. These blocks were interleaved with passive exposure to two F0-distinguished test stimuli presented in an oddball ratio to elicit a cortical mismatch negativity (MMN) response. F0 robustly influenced speech categorization when short-term regularities aligned with English but F0 exerted no influence in the context of the accent. The short-term regularities modulated event-related potentials evoked by F0-distinguished test stimuli across both N1 and P3 temporal windows and, for P3 amplitude, there was a strong correlation with perceptual down-weighting of F0. The influence of the short-term regularities persisted to impact MMN in interleaved passive listening blocks when regularities mirrored English but were absent when regularities conveyed the accent. Thus, cortical response is modulated as a function of statistical regularities of the listening context, likely reflecting both early dimension encoding and later categorization. Significance StatementSpeech perception is a quintessential example of how neurobiological systems flexibly adjust when input departs from the norm. Perception is well-tuned to native-language speech patterns. Yet it adjusts when speech diverges from expectations, as with a foreign accent. We observe that the effectiveness of specific cues in speech, like the pitch of a voice, in signaling phonemes like /b/ versus /p/ is dynamically re-weighted when speech violates native-language expectations. We find that this re-weighting is mirrored in cortical responses that reflect both early acoustic dimension encoding and also in later responses linked to phoneme categorization. The results implicate dynamic adjustments in the mapping of speech to cortical representations, as modulated by statistical regularities experienced across local speech input.