Repeated listening induces exposure-specific cortical tracking of intelligible continuous speech
Neural encoding of acoustic and linguistic features of continuous speech is sensitive to cognitive factors, such as attention and comprehension. We investigated whether neural tracking is also sensitive to the predictability of speech. Participants were repeatedly exposed to intelligible or unintelligible versions of the same audiobook segment while EEG was recorded. First, we fit encoding models to predict EEG responses from acoustic, sublexical, and lexical features of the presented speech. Model comparisons revealed no reliable improvement in model fit when lexical features were included; subsequent analyses were performed on models including only acoustic and sublexical predictors. Second, we compared prediction accuracy for models trained and tested on the same exposures with models trained and tested across different exposures. While we observed no overall change in prediction performance across exposures, we found that models were exposure-specific: prediction performance was highest within the same exposure and decreased with increasing temporal distance between the training and test exposure. This effect was observed for intelligible but not for unintelligible speech, suggesting that the effect depends on properties unique to intelligible speech, such as the ability to form increasingly specific predictions about upcoming linguistic input, rather than general, non-linguistic factors related to repeated exposure. This distance effect was associated with increased model weights from -90 ms to 130 ms, indicating an enhancement of familiar input during an early cortical processing stage. In summary, these findings indicate that cortical tracking of sublexical speech features is modulated by repeated exposure to intelligible speech, consistent with a role for linguistic predictability.