Prior exposure to speech rapidly modulates cortical processing of high-level linguistic structure

Area of Science:

• Neuroscience
• Linguistics
• Auditory Perception

Background:

• Neural activity tracks hierarchical linguistic units in connected speech.
• Speech intelligibility, influenced by spectral degradation, modulates neural tracking.
• Prior knowledge can enhance speech intelligibility.

Purpose of the Study:

• To investigate if enhanced speech intelligibility, via prior knowledge, strengthens neural tracking responses.
• To determine the effect of prior knowledge on cortical coherence at different linguistic levels (word, phrase, sentence).
• To examine the hemispheric lateralization of neural tracking modulation by intelligibility.

Main Methods:

• Recorded cortical magnetoencephalography (MEG) from 23 normal-hearing participants.
• Participants listened to intelligible speech followed by matched or unmatched unintelligible speech.
• Analyzed cortical coherence at word, phrase, and sentence rates.

Main Results:

• Prior knowledge enhanced cortical coherence at phrase and sentence rates, predominantly in the left hemisphere, compared to the unmatched condition.
• Cortical coherence at the word level, linked to acoustic onsets, was bilateral and unaffected by contextual information.
• No coherence enhancement was observed when unintelligible speech preceded intelligible speech.

Conclusions:

• Cerebral tracking of linguistic information is directly influenced by speech intelligibility, which is shaped by physical cues and prior knowledge.
• Enhanced intelligibility strengthens neural tracking of higher-order linguistic units.
• Findings provide a neural index for speech intelligibility and explain previous null findings regarding prior knowledge.