Perceptually relevant speech tracking in auditory and motor cortex reflects distinct linguistic features.

Area of Science:

• Neuroscience
• Auditory Perception
• Speech Processing

Background:

• The brain processes speech by tracking acoustic fluctuations across various timescales.
• Previous research often used generic frequency bands (e.g., delta, theta) for speech-time tracking, which can be ambiguous due to individual speaking variations.
• A direct link between specific speech statistical regularities and neural tracking timescales remains underexplored.

Purpose of the Study:

• To investigate neural tracking of speech at timescales derived directly from speech statistics, not generic bands.
• To determine the perceptual relevance of these specific timescale tracking mechanisms.
• To explore cross-frequency coupling in speech processing within the auditory-motor pathway.

Main Methods:

• Analysis of source-localized magnetoencephalography (MEG) data during online speech processing.
• Extraction of speech timescales directly from statistical regularities within the speech material.
• Examination of perceptual relevance by comparing neural tracking in correctly vs. incorrectly comprehended trials.

Main Results:

• Widespread significant neural tracking was observed at timescales corresponding to phrases (0.6-1.3 Hz), words (1.8-3 Hz), syllables (2.8-4.8 Hz), and phonemes (8-12.4 Hz).
• Stronger tracking in left premotor cortex (phrasal scale) and left middle temporal cortex (word scale) correlated with successful speech comprehension.
• Phase at the phrasal timescale coupled with beta-band power (13-30 Hz) in motor areas, suggesting top-down prediction.

Conclusions:

• Neural tracking of speech is specifically tuned to statistical regularities at multiple linguistic timescales.
• Distinct tracking mechanisms in auditory-motor regions play a crucial role in speech comprehension.
• Cross-frequency coupling supports predictive temporal processing during continuous speech perception.