Asymmetric function of theta and gamma activity in syllable processing: an intra-cortical study

Area of Science:

• Neuroscience
• Auditory Neuroscience
• Speech Processing

Background:

• Theta (4-8 Hz) and low-gamma (25-45 Hz) oscillations are hypothesized to integrate syllabic and phonemic information, respectively.
• The hemispheric functional division of these neural oscillations in speech processing remains largely unknown.

Purpose of the Study:

• To investigate the distinct roles of theta and gamma oscillations in the right and left auditory cortices during speech processing.
• To explore the hemispheric specialization of auditory processing at different frequency bands.

Main Methods:

• Stereotactic electroencephalography (EEG) was used to record cortical responses from an epileptic patient with bilateral electrode implantation in primary (A1/BA41, A2/BA42) and association (BA22) auditory cortices.
• Time-frequency analyses were applied to resting-state and syllable-processing data.

Main Results:

• A 5-6 Hz theta activity predominated in the right auditory cortex, while 25-45 Hz low-gamma activity was dominant in the left primary auditory cortex during both resting state and syllable processing.
• High-theta (7-8 Hz) resting activity was observed in the left primary and association auditory regions.
• Phase of left BA22 theta activity correlated with high-gamma induced power, indicating theta/gamma phase-amplitude coupling.

Conclusions:

• Theta and gamma activities in the right and left auditory cortices serve different functions in speech processing.
• The right auditory cortex appears to specialize in slow syllabic parsing (5-6 Hz theta), while the left auditory cortex handles faster phonemic extraction (~25-45 Hz gamma) and integrates it at a slower rate.
• Theta/gamma phase-amplitude coupling in the left auditory cortex may facilitate the neural shift from acoustic feature encoding to phonological encoding and abstract percept formation.