Theta-gamma phase-phase coupling during working memory maintenance in the human hippocampus

Area of Science:

• Neuroscience
• Cognitive Neuroscience
• Computational Neuroscience

Background:

• The theta-gamma neural coding theory proposes that working memory (WM) represents multiple items via superimposed gamma cycles on theta oscillations.
• Stable representation of multiple items in WM may involve phase-phase coupling, where gamma subcycles lock to specific theta phases.
• The hippocampus is crucial for multi-item working memory, suggesting its involvement in theta-gamma coding mechanisms.

Purpose of the Study:

• To investigate phase-phase coupling patterns in the hippocampus during multi-item working memory maintenance.
• To test predictions of the theta-gamma neural coding theory regarding hippocampal activity.
• To explore the relationship between phase-phase coupling, working memory load, and individual WM capacity.

Main Methods:

• Intracranial EEG recordings were obtained from presurgical epilepsy patients.
• Patients performed a variant of the serial Sternberg working memory task.
• Analysis focused on phase-phase coupling between theta and beta/gamma activity in the hippocampus during task performance.

Main Results:

• Increased phase-phase coupling between theta and beta/gamma activity was observed during WM maintenance compared to inter-trial intervals.
• This coupling was evident for two and four items, but not for a single item, which showed lower coupling.
• Load-dependent changes in coupling factors correlated with individual working memory capacities.

Conclusions:

• The findings support the theta-gamma neural coding theory by demonstrating increased hippocampal theta-gamma phase-phase coupling during multi-item WM.
• Hippocampal phase-phase coupling dynamics reflect the number of items maintained in working memory.
• These results highlight the role of hippocampal theta-gamma oscillations and phase-phase coupling in the neural mechanisms of multi-item working memory.