Subsequent memory effect in intracranial and scalp EEG.

Area of Science:

• Neuroscience
• Cognitive Science
• Electrophysiology

Background:

• Memory encoding is associated with gamma-band activity (30-100Hz) in widespread brain regions.
• Theta-band activity (3-8Hz) also modulates memory encoding, but findings on its directional effects are inconsistent across studies.
• Variations in memory tasks and methodologies may explain the divergent theta effects observed.

Purpose of the Study:

• To investigate the spectral correlates of successful memory encoding.
• To differentiate between global cortical and subcortical theta effects during memory processes.
• To validate the utility of scalp electroencephalography (EEG) for high-frequency activity detection.

Main Methods:

• Utilized intracranial EEG (iEEG) recordings in neurosurgical patients.
• Employed scalp EEG recordings in healthy controls.
• Analyzed spectral power in theta (3-8Hz) and high gamma (44-100Hz) bands during memory encoding tasks.

Main Results:

• Observed significant theta power modulations (both increases and decreases) in both iEEG and scalp EEG data.
• Detected high gamma power increases (44-100Hz) in both participant groups.
• Identified two distinct theta mechanisms: a widespread decrease across cortex and hippocampus, and an increase in the frontal cortex.

Conclusions:

• Two separate theta mechanisms contribute to memory success.
• Scalp EEG can effectively detect high-frequency gamma activity relevant to memory.
• Observed iEEG theta effects are unlikely attributable to epileptic activity.