Multiple Midfrontal Thetas Revealed by Source Separation of Simultaneous MEG and EEG

Area of Science:

• Neuroscience
• Cognitive Neuroscience
• Electrophysiology

Background:

• Midfrontal theta (∼6 Hz) is a recognized electrophysiological marker of response conflict, the competition between multiple actions.
• It is traditionally assumed to be a unitary signal, identifiable at a single electrode (e.g., FCz).
• This assumption may be flawed due to signal mixing from multiple neural sources.

Purpose of the Study:

• To investigate the dimensionality of midfrontal theta.
• To determine if conflict-related midfrontal theta represents a unitary or multidimensional neural process.
• To challenge the traditional view of midfrontal theta as a single signature.

Main Methods:

• Simultaneous magnetoencephalography (MEG) and electroencephalography (EEG) recording (328 sensors) in 9 human subjects.
• Application of feature-guided multivariate source-separation decomposition (generalized eigendecomposition).
• Analysis focused on spatial filters maximizing theta-band activity and midfrontal EEG topography.

Main Results:

• All subjects exhibited multiple (mean 5.89) midfrontal components contributing to sensor-level midfrontal theta power.
• These components were temporally uncorrelated and asynchronous, indicating unique neural origins.
• Conflict-related midfrontal theta activity is driven by multiple, distinct, and overlapping neural sources.

Conclusions:

• Midfrontal theta does not represent a unitary cognitive process.
• It reflects a multidimensional neural space with multiple origins.
• Signal demixing is crucial for understanding the neural dynamics of cognitive processes like response conflict.