Scalp electrodes record the combined electrical effects of multiple brain sources, so different source configurations can produce similar voltage patterns. This makes it difficult to identify one unique origin from the measurements alone. Constraints or regularization address this ambiguity by limiting or stabilizing the possible solutions, allowing researchers to obtain interpretable estimates of underlying neural activity.
The head model represents the biological system through which electrical signals travel, while the forward model predicts the scalp voltages expected from specified neural sources. Source estimation then works in the opposite direction, comparing recorded measurements with modeled predictions. Together, these models connect electrode-level observations to biologically meaningful estimates of brain activity.
Constraints and regularization guide the solution when scalp data do not uniquely determine the generating sources. Rather than accepting every mathematically possible configuration, the analysis imposes conditions that stabilize the estimate or restrict the solution space. The resulting localization is therefore an informed approximation, which helps researchers interpret spatial patterns without treating them as uniquely proven anatomical origins.
A typical workflow begins with scalp voltage measurements, followed by construction of a head model and calculation of the corresponding forward model. Researchers then solve the inverse problem using appropriate constraints or regularization. Finally, they examine the estimated activity across time or frequency, selecting analyses that match the biological process or research question under investigation.
After estimating likely source activity, researchers can examine how those estimates change over time or across frequency components. Time-resolved analysis preserves the method’s millisecond-level temporal strength, whereas frequency-based analysis helps characterize activity according to oscillatory content. These complementary views provide different ways to relate spatially estimated neural signals to biological processing.
The method is useful when researchers need spatial context for scalp-recorded electrical activity while retaining EEG’s millisecond-level temporal resolution. Supported applications include investigating sensory processing, cognition, brain connectivity, and neurological disorders. By linking estimated activity with biological function, it can extend EEG interpretation beyond electrode locations without discarding its principal temporal advantage.