Combined MEG and EEG suggest a limbic source network of the P3 including retrosplenial cortex and hippocampus

The event-related P3 is evoked by most task-relevant or salient stimuli, but its neural sources remain controversial. Here we reevaluate the role of hippocampus, unambiguously demonstrated in intracranial recordings, and retro-splenial cortex as sources of the P3 in magneto- and electroencephalography. Observers detected rare visual targets of a deviant shape and ignored rare non-targets of a deviant color. Source analysis was based on noise-normalized minimum-norm estimates in an individual, MRI-based source space. Critically, the cortical source space was extended to include the cornu ammonis of the hippocampus. Source analysis at the P3 peak showed strong sources in retrosplenial cortex and a weaker source in hippocampus. Activity in left somatosensory cortex was related to the button presses. Subsequent activity was observed in insula and anterior mid-cingulate cortex. Further simulation studies explored the spread of these sources and the plausibility of their combination to explain the data, and demonstrate that the combination of magneto- and electroencephalography increased the specificity of the source estimate. Overall, these data support an interpretation of the P3 as physiological indicator of activity in the limbic system for target and distractor stimuli.