Effects of temporal versus temporal plus extra-temporal lobe epilepsies on hippocampal ERPs: physiopathological implications for recognition memory studies in humans.

The present experiment was designed to investigate the incidence of temporal and extra-temporal epileptogenic lesions on hippocampal activity related to recognition memory. Hippocampal event-related potentials (ERPs) were recorded during a recognition memory task for pictures in patients with unilateral temporal lobe epilepsy and in patients with combined temporal and extra-temporal (frontal or parietal) epileptogenic foci. In the first group, the comparison between correctly recognized pictures and new distractors revealed that the usual ERP (N400/P600) 'old/new' effects were not dramatically affected by the presence of a temporal epileptogenic lesion. In contrast, these effects were absent in multifocal epilepsy patients, indicating that frontal and parietal brain regions play a part in the modulation of hippocampal ERP and related memory processing. These results suggest that ERP 'old/new' effects are dependent on interactions between frontal, parietal and medial temporal structures. Together with the results of other recent studies devoted to locating the neural sources of N400 and P600, the present findings provide physiopathological evidence that ERP 'old/new' effects are subtended by distributed yet interconnected brain regions that are known to play an important role in recognition memory processing.