Abstracts

Rationale: Intracranially recorded event-related potentials (ERPs) from electrodes within Medial Temporal Lobe (MTL) have been showed to correlate with verbal memory performance and are associated with novelty detection (1). Furthermore, correlations between intracranially recorded ERP abnormalities and epilepsy lateralization have been reported (2, 3). In patients with unilateral temporal lobe epilepsy (TLE) the N400 potentials are attenuated within the epileptic hippocampus, but they have not been explored as a biomarker to define the epileptogenic zone (2). Here we studied ERPs within seizure onset zone (SOZ) and other regions (NSOZ) of the medial temporal lobe during verbal memory encoding, to build a better understanding of the localization value of these cognitive potentials. Methods: We used intracranial EEG (iEEG) recorded during encoding of words in 11 epilepsy patients to analyze the intracranial ERPs recorded from the medial temporal lobes. Patients were implanted with depth electrodes for iEEG (0.1 – 120 Hz; sampling rate 500 Hz) monitoring as part of clinical evaluation for drug-resistant epilepsy. Lists of twelve words were presented on a laptop (each for 1600ms) for subsequent recall. We focused on a subset of contacts, which were active in the task showing induced responses to word presentation during encoding. ERP traces were generated by averaging over all raw data epochs for a given active contact. ERP sizes were estimated by calculating the absolute value of maximum amplitude in the first 350 ms, the 350 to 500 ms, and the latter 500 to 800 ms post-stimulus interval of the ERP trace. The amplitude component of the ERPs during the word presentation interval (encoding) were compared within SOZ and NSOZ electrodes across all subjects, in individual subjects, and in different temporal lobe structures. Results: We observed significant differences in the ERP amplitude when considering active electrodes in SOZ and NSOZ electrodes in medial temporal lobe. Conclusions: Limbic cognitive ERPs recorded during verbal memory task within the SOZ had different properties compared to those within NSOZ.  Evaluation of ERPs in individual patients and its correlation with hippocampal sclerosis is underway and will explore the feasibility of interictal, task related localization of the SOZ. These findings could prove clinically useful in surgery planning and for better understanding of memory in health and disease.References1. Grunwald, Thomas, and Martin Kurthen. "Novelty detection and encoding for declarative memory within the human hippocampus." Clinical EEG and neuroscience 37.4 (2006): 309-314.2. Grunwald, Thomas, and Manila Vannucci. "Electrophysiological studies of memory in epilepsy." Epilepsy and Memory (2012): 323.3. Puce, Aina, et al. "Limbic P3 potentials, seizure localization, and surgical pathology in temporal lobe epilepsy." Annals of neurology 26.3 (1989): 377-385. Funding: This work was supported by funding from the NIH R01-NS92882