Abstracts

Rationale: Electrophysiology and computer modeling studies show inhibitory processes are critically involved in the generation and synchrony of gamma oscillations, which are believed to play an important role in cognitive operations. In the epileptic brain, pathological changes in inhibitory processes especially within and between sites where seizures begin could alter the local and long-range synchrony of gamma. Therefore, we quantified the strength of gamma synchrony within and outside the seizure onset zone (SOZ) of surgical patients diagnosed with drug-resistant MTLE. Methods: Gamma (30-55Hz) event coupling (GEC) was computed from episodes of interictal EEG (0.1-100Hz; sampling 200Hz) recorded from bilateral depth electrodes positioned in gray matter of mesial and lateral temporal and frontal lobes of patients diagnosed with unilateral MTLE (n=25). All patients had standard anteromesial temporal resection and follow up to determine patients who were seizure-free and those who were not ("good" and "poor" seizure outcome respectively). Parametric statistical analysis was carried out on log transformed normalized GEC values with respect to anatomical region, hemisphere of SOZ, and seizure outcome. Results: Overall, compared to patients with good outcome (n=14), patients with poor outcome (n=11) had reduced GEC within the ipsilateral frontal lobe (p < 0.001) and between contralateral mesial and lateral temporal structures (p < 0.001), but no differences were observed in other anatomical regions. Contributing to the aforementioned differences were a significant asymmetry in GEC between ipsilateral and contralateral frontal lobe (p < 0.0001) as well as temporal lobe (p=0.001) of patients with poor outcome, whereas the strength of GEC was similar between these same anatomical regions in patients with good outcome. Conclusions: Results suggest reduced GEC could be associated with alterations in inhibitory as well as excitatory network activity. Moreover, differences in the spatial pattern and strength of GEC between seizure outcome groups could indicate a more extensive epileptogenic zone that involves ipsilateral frontal or contralateral temporal lobe structures or both in patients with poor seizure outcome. Funding: NINDS NS071048 (RS), EFA Fellowship for Clinicians (SW), NS065877 (AB), NS033221 (IF), NS033310 (JE)