Abstracts

Rationale: This study looked at spontaneous whole brain connectivity in MEG recordings of healthy controls and patients with drug resistant mesial temporal lobe epilepsy (MTLE) with hippocampal sclerosis. Methods: This study comprised of 40 MTLE (Right -20 M:F-10:10; Left -20, M:F-13:7) patients and 10 controls (M:F-5:5) who underwent MEG (Neuromag 306 channel system). Resting state record (Fs – 200 Hz, spikes excluded) obtained was preprocessed with MAXFILTER and converted to source space with LCMV beamformer. Amplitude envelopes by Hilbert transform was obtained from bandpassed data (0.3-40Hz) and demeaned. Concatenated amplitude envelopes of 3 groups (Controls, Left MTLE and Right MTLE) was run through a HMM (Hidden Markov Model) with a MVAR (Multivariate Auto Regression) observation model to identify discrete brain states. Individual state duration, transition probability, life time and fractional occupancy was calculated and compared. Parameters were compared between oligospikers (LMTLE-10; RMTLE-10) and frequent spikers (LMTLE-10; RMTLE-10). Results: HMM inference revealed 8 states among each cohort. Connectivity patterns similar to well-known resting state networks (RSN) were noted in controls. Consistent states seen across all groups included networks corresponding to DMN viz. sensorimotor, occipital and auditory networks. Attention networks were poorly defined in MTLE groups and decreased connectivity in anterior DMN (as in previous MRI/f-MRI studies). The mean lifetime of states was controls:0.123s±0.074s, right-MTLE:0.124±0.044s and left-MTLE:0.124±0.042s. Switching rate was higher in controls (0.031s) compared to MTLE (L-0.0824s, R-0.087s) i.e. reduced state stability in MTLE. MTLE patients showed reduced life times and greater intervals between subsequent visits in higher order cognitive states. Oligospikers showed greater bi-temporal connectivity (Inferior-middle temporal-fusiform gyrus) and decreased life times/increased interval period for higher order cognitive states compared to frequent spikers. There was no particular pattern of microstate predominance or state life times in those with specific neuropsychological deficits in temporal vs. in other lobes. Conclusions: This study examined the spatial and temporal characteristics of transient networks in MTLE depicting disturbances of basal cognitive networks and utility of HMM inference in studying RSN’s. Further studies into sub-band abnormalities are needed to improve understanding of RSN in healthy and MTLE. Funding: None