CORTICAL MODULARITY BREAKDOWN IN THE EPILEPTIC BRAIN

Sleep and deprivation of it are known methods to increase the yield of epileptic abnormalities. Whereas neurophysiological mechanisms underlying discharges increase were suggested, less is known of the topographical distribution patterns in wake and sleep. We compared interictal epileptic discharges (IEDs) scalp distribution using 256-channels EEG recordings in wake and sleep in people with pharmaco-resistant temporal lobe epilepsy (TLE). 14 adults with Right TLE, identified through clinical data, 3T-MRI, PET and electrical source imaging, recorded during a daytime-nap were included. IEDs were identified in wake and S2. A mean of 21 IEDs in wake and 39 in sleep were selected (p = 0.024) and processed to obtain the averaged spike. Digital signals from the averaged wake traces were then subtracted to the sleep ones (Geodesic-EGI, Oregon, USA) and a t-test between the two conditions run. Single subjects traces localized IEDs over the right temporal regions (rare zygomatic projection), topographically more circumscribed in sleep. EEG digital subtraction showed higher amplitude IEDs limited to the temporal mesio-posterior right areas, with a dipole inversion over the contralateral zygomatic leads. Student t-test confirmed a statistically significant signal increase over the same topographic region, with a slight posterior shift on temporal leads (p = 0.039). This is the first report of a progressive breaking up of cortical modularity during sleep, even S2 stage, in a pathologically hyperactive brain. A similar phenomenon has been previously described (Massimini et al., 2005) using TMS during sleep in healthy subjects. Our data exploit naturally occurring interfering stimuli (IEDs) in a naturally sleeping epileptic brain, demonstrating how they reverberate differently in sleep and in wake, with a sleep related amplitude increase. Interestingly, such phenomenon tends to be constrained to a cortical area partially overlapping with the waking one, as to point to the progressive disconnection of integrated areas.