Early magnetic field changes preceding the intracortical penicillin induced spikes

Events preceding interictal activity were studied using a combination of magnetoencephalography (MEG), electrocorticography (ECoG), and intracortical field potential recordings in rabbits. We measured MEG signals simultaneously with ECoG before and during interictal discharges induced by penicillin injected in the cortex (group 1: n = 12, medial cortex, regio retrosplenialis granularis; group 2: n = 4, cortical convexity, regio retrosplenialis agranularis; control group: n = 5); in group 3 (n = 12) a 16-channel depth electrode array was used to calculate the current source density in the cortical area exhibiting interictal epileptiform discharges. The modified Z-parameter as a lumped measure of magnetic field pattern changes and the global field power as a lumped measure of changes of field amplitude differences were calculated. In almost all recordings of both group 1 and 2, the Z-parameter of intra-individual MEG data became significantly larger than the control condition before the earliest change of the interictal spike recorded at the penicillin injection site (20-310 ms earlier, median: 91 ms, n = 151). The increase in Z-parameter in averaged MEG data of group 1 was significantly correlated with time as early as 790 ms before the spike (Pearson correlation coefficient, P < 0.05). After the start of the early increase of the Z-parameter, the global field power also began to increase before the ECoG spike. These results suggest a prespike field recruitment nearly 1 s before an interictal spike.