Origin and Spread of Periinfarct Depolarization in Rats Detected by ECoG and MEG

The knowledge about the origin and the spatio-temporal pattern of propagation of spreading depolarization (SD) after focal ischemic brain infarction is limited. Using the information of the simultaneously recorded ECoG and MEG it seems possible to describe the localization of underlying neurophysiological processes. We investigated in 8 rats 32 periinfarct depolarizations by simultaneously recorded ECoG and MEG. The ECoG was recorded by a grid of 4×4 electrodes with a spatial distance of 1.25mm between adjacent electrodes. The MEG was recorded by a 16-channel Micro-SQUID system built at the Biomagnetic Center (1st order asymmetric gradiometers, 6.7mm pick-up-coil diameter, 30mm baselength, covering an area of 3.2×3.2cm²). Infarction was initiated via occlusion of the right middle cerebral artery. From the ECoG the moment and location of the first detected negative deflection, the sequence of the involvement of cortical regions and the spatial distribution of the amplitude of depolarization were determined. From MEG the temporal pattern of main intracortical current was estimated. The frequency of SD was 5/hr (=12±6min). Negative deflection of electric potential could be detected at first over rostro-medial regions. The location of the maximal depolarization amplitude varied intra- and interindividually. In most cases SD waves propagated from rostro-medial to caudal regions. In 26 of 32 electric SD, magnetic field changes were detected. Often (n=12) long-lasting magnetic field changes started before electric changes. The mean duration of these changes varied considerably (205±163s). We suppose that the origin of periinfarct depolarization is frequently located in frontal cortical regions and that subcortical depolarization may contribute to the MEG signal.