Properties of spreading depression identified by EEG spectral analysis in conscious rabbits

Reflection of cortical waves of spreading depression in changes in high-frequency gamma activity was shown to improve significantly in conditions of bipolar recording of brain electrical activity. Intense suppression of the EEG gamma range (37-47 Hz) was seen in all cortical areas studied at the moment of onset of waves of spreading depression. The drop in the power of EEG gamma activity had clear leading and trailing fronts, allowing the passage time of the wave to be evaluated in each cortical area, as well as the sequence of propagation. Waves of spreading depression were characterized by normal, i.e., sequential movements across the cortex, only in the first 3-4 experiments. When experiments were continued, there were changes in the latent period and distortions of the propagation sequence. In some cases, waves of spreading depression responded to standard injections of KCl by developing rapidly (0.5-2 min) over the whole of the dorsal surface of the neocortex. More frequently, the wave was delayed (to 6-9 min) was completely blocked in some areas of the cortex. Different types of mild convulsive activity could accompany both the rapid propagation of waves across the cortex and inhibition of wave development. In most cases, there were no electrophysiological signs of changes in neocortical excitability. However, alterations to the spatial characteristics of waves of spreading depression and the spontaneous appearance of repeated waves provided evidence of changes in the functional state of the brain. Overall, EEG spectral analysis in conscious rabbits allowed analysis of spreading depression both in cases of sequential propagation across the cortex and in cases of atypical modes of development of this phenomenon.