Suppression of EEG gamma activity--an informative measure of spreading depression waves in the neocortex of the conscious rabbit.

Spreading depression can arise spontaneously in convulsions, migraine attacks, vascular lesions, and other pathological brain states. However, the methodological complexity of recording a direct current potential in the neocortex in humans and conscious animals significantly limits studies of the functional consequences of spreading depression. The present report describes a detailed analysis of the EEG dynamics at the moment of development of spreading depression waves and identifies specific signs allowing spreading depression to be recorded without recording changes in the direct current potential. At the moment of arrival of spreading depression, the interhemisphere equilibrium is highly disturbed because of large decreases in high-frequency activity. The time parameters of the dynamics of the gamma-1 and gamma-2 ranges were the most informative features of the development of spreading depression waves. Increases in power in the delta range developed with some delay on the background of a deep inhibition of high-frequency activity and are in essence a consequence of spreading depression. The encephalographic criteria of spreading depression waves identified here can significantly simplify the detection of this phenomenon both in experimental and clinical conditions in a number of pathological brain states.