SYNCHRONIZATION OF FAST (30-40 HZ) SPONTANEOUS CORTICAL RHYTHMS DURING BRAIN ACTIVATION

We investigated the synchronization of fast spontaneous oscillations ( mainly 30-40 Hz) in anesthetized and behaving cats by means of simulta neous extra- and intracellular recordings from multiple neocortical ar eas. Fast Fourier transforms, auto- and cross-correlations, and spike- or wave-triggered averages were used to determine the frequency and t emporal coherence of fast oscillations that outlasted the stimulation of ascending activating systems or that occurred naturally during beha vioral states of waking and rapid eye movement (REM) sleep but also ap peared during the depolarizing phases of slow sleep oscillations. In 9 0% of microelectrode tracks, the fast oscillations did not show field reversal at any depth of the cortex and were not observable in the und erlying white matter. The negative field potentials of the fast oscill ations were associated at all depths with neuronal firing. This field potential property of fast oscillations was in sharp contrast to the r eversal of slow sleep oscillation or evoked potentials at depths of 0. 25-0.5 mm. The coherence of fast spontaneous rhythms was spatially lim ited, being confined within a cortical column acid among closely locat ed neocortical sites, in contrast to the long-range synchronization of slow sleep rhythms. Depolarizing current pulses elicited spike-bursts (200-400 Hz) recurring at a frequency of 30-40 Hz. Our experiments de monstrate that the conventional notion of a totally desynchronized cor tical activity upon arousal should be revised as fast rhythms are enha nced and synchronized within intracortical networks during brain activ ation. Spontaneously occurring, subthreshold membrane potential depola rizing oscillations may bias cortical and thalamic neurons to respond synchronously, at fast frequencies, to relevant stimuli in the wake st ate or to internally generated drives in REM sleep.