Electrical activity of the isolated cerebral hemisphere and isolated thalamus

A surgical technique has been devised which permits complete neural isolation of the cortex of an entire cerebral hemisphere while preserving the blood supply and drainage. The status of the arterial circulation and venous drainage of the isolated hemisphere is evaluated at the termination of each experiment by comparison of the intact and isolated sides following aortic injection of a particulate dye. Such preparations show continuous or semicontinuous electrical activity consisting of high voltage aperiodic 0.5–4 cycle/sec oscillations and a superimposed lower voltage 25–70 cycle/sec rhythm. This activity may be interrupted by brief isoelectric episodes which may occur in all regions of the cortex simultaneously. The fast rhythm is essentially similar to that seen in the intact brain, and appears to constitute an autochthonic activity of the superficial neural feltwork of the cortex sustained in the absence of afferent neural influences. The high voltage aperiodic slow activity appears to be a consequence of the surgical procedure, but is probably related to neural isolation rather than to circulatory or mechanical insult. The isoelectric episodes are directly related to the physiological condition of the isolated hemisphere, and any factor which tends to compromise this state increases their incidence and duration. The isolated hemisphere is totally inactive only in the presence of significant hypoxia or ischemia. In contrast, the surgically isolated thalamus exhibits continuous rhythmic 6–9 cycle/sec activity of sinusoidal waveform. Thus, the aperiodic slow activity of isolated cortex is not a necessary consequence of deafferentation per se but reflects some characteristic peculiar to the cortex, perhaps a dependence of certain elements of its neural population upon subcortical influences for the development of an organized rhythmic behavior.