Randomness and Synchrony in the Generation of the Electroencephalogram

The question whether the EEG is dependent upon subcortical inputs or control has never been completely clarified. Burns (1950, 1951) demonstrated that cortical slabs which have been completely undercut are devoid of spontaneous activity and respond to direct shocks only by brief bursts of activity. However, there have also been conflicting experiments in which some residual activity appeared to have remained in undercut cortex (Kristiansen and Courtois 1949, Echlin et al. 1952, Henry and Scoville 1952, Ingvar 1955), although the possibility that this residual activity may be in the nature of injury discharge cannot be ignored. Essentially the same considerations apply to recordings from the frog brain. Gerard was able to record slow potential activity from small fragments of the olfactory bulb of the frog (Gerard 1936, Gerard and Libet 1940). Such potentials can indeed be recorded but, whereas the amplitude of EEG recorded from the intact frog brain even under the best of conditions does not appear to exceed 20–30, μV, the potentials recorded from small fragments of the brain are of the order of 200 μV and very significantly differ in their appearance from the normal EEG of the frog. (Indeed this seizure-like appearance is evident in the original records published by Gerard.) Thus the potentials recorded from the isolated olfactory lobe in the frog unquestionably represent bioelectric activity, but it seems much more dubious that these potentials may legitimately be identified as “EEG”.