perpolarizing potentials was related to the degree of depolarization produced by background excitatory synaptic bombardment or injury. Cells with 60 to 70 mv spikes exhibited minimum (3 to 5 mv) hyperpolarizations during paroxysmal EEG waves, whereas larger polarizing potentials (10 to 15 mv) were recorded in slightly depolarized cells with 40 to 50 mv spikes. Deteriorating cells depolarized to the level of spike inactivation exhibited augmented repolarizing potentials. In such neu? rons

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... e repolarizations during the EEG spikes had a time-course simi? lar to the hyperpolarizations observed prior to spike inactivation (Fig. 2, E or F). Changes in frequency and dura? tion of surface EEG spikes or sharp waves were associated with parallel changes in repolarizing potentials (Fig. 2, Gand#). Our study establishes that EEG spikes or sharp waves are temporally related to synchronously developing membrane depolarizations and hyper? polarizations in neurons in epilepto? genic foci. Depolarizations may or may not be associated with all-or-none discharges, whereas hyperpolarizations invariably produce inhibition of firing. The characteristics of these membrane potential changes are similar to those of prolonged postsynaptic potentials. This is particularly evident with respect to the hyperpolarizations or repolariza? tions which survive loss of the spikegenerating mechanism and exhibit a relationship to membrane potential level expected for inhibitory postsynaptic potentials (7). Although prolonged membrane po? tential changes and EEG discharges are closely related temporally, their relation with respect to polarity is variable as in the case of hippocampal seizure activities (5). The EEG spike or sharp wave in a lesion produced by freezing is a reflection of the sum of membrane depolarizations and hyperpolarizations in neurons located at various depths. Thus the polarity of the focal EEG dis? charge appears to be a consequence of variations in the magnitude and proportion of depolarizing, excitatory, and hyperpolarizing, inhibitory postsynaptic potentials generated in neurons in com? plex synaptic organizations (8, 9).