Excitability cycle of the direct cortical response studied with minimal stimuli and response averaging

1. 1. The direct response of suprasylvian gyrus of succinyl-cholinized unanesthetized cats was studied with minimal stimulation and responses averaging. Excitability cycles were determined by paired stimulation, the intervals between conditioning and testing stimuli being 0.25–500 msec. 2. 2. There is an initial period of augmented responsiveness, lasting about 5 msec, followed by a period of depression, maximal at about 10 msec, and with a variable rapid phase of recovery followed by a prolonged mild depression (occasionally augmentation) up to 500 msec. 3. 3. The absence of detectable latency and absence of early refractory period suggest that the dendrites are directly excited. 4. 4. A positive potential parallels the period of early depression. Both occur sooner with increased strength of stimulus, and the onset of depression occurs later after intravenous nembutal. It is suggested that both the depression and the positive potential represent activation of inhibitory mechanisms.