Potentials evoked by stimulation of the hippocampus: Low frequency suppression effect

Biphasic square wave pulses were applied to the hippocampus at low frequencies while the responses evoked in the amygdala, preoptic area and ventromedial nucleus of the hypothalamus were recorded. At a frequency of 1 cps, the responses were intitially large in amplitude and stable, but with repeated stimulation the responses became variable in amplitude and eventually disappeared. Recovery was complete within 1 hr. No suppression was found with low frequency stimulation of the amygdala. Fewer stimulation pulses were required to produce suppression at a frequency of 1 per 5 sec than at 1 per sec, but recruiting, rather than suppression, occurred at 3 per sec. Suppression occurred more rapidly at lower stimulation intensities. It was possible to produce a brief desuppression by temporarily increasing the stimulation frequency. Responses evoked within the hippocampus by amygdala stimulation were not altered after suppression of evoked responses had been produced by low frequency stimulation of the hippocampus. Several low frequency recruiting effects which took place in some subjects before suppression are reported. Also reported are correlations between ongoing behavior and evoked response amplitude during the period before complete suppression. These correlations between ongoing behavior and evoked response amplitude during the period before complete suppression. These correlations are similar to those reported between behavior and theta rhythms. The suppression phenomenon was not altered by amygdaloid kindling or by paralysis with curare. Possible mechanisms were discussed.