EVIDENCE THAT ELECTROCONVULSIVE SHOCK ALTERS MEMORY RETRIEVAL RATHER THAN MEMORY CONSOLIDATION

Four experiments reported here suggest that most experiments claiming that electroconvulsive shock (ECS) disrupts memory consolidation are confounded because of differences in activity levels and brain excitability states that exist between the experimental and control animals when they are tested for retention. Results of the first experiment showed that ECS increased open field activity levels. The effect of ECS upon brain excitability levels was determined in the second experiment by measuring the intensity of an electrical stimulus, delivered to a subcortical area, that would elicit a conditioned response. A transient decrease lasting 4 days was produced. Recovery of a learned response following ECS administration was investigated in the third experiment; the response was the avoidance of stepping off a platform. There was recovery when ECS induced increases in activity levels were controlled, and the recovery followed the same time course as the changes in brain excitability. A fourth experiment demonstrated that when activity levels and brain excitability states were equalized, ECS did not produce even a transient disruption of the avoidance response. These experiments show that ECS does not disrupt memory fixation which is dependent upon a neural reverberation process for consolidation, but suggest that memory retrieval may depend upon brain excitability states. The hypothesis is offered here that the neurological aspect of learning may involve changes in levels of brain excitability as reflected in the thresholds of functional neural systems, that retention implies a maintenance or reconstruction of these modifications of brain excitability, and that failure of retention occurs whenever brain excitability is modified away from that established by the training procedure. © 1968.