Comparison of the enhancement gradients of retention obtained with stimulation of the mesencephalic reticular formation after training or memory reactivation

Rats were conditioned to fear a tone by a single pairing of the tone with footshock. In Experiment 1, retention, as indexed by latency to drink in the presence of the tone, was enhanced by stimulation of the mesencephalic reticular formation (MRF) when given immediately, and to a lesser extent, 30 min after training. Stimulation at later intervals was without effect. In Experiment 2, animals were trained and, 4 days later, were given memory reactivation treatment and, at different intervals, stimulation of the MRF. This procedure resulted in a gradient of enhancement of retention very similar to that obtained in Experiment 1. Memory storage and memory retrieval explanations of these results are discussed and it is concluded that a demonstration of an enhancement gradient with treatment administered at various intervals after original learning can no longer serve as strong support for the notion that memory storage processes have been affected