Improvement of learning by mesencephalic reticular stimulation during postlearning paradoxical sleep

Evidences have been given which suggest that a newly formed memory trace is processed during paradoxical sleep (PS) following learning. The present experiments were aimed at testing the hypothesis that during postlearning PS the new memory trace is in a similar state as immediately after acquisition. For this purpose, a mild electrical stimulation of the mesencephalic reticular formation (MRF)--known to enhance retention performance when delivered just after learning--was administered during postlearning PS phases. Wistar rats were trained to run in a six-unit spatial discrimination maze for food reward. After each daily trial, extradural cortical electrodes (ECoG) activity was monitored polygraphically for 4 h. Half of the animals received nonawakening MRF stimulations during the first six phases of PS. Control rats received no stimulation. The learning results showed a marked improvement in performance, in terms of error number reduction, in the stimulated group. Results of a second experiment confirmed the facilitative effect of MRF stimulations given during postlearning PS. Moreover, they emphasized the specific role of PS, by showing that the same stimulations were ineffective when delivered, at the same time intervals after training, during six periods of waking or six periods of slow-wave sleep. These results lend support to the idea of a reactivation of the new memory trace during PS. They suggest that dynamic processes, similar to those immediately following acquisition or exposure to a reactivating treatment (i.e., a reminder), take place during postlearning PS.