Memory retention of stimulations during REM and NREM stages of sleep

In 51 normal young female subjects, stimulation by name calling or by intermittent photic stimulation was given during sleep. At different intervals after the stimulation, the subject was awakened and asked if she could recall it. If alpha activity had not been elicited by the stimulus, there was no recall. If the time occupied by alpha activity evoked by the stimulation was more than 30 sec, the stimulus could be recalled after a long period of sleep. When the evoked alpha activity lasted for less than 30 sec and the subject slept again, the longer the evoked alpha activity, the longer the sleeping time span with the memory retention of the preceding stimulation. With equal durations of evoked alpha activity, retention of the stimulus was better when the sleep following was REM stage than when it was NREM stage (stage 2). The results might be explained by the assumption that process of consolidation takes place most rapidly during wakefulness and is inhibited during sleep but to a lesser degree during REM stage than during NREM stage (stage 2).