Short-Term Memory During Stage-2 Sleep

A recently-proposed model of dream recall phenomena (Koulack & Goodenough, 1976) assumes a short-term memory store which functions during sleep, and from which either dreams or externally-delivered stimuli can be retrieved if the subject is awakened during the life of the short-term trace. To test this assumption, two experiments were designed to examine short-term memory for auditory stimuli over intervals of uninterrupted stage-2 sleep. A random series of single-digit numbers was presented at the rate of one every 30 sec throughout the night, at an intensity slightly above normal conversational levels. Subjects habituated to the sound and did not awaken spontaneously. Experimental awakenings took place either at 1 sec (Experiment 1) or at 1, 5, or 10 sec (Experiment 2) after selected target stimuli, and the subjects were asked to recall which number they had heard. Significant recall was obtained over intervals of up to 10 sec, and the likelihood of recall was inversely related to reaction times to the awakening stimulus. Repeating the target numbers at 30-sec intervals increased EEG alpha activity and K-complex amplitude, suggesting that some form of trace persists for at least 30 sec.