Mentation during sleep onset theta bursts in a trained participant: A role for NREM stage 1 sleep in memory processing?
NREM stage 1 sleep, known as a brief interval of transition from wake to sleep, is characterized by neurophysiological events and subjective sensory experiences that suggest the stage may be involved in memory processing. To examine this possibility, we conducted multiple awakenings with a trained participant during short bursts of theta activity defining the 5th sub stage of NREM stage 1 sleep (NREM1Ф). Awakenings provided frequent reports of vivid dreaming containing coherent scenes—as opposed to isolated objects often associated with general sleep onset imagery. Analyses of the temporal and semantic aspects of the memories associated with this imagery suggest that multiple memories are selected for incorporation in NREM1Ф imagery on the basis of their semantic proximity and temporal remoteness. Analyses also demonstrate a remarkable pattern in about a third of dreams in which distinct, semantically related memories dating from as little as 10 minutes to as much as 15 years ago were bound in close temporal and spatial proximity within the novel contexts of the imagery. This offline manipulation of semantic information, observed here at a phenomenological level, bears a resemblance to processes thought to underlie integrative encoding, i.e., the encoding of an association between events that were not experienced together but which are subsequently combined because they contain at least one overlapping element. This preliminary study provides evidence consistent with the notion that hippocampal-mediated memory processing occurs during the theta bursts found at sleep onset.