Life Goes on in Dreams

Abstract

WHAT GOES THROUGH OUR MINDS WHILE WE FALL ASLEEP? AT SLEEP ONSET, STUDENTS PREPARING FOR EXAMS COMMONLY RE-EXPERIENCE SOME OF the material they have been revising during the day; lovers have pervasive thoughts about their romantic partner; software developers may see bits of programming codes unfold in their mind's eye. These anecdotal observations suggest that, during the transition from wakefulness to sleep, elements from daytime activities may spontaneously re-emerge in dream-like mentation also called hypnagogic imagery. In a behavioral study published in this issue of SLEEP, Wamsley and colleagues1 provide support for the existence of such spontaneous cognitive “replay” at sleep onset. In this study, healthy young participants were trained on a highly motivating visuomotor game–the downhill skiing game Alpine Racer II. In this simulated skiing game, the player stands on a movable platform while holding two ski poles. The player's goal is to use his/her body to move the platform left or right to steer and thus improve the downhill skiing of an “avatar skier” presented on a computer screen. After training on the Alpine Racer, 30% of spontaneous mentation collected at different times during pre-sleep wakefulness and light NREM sleep (up to 300 sec after sleep onset) contained imagery (of any modality, 24%) or thoughts (6%) related to the skiing game. Wamsley et al.1 also found that imagery directly related to training on the game (unambiguous representations of the Alpine Racer or of skiing) declined across time. This time-course was paralleled by a tendency for game-related incorporations to become more abstracted from the original experience. These findings do not only provide empirical evidence for spontaneous memory replay during wakefulness and light NREM sleep (stages 1 and 2), but they show that reports of subjective experience offer valuable information about cognitive processes across changing brain states.2 Consistent with animal neurophysiology data,3 human imaging studies have demonstrated that brain activity associated with waking experiences may be reactivated during subsequent sleep, and can thus contribute to memory consolidation and improved performance.4–7 However, although incorporation of the Alpine Racing game was high in the Wamsley study, suggesting abundant memory replay at sleep onset, no relationship was found between introspective measures of memory replay and later improvement in performance on the game. Whether dreaming of some tasks or stimuli may enhance subsequent memory or performance, and whether dreaming actually relates to neural reactivation subsequent to learning, are fundamental questions that still remain unanswered.8–10 The popular belief that dreams reflect waking-life activities—the “continuity hypothesis of dreaming”—has found strong empirical support in the field of dream research.11–13 The recent study by Wamsley et al.1 appears to extent the validity of the continuity hypothesis to hypnagogic imagery and offers new insights into which daytime experiences may be good candidates for cognitive replay. High levels of incorporation of Alpine Racer are most plausibly related to the strong motivational and attentional involvement of the player during the game. Consistent with this interpretation, a few participants who only observed those playing Alpine Racer also incorporated elements of the game into their sleep-onset mentation, at rates similar to the participants who were actively engaged in the game. These effects and their time-course suggest that novelty may be a critical factor for the selection of material to be mentally replayed (Figure 1). Moreover, many baseline night reports incorporated thought or imagery related to the game (compared to a control set of sleep-onset mentation reports), indicating that the mere anticipation of the task could trigger prospective memory processes that emerged at sleep onset. It is tempting to speculate that hypnagogic imagery may contribute to the integration of recent experiences with long-term memories and future goals. How this is achieved at the cognitive and neurophysiological levels is a fascinating and challenging topic for future investigations. Figure 1 Impact of novelty on dream experience. Five days after seeing a new model of an airplane in a newspaper (original photograph on the left), the dreamer reported a very similar airplane in a dream. The drawings (on the right) that accompanied the dream ... Notably, while consciousness dissipates, waking concerns may sometimes also prevent us from falling asleep. For example, pre-sleep cognitive activity has been implicated in the maintenance of sleep-onset insomnia.14,15 Therefore, manipulating hypnagogic imagery at sleep onset may also have important clinical implications for the management of sleep disorders.