Sleep research: observing dreams and inducing hypnagogic images

Abstract

Sleep research is also about the fascination with dreams and consciousness. The current issue of the Journal of Sleep Research contains papers relevant to evolving research trends in this area. Collecting dream reports after induced or spontaneous awakenings is a standard approach in dream research. This approach has established that dreams occur primarily, but not exclusively, during paradoxical sleep (PS). PS is characterized by muscle atonia and little behaviour can be observed during this state, except rapid movements of the eyes; hence the alternative name: rapid eye movement (REM) sleep. In general, collecting dream reports is the only way to gain a glimpse into the dreams of others; but what about those conditions in which muscle atonia during PS is incomplete, and behaviours can be observed? Can we link the observed behaviours to the dream reports? Establishing a link between the ‘self-reported dream content’ and ‘observed behaviour’ would suggest that conscious experiences during dreams, or at least their recollections, are related to motor manifestations. Valli et al. (2011) addressed this question in a study of patients with REM sleep behaviour disorder, which is thought to represent the acting-out of dreams while asleep and unaware of the environment. The authors used video-polysomnographic recordings and collected structured and free-worded dream reports following spontaneous and induced awakenings from REM sleep in patients. They next randomly assigned four dream reports to each video-clip of the 5-min period preceding the awakening and asked seven judges (neurologists) to identify the dream report matching the video-clip. The neurologists performed better than chance although well below perfect. Reading the dream reports presented in these papers makes one wonder what the video-clips may look like. This fascinating research is one example of the application of modern dream research in the context of our efforts to understand mind–brain–behaviour interactions (Valli, 2011). Other examples include the use of lucid dreamers and neuroimaging approaches (Dresler et al., 2011), and there is little doubt that we will see much more of this research in the future. Whereas in the paper by Valli et al. the presumed direction of causality was from dream experience to behaviour, Kusse et al. (2011) investigate how the waking experience may persist or reappear at sleep onset. Hypnagogic images can appear at the transition from wakefulness to sleep and the simple question is to what extent the experience during wakefulness preceding sleep onset influences the contents of these ‘hallucinations’. The importance of this question relates to the continuity of the conscious experience across vigilance states. The authors follow previous approaches (Stickgold et al., 2000) using the computer game Tetris. Again, the approach is to wake the participants, but after the electroencephalograph indicates entry into sleep Stage 1. Important changes and improvements relative to previous investigations are the time of day at which the reports were collected (daytime naps) and the use of an anticipation control group. The absence of a large number of Tetris elements in those participants who were told that they would play the game after the nap suggests that it is the actual experience and not the anticipation of an experience, that determines the images during these hypnagogic hallucinations. Furthermore, those who reported more Tetris-related experiences performed better. The authors are careful to point out that this correlation is likely to reflect focused attention and motivation during practice, rather than memory processes during sleep. Whereas the papers on REM sleep behaviour disorder and Tetris specifically address the relation between conscious experience during sleep and current or previous behaviour, there are additional papers in this issue of the Journal describing the interaction between sleep/time of day and the waking experience. For example, Zhou et al. (2011) describe how time of day/the circadian timing system may cloud our judgement of how well or poorly we perform. Other papers show that sleep is important for discovering rules to guide decision-making (Pace-Schott et al., 2011) and that sleep spindles can be induced by exposure to electromagnetic fields, such as those emitted by mobile telephones (Schmid et al., 2011). Overall, these papers provide evidence for the two-way interaction between sleep and the waking experience.