Hypnagogic hallucinations are conscious mental representations that emerge spontaneously at sleep onset. They typically consist of florid and vivid perceptions of various modalities, with a predominance of visual, auditory or kinaesthetic perceptions over olfactory or gustatory hallucinations (Foulkes and Vogel, 1965; Mavromatis, 1987). They commonly incorporate thoughts (Mavromatis, 1987), but relatively little emotion (Foulkes and Vogel, 1965). Early on, some of them were recognized as ‘recurrent sensations’ or ‘perseverative images’, namely representations inspired by recently seen objects (Mavromatis, 1987; Schacter, 1976). Recent studies experimentally confirmed that incorporation of stereotypical images into night-time sleep-onset mentation can be induced by prior practice of video games, such as Tetris (Rowley et al., 1998; Stickgold et al., 2000) or Alpine Racer, a downhill skiing game (Wamsley et al., 2010). Several features of this intriguing phenomenon are still incompletely characterized. First, experience-related hypnagogic hallucinations were exclusively reported during night-time sleep onset (Stickgold et al., 2000; Wamsley et al., 2010). Although it is known that other sleep-related spontaneous mental experiences, such as dreams, follow a circadian modulation (Chellappa et al., 2009), the occurrence of hypnagogic hallucinations during daytime sleep, beyond individual experience and some anecdotal evidence (Stickgold et al., 2000), has not yet been experimentally assessed. Second, the vigilance state in which experience-related hypnagogic hallucinations can be induced is still to be firmly characterized. As a rule, hypnagogic hallucinations occur in a specific state of vigilance at the transition of wakefulness to sleep, which was shown to correspond to sleep stage 1 (S1; Foulkes and Vogel, 1965; Hori et al., 1994; Rowley et al., 1998). At odds with these findings, it was recently suggested that experience-related hypnagogic hallucinations might also occur during waking and sleep stage 2 (S2; Stickgold et al., 2000; Wamsley et al., 2010). However, these conclusions were not derived from standard polygraphic recordings, and the vigilance state associated with experience-dependent hypnagogic hallucinations remains to be firmly established. Third, the nature of these experience-related hallucinations has not yet been thoroughly investigated. After Tetris practice, they were categorized in images and thoughts about the game (Stickgold et al., 2000). After Alpine Racer training, they consisted of thoughts or visual and kinaesthetic perceptions, directly or indirectly related to the task (Wamsley et al., 2010). A more detailed characterization of the hallucinatory material is important because it might shed some light on the underlying organization of brain function (Schwartz and Maquet, 2002). Fourth and finally, the origin of hypnagogic hallucinations has to be experimentally confirmed in different groups of volunteers: does anticipation on task practice, as reported by Wamsley et al. (2010), have a similar effect to genuine task experience on the mental content at sleep onset? Moreover, it remains uncertain whether experience-related hypnagogic hallucinations reflect active memory processes or simply indicate the increased readiness to fire of neural populations whose synaptic connections were recently strengthened. In support of the former assumption, the probability of induction of hallucinations after playing Tetris tended to be negatively correlated with initial performance, suggesting that individuals who reported most hallucinations at sleep onset were those with the largest potential learning range (Stickgold et al., 2000).
Here we address these different issues and characterize hypnagogic hallucinations in three groups of volunteers who practiced Tetris (experimental and anticipation groups) or not (control group) using iterative awakenings during polygraphically monitored daytime naps.