Abstract
The transition from a relaxed to a drowsy state of mind is often accompanied by hypnagogic experiences: most commonly, perceptual imagery, but also linguistic intrusions, i.e., the sudden emergence of unpredictable anomalies in the stream of inner speech. This study has sought to describe the contents of such intrusions, to verify their association with the progression of sleep onset, and to investigate the electroencephalographic processes associated with linguistic intrusions as opposed to more common hypnagogic perceptual imagery. A single participant attended 10 experimental sessions in the EEG laboratory, where he was allowed to drift into a drowsy state of mind, while maintaining metacognition of his own experiences. Once a linguistic intrusion or a noticeable perceptual image occurred, the participant pressed a button and reported it verbally. An increase in the EEG-defined depth of drowsiness as assessed by the Hori system of sleep onset was observed in the last 20 s before a button press. Likewise, EEG Dimension of Activation values decreased before the button press, indicating that the occurrence of cognitively incongruous experiences coincides with the rapid change of EEG predictability patterns. EEG hemispheric asymmetry analysis showed that linguistic intrusions had a higher alpha and gamma power in the left hemisphere electrodes, whereas perceptual imagery reports were associated with a higher beta power over the right hemisphere. These findings indicate that the modality as well as the incongruence of drowsiness-related hypnagogic experiences is strongly associated with distinct EEG signatures in this participant. Sleep onset may provide a unique possibility to study the neural mechanisms accompanying the fragmentation of the stream of consciousness in healthy individuals.
Highlights
Temporal and spatial integration of experiences is a fundamental property of the stream of consciousness (James, 1890), which seems to hold for the perception of relatively stable properties in the physical world, and for internally generated experiences, such as inner speech
In addition to the increase of Hori scores, Dimension of Activation (DA) analysis demonstrated a decrease of EEG signal complexity, which typically occurs during sleep onset (Magnin et al, 2010), in a time window preceding linguistic intrusions and perceptual imagery reports
We found a decrease of theta and alpha power in the last 20 s compared to 40–20 s time window before perceptual imagery reports, which conceptually replicates a previous report of theta and alpha power decrease during 9–0 s compared to 35–10 s before a report of visual and kinaesthetic imagery during sleep onset (Germain and Nielsen, 2001)
Summary
Temporal and spatial integration of experiences is a fundamental property of the stream of consciousness (James, 1890), which seems to hold for the perception of relatively stable properties in the physical world, and for internally generated experiences, such as inner speech. A wide range of behavioral, perceptual, and cognitive alterations that lead to the fragmentation of consciousness seems to take place in the transition between wakefulness and sleep (Goupil and Bekinschtein, 2011), and – as we show here – they can be successfully studied in a drowsy participant in the sleep lab. Subjective experiences during this transition are sometimes referred to as hypnagogia (Mavromatis, 1987), with hypnagogic imagery being the most paradigmatic alteration of consciousness during sleep onset (McKellar and Simpson, 1954). In addition to hypnagogic imagery, other types of hypnagogia may include awareness of sleep onset (Kaplan et al, 2007), distorted perception of space (Bareham et al, 2014), and time (Minkwitz et al, 2012), as well as linguistic alterations such as those we describe below
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