SP is a transient, conscious state of involuntary immobility occurring immediately prior to falling asleep or upon wakening and is classified as a parasomnia associated with REM (ASDA, 1990). Although individuals are unable to make gross bodily movements during SP they are able to open their eyes and subsequently to report accurately on events in their surroundings during the episode (Hishikawa & Kaneko, 1965). Approximately 25 to 40% of people report some SP experience (Cheyne, Newby-Clark, & Rueffer, in press; Fukuda, Ogilvie, Chilcott, Vendittelli, & Takeuchi, 1998; Spanos, McNulty, DuBreuil, Pires, & Burgess, 1995), although the incidence may vary across cultures (Fukuda, Miyasita, & Ishihara, 1987; Ness, 1978).
SP has traditionally been linked with narcolepsy and cataplexy as part of the ‘‘narcoleptic tetrad,’’ but is considerably more common than the latter disorders, the incidence of which range from .03 to .16% (Hishikawa & Shimizu, 1995). A number of anomalous sensory experiences frequently accompany SP. In the present study, these are referred to, collectively, as hypnagogic and hypnopompic experiences (HHEs). The HHEs include an acute sense of a monitoring ‘‘evil presence,’’ combinations of auditory and visual hallucinations, pressure on the chest, as well as suffocating, choking, floating, out-of-body, and flying sensations (Hishikawa, 1976; Hufford, 1982). Although these experiences bear some similarity to non-SPrelated pre- and postdormital hypnagogic images and sensations (Foulkes & Vogel, 1965; Mavromatis, 1987; Rowley, Stickgold, & Hobson, 1998; Schacter, 1976), HHEs accompanying SP appear to be substantially more vivid, elaborate, multimodal, and terrifying (Hufford, 1982; Takeuchi, Miyasita, Inugami, Sasaki, & Fukuda, 1994).
It has been conjectured that complex combinations of SP-related HHEs form the basis of diverse worldwide cultural accounts of nocturnal incubus/succubus assaults, spirit possessions, old hag attacks, ghostly visitations, and alien abductions (Adler, 1994; Bloom & Gelardin, 1976; Firestone, 1985; Fukuda, 1989; Hufford, 1976, 1982; Liddon, 1967; Ness, 1978; Wing, Lee, & Chen, 1994). In these accounts, a dreadful and evil presence in the form of a vampiric lamia, demon, spirit, or hag sits on the victim’s chest and smothers or chokes the helpless sleeper.
SP has been experimentally linked to REM states, particularly with sleep-onset and sleep-offset REM (Hishikawa & Kaneko, 1965; Nan’no, Hishikawa, & Koida,
1970). Hishikawa and Shimizu (1995) speculate that SP may be produced by hyperactivation of cholinoceptive and/or cholinergic Sleep-on neural populations or, they deem more likely, hypoactivation of noradrenergic or serotonergic Sleep-off populations in the pons. Thus, SP may reflect an anomaly of the functioning of the monoaminergic systems and/or their inhibition of cholinergic systems (Hishikawa & Shimizu, 1995). Sensory thresholds for awakening are relatively high during REM, suggesting that there may be, at best, weak and inconsistent cortical sensory processing during REM (Llina´s & Pare´, 1991). REM associated with the night-mare, however, appears to differ from dream-related REM in that there is little or no blocking of exteroceptive stimulation and no loss of waking consciousness (Hishikawa, 1976; Hishikawa & Kaneko, 1965). In any case, the throughput of sensory information along thalamocortical pathways during REM may be quite variable and, at times, exceed that during waking states (Inoue, Duysens, Vosser, & Coenen, 1993; van Hulzen & Coenen, 1984). During phasic SP, periods of high thalamic ‘‘transfer ratio’’ (Coenen & Vendrick, 1972) may result in high levels of both exteroceptive input and quasi-random activation originating in the brain stem. A major and distinctive feature of SP, we will argue, is the anomalous combination of high levels of exogenous and endogenous sources of cortical activation. Finally, the immobility of SP is also consistent with the general atonia maintained during REM by marked and sustained hyperpolarization of the spinal motoneurons (Chase & Morales, 1989). (1)
Sleep paralysis occurs immediately prior to falling asleep or upon waking. During these episodes, individuals are conscious of their surroundings and able to open their eyes, but unable to move (Hishikawa 1976), An acute sense of fear and various hypnagogic and hypnopompic experiences often accompany sleep paralysis, although little systematic evidence is available
on the prevalence of different experiences within a sleep paralysis episode.
Sleep paralysis may occur during sleep onsetor offset-REM (Fukuda 1994; Hishikawa and Shimizu 1995) and the hallucinoid experiences may result from neurological events associated with REM dream imagery (Hishikawa and
Shimizu 1995), These experiences include a sensed presence, auditory and visual hallucinations, unusual bodily sensations including floating, and feelings of pressure on the body (Hishikawa 1976; Hufford 1982),
The Waterloo Sleep Experiences Scale was designed to assess the prevalence of sleep paralysis and associated experiences, as well as to permit the analysis of relations among such experiences. Related research with this instrument has discovered a high degree of structure in the
patterning of hypnagogic and hypnopompic experiences (Cheyne et al. in press). This structure is consistent with cultural accounts of the 'Old Hag', 'Kanashibari', and ghost oppression that have been linked to sleep paralysis and associated experiences...
In our investigation, one-quarter of respondents who endorsed the basic paralysis question reported no additional hallucinatory features, and about 5% of the entire sample reported experiencing the full range of hallucinoid experiences associated with cultural phenomena such as the 'Old Hag.'
This latter finding suggests that instruments that fail to assess hallucinoid experiences along with prevalence rates of sleep paralysis may be misleading.
For example, reports of 30-40% in the literature may give the false impression that very large proportions of the population are experiencing the equivalent ofthe 'Old Hag' phenomenon (Hufford 1982; Ness 1978). The results also indicate that fear is associated with the reporting of hallucinoid experiences over and above the occurrence of sleep paralysis itself. Thus, at least some degree of fear is a response to something more than paralysis itself.
Indirect evidence was provided for a model in which sensed presence during sleep paralysis is associated with a fear reaction, possibly associated with amygdaloid and temporal lobe activation (Cheyne et al. in press) which, in turn, motivates additional experiences. Sensed presence and associated fear
may increase vigilance, detection, and interpretation of both endogenous (perhaps involving REM-related, oculomotor, middle ear, and primary sensory cortical activation) and exogenous environmental events. A sensed presence associated with fear may also initiate a search for an external threat and
focus attention on information in the visual and auditory systems. Visual experiences in particular may 'flesh out' the sense of presence and give it apparent form and substance.
The sense of a disembodied presence may subsequently fade and be replaced with more substantive perceptual experiences. This may explain why, in contrast to some of the other hallucinoid experiences, sensed presence was somewhat less frequent in the most elaborate reports than in the sparser accounts. (2)
1. Hypnagogic and Hypnopompic Hallucinations during Sleep Paralysis: Neurological and Cultural Construction of the Night-Mare
J. Allan Cheyne, Steve D. Rueffer, and Ian R. Newby-Clark
2. Relations among hypnagogic and hypnopompic experiences associated with sleep paralysis*
J, ALLAN CHEYNE, IAN R. NEWBY-CLARK andSTEVE D, RUEFFER
