Associated Hypnagogic and Hypnopompic Experiences
Sleep paralysis most often has an adolescent onset. Earlier research had generally studied student populations leaving open the possibility that this might have been an artefact of the fact that only young people were surveyed. In several surveys with older samples, (Mean age of approximately 30) we have corroborated a very clear tendency for people at all ages to report an adolescent onset for their episodes. Several large samples have produced consistent means of 17 years of age, with a sharp increase after 10 and an even sharper decline from 17 to the mid-twenties. The results do suggest, however, that sleep paralysis episodes can begin at virtually any age, although it is rare for this to happen after 30.
Our research has led us to conclude that hallucination is probably not too strong a term for the experiences associated with sleep paralysis. We take our definition of hallucination from Slade & Bentall (1988). A hallucination is an experience of perception in the absence of an appropriate stimulus, but which has the impact of a conventional perception and is not under the control of the experient. A hallucination has the quality of being a sensation related to external event rather than merely a product of the imagination. It does not seem to be merely an idea. It has the quality of objectivity, that is, something beyond the willing and wishing of the experient. The "object" of the hallucination" is taken to exist independently of the will of the experient. The experience is, in principle, a publicly available phenomenon. The hallucinator should also believe that any appropriately situated person should be able to confirm these experiences. These qualities of sensation, objectivity, existence, and independence, are among the defining qualities of hallucinations (Aggernaes, 1972).
There are probably several degrees of a hallucinatory experience,
distinct from illusions and normal or conventional sensations. A
"full-blown" hallucination seems like a real experience and is
believed to be a real experience. One might say the individual is both
hallucinating and is deluded by the hallucination in to accepting it
real experience. A hallucination proper may be said to have
the sensation seems quite authentic even if the experient judges the
experience to be, for some reason, suspect. It seems
real but there is also
something counterfeit about the experience. A pseudo-hallucination
this counterfeit quality but it also lacks the fullness of a
sensation. It has an ethereal, "as-if" quality, lacking the
richness of a true sensation. An illusion is simply a
misinterpretation of a
Depending on the nature of the question and the populations surveyed between 20 - 40 % of people report having had such an experience. For about a third of these people (Figure 1) that is about the extent of the experience and other than a momentary concern about being paralyzed many of these people do not appear to give the matter much thought. It is entirely possibly that almost everyone has experienced such a state but has scarcely noticed and soon forgotten the experience. Another two-thirds of those experiencing sleep paralysis, however, have associated experiences sometimes referred to as hypnagogic and hypnopompic hallucinations. These hallucinations may be tactile, kinaesthetic, visual, or auditory. The most common of these experiences is the "sensed" presence accompanied by fear. Individuals vary considerably in the extent to which they report such symptoms. A rather small proportion (>5%) report all the associated components (Figure 1).
REM and Dreaming: A major distinction of sleep states, for close to a half century, has been accepted between REM and NREM sleep (Aserinsky & Kleitman, 1953; Jouvet, 1967). REM periods are characterized by desynchronized cortical characterized by low-voltage fast EEG patterns with synchronized hippocampal activity characterized by slow (4-8 Hz) theta activity (e.g., Culebras, 1994). It is also widely accepted that dreaming is more common and more vivid during REM than during NREM sleep (Dement & Kleitman, 1957). In addition to the characteristic desynchronized cortical low-voltage fast EEG activity, there are numerous physiological, behavioral, and sensory features associated with REM such as muscle atonia, gating of sensory input, rapid eye and middle ear movements, as well as heart rate and respiration changes (Carskadon & Dement, 1989; Symons, 1993).
Within REM periods a distinction is sometimes made between a background tonic state (TREM) and bursts of phasic REM (PREM) every 16-120 seconds and lasting from 2-9 seconds (Aserinsky, 1971, Molinari & Foulkes, 1969). Specifically, PREM is characterized by bursts of rapid eye and middle ear movements and characteristic cortical and hippocampal EEG patterns. PREM is associated with, and may be preceded by, ponto-geniculo-occipital EEG waves (PGO spikes in animal preparations) originating in the bilateral, dorsolateral pons and projecting rostrally through the lateral geniculate nucleus and other thalamic nuclei (Hobson, Alexander, Frederickson, 1969). It has been conjectured that the most vivid dreams, or most vivid events within dreams, are associated with PREM (Molinari and Foulkes, 1969).
REM and SP: SP has also been associated with REM states, particularly with sleep-onset and sleep-offset REM (SOREM) (Nan'no, Hishikawa, & Koida, 1970). In both REM dreams and SP hallucinations a general atonia is maintained during REM by marked and sustained hyperpolarization of the motoneurons (Chase & Morales, 1989). One likely function of the general atonia is the prevention of the physical enactment of the motor components of dreaming. There are at least two major traditional hypotheses concerning the connection between neurophysiological events and visual imagery in dreams. The visual imagery of dreaming may arise either from the direct stimulation of visual areas of the cortex during the PGO spike, in which case the rapid eye movements may reflect attempts to scan the images (Ladd, 1892; Roffwarg, Dement, & Muzio, 1962), or conversely, the mages may be produced by the oculomotor impulses in response to direct stimulation from the gigantocellular pontine reticular field (Hobson & McCarley, 1977; McCarley & Hobson, 1979).
REM is thought to be generated in the lateral portions of the
reticularis pontis oralis (RPO) immediately ventral to the locus
the pontine reticular formation. The neurotransmitters in this region
not been clearly determined, but are neither cholinergic nor
The RPO receives projections from cholinergic regions in the
tegmental nucleus (LDT) and the pedunculopontine tegmental nucleus
well as from ventromedial portions of the medulla. The RPO, LDT, and
collectively thought to be part of the REM-on neural population
& McCarley, 1990). These populations are hypothesized to interact
REM-off noradrenergic neurons in the locus ceruleus and seratonergic
in the raphe system. These latter populations are most active during
and least active during REM. Interactions between the REM-on and
populations are thought to control REM onset and offset (Steriade
REM SP with HHEs differs from REM dreams in that during SP there is little or no blocking of exteroceptive stimulation and there is no loss of waking consciousness. SP with HHEs differs from dream experience in that the sensory cortex may be receiving both externally and internally generated information. The peculiarity of the HHEs in SP may, in part, be a result of the brain's attempts to integrate endogenous cortical arousal originating in the pons with normal sensory input. A similar peculiarity may exist for motor pattern arousal during SP. McCarley and Hobson argue that, during dream generation by internal stimulation of motor programs, we interpret the activity of the pattern generators and their corollary discharge as movement. The lack of peripheral feedback, though not normally necessary for effective control, may contribute to a sense of unreality to the apparent movement and hence to the "bizarreness" of dreams. Pontine activation of motor patterns during SP appears to be less common in SP than in dreams, if subjective reports of illusory movement are to be taken as evidence. Volitional attempts at movement during SP are common, however, and the absence of feedback is most often, though not always, experienced as paralysis rather than illusory movement. Thus it appears that, during SP, the frontal cortex is more sensitive to the absence of feedback than during dreaming. When motor programs are spontaneously activated during SP these might be extremely resistant to coherent interpretation am may be experienced as very unusual bodily states. In concluding sections we will relate more specifically the phenomenology of various HHEs to the underlying neurophysiology.