There are probably several degrees of an hallucinatory experience, as
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 as a real experience. An
hallucination proper may be said to have occurred if the sensation seems
quite authentic even if the experiencer judges the experience to be, for
some reason, suspect. It seems real but there is also something counterfeit
about the experience. A pseudo-hallucination also has this counterfeit
quality but it also lacks the fullness of a conventional sensation. It
has an ethereal, "as-if" quality, lacking the richness of a true sensation.
An illusion is simply a misinterpretation of a conventional stimulus.
|Frequency of Associated Experiences
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, kinesthetic, 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).
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) (Naníno, 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 nucleus
reticularis pontis oralis (RPO) immediately ventral to the locus ceruleus
in the pontine reticular formation. The neurotransmitters in this region
have not been clearly determined, but are neither cholinergic nor monoaminergic.
The RPO receives projections from cholinergic regions in the laterodorsal
tegmental nucleus (LDT) and the pedunculopontine tegmental nucleus (PPT)
as well as from ventromedial portions of the medulla. The RPO, LDT, and
PPT are collectively thought to be part of the REM-on neural population
(Steriade & McCarley, 1990). These populations are hypthesized to interact
with REM-off noradrenergic neurons in the locus ceruleus and seratonergic
neurons in the raphe system. These latter populations are most active during
waking and least active during REM. Interactions between the REM-on and
REM-off populations are thought to control REM onset and offset (Steriade
& McCarley, 1990).
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