Does Recurrent Isolated Sleep Paralysis Involve More than Cognitive Neurosciences? By Jean-Christophe Terrillon and Sirley Marques Bonham

 John Anster Fitzgerald: The Nightmare, (1857-58), private collection

The commonly experienced sleep paralysis may last from a few seconds to several minutes, occasionally longer, and is a pure motor inhibition: individuals have a subjective impression of wakefulness, a veridical perception of the actual environment (in most cases they can open their eyes and look at their surroundings), and they are unable to move despite concerted efforts to do so. At a physiological level, the motor inhibition can be explained in terms of a transient shift of electrolytes, as we have seen in the preceding section. In the case of RISP, in addition to the primary feature of paralysis, a large ensemble of secondary features called hypnagogic and hypnopompic experiences occur. The terms "hypnagogic" and "hypnopompic" refer to experiences that take place while drifting to sleep and during the process of waking up respectively. Finally, more rarely, other features (tertiary features) that go beyond the hypnogogic and hypnopompic experiences occur during "full-blown" episodes of RISP, and they will be examined in more detail in the next section. The experiences that are part of both the secondary and tertiary features are described as "hallucinations" in the conventional scientific literature on sleep paralysis. As stated in [53a]:

A hallucination is an experience of perception in the absence of an appropriate stimulus (from the actual world), but which has the impact of a conventional perception and is not under the control of the experiencer. It is generated by internal stimuli inside the brain. A hallucination has the quality of being a sensation related to an 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 experiencer. The "object" of the hallucination is taken to exist independently of the will of the experiencer. 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. There are probably several degrees of a hallucinatory experience, besides 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. A hallucination proper may be said to have occurred if the sensation seems quite authentic but 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.

Sleep paralysis seems to embody all the degrees of hallucinatory experience. However, In the case of full-blown episodes of RISP, individuals are convinced that these experiences have objective external sources. They are unlikely to describe their experience as one of sleep paralysis, but rather, for instance, as one of demonic possession or of spirit encounter, and the perceived reality of the "events" that occur during RISP episodes enhances their fear or even "terror." Hypnogogic and hypnopompic hallucinations include visual, auditory, haptic (tactile), and more rarely olfactive hallucinations. The individual perceives vivid dream-like imagery, hears voices, song or music, and sounds such as footsteps or static from a radio, for example. The increasing buzzing or ringing in the ears as described in section 2 may also qualify as an auditory hallucination although it is part of a real condition called tinnitus. The tactile hallucination includes a pressure on the chest, a sensation of choking, or of touch on different parts of the body, and occasionally of genital stimulation. Another very intriguing type of hallucination that occurs commonly is feeling a presence in the room. The presence is usually perceived as threatening or evil, occasionally ugly, but also sometimes benign. As described in section 2, it may be human, animal, or neither, or even some combination, or undefinable. It may be simply watching, or it may be speaking, or even attacking, for example by pulling the individual's legs or by trying to smother the individual (this experience is associated with the choking sensation). The pressure on the chest may procure the sensation that the presence (or "entity") is sitting on the chest.

The "hallucinations" that are part of the tertiary features and thus occur more rarely, are proprioceptive and autoscopic hallucinations. In the case of proprioceptive hallucinations, the individual feels that he, or part of himself, is at a different location from the physical body: he might feel phantom limbs or he has the subjective experience of slipping away from the physical body in what appears to be a phantom body. Subjective experiences of floating, rising and occasionally rolling also occur. In addition, when experiencing an autoscopic hallucination, the proprioceptive hallucination is coupled with visual hallucinations: the individual, while in a floating state, can see the actual room, and eventually his physical body lying motionless on the bed. Or he can see a fictitious, dream-like environment characterized by vivid imagery, or even what seems to be a superposition of both the physical world and a dream-like world. In all three cases he has a subjective experience of awareness and his experience is perceived to be real. These types of experiences are part of the so-called "Out-of-Body Experiences" (OBEs) phenomenon. We should point out here that the autoscopic hallucinations experienced during a full-blown RISP episode are not to be confused with the "classical" autoscopic hallucinations, in which case an individual, while in a fully awake state and moving, "sees" a (phantom) double of himself or "Doppelg nger". Interestingly, in the case of sleep paralysis the opposite situation seems to occur : the phantom body "sees" the physical body from some vantage point. The term "autoscopic" may still be used since there is a dissociation of a physical body and of a perceived (phantom) body double.

In addition, the tertiary features include other types of experiences such as derealization (the feeling that the surroundings of the individual are unreal), depersonalization (a loss of sense of personal identity), a subjective continuity of conscious experience as opposed to the shifting images of a usual dream, a dissociation of modalities, distortions of body image, and tingling or vibrating sensations. Usually, these tertiary features seem to occur later in a RISP episode than the secondary features, in what we call a second phase of the episode. Finally, in a third phase, the phantom body reenters the physical body, slowly or abruptly. The individual might then wake up or fall back into another episode, not uncommonly directly in the second phase. Based on the above description of the different features of RISP, we now attempt to connect some of the basic features with what is known at present on the subject of awareness during sleep and hallucinations in the field of cognitive neurosciences.

Already in the 1960's, the Canadian neurologist W. Penfield [29] had showed that auditory hallucinations can be induced in the wake state by electrical stimulation of the temporal lobe. Moreover, It has recently been shown that tinnitus, a condition that affects tens of millions of people worldwide and is in some cases permanent [30], is closely associated with the activity of the auditory cortex and involves the temporal lobe (Lockwood et al., 1998 [31]). We therefore may conjecture that the tinnitus (buzzing/ringing sound in the ears) that increases in intensity during the first phase of a RISP episode as well as other auditory hallucinations (such as voices or a variety of sounds) are associated with the temporal lobe and that the activity of the temporal lobe increases significantly at the beginning of RISP episodes. In the same way, internally generated visual stimuli (via the visual cortex) would be responsible for the visual hallucinations, at least during the first phase of a RISP episode, and would have REM-like qualities despite the fact that there still is a subjective experience of awareness. Very little, if nothing, is known at present about the specific areas in the brain which would be involved in the other types of experiences, in particular the tertiary features of experiences. If we now address the issue of awareness, it has been shown that, whereas higher-order mental functions associated with the activity of such brain centers as the prefrontal cortex are essentially shut down during normal REM sleep and the limbic system (an older part of the brain) that is responsible for emotional and visceral phenomena is activated (A.R. Braun et al., 1998 [32]), those same higher-level functions are still operating during lucid dreams (S. Laberge, 1985 [14]).

While lucid dreaming, loss of critical insight, diminished self-reflection and impaired logic that is typical of normal dreaming does not seem to occur. Critical thinking can still take place. The awareness that is retained during the second phase of a RISP episode and some characteristics of this phase (the dream-like surroundings) tends to indicate that a close connection probably exists between RISP and lucid dreaming, although it is improbable that a whole RISP episode is itself a lucid dream. Rather, lucid dreaming could be one of several different components of a RISP episode. Indeed, Takeuchi et al. (1992, [33]) elicited ISP from normal subjects by a nocturnal sleep interruption schedule. All of the subjects with ISP experienced inability to move and were simultaneously aware of lying in the laboratory. All but one reported auditory/visual hallucinations and unpleasant emotions. Judging from the polysomnogram recordings they obtained, ISP differs from other dissociated states such as lucid dreaming, nocturnal panic attacks and REM sleep behavior disorders. However, the polysomnogram might have recorded only the first phase of a RISP episode, and recordings of a second phase might have shown similarities with lucid dreaming.