Hypnagogia

In 1848, Alfred Maury introduced the term "hypnagogic" from the Greek words ύπνος ("hypnos"), meaning "sleep", and αγωγός ("agōgos"), meaning "conductor" or "leader". Later, in 1904, Frederic Myers coined the term "hypnopompic", with its word-ending originating from the Greek word πομπός ("pompos"), meaning "sender". ==Definitions==

The word hypnagogia is sometimes used in a restricted sense to refer to the onset of sleep, and contrasted with hypnopompia, Frederic Myers's term for waking up. However, hypnagogia is also regularly employed in a more general sense that covers both falling asleep and waking up. Indeed, it is not always possible in practice to assign a particular episode of any given phenomenon to one or the other, given that the same kinds of experience may occur in both as people drift in and out of sleep. Other terms for hypnagogia, in one or both senses, that have been proposed include "presomnal" or "anthypnic sensations", "visions of half-sleep", "oneiragogic images" and "phantasmata", "the borderland of sleep", "praedormitium", "sleep onset dreams", "dreamlets", and "wakefulness-sleep transition" (WST). Threshold consciousness (commonly called "half-asleep" or "half-awake", or "mind awake body asleep") describes the same mental state of someone who is moving towards sleep or wakefulness but has not yet completed the transition. Such transitions are usually brief but can be extended by sleep disturbance or deliberate induction, for example during meditation. ==Signs and symptoms==

Transition to and from sleep may be attended by a wide variety of sensory experiences. These can occur in any form, individually or combined, and range from the vague and barely perceptible to vivid hallucinations. Sights Among the more commonly reported at about 86%, i.e., from simple Eigenlicht to whole imagined scenes. Descriptions of exceptionally vivid and elaborate hypnagogic visuals can be found in the work of Marie-Jean-Léon, Marquis d'Hervey de Saint Denys. Tetris effect People who have spent a long time at some repetitive activity before sleep, in particular one that is new to them, may find that it dominates their imagery as they grow drowsy, a tendency dubbed the Tetris effect. This effect has even been observed in amnesiacs who otherwise have no memory of the original activity. When the activity involves moving objects, as in the video game Tetris, the corresponding hypnagogic images tend to be perceived as moving. The Tetris effect is not confined to visual imagery but can manifest in other modalities. For example, Robert Stickgold recounts having experienced the touch of rocks while falling asleep after mountain climbing. 8-36% of hypnagogic hallucinations are auditory, making up a small number of cases. Other sensations Gustatory, olfactory and thermal sensations in hypnagogia have all been reported, as well as tactile sensations (including those kinds classed as paresthesia or formication). Many people with synesthesia report seeing a flash of light or some other visual image in response to a real sound. Proprioceptive and somatic effects may be noticed, with numbness and changes in perceived body size and proportions, Perhaps the most common experience of this kind is the falling sensation, and associated hypnic jerk, encountered by many people, at least occasionally, while drifting off to sleep. 25% to 44% of hallucinations are somatic. Cognitive and affective phenomena Thought processes on the edge of sleep tend to differ radically from those of ordinary wakefulness. For example, something that one agreed with while in a state of hypnagogia may seem completely ridiculous while in a waking state. Hypnagogia may involve a "loosening of ego boundaries ... openness, sensitivity, internalization-subjectification of the physical and mental environment (empathy) and diffuse-absorbed attention". Hypnagogic cognition, in comparison with that of normal, alert wakefulness, is characterized by heightened suggestibility, illogic and a fluid association of ideas. Subjects are more receptive in the hypnagogic state to suggestion from an experimenter than at other times, and readily incorporate external stimuli into hypnagogic trains of thought and subsequent dreams. This receptivity has a physiological parallel; EEG readings show elevated responsiveness to sound around the onset of sleep. Herbert Silberer described a process he called autosymbolism, whereby hypnagogic hallucinations seem to represent, without repression or censorship, whatever one is thinking at the time, turning abstract ideas into a concrete image, which may be perceived as an apt and succinct representation thereof. The hypnagogic state can provide insight into a problem, the best-known example being August Kekulé's realization that the structure of benzene was a closed ring while half-asleep in front of a fire and seeing molecules forming into snakes, one of which formed an ourobouros. Many other artists, writers, scientists and inventors – including Beethoven, Richard Wagner, Walter Scott, Salvador Dalí, Thomas Edison, Nikola Tesla and Isaac Newton – have credited hypnagogia and related states with enhancing their creativity. A 2001 study by Harvard psychologist Deirdre Barrett found that, while problems can also be solved in full-blown dreams from later stages of sleep, hypnagogia was especially likely to solve problems which benefit from hallucinatory images being critically examined while still before the eyes. A feature that hypnagogia shares with other stages of sleep is amnesia. But this is a selective forgetfulness, affecting the hippocampal memory system, which is responsible for episodic or autobiographical memory, rather than the neocortical memory system, responsible for semantic memory. but the evidence for this has been disputed. In particular, suppression of REM sleep due to antidepressants and lesions to the brainstem has not been found to produce detrimental effects on cognition. Hypnagogic phenomena may be interpreted as visions, prophecies, premonitions, apparitions and inspiration (artistic or divine), depending on the experiencers' beliefs and those of their culture. ==Physiology==

Physiological studies have tended to concentrate on hypnagogia in the strict sense of spontaneous sleep onset experiences. Such experiences are associated especially with stage 1 of NREM sleep, but may also occur with pre-sleep alpha waves. Davis et al. found short flashes of dreamlike imagery at the onset of sleep to correlate with drop-offs in alpha EEG activity. To identify more precisely the nature of the EEG state which accompanies imagery in the transition from wakefulness to sleep, Hori et al. proposed a scheme of 9 EEG stages defined by varying proportions of alpha (stages 1–3), suppressed waves of less than 20μV (stage 4), theta ripples (stage 5), proportions of sawtooth waves (stages 6–7), and presence of spindles (stages 8–9). Germaine and Nielsen found spontaneous hypnagogic imagery to occur mainly during Hori sleep onset stages 4 (EEG flattening) and 5 (theta ripples). The "covert-rapid-eye-movement" hypothesis proposes that hidden elements of REM sleep emerge during the wakefulness-sleep transition stage. Support for this comes from Bódicz et al., who notes a greater similarity between WST (wakefulness-sleep transition) EEG and REM sleep EEG than between the former and stage 2 sleep. resulting in impaired cognition and even amnesia. ==History==

Early references to hypnagogia can be found in the writings of Aristotle, Iamblichus, Cardano, Simon Forman, and Swedenborg. Romanticism brought a renewed interest in the subjective experience of the edges of sleep. In more recent centuries, many authors have referred to the state; Edgar Allan Poe, for example, wrote of the "fancies" he experienced "only when I am on the brink of sleep, with the consciousness that I am so". In Charles Dickens's Oliver Twist there is a pair of episodes, one that portrays the title character experiencing a hypnopompic state, which precedes the following episode, which describes him in a hypnagogic state: There is a kind of sleep that steals upon us sometimes, which, while it holds the body prisoner, does not free the mind from a sense of things about it, and enable it to ramble at its pleasure. … It is an undoubted fact, that although our senses of touch and sight be for the time dead, yet our sleeping thoughts, and the visionary scenes that pass before us, will be influenced and materially influenced, by the mere silent presence of some external object; which may not have been near us when we closed our eyes: and of whose vicinity we have had no waking consciousness". Serious scientific inquiry began in the 19th century with Johannes Peter Müller, Jules Baillarger, and Alfred Maury, and continued into the 20th century with Leroy. Francis Galton in his The Visions of Sane Persons (1881), wrote: : [B]efore I thought of carefully trying, I should have emphatically declared that my field of view in the dark was essentially of a uniform black, subject to an occasional light-purple cloudiness and other small variations. Now, however, after habituating myself to examine it with the same sort of strain that one tries to decipher a sign-post in the dark, I have found out that this is by no means the case, but that a kaleidoscopic change of patterns and forms is continually going on, but they are too fugitive and elaborate for me to draw with any approach to truth. Maria Mikhaĭlovna Manaseina wrote "that for these phenomena to attract attention a certain power of observation is required; that is why they are chiefly found in intelligent persons". Manaseina also noted that children are more likely to take interest in them: "[M]any children are accustomed to press their heads into the pillow and adopt an attitude of expectant attention towards the visions that then begin to form…" Havelock Ellis later recalled such a childhood interest in these visions: : I should myself be inclined to deny that I had ever had any such visionary faculty [for hypnagogic hallucination] if it were not that I can recall one occasion of its presence, at about the age of seven, when sleeping with a cousin of the same age; we amused ourselves by burying our heads in the pillows and watching a connected series of pictures which we were both alike able to see, each announcing any change in the picture as soon as it took place. The advent of electroencephalography (EEG) supplemented the introspective methods of these early researchers with physiological data. The search for neural correlates for hypnagogic imagery began with Davis et al. in the 1930s, and continues with increasing sophistication. While the dominance of the behaviorist paradigm led to a decline in research, especially in the English speaking world, the later twentieth century has seen a revival, with investigations of hypnagogia and related altered states of consciousness playing an important role in the emerging multidisciplinary study of consciousness. Nevertheless, much remains to be understood about the experience and its corresponding neurology, and the topic has been somewhat neglected in comparison with sleep and dreams; hypnagogia has been described as a "well-trodden and yet unmapped territory". Important reviews of the scientific literature have been made by Ghibellini & Meier, Leaning, Schacter, Richardson and Mavromatis. ==Research==

Self-observation (spontaneous or systematic) was the primary tool of the early researchers. Since the late 20th century, this has been joined by questionnaire surveys and experimental studies. All three methods have their disadvantages as well as points to recommend them. Naturally, amnesia contributes to the difficulty of studying hypnagogia, as does the typically fleeting nature of hypnagogic experiences. These problems have been tackled by experimenters in several ways, including voluntary or induced interruptions, and training in the art of introspection to heighten the subject's powers of observation and attention. Another method is to induce a state said to be subjectively similar to sleep onset in a Ganzfeld setting, a form of sensory deprivation. But the assumption of identity between the two states may be unfounded. The average EEG spectrum in Ganzfeld is more similar to that of the relaxed waking state than to that of sleep onset. Wackerman et al. conclude that "the Ganzfeld imagery, although subjectively very similar to that at sleep onset, should not be labelled as 'hypnagogic'. Perhaps a broader category of 'hypnagogic experience' should be considered, covering true hypnagogic imagery as well as subjectively similar imagery produced in other states". ==See also==