![]() As it becomes darker outside, images of the objects in the room behind the person are reflected dimly in the window glass. The person may be thoroughly absorbed by the view of the outside world and fail to visualize the interior of the room at all. At the time of awakening, the typical sleeper again passes through a period of perceptual release, often experiencing dreams that increase in intensity, and perhaps may have the hallucinations of “partial sleep,” these now being called hypnopompic (associated with a semiconscious state before awakening) experiences.Ī simplified but perhaps helpful way of characterizing these conditions might be to imagine someone standing at a window opposite a fireplace, looking out at a garden in the sunset. ![]() This state (with specific EEG signs and rapid movements of the closed eyes) probably occurs several times every night, so that more than 20 percent of an average person’s sleep is taken up with several dreams, each of about 10 to 15 minutes’ duration. Sensory stimuli from without (such as noise or cold) or stimuli from within the body (such as dyspepsia or anxiety), plus a somewhat regular spontaneous fluctuation in the depth of sleep, will periodically (perhaps every 90 minutes or so) bring the sleeper into a state that favours perceptual release, in which case dreaming tends to take place. During this phase, measurements taken by an electroencephalograph (EEG) will record the sleeper producing large, slow brain waves. The first stages of sleep appear to be dream-free. A frequently occurring hypnagogic hallucination is the sensation of loss of support or balance, perhaps accompanied by a fragmentary “dream” of falling, followed immediately by a jerking reflex recovery movement (the myoclonic jerk) that may jolt the sleeper back into wakefulness. Hypnagogic hallucinationsĬommon hypnagogic hallucinations may be visual (e.g., scenes from the previous few hours appear) or auditory (e.g., one seems to hear one’s name called). Thus, the so-called hypnagogic (induced by drowsiness) phenomena occur. In the process of falling asleep, a person passes through a period of “partial sleep” in which awareness of the environment drops rapidly but in which the level of cortical arousal (which falls less rapidly) remains sufficiently high to permit some appreciation of external stimulation. Since levels of brain arousal during sleep and wakefulness also are mediated via reticular formation activity, sleeping and dreaming merit consideration as hallucinatory activities. The ways in which the reticular network of cells in the brainstem acts as a regulatory and integrating system for these relationships remain under intensive study. These inhibiting forces require for their maintenance a relatively high level of sensory input of appropriate quality and frequency. Under circumstances other than surgery or such situations as, for example, some forms of epilepsy, however, there must be a decrease in the forces that ordinarily dominate consciousness before hallucinations occur. Hallucinatory experiences can be brought about through direct stimulation of the exposed brain, as in cases of local cortical arousal under the neurosurgeon’s stimulating electrode. Induction of hallucinations Direct brain stimulation SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.This Time in History In these videos, find out what happened this month (or any month!) in history.#WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.Britannica Classics Check out these retro videos from Encyclopedia Britannica’s archives.Britannica Explains In these videos, Britannica explains a variety of topics and answers frequently asked questions.
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