Articles

Particular sensations that occur at the threshold between waking and sleeping do not belong in the realm of dreams. Typical transitional manifestations during this intermediate stage are extremely fleeting and vanish as soon as any attempt is made to register them consciously. As early as 1848, the French psychiatrist Maury was intensely concerned with experiences made during sleep onset, and he coined the term, hypnagogic, meaning "leading to sleep," to cover these sensations.

Sensations occurring during sleep onset consist of optical or acoustic sensory experiences that are not the result of any external stimulus. They include flashes of light, colored rings, waves, and other geometric figures, partly disfigured or fragmentary. Whole scenes with

action sequences similar to visions also occur. Acoustic phenomena include the perception of voices, hearing one's name called, or hearing entire fragments of conversation or music. The last-mentioned phenomena already belong, however, in the realm of complex hallucinations. One difference between hypnagogic hallucinations and dreams, which can usually be recalled, is their brevity. In addition, the person falling asleep remains an impartial observer of these phenomena. He does not experience himself as the acting person, as is frequently the case in dreams. Usually he can spontaneously distinguish between a dream and a hypnagogic hallucination. The person experiences himself as not yet sleeping, while in the course of a dream the person usually knows that he is sleeping or dreaming (Dittrich).

Another symptom occurring during sleep onset is a sudden, vigorous, jerky movement (myoclonus). It may be so strongly felt that it wakes the person up or flows right into typical dreams of falling. It is assumed to be caused by structures of the brain near the brain stem that are able to get past impulses from the cerebral cortex usually blocking them off. The result is the coordinated motor symptoms of the myoclonic jerk (Kugler).

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