Direct brain stimulation

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. 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. These inhibiting forces require for their maintenance a relatively high level of sensory input of appropriate quality and frequency.

Sleep

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. Since levels of brain arousal during sleep and wakefulness also are mediated via reticular formation activity, sleeping and dreaming merit consideration as hallucinatory activities. 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. Thus, the so-called hypnagogic (induced by drowsiness) phenomena occur.

Hypnagogic hallucinations

Common 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). 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.