A hallucination is a vivid sensory impression generated by the mind rather than by any external stimuli. It may be seen (vision), heard (sound), felt (haptic), smelled, or tasted.
The person must be awake and of sound mind. Hallucinations occur when emotional, chemical, or physical factors such as sleep deprivation, mental illness, medication side-effects cause the mechanism within the brain the helps distinguish external sensory perceptions from internal memory-based perceptions to misfire. Hallucinations must be distinguished from dreaming, illusion, imagery and delirium. One interesting form that often is a normal finding for individuals is called Hypnogogic Hallucinations. These are hallucinations that occur upon falling asleep. However, if these occur in the setting of daytime sleepiness then the individual should seek further evaluation from their physician as it could be one manifestation of a larger underlying treatable neurological disease called Narcolepsy.
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