Hallucinations feel real while stimuli are missing
A hallucination is a perception with the full force of reality but nothing outside to cause it. It differs from dreams, because you are awake, from illusions, where something real is misread, and from pseudohallucinations, which the person recognizes as unreal. Five cups of coffee a day may be enough to invite one.
Any sense can hallucinate: sight, sound, smell, taste, touch, body position, balance, pain, temperature, even the sense of time, and episodes that hit several channels at once are called multimodal. Milder disturbances include flickers at the edge of vision or faint voices. Hallucinations also differ from delusional perception, where a real stimulus is correctly sensed but loaded with extra meaning. Sir Thomas Browne brought the word into English in 1646 from Latin alucinari, to wander in the mind.
Hearing things is the most common type. Simple forms are hisses, whistles, or steady tones, and some tinnitus fits here, though pulsatile tinnitus can be the real sound of blood near the ear. Complex forms bring voices or music. Hallucinated voices are frequent in schizophrenia, and about 55% of them are hostile, often talking about the person rather than to them. In schizophrenia voices usually seem to come from outside; in dissociative disorders they seem to speak from within. A La Trobe University study linked roughly 500 mg of caffeine a day, about five cups of coffee, to a higher chance of hearing things.
Voices do not automatically mean mental illness. Temporal lobe epilepsy, Wilson’s disease, lupus, multiple sclerosis, and metabolic disorders can all produce psychosis, and the Hearing Voices Movement supports people who hear voices without other impairment. Command hallucinations, which issue orders, are more often obeyed when the instruction is non-violent.
Other senses have their own versions. Simple visual hallucinations are lights and shapes; complex ones are lifelike scenes. Phantosmia means smelling odours that are absent, usually burnt or rotten and often harmless, while formication feels like insects under the skin and is tied to prolonged cocaine use. Images while falling asleep or waking are considered normal.
Source: Hallucination
