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Perceptions that occur in the absence of external stimuli:
Auditory: Occur most frequently in the psychoses of schizophrenia, mania, and depression.
Visual: Occur more frequently in psychotic episodes due to medical, neurologic, and toxin-induced disorders although are often associated with primary psychiatric disease.
Olfactory and Gustatory: Often occur as prodromal symptoms of complex partial seizures.
Tactile: Frequently occur in delirium i.e., formication is the sensation of bugs crawling on the skin.
Misperception or misinterpretation of existing stimuli, often seen in patients with delirium. For example, a patient describes seeing robots in the room, but on further questioning he points to the vent in the ceiling and says, “There’s his mouth, those are his eyes..."
Normal perception of external stimuli.
Feeling that one is falling apart, fragmenting, not one’s self, becoming unreal, or detached. Seen more frequently in anxiety disorders or delirious states.
Feeling that the world is not real, people are not real, or things are becoming distant, alien, or strange.
Inability to recognize people or objects even when basic sensory modalities are intact. Usually suggests an organic problem.
Sensation or hallucination caused by another sensation (e.g. a sound experienced as being seen or a visual experience experienced as heard); common with hallucinogenic drugs.