Background: There are several clinical phenomena that resemble hallucinations which are inadequately studied because the terminology defining them is inadequate. Methods: A review of the relevant literature, identified by searches of Ovid and PubMed databases. Results: A historical review reveals that the term pseudohallucination has been used to describe several separate and unrelated phenomena. Herein this term is redefined, and an additional term, parahallucinations, is introduced. Hallucinations are defined as sensory perceptions that have the compelling sense of reality of true perceptions but that occur without external stimulation of the relevant sensory organ and are experienced as following the sensory path, that is, can be localized in three-dimensional space outside the body. Pseudohallucinations are defined as hallucinatory phenomena that do not follow the sensory path and are experienced predominantly by psychiatrically ill subjects. Parahallucinations are defined as hallucinatory phenomena that occur due to an injury or abnormality to the peripheral nervous system. Insight into the reality of these experiences-the realization that they are not real-is not felt to be important in their phenomenology. Conclusion: Utilization of this classification system during clinical investigations will yield greater insight into the pathophysiology, course, treatment, and prognosis of psychiatric and neurologic disorders.
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