| According to Dorland's Medical Dictionary,
Edition 28, hallucinations
(common manifestations of neuropsychiatric illnesses) are "a sense
perception without a source in the external world; a perception of an external
stimulus object in the absence of such an object".
All the senses can be subject to hallucinatory
activity. Thus we classify hallucinations as auditory, visual, olfactory,
gustatory, haptic (touch), kinesthetic (involving the sense of bodily movement)
and somatic (involving the perception of physical experiences occurring
with the body).
Hallucinations may be formed, well defined
and recognizable sensations (for example, auditory hallucination of voices
talking to or about oneself) or unformed elementary sensations (strange
sounds, unrecognizable voices speaking unintelligible things).
Hallucinations are the expression of dysfunctional
brain activity due to tissue damage (tumors, strokes, scars, parasites,
infections), intoxication (drugs or metabolic products) and the action
of not yet identified factors (for example, the hallucinations of schizophrenia
and affective disorders).
The utilization of modern techniques like PET
brain scan allowed the investigators to identify physiological correlates
to hallucinatory activity like, for instance: activation of interconnected
areas deep within the brain's core (bilateral thalamus, left hippocampus,
parahippocampal gyrus, right anterior cingulate and left orbitofrontal
cortex) and of other areas on the brain's surface (visual and auditory
cortex). reduced activation in the left middle temporal gyrus, the rostral
supplementary motor area and the left medial prefrontal cortex.
Additional information can be found in the
following sites:
http://www.schizophrenia.com.newsletter/697/697halluc.htm
http://www.apa.org/monitor/jan97/image.html
http://www.futur.com/webtrack/sep96/sept2.htm
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