![]() ![]() Further, visual hallucinations are common in those with certain sleep disorders, and occur more frequently in those who are drowsy. Lesions of the brainstem have led to visual hallucinations (as in peduncular hallucinosis). 7įinally, due to its role in the maintenance of arousal, the reticular activating system has been implicated in the genesis of visual hallucinations. One study reported visual hallucinations in 10 of 13 healthy subjects blindfolded for a period of 5 days this finding lends strong support to the idea that the simple loss of normal visual input is sufficient to cause visual hallucinations. 6 Of note, visual hallucinations may be induced by prolonged visual deprivation. 5Ī multitude of lesions can cause this loss of input and inhibit other cognitive functions. It has been further suggested that deafferented neurons undergo specific biochemical and molecular changes that lead to an overall increase in excitability (similar to the denervation hypersensitivity seen in phantom limb syndrome experienced by amputees). 4 Normal inputs are thought to be under the control of inhibitory processes that are effectively removed by deafferentation. Lesions that cause deafferentation of the visual system may lead to cortical release phenomenon, including visual hallucinations. 3 These data are supported by both electroencephalographic (EEG) recordings and direct stimulation experiments. Irritation of the primary visual cortex (Brodmann's area 17) causes simple elementary visual hallucinations, while irritation of the visual association cortices (Brodmann's areas 18 and 19) causes more complex visual hallucinations. The first mechanism involves irritation (e.g., seizure activity) of cortical centers responsible for visual processing. Manford and Andermann 2 summarized 3 pathophysiologic mechanisms thought to account for complex visual hallucinations. To date, no single neural mechanism has explained all types of visual hallucinations however, the similarity of visual hallucinations that are associated with seemingly diverse conditions suggests a final common pathway. ![]() Visual hallucinations can be the result of all 3 processes, given the interplay among disturbances of brain anatomy, brain chemistry, prior experiences, and psychodynamic meaning. These have been summarized and categorized by Asaad and Shapiro 1: psychophysiologic (i.e., as a disturbance of brain structure), psychobiochemical (as a disturbance of neurotransmitters), and psychodynamic (as an emergence of the unconscious into consciousness). Numerous hypotheses have been suggested to explain the genesis of visual hallucinations. ![]()
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