Positive visual phenomena

Lesions in the visual pathway affect vision most often by creating deficits or negative phenomena, such as blindness, visual field deficits or scotomas, decreased visual acuity and color blindness. On occasion, they may also create false visual images, called positive visual phenomena. These images can be a result of distortion of incoming sensory information leading to an incorrect perception of a real image called an illusion. When the visual system produces images which are not based on sensory input, they can be referred to as hallucinations. The visual phenomena may last from brief moments to several hours, but they also can be permanent. They are generally associated with other symptoms but occasionally are isolated. Conditions causing these phenomena include disruptions in the visual input along the pathways (retina, optic nerve, chiasmal and retrochiasmal lesions) lesions in the extracortical visual system, migraines, seizures, toxic-metabolic encephalopathy, psychiatric conditions and sleep apnea, among others. The mechanisms underlying positive visual phenomena are not yet well understood. Possible mechanisms may be: 1) defect in the sensory input causing compensatory upregulation of the visual cortex, 2) faulty visual processing in which inputs are normal but lesions result in an inappropriate pattern of cortical excitation, 3)variants of normal visual processing. Of all forms of hallucination, visual hallucinations are the least likely to be associated with psychiatric disorders. For example most patients with visual hallucinations do not have schizophrenia and most patients with schizophrenia do not have visual hallucinations.

As described above, illusions can consist in a misinterpretation of a real sensory input, such as a recurrence, persistence, duplication or change in the size of images.

A persistence of a visual image of an object in time after the actual object has disappeared. There are two forms of palinopsia, an immediate and a delayed type. In the immediate type the image continually persists in the visual field after actually disappearing. On the other hand, in the delayed type, the image reappears after an interval of minutes to hours after disappearing. The pathophysiology of palinopsia remains unclear. The immediate type may be an exaggeration of the afterimage whereas the delayed type may indicate that there is cerebral involvement, such as an ictal manifestation or a structural lesion, but has also been shown to be inducible by drugs. The differential diagnosis includes toxins, metabolic disorders and psychiatric conditions.

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