The perceptual consequences of visual loss: `positive' pathologies of vision

Fifty patients with visual hallucinations and illusions secondary to degenerative eye disease reported remarkably stereotyped experiences. Questionnaire responses revealed five previously recognized categories of pathological vision (perseveration, illusory visual spread, polyopia, prosopometamorphopsia and micro/macropsia) and three novel categories (tessellopsia, hyperchromatopsia and dendropsia). Identical pathologies of vision occur in a range of clinical and experimental settings, suggesting that they reflect fundamental visual processes. The known neurophysiology of the visual cortex helps explain the phenomenology of the experiences and provides the basis for a neurobiologically based classification of positive and negative visual perceptual disorders.