Simple formed hallucinations confined to the area of a specific visual field defect

Thirteen patients with transient or permanent homonymous visual field defects experienced formed hallucinations localized to the affected part of the visual field. The lesion was occipital in 8 instances (infarction 7, porencephalic cyst 1), parietooccipital in 3 (infarction 2, angioma 1) and probably parietal in 2 (epilepsy 1, encephalitis 1). The disorder involved the right hemisphere in 9 cases, the left hemisphere in 3 cases and both hemispheres sequentially in one patient. Hallucinations were accompanied by palinopsia in 2 cases, metamorphopsia in one case and constriction of one pupil in another case. This particular type of hallucination is considered as an irritative phenomenon of the visual association cortex which can be symptomatic of a parieto-occipital lesion and does not necessarily implicate the temporal lobes. Distinctive features about the visions were that they consisted of people, animals or objects. There was no auditory accompaniment and any action that took place was stereotyped and did not tell a story. In most cases, the hallucinations were not clearly related to any visual memory. It is suggested that the visual association cortex amy be responsible for the organization of visual percepts into broad categories of which people, animals and objects are representative. The occurrence of such hallucinations with a visual field defect suggests that the cells of the association cortex are more likely to discharge spontaneously once they are deprived of their normal afferent inflow from the calcarine cortex.