Excerpt

Hallucinations are sensory images “seen” in the absence of external stimuli. They occur on falling asleep or waking up, during hypoglycemia, delirium, epilepsy, psychotic episodes, advanced syphilis, sensory deprivation, and migraine (Siegel and West 1975). They can be triggered by photic or electrical stimulation and by a variety of hallucinogenic drugs. It has been suggested that “many important human experiences (such as dreams and visions of biblical prophets and the creative imagery of great artists) are . . . related to hallucinations” (Siegel and West 1975). 

Klüver (1967) made many studies of such hallucinations, especially visual ones, mainly by ingesting the drug mescaline, derived from the peyote cactus, and concluded that four types of patterns are usually observed: (1) gratings, lattices, fretworks, filigrees, honeycombs, or chessboard designs; (2) cobwebs; (3) tunnels, funnels, alleys, cones, and vessels; and (4) spirals. Klüver termed these four types form constants. More recent observations by R. Siegel (1977) have confirmed Klüver’s classification. Lattice, spiral and funnel hallucinations are shown in figure 1.

Visual hallucinations appear to be stabilized against eye movements, in much the same way that external images are stabilized. This suggests that they are centrally, rather than peripherally, located. This conclusion is supported by other observations, for example, hallucinations induce form constants even in total darkness, and even in blind subjects (Krill, Alpert, and Ostfield 1963). 

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