Elementary Visual Hallucinations and Their Relationships to Neural Pattern-Forming Mechanisms Vincent A. Billock and Brian H. Tsou U. S. Air Force Research Laboratory An extraordinary variety of experimental (e. g., flicker, magnetic fields) and clinical (epilepsy, migraine) conditions give rise to a surprisingly common set of elementary hallucinations, includingspots, geometric patterns, and jagged lines, some of which also have color, depth, motion, andtexture. Many of these simple hallucinations fall into a small number of perceptual geometries—the Klu¨ver forms—that (via a nonlinear mapping from retina to cortex) correspond to even simpler sets of oriented stripes of cortical activity (and their superpositions). Other simple hallucinations(phosphenes and fortification auras) are linked to the Klu ¨ver forms and to pattern-forming cortical mechanisms by their spatial and temporal scales. The Klu ¨ver cortical activity patterns are examples of self-organized pattern formation that arise from nonlinear dynamic interactions between excit-atory and inhibitory cortical neurons; with reasonable modifications, this model accounts for a widerange of hallucinated patterns. The Klu ¨ver cortical activity patterns are a subset of autonomous spatiotemporal cortical patterns, some of which have been studied with functional imaging tech-niques. Understanding the interaction of these intrinsic patterns with stimulus-driven corticalactivity is an important problem in neuroscience. In line with this, hallucinatory pattern formationinteracts with physical stimuli, and many conditions that induce hallucinations show interestinginteractions with one another. Both types of interactions are predictable from neural and psycho-physical principles such as localized processing, excitatory-inhibitory neural circuits, lateral inhi-bition, simultaneous and sequential contrast, saccadic suppression, and perceptual opponency. Elementary hallucinations arise from familiar mechanisms stimulated in unusual ways. Keywords: intrinsic neural activity, Klu ¨ver form constant, migraine fortification aura, phosphene, spatiotemporal pattern formation What are these Geometrical Spectra? and how, and in what depart-ment of the bodily or mental economy do they originate?... there isa kaleidoscopic power in the sensorium to form regular patterns by thesymmetrical combination of causal elements.... —The astronomer Sir John F. W. Herschel (1867) on hallucinationsseen during migraine and during surgery “under the blessed influ-ence of chloroform. ” I had two days spoiled by a psychological experiment with mescal, an intoxicant used by some of our Southwestern Indians in their religiousceremonies; a sort of cactus bud, of which the U. S. Government haddistributed a supply to certain medical men, including Weir Mitchell, who sent me some to try. He himself had been in “fairyland. ” It givesthe most glorious visions of color.... I took one bud three days ago, was violently sick for 24 hours, and had no other symptoms.... I will take the visions on trust! —William James (1920) in a June 11, 1896, letter to his brother Henry. For many, mention of visual hallucinations brings to mind complex visual imagery: the stuff of waking dreams. This ismisleading; simple forms and patterns are more typical than Freud-ian drama. Elementary hallucinations—generated by a host ofclinical and laboratory-induced states—range from simple spots tothe flashing serrated arcs that accompany migraines to the kalei-doscopic patterns induced by some drugs. Until recently, this sheervariety made hallucinations seem like a collection of disparatecuriosities ill-posed for serious study. However, there are at leastfour reasons why this view is mistaken. First, there is much moreorder in these conditions than is generally appreciated. There aremany links and interactions between hallucinatory conditions. Moreover, even very different-seeming elementary hallucinationstake place on common spatial and temporal scales that correspondto identifiable neural mechanisms. Indeed, some different-seeminghallucinations can have the same pattern of cortical activity, dif-fering only by cortical position and orientation. The strong con-nections between elementary visual hallucinations and particularneural systems led Frances Wilkinson (2004) to call hallucinations This article was published Online First March 26, 2012. Vincent A. Billock, National Research Council, U. S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio; Brian H. Tsou, U. S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio. This research was funded in part by a National Research Council/Air Force Office of Scientific Research Senior Research Award to Vincent A. Billock. We thank Scott Kelso and Mingzhou Ding for introducing us to this topic;Oliver Sacks for suggesting that we look at relationships between patternsformed under migraine and image stabilization; Angela Brown, Lynn Olzak,Keith White, and Ewen King-Smith for helpful discussions on migraine,functional imaging limitations, and retinal sampling; and Bard Ermentrout and Whitman Richards for comments on an earlier version of the manuscript. Correspondence concerning this article should be addressed to Vincent A. Billock, NRC, Room 210, Building 248, Wright-Patterson Air Force Base, OH 45433. E-mail: vincent. billock. ctr@wpafb. af. mil Psychological Bulletin In the public domain 2012, Vol. 138, No. 4, 744-774 DOI: 10. 1037/a0027580 744