model describes the overall changes in arc shape and apparent speed as it propagates across cortex; this behavior is generic forweakly excitable media and can be mimicked by reaction-diffusionmodels (Dahlem & Hadjikhani, 2009). A temporary scotoma(blind region) is left in the wake of the fortification arc's move-ment. Mapping the serrated arcs to cortical coordinates reveals thateach serration covers about 1 mm of cortex and that the arc movesat a constant and rather stately speed of 2-3 mm/min on cortex(Gru¨sser, 1995; Lashley, 1941; Richards, 1971; Wilkinson, Fein-del, & Grivell, 1999), requiring 20-25 min to traverse one entireside of striate cortex. Richards (1971) suggested that the angularity of fortifications was consistent with activation of hexagonally packed orientationhypercolumns as a trigger wave swept through them (see Figure 2). The packing that Richards predicted is strikingly similar to thecortical iso-orientation pinwheel organization ultimately revealedby neuroanatomy (Bonhoeffer & Grinvald, 1991; Swindale, Mat-subara, & Cynader, 1987). For other developments in fortificationmodels, see Dahlem, Engelman, Löwel, and Mu ¨ller (2000); Reggia and Montgomery (1996); and Schwartz (1980). When the psycho-physics of migraine are compared to the topographical mappingqualities of visual cortices, the likely cortical loci of migrainepercepts are Areas V1, V3a, and V8. Jagged arcs are consistentwith the orientation processing in V1, but there is no reason toassume that other visual areas cannot be activated during migraine,and if activated, there is no reason to assume that this activitycould not affect V1 via feedback. (Indeed, based on studies ofcortical spreading depression, the condition could spread over theentire occipital lobe of the affected hemisphere but could havedifficulty crossing prominent fissures between cortical areas, likethe parieto-occipital sulcus. ) Sacks (1995a) reported a range ofphenomena consistent with the activation of many sensory areas. Interestingly, Hadjikhani et al. (2001) had a subject with an un-usual exercise-induced aura—a drifting crescent-shaped cloud of TV-like noise—shown by functional magnetic resonance imaging(f MRI) to originate in V3a. (This percept resembled the twinklingtextures induced adjacent to a centrally viewed patch of TV noise;Tyler & Hardage, 1998. ) Hadjikhani et al. suggested that classicfortification illusions may arise in V1 and color effects in V8. Functional imaging also shows cortical thickening abnormalities inareas V3a and MT of the brains of migraineurs, which is interest-ing because MT is important in motion perception and migraineursare especially susceptible to visual motion-induced sickness(Granziera, Da Silva, Snyder, Tuch, & Hadjikhani, 2006). The slow movement of the fortification arcs suggests a diffusive-triggering process. The closest physiological analogue tothe spread of a migraine fortification arc (and its accompanyingscotoma) is a wave of cortical spreading depression, triggered inanimal preparations by an infusion of potassium. The depressionaspect is a matter of temporal scale: Initially, the spreading waveof extracellular potassium renders affected neurons briefly hyper-excitable, but as potassium concentration increases, the neuronsbecome so depolarized that further action is suppressed for alonger period. In humans, Wilkinson (2004) suggested that a“wavefront of neural excitation operating on intrinsic corticalnetworks is presumed to underlie the positive hallucinations andthe subsequent neuronal depression, the scotoma” (p. 308). Had-jikhani et al. (2001) found eight aspects of f MRI imageryduring migraine corresponding to known aspects of cortical Figure 1. Some characteristic elementary visual hallucinations. A-D: These LSD flashbacks painted by Oster (1970) come in circular, radial and spiralgeometries, three of the most common percepts cataloged by Klu ¨ver (1966) for many hallucinatory conditions. E: A proliferation of identical phosphenes (poly-opia) induced by THC and arranged in a spiral geometry (Siegel & Jarvik, 1975). F-G: Some more complicated lattice-like patterns produced by THC intoxication(Siegel & Jarvik, 1975) and by binocular pressure on the eyes (Tyler, 1978). H:Superposition of fortification patterns produced by migraine; actual patterns flashand move across retina (Richards, 1971). Panels A-D from “Phosphenes,” by G. Oster, 1970, Scientific American, 222 (2), p. 82. Reprinted with permission. Copy-right 1970 Scientific American, a division of Nature America, Inc. All rights reserved. Panels E-F from “Drug-Induced Hallucinations in Animals and Man,”by R. K. Siegel and M. E. Jarvik, in R. K. Siegel and L. J. West (Eds. ),Hallucinations (pp. 117 & unnumbered page [Color Plate 6] following p. 146), 1975, New York, NY: John Wiley & Sons. Copyright 1975 by John Wiley &Sons. Reprinted with permission. Panel G from “Some New Entopic Phenomena,”by C. W. Tyler, 1978, Vision Research, 18, p. 1637. Copyright 1978, with permission from Elsevier. Panel H from “The Fortification Illusions of Migraines,”by W. Richards, 1971, Scientific American, 224 (5), p. 90. Copyright 1971 by W. Richards. Reprinted with permission. 746 BILLOCK AND TSOU