hot and falling cold fluid, which from above appear like the stripes in the Ermentrout-Cowan model. As in the Ermentrout-Cowanmodel, there is no preferred orientation to these stripes. However,even a single line of injected fluid can dictate the final orientationof the entire system (Bestehorn & Haken, 1991). Billock and Tsou(2007) adapted this idea to flicker-induced hallucinations, by plac-ing small fan-shaped or bull's-eye patterns in the midst of auniform flickering field. They expected the hallucinatory patternsto grow from the biasing stimulus, like a geometric seed dictating crystal growth in a supersaturated solution. Instead, they found aninteresting perceptual opponency between orthogonal hallucina-tory geometries: Flickering around a small physical bull's-eyepattern induces a hallucinatory rotating fan shape in the flicker,whereas flickering around a small fan shape induces hallucinatoryconcentric circular forms that can wobble and pulsate (see Figure11). Some patterns are more visible than others: Induced fan-Figure 10. Spontaneous cortical pattern formation in cats. Top: Single-frame images of voltage-dependent dyes on the visual cortex evoked by oriented physical stimuli (Ringach, 2003). Bottom: Similarity of sponta-neous states arising in sensory-deprived cats to the activity evoked by oriented stimuli (Kenet, Bibitchkov,Tsodyks, Grinvald, & Arieli, 2003). a: Cortical activity evoked by presentation of vertical gratings, averagedover 160 frames. b: Single frame of spontaneous activity resembling the activity in Panel a. c: Single frame ofimaged neural activity evoked by vertical grating, shown for comparison. Top panel from “Neuroscience: Statesof Mind,” by D. L. Ringach, 2003, Nature, 425, p. 913. Reprinted by permission from Macmillan Publishers Ltd., copyright 2003. Bottom panels from “Spontaneously Emerging Cortical Representations of Visual Attributes,” by T. Kenet, D. Bibitchkov, M. Tsodyks, A. Grinvald, and A. Arieli, 2003, Nature, 425, p. 954. Reprinted by permission from Macmillan Publishers Ltd., copyright 2003. 760 BILLOCK AND TSOU