some theoreticians use the mapping from geometric patterns on retina to stripes on V1 to argue that these mappings evolved toallow the cortex to exploit certain regularities and invariancesin image processing (Caelli, 1977; Dodwell, 1991; Schwartz,1977, 1980); if other sensory cortices exploit similar ap-proaches, it should be possible to derive what forms would beof special significance to the information processing takingplace in these sensory cortices. Third, the insights from the firsttwo lines of research could be used to design functional imagingstudies of nonvisual cortices, using nonvisual stimuli to revealcortical stripe formation, as Tootell, Switkes, Silverman, and Hamilton (1988; Tootell et al., 1998) did with visual cortex. This then would provide feedback to the researchers engaged inthe other two lines of research. Conclusions It is in this sense, finally, that migraine is enthralling; for it shows us, in the form of a hallucinatory display, not only an elemental activityof the cerebral cortex, but an entire self-organizing system, a universalbehavior, at work. It shows us not only the secrets of neuronalorganization, but the creative heart of Nature itself. —Oliver Sacks (1995b) The variety of hallucinatory patterns generated by self-organized cortical pattern formation is dauntingly broad, and ourunderstanding of these phenomena is still incomplete, but thatshould not keep us from celebrating the remarkable multidisci-plinary convergence described above. Until recently, hallucina-tions were treated as disparate curiosities of vision, ill-conditionedfor serious study. Today, all evidence suggests that elementaryhallucinations are grounded in nonlinear dynamic neuronal net-works and that hallucinations provide a window into the innerworking of those networks. Geometric hallucinations are linked tophosphenes and migraine fortifications by their common spatialand temporal scales, by various theoretical commonalities, byinducers that lead to various combinations of these hallucinations,and by interactions between the many conditions that induce them. Further advances are expected from developments in functionalimaging and sophisticated mathematical models. These studiesshould illuminate ordinary visual processing as well; the interac-tions of geometric hallucinations with one another and with phys-ical stimuli obey many simple rules found in ordinary stimulus-driven vision and operate on familiar spatial and temporal neuralscales. Specifically, the behavior of hallucinatory percepts sug-gests that neural pattern formation in visual cortex is governed bythe same cortical properties of localized processing, lateral inhi-bition, simultaneous and sequential contrast, saccadic interference,and perceptual opponency that govern ordinary vision. Hallucina-tory patterns arise from ordinary cortical mechanisms driven inextraordinary ways. References Abraham, H. D., & Duffy, F. H. (2001). EEG coherence in post-LSD visual hallucinations. Psychiatry Research: Neuroimaging, 107, 151-163. doi: 10. 1016/S0925-4927(01)00098-1 Aleman, A., & Larøi, F. (2008). Hallucinations: The science of idiosyn-cratic perception. Washington, DC: American Psychological Associa-tion. doi:10. 1037/11751-000Allefeld, C., Pu ¨tz, P., Kastner, K., & Wackermann, J. (2011). Flicker-light induced visual phenomena: Frequency dependence and specificity ofwhole percepts and percept features. Consciousness and Cognition, 20, 1344-1362. doi:10. 1016/j. concog. 2010. 10. 026 Amari, S. (1977). Dynamics of pattern formation in lateral-inhibition type neural fields. Biological Cybernetics, 27, 77-87. doi:10. 1007/ BF00337259 Arieli, A., Sterkin, A., Grinvald, A., & Aertsen, A. (1996, September 27). Dynamics of ongoing activity: Explanation for the large variability inevoked cortical responses. Science, 273, 1868-1871. doi:10. 1126/ science. 273. 5283. 1868 Aurora, S. K., Ahmad, B. K., Welch, K. M., Bhardhwaj, P., & Ramadan, N. M. (1998). Transcranial magnetic stimulation confirms hyperexcit-ability of occipital cortex in migraine. Neurology, 50, 1111-1114. Re-trieved from http://neurology. org/content/50/4/1111. full Aurora, S. K., & Wilkinson, F. (2007). The brain is hyperexcitable in migraine. Cephalalgia, 27, 1442-1453. doi:10. 1111/j. 1468-2982. 2007. 01502. x Babb, T. L., Halgren, E., Wilson, C., Engel, J., & Crandall, P. (1981). Neuronal firing patterns during the spread of an occipital lobe seizure tothe temporal lobes in man. Electroencephalography and Clinical Neu-rophysiology, 51, 104-107. doi:10. 1016/0013-4694(81)91513-3 Bak, M., Girvin, J. P., Hambrecht, F. T., Kufta, C. V., Leob, G. E., & Schmidt, E. M. (1990). Visual sensations produced by intracorticalmicrostimulation of the human occipital cortex. Medical and Biological Engineering and Computing, 28, 257-259. doi:10. 1007/BF02442682 Baker, T. I., & Cowan, J. D. (2009). Spontaneous pattern formation and pinning in the primary visual cortex. Journal of Physiology (Paris), 103, 52-68. doi:10. 1016/j. jphysparis. 2009. 05. 011 Becker, C., & Elliot, M. A. (2006). Flicker-induced color and form: Interdependencies and relation to stimulation frequency and phase. Con-sciousness and Cognition, 15, 175-196. doi:10. 1016/j. concog. 2005. 05. 004 Becker, C., Gramann, K., Muller, H. J., & Elliott, M. C. (2009). Electro-physiological correlates of flicker-induced color hallucinations. Con-sciousness and Cognition, 18, 266-276. doi:10. 1016/j. concog2008. 05. 001 Bestehorn, M., & Haken, H. (1991). Associative memory of a dynamical system: The example of the convection instability. Zeitschrift fu ¨r Physik B, 82, 305-308. doi:10. 1007/BF01324341 Bexton, W. H., Heron, W., & Scott, T. H. (1954). Effects of decreased variation in the sensory environment. Canadian Journal of Psychology/ Revue canadienne de psychologie, 8, 70-76. doi:10. 1037/h0083596 Billock, V. A., Cunningham, D. W., Havig, P. R., & Tsou, B. H. (2001). Perception of spatiotemporal random fractals: An extension of colori-metric methods to the study of dynamic texture. Journal of the Optical Society of America: A. Optics, Image Science & Vision, 18, 2404-2413. doi:10. 1364/JOSAA. 18. 002404 Billock, V. A., Gleason, G., & Tsou, B. H. (2001). Perception of forbidden colors in retinally stabilized equiluminant images: An indication ofsoftwired cortical color opponency? Journal of the Optical Society of America: A. Optics, Image Science & Vision, 18, 2398-2403. doi: 10. 1364/JOSAA. 18. 002398 Billock, V. A., & Tsou, B. H. (2004). What do catastrophic visual binding failures look like? Trends in Neurosciences, 27, 84-89. doi:10. 1016/ j. tins. 2003. 12. 003 Billock, V. A., & Tsou, B. H. (2006). Fechner-Benham subjective colors do not induce Mc Collough after-effects. Spatial Vision, 19, 161-172. doi:10. 1163/156856806776923443 Billock, V. A., & Tsou, B. H. (2007). Neural interactions between flicker-induced self-organized visual hallucinations and physical stimuli. PNAS: Proceedings of the National Academy of Sciences, USA, 104, 8490-8495. doi:10. 1073/pnas. 0610813104 Billock, V. A., & Tsou, B. H. (2010, February). Seeing forbidden colors. 766 BILLOCK AND TSOU