170mm x 244mm Collerton c10. tex V3-11/21/2014 5:28 P. M. Page 219 10 Geometric visual hallucinations and the structure of the visual cortex Jack D. Cowan Department of Mathematics, Department of Neurology, Committee on Computational Neuroscience, University of Chicago, Chicago Il 60637... the hallucination is... not a staticprocess but adynamic process, theinstabilityof which reflects an instabilityinitsconditions of origin Klüver(1966)inacommenton Mourgue(1932) 10. 1 Introduction Geometric visual hallucinations are more or less regular patterns seen in the visual field, that move with the eyes. They have been studied scientifically for almost 200 years, beginning with the work of Purkinje on flicker phosphenes in 1819, later reported in Purkinje (1918). However, the first major modern study was carried out by Klüver in the 1920's and summarized in Klüver (1966). Klüver observed first-hand the effects of peyote (otherwise known as mescal) and orga-nized the various images he saw into four classes, which he called form-constants (Figure10. 1). Theseclassescomprised(i)grating,lattice,fretwork,filigree,honey-comb,orchessboard;(ii)cobweb;(iii)tunnel,funnel,alley,cone,orvessel;and(d) spiral. Figures10. 2-10. 4showexamplesoftheseformconstants,whileexamplesof the other classes are shown in figures 10. 3-10. 5 Klüver also found that the colour, brightness, and movement of the imagery were particularly vivid and striking. The Neuroscienceof Visual Hallucinations, First Edition. Edited by Daniel Collerton,Urs Peter Mosimann and Elaine Perry. © 2015John Wiley& Sons, Ltd. Published 2015by John Wiley& Sons, Ltd.