propeller shape rotating in time with the illusory movements of the flashing icons (see Figure 3). Mac Kay effects. Donald Mac Kay (1957a, 1957b, 1965, 1978) described a series of phenomena that have become known as Mac Kay effects, without much separate consideration of theirdiverse nature. Most of these effects involve some kind of oppo-nency between certain spatial forms. If a pattern of concentriccircles is steadily fixed for many seconds and then replaced with ablank field, a faint noisy fan-like shape can be seen as an after-image. Similarly, if the same pattern is viewed in flickering lightin an otherwise dark room, a fan-shaped pattern will be seensuperimposed on the concentric circles. Mac Kay treated fan shapesand concentric circles as complementary (opponent forms) insequential contrast with one another and interpreted the effects offlickering light as two phases of perception superimposed inone another: The physical concentric circles are seen during theflashes, and their complementary afterimage is seen during thedark phase. Similar superimposed patterns can be seen whileviewing closely spaced patterns under steady room light. Here, aflicker-like temporal stimulation is supplied by eye movementsacross the closely spaced features. This interpretation is reinforcedby the absence of this third form of the Mac Kay's hallucinatoryeffect under retinal image stabilization. Another variation of the Mac Kay effect was seen when the physical patterns were printedon transparencies and illuminated by dynamic spatial noise (e. g.,TV static) from behind. Mac Kay reported two forms of motion forthis stimulation: one that seems to flow along the transparencies'contours and one that seems to flow at right angles to thosecontours. The latter is like a Mac Kay opponent effect, but actualillusory contours are weak or absent. The strengths of the twoeffects seem to vary with the channel the TV noise is taken from,suggesting that the statistics of the noise matter. Billock and Tsou(2007) were able to strengthen the Mac Kay noise effect usingfractal noise; when viewed through concentric circles, hallucina-tory rotating fan shapes are seen; when viewed through fan-shapepatterns, pulsating hallucinatory circles are seen. Finally, Mac Kay(1965) and Fiorentini and Mac Kay (1965) noted that noise canaggregate into moving shapes under certain conditions: If framesof spatial noise are interspaced with frames of uniform luminance,the noise pixels seem to aggregate into maggot-like wrigglingforms. The effect is strongest in binocular vision and does notoccur if the noise is viewed by one eye and the blank frames by theother eye. Drug-Induced Hallucinations Hallucinations seen in the early stage of drug intoxications range from simple phosphenes to patterns as complicated as intri-cate Persian rugs. Klu ¨ver (1966) compared reports on drug-based hallucinations and found that most hallucinations fit into four basiccategories: (a) lattices (rhomboidal and hexagonal), (b) cobwebs,(c) radial and concentric forms (fan shapes and bull's-eye-likepatterns) whose three-dimensional versions resemble tunnels andfunnels, and (d) spirals. Figures 1A-1G show a variety of these Klu¨ver geometries. Klu ¨ver also noted that these same forms can be seen under many other conditions. Drug hallucinations are oftenreported to be very bright (similar in this respect to migrainefortification patterns) and vividly colored (in contrast to the pastelcolors usually seen in flicker-induced hallucinations). Paintingsand drawings of these vivid images show that the same forms are seen by independent observers across different cultures (Siegel &Jarvik, 1975). These drawings also provide evidence of a charac-teristic spatial scale (roughly 2 mm on cortex) for drug hallucina-tions, estimated by comparing the visual cortex's dimensions to thenumber of illusory periodic features. Hallucinogenic drugs gener-ally modulate one of four neurotransmitter-based mechanisms:acetylcholine (e. g., scopolamine), dopamine (amphetamine, co-caine), glutamate (ketamine, PCP) or serotonin (LSD, mescaline,psilocybin). For a useful recent review of hallucinogens and neu-rotransmitters, see Aleman and Larøi (2008). Not surprisingly,clinical conditions that affect these neurotransmitters can alsoinduce hallucinations; for example, about 25% of patients with Parkinson's disease (a dopamine defect; Wolters & Berendse,2001) report hallucinations, as do 60% of patients with Lewy bodydementia (an acetylcholine defect; Aleman & Larøi, 2008). As inepilepsy, the locations of neural defects and lesions matters: Lewybody dementia patients who have visual hallucinations tend toform more Lewy bodies in visually specialized areas of temporallobe than patients who are hallucination free (Harding, Broe, &Halliday, 2002). Sensory Deprivation and Charles Bonnet Syndrome In the 1950s, Donald Hebb was interested in the effects of conditions rumored to have been employed in brainwashing andcoerced confessions (Heron, 1961). Students in his lab discoveredthat systematically depriving the brain of differentiated sensoryinputs could lead to hallucination (Bexton, Heron, & Scott, 1954;Heron, Doane, & Scott, 1956). For example Heron et al. 's (1956)subjects wore translucent goggles to provide spatially unstructuredstimulation, had audition masked by ventilation fan noise, andwore cardboard sleeves to limit tactile stimulation. All 14 subjectsreported visual hallucinations—at first phosphenes and geometricforms but later some more complex hallucinations as well (thesame evolution reported for hallucinogenic drugs). Interestingly,studies in which the subjects were in darkness reported fewerhallucinations, and more time was often required before halluci-natory onset (cf. Vernon, 1963; Zubek, 1969), but other differ-ences between studies may have influenced the incidence of hal-lucination (Zuckerman & Cohen, 1964). Sometimes, these sensorydeprivation effects occurred as a side effect: About 39% of direc-tors of medically protective environments like laminar flow units(for treatment and prevention of infection) reported hallucinationsin their patients—the fourth most common side effect in this study(Kellerman, Rigler, & Siegel, 1977). Nature provides a clinicalversion of a tightly controlled sensory deprivation experiment—Charles Bonnet syndrome (CBS)—in which subjects who havevisual losses experience both simple and complex visual halluci-nations (ffytche, 2005; Wilkinson, 2004). In the literature, thecomplex illusions get more emphasis, but actually, the simplehallucinations seem to predominate. For example in Lepore's(1990) 59 patients with CBS, 63% had only elementary halluci-nations, and 27% had both elementary and complex hallucinations. Of the simple percepts, grids and lattices are common, especiallyfor losses of central vision (e. g., macular degeneration). Functionalimaging (f MRI) during these elementary hallucinations showsincreased activity in occipital cortex (ffytche, Howard, Brammer,Woodruff, & Williams, 1998); patients who hallucinate in color751 HALLUCINATIONS AND CORTICAL PATTERN FORMATION