ular vision, thresholds were higher, and the function's minimum was (for two thirds of observers) shifted to higher frequencies. Slightly lower thresholds for binocular vision are expected oninformation theoretic grounds (by a factor of √2), which con-trasts with the near absolute requirement of binocular pressurefor ocular pressure hallucinations. Remole's results lie mostlybetween these extremes: At some frequencies, binocular thresh-olds are an order of magnitude lower than for monocular vision,while at other frequencies, the binocular advantage drasticallynarrows. Remole also examined wavelength effects: Flickeringyellows and reds required less luminance than blues and greensto induce hallucinations, and the threshold differences for redversus green and blue versus yellow were highest at about 14Hz; Remole believed this implicates color opponency in hallu-cinatory pattern formation. Reports vary on the qualia of col-ored hallucinations, with some subjects finding the colors un-earthly (W. G. Walter, 1956) and either too vivid or not vividat all (we have found pastel colors in our experiments, likethose induced in Fechner-Benham subjective color; Billock &Tsou, 2006). As W. G. Walter (1956) pointed out, on some levelthe colors are real enough: Brady (1954) conditioned subjects togive a galvanic skin response to red lights and found thatflicker-induced red led to the conditioned response. Although W. G. Walter (1956) made much of the strobo-scope's very short pulse width, Smythies (1960) tested variouswaveforms and light-to-dark pulse widths and concluded thatthe waveform's exact shape has little effect, if temporal alter-ation of light and dark is vigorous. In a foreshadowing ofexperiments using visual noise (Billock & Tsou, 2007; Mac-Kay, 1965), Smythies (1960) noted that random flicker alsoworks well. Smythies reported that sudden changes in theflicker frequency while hallucinating can cause the hallucinatedfeatures to become finer/more numerous with higher temporalfrequency and coarser/less numerous with lower temporal fre-quency. Smythies (1960) also tried stimulating the two eyeswith out-of-phase flicker and noted that the perceptual effectswere similar to doubling the frequency of stimulation. Given Remole's (1973) results, it would be interesting to know whateffect this has on the pattern-formation thresholds. When onlyone eye is stimulated, hallucinations vary with the state ofbinocular rivalry. Light-phase hallucinations (the geometricpercepts addressed throughout this article) occur during rivalryperiods when the stimulated eye is dominant, while dark-phasepercepts (swirling amorphous patterns, boiling, ameba-likeblobs, etc. ) occur when the patched eye is dominant (Brown &Gebhard, 1948; Smythies, 1959a). Smythies (1959a) arguedthat the patterns are visualizations of the noise in the patchedeye. This is supported by one study of spatiotemporal fractalnoise, which gave rise to percepts of similar character (Billock,Cunningham, Havig, & Tsou, 2001). They also resemble someof the smoke-like contours reported for some apparent-motion-induced hallucinations (Billock & Tsou, 2007) and for somenoise stimuli (Fiorentini & Mac Kay, 1965; Mac Kay, 1965). Other Photopic-Induced Hallucinations Motion-induced hallucination. There are at least two other kinds of photopically induced patterns: motion-induced hallucina-tions and Mac Kay effects. Among motion-induced illusions are theillusory and supernumerary spokes reported by Tynan and Sekular (1975); Holcombe, Macknik, Intrilgator, Seiffert, and Tse (1999);and Purves, Paydarfar, and Andrews (1996). A particularly spec-tacular motion-induced pattern-forming effect was discovered by Mayzner (1975), while manipulating the marquee light illusion. Mayzner linked flashing lights into a closed circular or squarecircuit (see, e. g., Figure 3); the flashing icons appear to moveabout the circuit, taking corners like ducks in a shooting gallery. When four or five equally spaced illusory-moving lights areviewed in the dark, the imaginary space defined by the movementfills with illusory colors and rotating shapes. The two studies toaddress this phenomenon only differ in the nature of the color. Mayzner described a succession of vibrant colors filling the space. Billock and Tsou (2007) found smoky swirls of more pastel colors,often green, purple, and gray, interlaced into a pulled-taffy-liketexture. This texture evolves into a storm-like pattern, with grayarms reaching out toward the icons. The arms straighten out into a Figure 3. An artistic rendition of a Mayzner illusion induced by illusory motion. Illusory motion is induced by turning icons on and off in turn (themarquee light illusion). Here, a square is defined by 20 such icons. If everyfourth icon is lit and then extinguished, followed by the icons clockwise tothem, and so on, the percept is of five icons moving around an imaginarysquare. Active icons are depicted in white, newly extinguished icons (stillfaintly visible due to visual persistence) are depicted in gray, and othericons are outlined in dashes. When viewed in the dark, the center of thesquare fills with a smoky or taffy-like colored texture that resembles astorm pattern. During viewing, radial arms appear in the storm pattern andreach out toward the moving icons, forming a five-bladed hallucinatorypropeller. However, the number of blades may be as great as the numberof visible icons (both the active icons and the ones visible because of visualpersistence). From “Neural Interactions Between Flicker-Induced Self-Organized Visual Hallucinations and Physical Stimuli,” by V. A. Billockand B. H. Tsou, 2007, PNAS: Proceedings of the National Academy of Sciences, USA, 104, p. 8493. Copyright 2007 National Academy of Sci-ences, USA. Reprinted with permission. 750 BILLOCK AND TSOU