mation stretches the horizontal cell layer, depolarizing on-center bipolar cells and hyperpolarizing off-center cells. Continued deepocular pressure (for more than about 40 s) results in a temporaryblindness via ischemia (like applying a choke hold directly to theretina)—a useful technique often employed by psychophysicists. In the time between phosphene induction and eventual blindnesslies an interesting perceptual phenomenon: Sustained simultaneousbinocular pressure evokes complicated Klu ¨ver-like percepts, like the lattice spiral in Figure 1G (Tyler, 1978). The requirement forbinocular stimulation is intriguing; the instructive exception is thatfor some amblyopic subjects, stimulation of the good eye is suf-ficient to induce geometric forms (Gru ¨sser & Landis, 1991. ) Most pressure percepts are not consistent with the entopic (intrinsicanatomical) retinal structures posited by Purkinje (1819/1823), andtheir commonality with other geometric hallucinations suggeststhat pressure-induced excitation drives a cortical pattern-formingmechanism. Electrical Stimulation of Retina After Galvani's dramatic demonstrations of electrically in-duced muscle contraction and Volta's invention of the battery,it was not long until electric current was applied to the head andespecially the eye. As for so many visual effects, Jan Purkinje(1819/1823) led the way. Light sensations were generally re-ported at onset and offset of current. Some studies placed one electrode on the roof of the mouth and the other on the fore-head; others applied current directly to the eye, sometimesimmersing the eyes in salt water and passing the current thoughthe solution. Vivid colors, phosphenes, and patterns were pro-duced in these experiments. At high frequencies, large coloredcrescent, horseshoe, smoke-ring and ameba-shaped phosphenesappear, which persist to stimulation rates of 100 Hz in the darkand up to 210 Hz in light-adapted eyes (Wolff, Delacour,Carpenter, & Brindley 1968); the retina can follow higher rateswith electricity than light, in part because the photochemicalbottleneck in the photoreceptors is bypassed. Wolff et al. (1968)found that geometric patterns seen for 10-40 Hz sinusoidalelectrical stimulation were equivalent to those seen for flicker-ing light. There is an understated heroic quality to many ofthese early experiments, which mostly used the investigators assubjects. Brindley (1955) noted that his experiment, whichapplied the electrodes directly to the conjunctiva of the eye,required topical anesthesia; despite this, the experiment wasdifficult because the current through his anesthetized eye spreadto other nerves and made his teeth ache severely. Alternately, electric retinal stimulation can produce effects unique to retinal dynamics. Carpenter (1973) electrically stimu-lated the eye with 100 Hz AC current and found that if a darkobject passes through the visual field, a series of thin light illusorycontours form in the passing object's wake. These contours obeyfour rules: (a) The illusory contours disappear promptly when thecurrent is turned off. (b) One contour forms for each cycle of thealternating current completed while the moving object transitsthe visual space. (c) Half of the contours move with the wake; theother half move against it. (d) If two contours collide, they neitherpass through one another nor mutually annihilate, but rather, theymerge to form loop-like structures. Drover and Ermentrout (2006)modeled the formation of such contours by populations of gap-coupled neurons responding to every other cycle of the drivingfrequency: Some contours are generated by even-numbered stim-ulus cycles and others generated by odd-cycles, each with a dif-ferent motion bias as they form traveling waves. These wakecontours are reminiscent of Charpentier's bands (Mc Dougall,1904): a series of three to four afterimages produced over about 50ms when a bright bar is moved in the dark. This periodicitycorresponds to a sampling rate of at least 60 Hz. Drover and Ermentrout's half-sampling mechanism for wake contours, if ap-plied to the 120 Hz oscillations found in the electroretinogram(ERG; King-Smith, Loffing, & Jones, 1986), might account for Charpentier's bands as well. Electric Stimulation of Cortex (Including TMS) High voltages applied to the scalp near visual cortex produce diffuse cortical stimulation, creating textured phosphenes sev-eral degrees across (Merton & Morton, 1980). Knoll, Kugler,Eichmeier, and Höffer (1962) used temporal electrodes drivenby 5-100 Hz electric current to induce geometric patterns in 46observers similar to patterns seen in flickering lights and indirect electric cortical stimulation experiments (Penfield &Rasmussen, 1950). Many of Knoll et al. 's subjects saw thehallucinatory patterns better when light-adapted, a fact notedrepeatedly for other hallucinatory inducers. Eichmeier and Nie-Figure 2. An interesting interpretation of a migraine attack like that depicted in Figure 1H (a superposition of several migraine fortificationpatterns seen by one subject during the course of an attack). Each verticalarc in Figure 1H represents one snapshot of the illusion. Here, Richards(1971) has connected features from successive fortification patterns, re-vealing a hexagonal-like pattern of underlying activity that closely resem-bles the hexagonal packing of cortical orientation hypercolumns revealedby neuroanatomy. From “The Fortification Illusions of Migraines,” by W. Richards, 1971, Scientific American, 224 (5), p. 93. Reprinted with permis-sion. Copyright 1971 Scientific American, a division of Nature America, Inc. All rights reserved. 748 BILLOCK AND TSOU