gratings; Kelly, 1983). To reemphasize, unlike achromat-ic information, which is also transmitted by magno units,color is carried by P cells with sustained temporalproperties. If retinogeniculate mechanisms are solelyresponsible for fading of stabilized images, one wouldexpect color to be affected less by stabilization relative tostabilized luminance signals, not more. (3) Fading of Dynamic Images is Paradoxical to a Peripheral Mechanism Explanation Empty figures defined by a twinkling random dot back-ground fade if fixated steadily (Spillman & Kurtenbach,1992). Dynamic image fading cannot be due to transientperipheral mechanisms; fading is actually faster fordynamic stimuli than for static noise. This suggests a central mechanism that requires coherent modulation to extract kinetic edges. Four Lines of Evidence that Central Mechanisms Mediate Stabilized Image Phenomena (1) Effects of Binocular and Other Sensory Interactions(a) Stabilized monocular image fading can be reversed by similarly patterned stimulation of the other eye(Cohen, 1961), especially if placed on correspondingpoints of the stabilized and unstabilized retinae (effec-tiveness drops off monotonically with retinal disparity). In dichoptic presentation, modulating the background eye slows fading of the image in the stabilized eye(Gerling & Spillmann, 1987). (b) Conversely, fading ofa stabilized image makes unstabilized stimuli in theother eye less visible (Krauskopf & Riggs, 1959). (c)Even after a stabilized image has disappeared in one eye,it can be combined with an unstabilized duplicatemoving image in the other eye, to yield a sensation of motion in depth (Crane, 1994). (d) Some stabilized images induce an abrupt absolute blindness (Billock,Gleason, & Tsou, 2001; Ditchburn, 1973). Both blind-ness and recovery are binocularly simultaneous, imply-ing a central mechanism. (e) Visibility of stabilizedimages can be maintained by listening to auditorystimuli; the effect decays to baseline in about 15 min ifthe stimulation is not varied, suggesting a role for attention (Ditchburn, 1973). (2) Fragmentation Davies (1973), Evans (1965), and Pritchard, Heron, and Hebb (1960) find complex stabilized images dis/reap-pear, not as wholes but in fragmented forms (forillustrations of fragmentation percepts, see Billock & Tsou, 2004). This image fragmentation is not explained by the properties of peripheral neurons (e. g., fragmen-tation can eliminate contours that are present only in abinocular image, but not in the separate retinal images;Evans & Wells, 1967). Moreover, image fragments that wax and wane together obey Gestalt-like rules (Evans,1965, 1967): (a) Short lines appear and disappear as aunit. Fragmentation probability increases with linelength; for foveal vision fragmentation is likely for linelengths of roughly 45 arc min (an order of magnitudelarger than foveal retinogeniculate receptive fields),suggesting a range for the cortical mechanisms in-volved. (b) Disappearance and reappearance of parallel lines are correlated. (c) Random patterns are moreunstable than meaningful ones. (d) Angular patternsare more fragmented and perceptually unstable thanrounded patterns. (e) Fragmentation and fading in oneregion of a field is strongly affected by activity inneighboring regions. (f) Complex patterns are morelikely to fragment than simple ones. Simulations show that Gestalt-like grouping rules are emergent properties of cooperative segmentation and binding networks(Horn & Opher, 2000; Wang & Terman, 1997; Sporns,Tononi, & Edelman, 1991). Interestingly, some modelsof synchronized segmentation tend to fragment imagesif the spatial properties of the model do not pool over alarge enough set of units to discount noisy stimulusinputs (Terman & Wang, 1995). Similar fragmentation is reported by some subjects with migraine (mosaic vi-sion; Sacks, 1995) and amblyopia (especially for highspatial frequency stimuli; Hess, Field, & Watt, 1990); alink between amblyopia and neural synchronization issuggested by Roelfsema, Ko ¨nig, Engel, Sireteanu, and Singer (1994) finding that cortical cells are normal inspatial frequency selectivity, but cells driven by theamblyopic eye do not synchronize well, especially for high spatial frequency stimuli. (3) Filling-in of Stabilized Images If a broken figure is stabilized and fades—then regener-ates—the reappearance of the figure is often marked bycompletion of the break (Ditchburn, 1973). Moreover,Cardu, Gilbert, and Stabel (1971) find that objects made up of dashed lines often (45% of the time) exhibit completion just before fading. This effect resemblesthe completion of images broken by scotomas and iscompatible with binding algorithms that incorporatecooperative interactions. (4) Stabilization Effects on EEG Correlated to Perceptual Phenomena Although no stabilization experiment has looked for synchronization of g-band activity, several studies find that the relative level of a-rhythm activity increases when stabilized images disappear or when perceptualblanks occur during Ganzfeld viewing (Keesey & Nich-ols, 1967, 1969; Lehmann, Beeler, & Fender, 1967,Lehmann, Beeler, & Fender, 1965; Evans & Smith, 1964). Following stabilization, power in the EEG a-band Billock and Tsou 1043