derived from segmentation models that rely on balancing cooperation and competition [15-18]. Competitive net-works deﬁne the locations of borders and edges; coopera-tive networks reinforce mechanisms that agree about thepresence of a border, and ﬁll-in areas between borders. Ifthe balance between these mechanisms breaks down, part-binding should fail in distinctive ways. If cooperativemechanisms are too strong, then the network will settleon a common value for all points in an image. If this occurs slowly enough in an already segmented image, the image would appear to fade, or gray-out, or melt. Conversely, if competition is too strong or cooperation istoo weak, the image should appear to fragment. Thisoccurs for some neural networks that use nearest neighborconnectivity [17,18]. Binding-like failures in retinal stabilization: fading and blackout Images can be mechanically stabilized from eye move-ments, either by mechanical coupling to a contact lens, orvia an eye-tracker [14,19]. When an image is stabilized, the most common perceptual effect is a gradual fading ofthe image. Similar effects can be obtained using steadyﬁxation, afterimages or ganzfelds. If the image starts outas a rather textureless surface (a ganzfeld), then gradually all aspects of the percept are lost and the subject perceives a uniform, depthless gray (eigengrau) even if the ganzfeldwas initially colored. Fading is usually attributed to thetransient properties of certain retinogeniculate units-thetransient M cells. This explanation is inadequate on atleast two grounds: (i),80% of retinogeniculate units belong to the P cell pathway, which has a sustained response to both achromatic form and color stimuli [20], and (ii) most visual mechanisms (including achromaticform, color and depth) receive inputs from P cells and canbe driven by P cells alone, if M cells are silenced [21]. And this information is utilized by higher mechanisms; forexample, the depth and multistability of a Necker cubeare just as salient when stabilized, so long as it remains visible [22]. Although P cells signal the attributes of stabilized images to cortex (a necessary condition for corticalprocessing), this is not always a sufﬁcient condition forveridical perception. Consider some properties of stabil-ized images, paradoxical to P cell function, implicatingcentral mechanisms in stabilized-image fading. For example, P cells have sustained responses for high spatial frequencies and high contrasts (unlike M cells, whichbecome more transient at high contrasts [23]); yet Purkinje's tree (the high-contrast pattern of blood vesselshadows on the retina) fades unusually rapidly, and fadesfastest (as fast as 80 ms) in the parafovea, where P cells arepresent in high densities [24]. Similarly, although P cells have strong sustained responses to chromatic stimuli, stabilized color images fade faster and more completelythan achromatic stimuli [25,26]. Actually, many attributes of visual fading are more consistent with central than peripheral mechanisms. Forexample, the fading of a stabilized image in one eye can beslowed or reversed by modulating the stimulation of theother eye [27]. Conversely, fading of a stabilized image inone eye makes images viewed by the other eye less visible. Remarkably, even if the stabilized image in one eye has completely disappeared, it can still be binocularly fused with a duplicate moving image in the other eye to yield aparadoxical percept of motion in depth [14]. Finally, fading of images can be slowed or reversed by stimulatingattention or stimulating the other senses [19]. Blackout is a related, albeit more dramatic effect. Sometimes imagestabilization, even if only in one eye, induces an extremely abrupt, binocularly simultaneous visual blackout. Recov-ery can be initiated by a blink. Descriptions of this 'blackﬁeld' as 'more black than black' do not do it justice [19]. Our subjects describe it as 'like someone cut my optic nerves,and I don't have a visual system anymore' [13]. Ditchburn attributes the black-ﬁeld effect to a fading mechanismcombined with a cortical inhibitory feedback loop that runs out of control [19]. Binding-like failures in retinal stabilization: image fragmentation From a binding failure perspective, the most interestingretinal stabilization effect is fragmentation [28-30] (Figures 1,2 ). In general, complex patterns are more likely to fragment than simple ones, angular patterns fragmentmore than round ones, and parallel lines tend to disappear and reappear together [31,32]. Sometimes the fragmenta-tion takes the form of a series of breaks in the image(Figure 1 ), whereas at other times, entire portions of the ﬁgure disappear and reappear ( Figure 2 ). The probability of fragmentation versus unitary fading grows with linelength and reaches equal probability for lines subtending45 0, suggesting a range for the neural processes involved [32]. Interestingly, if a stabilized image with gaps in its structure fades and then reappears, the gaps in the imageare often ﬁlled-in [19]. Moreover, ﬁlling-in can occur even as the overall image is fading [33]. When images fragment, they tend to fade and revive in clusters that are cognitivelymeaningful, and follow Gestalt-like rules of association(Figure 2 ). These processes act on the central image; fragmentation can eliminate contours present only in a binocular image and not in its retinal precursors [34]. Similar fragmentation can occur during migraine attacks[35] and in strabismic amblyopia (a cortical visual defect induced by poor coordination of the eyes during visualdevelopment), especially for high spatial frequency stimuli[36](Figure 1 ). The connection of fragmentation phenom-ena to amblyopia is especially interesting because, in cat, cortical cells driven by the amblyopic eye are poorlysynchronized compared with cell populations driven by thenormal eye-especially for high spatial frequency stimuli[37]. No studies of synchronization during stabilization have been made. However, some early electroencephalo-gram (EEG) studies report that alpha-rhythm (9-10 Hz) power increased,0. 7-1. 0 s before disappearances; higher frequencies (including the gamma band, which is nowimplicated in binding) were suppressed until,1 s before spontaneous reappearances [38,39]. Oddities of equiluminant images Like stabilized images, pictures in which hues have equalluminance are less perceptually salient than natural Opinion TRENDS in Neurosciences Vol. 27 No. 2 February 2004 85 www. sciencedirect. com