stereogram, when a group of dots is segmented from the background, the pattern of dots deﬁnes a textured surfaceof common depth. Near equiluminance, depth is seen for the individual colored pixels but, unlike ordinary random dot stereopsis, the pixels seem to be separate rather thanpart of the same textured surface [41]. This is not a curiosity of stereopsis: 2D surfaces are not linked together when deﬁned only by hue. An image composed ofequiluminous patches does not appear uniﬁed but, rather, appears as patches of one color ﬂoating on the other [41,42]. In other cases, all sense of object and surface can be lost and the target is perceived as a 'jumble of lines' [41] or 'extremely confusing and hard to describe' [40]. More-over, the contrast of equiluminant images can seemunstable-Gregory [40]describes such images as 'jazzy'. In 1927, Liebmann (translated in Ref. [42]) reported that there is a 'critical zone [where] everything ﬂows... glim-mers... most everything is soft, jelly-like, colloidal. Often... parts which belong together in the normal ﬁgure now have nothing to do with one another. [It is] a world without ﬁrm things, without solidity. ' Such effects are mostsevere when the borders are deﬁned by tritan (S-cone) modulation: the borders of the images can appear to melt like hot wax and distort like pulled taffy (reminiscentof Liebmann's comment), and the colors diffuse into oneanother in the same way as spilled inks, blending into continuous color gradients [13,44]. There are interesting local boundary effects: fading is sometimes localized to particular features or regions [40]and when equiluminant features or edges fade away, they are sometimes replacedwith an illusory achromatic border [40,41] ). Cross-modal reinforcement: what is lost in stabilized and equiluminant images?We have discussed how a balance of cooperation and competition is required to produce reliable segmentation networks, and how these networks sometimes fail,especially for nearest-neighbor connectivity [17,18]. One way to avoid this is to allow the activity of a secondnetwork to reinforce the ﬁrst-in effect, the secondnetwork can act as a second set of nearest neighbors. Inthe segmentation literature, this reinforcement between different sensory modalities-called 'cross-modal con-struction' [45]-improves the precision and likelihood of both correct segmentation and feature binding [45-47]. I f part-binding mechanisms evolved to exploit this reinforce-ment, then its loss could compromise part-binding. Theevidence described in this article is consistent withreliance on cross-modal reinforcement of color segmenta-tion mechanisms by achromatic form mechanisms, and reliance on reinforcement of achromatic form segmenta-tion systems by mechanisms sensitive to motion ortemporal modulation. It takes two: combining equiluminance and stabilization makes everything worse If this cross-modal construction hypothesis is correct, thenwe would expect that compromising more feature systems would further worsen part-binding. This seems to be the case. A familiar example is the minimum border effect-the weakened border formed by an equiluminant bipartiteﬁeld can disappear completely when eye ﬁxation is steady[44]. A more dramatic example is our own experiment using stabilized opponent colors [13]; unlike the equilu-minant experiments already discussed, binding failures occur for any color combination. Perhaps the most estab-lished law in experimental psychology is that certain colorcombinations such as reddish green and bluish yellow arenever allowed; this observation formed the basis for Hering's theory of color opponency and much of moderncolor science. However, it had been reported that somesubjects saw these forbidden colors when colored bipartite ﬁelds such as those shown in Figure 3 were stabilized [14]. When we attempted to replicate these results, we foundthat the effect depended on the combination of equilumi-nance and stabilization. When we stabilized non-equilu-minant red-green or blue-yellow borders, we saw avariety of part-binding failures, which included effectswhere the border would disappear in places; the unbound colors both diffuse and compete, resulting in illusory textures such as red glitter on a ﬁeld of green or bluestreaks on yellow [13]. Sometimes, the two colored sides of the bipartite ﬁeld would appear to switch places or evenrotate 90 8(an effect reminiscent of Treisman's illusory conjunctions). Only when we made the two sides of thebipartite ﬁeld equiluminant did subjects report forbidden Figure 2. Some gestalt-like attributes of image fragmentation in retinal stabilization that implicate central mechanisms. (a-d) Fragmentation of monocular stabilized images. Reproduced, with permission, from Ref. [29]. Pictures on the left show unstabilized images and pictures to the right show stabilized percepts. Note that images tend to fragment and fade, such that the visible fragments follow rules. In (a) and (b), meaningful fragments are preserved. In (c), parallel fragments are pre-served. In (d), gestalt grouping rules are mostly followed. (e)Effects of stabilizing the monocular parts (left-eye image and right-eye image) of a binocular (fused) percept. Note that disappearance of image fragments is characteristic of the fused image, not the monocular forms, ruling out peripheral explanations. Reproduced,with permission, from Ref. [34]. (a) (b) (c) (d) (e) Left-eye image Right-eye image Fused image Opinion TRENDS in Neurosciences Vol. 27 No. 2 February 2004 87 www. sciencedirect. com