colors. The border collapsed completely and the two colored sides appeared to run together, like diffusing inks or melting waxes, creating new color combinationsundreamt of in Hering's philosophy. In some cases, thesemanifest as a gradient which ran from red on one side togreen on the other, with every possible shade of reddishgreen in between. In other cases, subjects report seeing aspatially homogeneous reddish green, the red and greencomponents of which were as compelling as the red andblue components of a purple. We modeled this opponencyloss in terms of a cortical competition model whosecompetition (between color-labeled neurons) is disabled[13]. Why this should occur is not clear, but there is a model of cortical competition, whose competition is gated bygamma-band oscillations [48]; this is the frequency range that is implicated in many binding studies and thefrequency range in EEG that is lost during stabilization-induced fading and fragmentation [38,39]. Electrophysiological testing of part-binding failures We argue that phenomena such as border disappearance, fragmentation, diffusive color spreading, and linkingfailures of equiluminous and stabilized images are examples of part-binding failures, and that these particu-lar failure conditions are clues that part-binding relies onachromatic mechanisms reinforcing segmentation bycolor, and on motion-driven mechanisms reinforcingsegmentation by achromatic form. Testing these ideaswill require some notion of what the electrophysiologicalcorrelates of binding are and how they could be detectednon-invasively in intact responding humans. Becausegamma-band activity is implicated in binding processes,the loss of high-frequency EEG power that accompaniesstabilized-image fragmentation [38,39] is suggestive but not deﬁnitive. One potential problem is that some sourcesof gamma-band power in EEGs might be unrelated tobinding. Another potential problem is that our bindingfailures might actually be incorrect bindings and thereforenot distinguishable by EEG. Recent experiments have castlight on both questions, by using fragmented and camou-ﬂaged images in which subjects could eventually learn to see coherent embedded objects [10,12]. (These studies are especially relevant here because the fragmentation-to-order shift is the complement to the order-to-fragmenta-tion phenomena we studied. ) There was gamma-bandactivity whether subjects saw the coherent picture or not,but there were two differences in EEG when gestaltperception was obtained: the temporal output of widelyspaced electrodes became more correlated (consistent withsynchronization of widely spaced neural mechanisms [12]) and a second kind of gamma-band response, the 'inducedgamma response', was recorded. On every physicallyidentical stimulus presentation there is a gamma-bandresponse that is phase-locked to the camouﬂaged orfragmented stimulus, but only on trials where the gestaltpercept is seen does a second signature emerge thatconsists of gamma-band bursts in variable phase to theactual stimulus [10,12]. We therefore predict that EEG signatures of binding, such as the induced gamma response, will be reduced during binding-like failures induced by retinal stabilization or equiluminance, andthat these signatures will be further minimized oreliminated during the more dramatic perceptual effectscreated by combining stabilization and equiluminance[13]. A sophisticated version of this experiment could be done using the methods of Billock et al. [13], but the simpler methods of Buck et al. [44]might well sufﬁce. Acknowledgements We thank Kenneth Blum, Scott Kelso, Oliver Sacks, Wolf Singer and Anne Treisman for particularly helpful suggestions and remarks. References 1 Treisman, A. (1998) Feature binding, attention and object perception. Philos. Trans. R. Soc. Lond. B Biol. Sci. 353, 1295-1306 2 von der Malsburg, C. (1999) The what and why of binding: the modeler's perspective. Neuron 24, 95-104 3 Horn, D. and Opher, I. (2000) Temporal segmentation and binding in oscillatory neural systems. In Oscillations in Neural Systems (Levine, D. S. et al., eds), pp. 201-216, Erlbaum Figure 3. An image used in the stabilized-image studies of Billock et al. [13]. The image was stabilized on the retina and viewed through an unstabilized aperture. This arrangement keeps the whole image from fading [14]but does not protect the border between red and green ﬁelds. Typically, portions of the border melt or fade and the opponent colors tend to ﬂow and compete, creating complex illusory textures. Sometimes the two sides will switch colors, or even rotate 90 8. This phenomenon resembles an illusory conjunction of location and color (a feature-binding failure). If the two sides are made equal in luminance, the border disap-pears and the colors ﬂow together to form mixture colors of reddish-green, inviolation of Hering's law of color opponency. Sometimes these colors are uniform, other times they form a smooth gradient ranging from reddish green to greenish red. Similarly, stabilizing an equiluminant blue-yellow bipartite ﬁeld results inperception of bluish-yellow forbidden colors. These phenomena can be modeled by a breakdown of competitive interactions between some color-labeled cortical units [13]. 1 4 8and 24 8are degrees of visual angle for which the stabilized stimulus subtended horizontally and vertically, respectively. Opinion TRENDS in Neurosciences Vol. 27 No. 2 February 2004 88 www. sciencedirect. com