What do catastrophic visual binding failures look like? Vincent A. Billock1and Brian H. Tsou2 1General Dynamics, Suite 200, 5200 Springﬁeld Pike, Dayton, OH 45431, USA 2US Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, USA Ordinary vision is considered a binding success: all the pieces and aspects of an image are bound together,despite being processed by many different neurons in several different cortical areas. How this is accom-plished is a key problem in visual neuroscience. The study of visual binding might be facilitated if we had ways to induce binding failures. A particularly interest-ing failure would involve a loss of the physical integrity of the image. Here, we identify conditions that induce such perceptual failures (e. g. the melting together ofequiluminant colored images and the fragmentation of retinally stabilized images) and we suggest that these should studied using electrophysiological measuresof binding. In visual cortex, many neurons are specialized for detecting different visual attributes such as form, color, motion and depth. Often these cells are physically separated-in some cases, they seem to aggregate by stimulus preference in different cortical areas or at different spots of the same cortical area. So how does thevisual system combine these responses to create uniﬁed and veridical percepts? A leading theory suggests that the stimulus-based responses of widely spaced neurons can bebound together by the temporal synchronization of their gamma band (30-90 Hz) activity. Although controversial, an impressive body of theoretical and electrophysiologicalevidence for this theory has accumulated [1-12]. Some of the most compelling binding studies compare perceptual and electrophysiological measures in humans [10-12]. T o further such efforts, it is necessary to understand what perceptually constitutes a binding success or failure, so that its electrophysiological correlates can be sought. There are two especially interesting aspects of the 'binding problem': feature-binding and part-binding. Feature-bind-ing correctly associates various features of imagestogether, for example so that we see a yellow school bus moving towards us, rather than yellowness, curves and angles, and movement, as in a dynamic abstract painting. Part-binding refers to correct construction of spatially coherent percepts, which need to be extracted (segmented)from the noisy and often ambiguous retinal image. To study feature-binding, Treisman introduced the notion of feature-binding failures called illusory conjunctions-under some circumstances, especially in peripheral vision,two features such as shape and color can be incorrectly associated with one another-and this has been very useful for psychophysical studies of feature-binding [1]. However, there have been no comparable studies of part-binding failures (except in brain lesions [8]). For electro-physiological studies it would be useful to be able to produce part-binding failures in normal humans using reversible and noninvasive methods. So, what would a really spectacular part-binding failure look like? Has anyone ever seen one? Do they occur in normal (brain-undamaged) observers? In addressing these questions, we realized that we have seen candidates for such bind-ing failures. Recently, we studied the perception of retinally stabil-ized colored images [13]. We coupled a motorized mirror to an optical eye-tracker (which measures eye movements using infrared reﬂections from the cornea and lens). Subjects view the stimulus as a reﬂection in the mirror. By deﬂecting the mirror to compensate for eye movements, the reﬂected image remains stationary on the retina [13,14]. The textbook account of these stabilized images is that they fade (because of transient temporal responses of some retinal mechanisms), and fade they do. However, they also become unstable and lose their spatial coherence: their internal structure can ﬂow about like ﬁgures made from melting wax, or their internal borders can fade away altogether so that the colors of the image diffuse into oneanother. These and similar bizarre phenomena are not explicable in terms of peripheral neural mechanisms. But what central mechanism could exhibit such behaviors? It occurred to us that some of the behaviors that we (and many earlier investigators of equiluminant and stabilized images) found were good examples of part-binding fail-ures. Because the vast majority of these studies were made in an era innocent of binding theory, we have a rare opportunity to harness several decades of misunderstood research in the goal of advancing the current research in binding. Expected failure modes of part-binding There is an extensive literature of algorithms for part-binding. In general, these models need to bind things that belong together and segment them from backgrounds and other objects. At this level, one could imagine binding failures that result from an inability to link parts into a whole (such a binding failure will be discussed in a later section of this article). Additional possibilities can be Corresponding author: Vincent A. Billock (vince. billock@wpafb. af. mil). Opinion TRENDS in Neurosciences Vol. 27 No. 2 February 2004 www. sciencedirect. com 0166-2236/$-see front matter. Published by Elsevier Ltd. doi:10. 1016/j. tins. 2003. 12. 003