images. Luminance differs slightly from brightness and is usually deﬁned operationally; for example, if two colors are shown in the same place in rapid sequence (a procedure known as heterochromatic ﬂicker photometry), the sen-sation of ﬂicker is drastically reduced for a certain ratio of the radiances of the colors-this is deﬁned as equilumi-nance. Slightly harder to use in practice, but of morerelevance to this discussion, is the minimum border criterion: at equiluminance, the apparent border contrast between differently colored image parts is minimized. Similarly, perception of form, depth, motion and color are all degraded for equiluminant stimuli [40-42]. Sometimes these phenomena are attributed to a lack of chromaticinput to these mechanisms, but several studies show thatthese systems can be driven by P cells in general and by hue-speciﬁc mechanisms in particular [43]. Indeed, even some putatively chromatic functions (e. g. chromaticrivalries) are disrupted by equiluminance [41]. Othershave attributed some effects to the lower acuity of chromatic mechanisms. This can explain some effects[42]but the most interesting effects are out of proportion to the acuity loss [43], or are seemingly unrelated to it [40,41]. From a binding point of view, the most signiﬁcant of these are linking failures and image melting. Binding-like failures at equiluminance: linking failures and perceptual melting A variety of symptoms suggest that there are part-binding failures in equiluminance. Included in these are failures tolink pieces of a surface. Consider random dot stereopsis. Itis possible to build a stereogram in which the two images contain only random dots. The two images are near duplicates; one image has a group of dots shifted overseveral pixels. Binocular fusion of the two images yields apercept of a group of random dots ﬂoating above or belowthe random dot background. In an ordinary random dot Figure 1. Examples of fragmentation percepts. (a)A square (left) breaks up during retinal stabilization, with fragmentary losses and fading starting at the corners. Repro-duced, with permission, from Ref. [29]. (b)The effects of stabilizing a long bar (left). The bar fragments in various places and the fragments fade. Reproduced from Ref. [19] by permission of Oxford University Press. The probability of such fragmentation increases with bar length [28]and some studies show a loss of high-frequency electro-encephalogram power during such stabilization-induced fragmentation and fading [38,39]. (c)Mosaic vision. An odd fragmentation phenomenon reported by a small min-ority of patents during epileptic seizures and migraine. According to Sacks, 'mosaic vision denotes the fracture of the visual image into irregular, crystalline, polygonal facets, dovetailed together as a mosaic. The size of the facets may vary greatly. If they are extremely ﬁne, the visual world presents an appearance of c rystalline iridescence or “graininess”, reminiscent of a pointillist painting... If the facets become larger, the visual image takes on the appearance of a classical mosaic... o r even a “cubist” appearance. ' Reproduced, with permission, from Ref. [35]. (d)A possibly related phenomenon, described by subjects with strabismic amblyopia. Vision is normal for low spatial frequency gratings but distorted at high spatial frequencies, as if the features were improperly bound by location. At the highest spatial fr equencies, breaks appear in the features. The phenomenon is not due to an inability to transduce the features because contrast sensitivity is normal (graph). 'c/d' (cycles per degree) refers to the periodicity of the coarse and ﬁne gratings (e. g. 1 c/d mean that one black and one white bar ﬁt into one degree of visual angle, which is roughly the angle s ubtended by the width of the thumb held at arms length; the best human acuity-20/10-is equivalent to being able to see a 60 c/d grating). Reproduced, with permission, f rom Ref. [36]. Normal Pointilliste Mosaic Cubist Vertical Horizontal Contrast sensitivity 110100300 0. 1 1 10 100 Spatial frequency (c/d)1c/d 4c/d 10c/d 15 c/d4o(a) (b) (c) (d)Opinion TRENDS in Neurosciences Vol. 27 No. 2 February 2004 86 www. sciencedirect. com