Theory's cross-ambiguity function). The spectral corre-lation between A(fs,ft) and C(fs,ft)i s Q1;2¼Aðfs;ft Þ2Cðfs;ft Þ ¼Z Aða;b ÞCðfsþa;ftþb Þdadb ð10Þ where a,bare dummy variables of integration. For an equiluminant stimulus, the achromatic signal to theachromatic mechanism is zero and the correlatedcrosstalk consists of the correlation between the chromatic signal in the chromatic pathway [the bold term in the C(f s) equation] and the chromatic crosstalk extracted by the achromatic pathway [the plain typeterm in the A(f s) equation]. Thus, at equiluminance, the visual system attempts to bind something to nothing,based on the correlation between something andbandpass filtered noise. This situation corresponds tosome very odd perceptual effects. Effects of Equiluminance on Perception At equiluminance perception of form, depth and mo-tion are degraded (Cavanagh, 1991; Livingstone & Hubel, 1987; Gregory, 1977). Equiluminance can alsodisrupt segmentation and binding. Segmentation bybinocular disparity is severely degraded at equilumi-nance for random dot stereograms (Lu & Fender,1972). Surfaces are not linked together if their featuresare defined only by color; an image made of equilu-minant colors appears as patches of those colors, not as a unified whole (Cavanagh, 1991; Livingstone & Hubel, 1987). Moreover, perception of equiluminousforms can be unstable. Gregory (1977) found equilu-minant images ''looked uns table in contrast and 'jazzy'. '' This instability is sometimes attributed to aninability of ocular accommodation to use color infor-mation, but this is contradicted by Kotulak, Morse, and Billock (1995). Liebmann (1927) found that there is a critical luminance zone within which ''everything flows... glimmers... everything is soft, jelly-like, colloidal. Often... parts which belong together in the normal figure now have nothing to do with each other. (It is) aworld without firm things, without solidity'' (translatedin Cavanagh, 1991). Studies using luminance minimizedborders show these effects are particularly severe forborders defined by S-cone (tritan) modulation; the borders collapse and the color fields blend into a continuous color gradient (Buck, Frome, & Boynton,1977). No Evidence for Explanations Based on Lack of Parvo Inputs to Central Pathways Livingstone and Hubel (1987) ascribe the detrimental effects of equiluminance to a lack of parvo inputs tomotion, form, and depth channels, but evidence con-tradicts this (Merigan & M aunsell, 1993; Ingling & Grigsby, 1990; Schiller, Logothesis, & Charles, 1990). Moreover, there are chromatic mechanisms sensitive toform, motion, and depth (Regan, 2000; Cavanagh,1991; Mullen & Kingdom, 1991). Indeed, paradoxically,some chromatic phenomena are adversely affected byequiluminance, including color discrimination, color contingent aftereffects, and color rivalries (for review, see Mullen & Kingdom, 1991; Livingstone & Hubel,1987), suggesting a more subtle origin for the effectsof equiluminance. Several possibilities are consideredbelow. Evidence Contradicting Explanations Based on Low Chromatic Acuity Although the low acuity of chromatic mechanisms af-fects some visual phenomena (Cavanagh, 1991), it does not explain the poor and unstable contrast of some equiluminous borders. Border sharpness is not strictly ahigh spatial frequency phenomenon; a blurred edgelooks sharp if the missing harmonics of its Fourier seriesare below detection threshold (Campbell, Hopwell, &Johnstone, 1978). Similarly, reducing acuity by dimmingillumination has little effect on contrast over a largerange and edges modulated at 15 Hz appear sharp, even though acuity is reduced by a factor of 2. 5 (Livingstone & Hubel, 1987). Finally, as Mullen and Kingdom (1991)put it, ''it would be surprising if the lower borderdistinctness rated for S cone mechanisms compared to M-L cone ones was due to their differences in acuitysince a greater difference in acuity occurs betweenluminance and M-L chromatic mechanisms with no lossof border distinctness. '' Luminance as a Master Signal? Gregory (1977) posits that luminance is a master signal necessary for demarcating borders. This fits luminancecaptures color phenomena but does not explain whytritan equiluminous borders suffer excessively relative toequiluminous borders that stimulate the red/green sys-tem. Nor can the vulnerability of tritanopic borders bedue to a lack of S-cone driven color contrast mecha-nisms; double opponent b-y cells (Livingstone & Hubel, 1984), are well suited for transducing S-cone modulatedchromatic contrast and multiple S-cone driven spatialfrequency channels are found psychophysically (Human-ski & Wilson, 1993). A Binding-level Explanation? A possible explanation for the insalience of tritanopic borders stems from the origins of the r/C0g,r+g (luminance) and b-y signals. The major input of r/C0g 1040 Journal of Cognitive Neuroscience Volume 16, Number 6