Note that the matched filters are only partially suc-cessful in extracting signals tuned along one stimulusdimension. In both cases, the undesired portions of thesignal—tuned along both space and time—are identical,and so spectral correlation between the mechanisms isdominated by the crosstalk. As before, we use the boldterms of Equations 12 and 13 as input signals INPUT 1, INPUT 2in Equation 1 and estimate correlated crosstalk (Q1,2in Equation 1) by computing the spectral correla-tion between Equations 12 and 13 (neglecting unknownsources of correlated noise; e. g., Horn et al., 1991) Q 1;2¼SPðfs;ft Þ2TEðfs;ft Þ ¼ZZ SPða;b ÞTEðfsþa;ftþb Þdadb ð14Þ where aand bare dummy variables of integration. These spatio-temporal interactions present two possi-bilities for crosstalk-mediated segmentation: (1) Mutualfeedback between correlated form and motion mecha-nisms both responding to moving edges (the cross-modal construction model discussed above for form/color interactions); (2) Motion-correlated segmentationwithin the form pathway; that is, stimulus motion givesrise to two responses in the form pathway: the sustained form signal (the first term in Equation 12) and the trans-ient crosstalk signal (the second term in Equation 12). Because these crosstalk signals arise only in formdetectors stimulated by the target's motion, they are apotential cue for binding by correlated motion. A Binding Failure—Perception of Stabilized Images During normal vision images are in constant motion; microtremors of the eye make volitional stabilizationalmost impossible. However, images can be stabilized mechanically or by producing afterimages on the retina. These stabilized images can fade away rather quickly, aneffect often attributed to the temporal response prop-erties of retinogeniculate neurons. We argue that thisexplanation is grossly inadequate and that a failure of cor-tical segmentation mechanisms is indicated. However,the cortex can only operate on what afferent mecha-nisms send it, so next we analyze what early and cortical mechanisms contribute to stabilized image perception. What Early Mechanisms Do to Perception of Stabilized Images Some stabilized image percepts are consistent with retinogeniculate cell properties. Low contrast, low spa-tial frequency stimuli (favoring transduction by transientmagno [M] cells) usually fade faster than high contrast,high spatial frequency stimuli (favoring transduction bysustained parvo [P] cells; Ingling & Grigsby, 1990). Moreover, nonbleaching chromatic afterimages elevateboth achromatic and chromatic thresholds, suggesting a common P cell-like pathway (Kelly & Martinez-Uriegas,1993). There are some sustained M cells that respond tostatic images for a few seconds, but their temporalresponse becomes transient for high contrast stimuli(Benardete, Kaplan, & Knight, 1992), leaving only P cellsto mediate perception of afterimages. Moreover, King-Smith, Rosten, and Alvarez (1980) describe a subject who (on psychophysical grounds) appears to be missing the parvo system; this subject was also unable to per-ceive afterimages. Hence, transduction by P cells is anecessary condition for perception of most stabilizedimages. However, retinogeniculate properties cannotaccount for at least seven lines of evidence that centralmechanisms are responsible for some binding-failure-like oddities of stabilized images. Three Properties of Stabilized Image Perception Paradoxical to P-and M-Cell Properties (1) Anomalously Rapid Fading of Images that Should Tap High Acuity Sustained Channels Recall that Equation 2 shows that P cells have a sustained response for high spatial frequencies. Near the fovea,these cells have midget receptive fields capable oftransducing spatial frequencies up to 60 c/deg (conesampling limit). Additionally, these P cells (unlike M cells) are sensitive to high contrasts (Benardete et al., 1992). So, it is odd that the stabilized pattern of highcontrast shadows cast by blood vessels on the retinafades faster than other stabilized images (Coppola &Purves, 1996). Moreover, the closer the blood vessels areto the fovea, the faster they fade (as fast as 80 msec forthe highest spatial frequency components), in contra-diction to the response characteristics and retinal distri-bution of P cells. (2) Anomalously Rapid Fading for Stimuli that Tap Chromatic Sustained Mechanisms P cells have a chromatic response that is lowpass in both space and time (Equation 2 shows a bandpass chromaticcomponent as well, but this merely adds to the responseat moderate spatial and temporal frequencies without driving down the response to low frequencies). Al-though Equation 2 must hold if P cells obey linearsuperposition, it does not model psychophysical chro-matic contrast sensitivity for very low temporal frequen-cies (<0. 2 Hz), where the temporal CSF slope isconsistent with a first-order temporal differentiation(Kelly, 1981); this extra temporal derivative is probablya cortical process acting on P-cell inputs. A perceptual manifestation of this is the dramatic elevation of stabi-lized chromatic grating detection thresholds (a factor ofat least 45 greater than for chromatic unstabilized gra-tings and much higher than the elevation for achromatic 1042 Journal of Cognitive Neuroscience Volume 16, Number 6