signal to the cortex comes from spatially opponent P cells that multiplex luminance and color signals, induc-ing crosstalk between cortical luminance and r/C0gcolor mechanisms. Conversely, it appears that a majorsource of retinogeniculate y-b signal is from cells withspatially nonopponent Type II receptive fields (de Monasterio & Gouras, 1975; Wiesel & Hubel, 1966). These cells carry no multiplexed luminance signal and therefore do not induce correlations between cortical luminance and color detectors. The paucity of S-coneinputs to luminance (for review, see Cavanagh, 1991)removes another source of correlations between y-band luminance mechanisms. If correlations betweenmechanisms responding to borders are exploited forbinding, perhaps the y-b system, deprived of correla-tions to luminance borders, evolved only a weak input to the border contrast system. We suggest that binding allows achromatic mechanisms to reinforce r/C0gmech-anisms involved in segmentation and border formation(the luminance-driven form mechanism may need lessreinforcement from the chromatic system because—asdiscussed later—it receives reinforcement from motionmechanisms for unstabilized stimuli). CORRELATED CORTICAL SPATIAL/TEMPORAL MECHANISMS Why Form and Motion (or Space and Time) Are Not Independent There is evidence for separate mechanisms mediating detection of spatial and temporal variation (for review,see Zeki, 1993). There is also evidence for interactions between spatially and temporally tuned mechanisms (Burt, 1987; Breitmeyer & Ganz, 1976). Such interac-tions—as unavoidable as chromatic/achromatic interac-tions—are due to the physical entanglement of spatialand temporal information, and to spatio-temporal mul-tiplexing in retinogeniculate neurons. The physicalentanglement of motion and form is obvious; thevelocity (deg/sec) of a moving grating is V=f t (cycles/sec)/ fs(cycles/deg), where fs,ftare spatial and temporal frequency (each spatial and temporal fre-quency in the stimulus's Fourier representation canbe so treated). The neural entanglement of spatialand temporal information is more complicated, butcan be analyzed analogously to chromatic/achromaticinteractions. Consider the psychophysical achromaticspatio-temporal contrast sensitivity function. In princi-ple, if the function is separable (decomposable into the product of spatial and temporal functions), then inde-pendent spatial and temporal information can be ex-tracted by a homomorphic filter (a log transformfollowed by a matched filter). Unfortunately, contrastsensitivity is inseparable. However, Burbeck and Kelly(1980) showed that an Ingling-Martinez identity couldachieve a limited separati on, decomposing contrast sensitivity into the sum of two separable surfaces. Fora stimulus Z(f x,ft) the frequency response (assuming equal integrated excitatory/inhibitory sensitivity) is Xðfx;ft Þ¼0:5Zðfx;ft Þ1/2Seðfx Þ/C0Siðfx Þ/C1381/2Teðft Þþ Tiðft Þ/C138 Sustained (spatially tuned) response þ0:5Zðfx;ft Þ1/2Seðfx Þþ Siðfx Þ/C1381/2Teðft Þ/C0Tiðft Þ/C138 Transient (temporally tuned) response ð11Þ where Se,Te,Si,Tiare the excitatory and inhibitory spatial and temporal response functions defined in Equation 2. In Equation 11, the terms are labeledsustained and transient. The sustained term hasbandpass tuning for spatial frequencies and lowpasstuning for temporal frequencies. The transient term has lowpass tuning for spatial frequency and bandpass tuning for temporal frequency. In the psychophysicalliterature, the terms sustained and transient usually referto separate retinogeniculate spatial and temporalprocessing pathways. Yet, Burbeck and Kelly's analysisshows that both kinds of responses are embedded in allretinogeniculate cells with center/surround receptivefields, and are well modeled by Equation 11 for cells that obey superposition (P cells and X-like M cells). More-over, although the sustained and transient signals can beseparated algebraically, there is no plausible spatio-temporal filter that enables a strict spatial and temporalseparation by physiological means; the spectral contentof the terms overlap and attempts to extract one signalwill extract a small crosstalk signal as well. Quantifying Stimulus-induced Spatial and Temporal Mechanism Correlations Here, we estimate the stimulus-driven correlation be-tween temporally and spatially tuned pathways, createdby cortical filtering of the spatio-temporal signals carriedby LGN afferents. To extract the maximum signal fromthe spatially tuned (sustained) component of an array of active X-cells, we apply a filter matched to the sustained component, yielding a frequency response SPðfx;ft Þ¼0. 5Z( fx,ft)[Se(fx)--Si(fx)]2[Te(ft)+Ti(ft)]2 (Spatially tuned term) þ0:5Zðfx;ft Þ1/2Seðfx Þ2/C0Siðfx Þ2/C1381/2Teðft Þ2/C0Tiðft Þ2/C138 (Common information term) Similarly, a matched filter can be used to attempt to extract the temporally tuned (transient) component ofthe X-cell signals, yielding a frequency response TEðfx;ft Þ¼0:5Zðfx;ft Þ1/2Seðfx Þ2/C0Siðfx Þ2/C1381/2Teðft Þ2/C0Tiðft Þ2/C138 (common information term) þ0. 5Z(fx,ft)[Se(fx)+Si(fx)]2[Te(ft)--Ti(ft)]2 (temporally tuned term)(12) (13) Billock and Tsou 1041