Disparity and texture gradients are combined in two ways

Combining information from multiple cues can improve reliability, but may come at a cost: loss of single-cue information. We have found (Hillis et al., Science, 298, 1627, 2002) that combining disparity and texture gradients improves the precision of slant estimation, but that single-cue information (disparity alone and texture alone) can be lost in the combination. However, different combinations of disparity and texture cues are relevant for different types of perceptual judgments. For judgments of slant, disparity and texture gradients should be combined in a weighted sum. For judgments of texture homogeneity (i.e., shape constancy), slants specified by disparity and texture should be compared and this can be accomplished by subtracting one from the other. We asked whether disparity and texture gradients are used in both of these judgments, by weighted summation for one and by weighted differencing for the other. We presented planes whose slants were defined by disparity and texture gradients. On each trial, three stimuli appeared sequentially. Two (or one) standard stimuli had a constant slant specified by both disparity and texture. One (or two) comparison stimuli had disparity and texture signals that indicated different slants from one another. Observers made three perceptual judgments (in separate sessions): (1) "Oddity," in which they indicated on any basis the one that was different than the other two (as in Hillis et al.), (2) " Slant," in which they indicated the one that had a different slant than the other two, and (3) "Homogeneity," in which they indicated the one whose texture elements had a different distribution on the simulated surface than the other two. We found that the envelope of the slant and homogeneity thresholds predicted the oddity thresholds. Thus, disparity and texture cues are combined to allow precise judgments of slant and texture homogeneity. But in so doing, the nervous system loses access to the individual cues' slant estimates.