Are discrimination thresholds a valid measure of variance for judgments of slant from texture?

There have been many experiments reported in the literature that have employed discrimination procedures to estimate the variance of observers' slant judgments from texture and binocular disparity, both individually and in combination. The research described in the present article identifies two serious methodological flaws in these studies. Although discrimination thresholds can be influenced by the variance of observers' slant estimates, they can also be affected by systematic biases in observers' judgments, and the presence of 2D cues that are irrelevant to the perception of slant. A series of five experiments is reported to show that: (1) the slants of surfaces specified by texture gradients can be systematically underestimated; (2) surfaces specified by texture gradients appear significantly less slanted than equivalent surfaces specified by binocular disparity; (3) the difference in bias between observers' slant judgments from stereo and texture may be more important than their relative variance in determining how these cues are weighted when presented in combination; (4) observers may be less sensitive to variations in apparent slant from texture than they are to variations in 2D cues that are unrelated to the perception of slant; and (5) these 2D cues may be the primary source of information for discriminating images of textured surfaces. These findings provide strong evidence that the results of prior discrimination studies have been misinterpreted because of the confounding effects of bias and/or 2D cues, and that this has resulted in several questionable conclusions that have been broadly accepted within the field.