The accuracy of metric judgements : perception of surface normal

Observers adjusted a probe (a short rod) to appear normal to a planar surface slanted in depth. In Experiment 1, observers (N = 12) performed this metric task in two conditions: with reduced cues to calibration of binocular viewing parameters and with full cues. The results provided evidence for the use of an internal working metric in metric tasks because they confirm predictions that (i) errors should be largely systematic and accounted for by assuming an inaccurate working metric and (ii) this metric should be consistent with miscalibration of relevant viewing parameters. The data support the prediction that performance errors decrease in a manner consistent with improved binocular calibration, when better cues to relevant viewing parameters are provided. We performed two additional control experiments as further tests of the binocular miscalibration account, to determine whether performance in Experiment 1 could be explained instead by the use of monocular cues. We found that monocular performance was significantly poorer than binocular performance in reduced-cue conditions (Experiment 2) and full-cue conditions (Experiment 3). These control experiments provide confirmation that binocular cues contribute to performance in the full-cue conditions of Experiment 1, and that disparity was the only effective cue to slant in reduced-cue conditions.