Failure of facial configural cues to alter metric stereoscopic depth

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J. Burge, M. A. Peterson, and S. E. Palmer (2005) reported that an ordinal cue to depth can influence the perception of metric depth in stereoscopic displays. They argued that when a familiar figure--a face--is placed stereoscopically closer than a background there is greater perceived depth relative to the ground than when the face shape is placed stereoscopically further and becomes the ground. This result suggests the possibility that a non-metric depth cue--the familiarity of a figure--can influence the perception of metric depth in stereoscopic displays. However, the method leaves open the possibility that these results were due to a response bias, rather than from a genuine change in perceived depth. To assess this possibility, we used the same basic stimulus but directly measured the perceived depth difference between the face and non-face surfaces when arranged as figure and ground or ground and figure respectively using a separate double depth probe to measure perceived depth. We found no difference between the perceived depth of familiar and unfamiliar figures as a function of whether they were stereoscopically figure or ground. We conclude that the J. Burge et al. (2005) result depends on their particular task and is likely to reflect a response bias. It is premature to conclude that facial configural cues distort perception of metric depth although we argue that there are circumstances in which ordinal cues do influence metric depth.