The Effects of Common Fate Grouping on Suprathresold Depth Estimates

When two vertical lines are perceived to form the boundaries of a common object, observers underestimate their separation in depth. This disruption in perceived depth depends directly on the perceived grouping via closure of the resultant figure. Here I tested if this effect generalises to motion based grouping cues, specifically common fate. Stimuli were presented on a mirror stereoscope and an on screen ruler was used to record depth magnitude estimates. It was found that depth estimates were accurate over a range of suprathreshold disparities, for both static and moving stimuli. Attempts to strengthen the grouping cues had no impact on depth magnitude estimates. This was true even when a more complex biological motion stimulus was used. These results show that previously reported reductions in perceived depth are not generalizable to grouping via common motion. It appears that this phenomenon only occurs when the spatial layout suggests a common object.