Size matters: Perceived depth magnitude varies with stimulus height

Both the upper and lower disparity limits for stereopsis vary with the size of the targets. Recently, Tsirlin, Wilcox, and Allison (2012) suggested that perceived depth magnitude from stereopsis might also depend on the vertical extent of a stimulus. To test this hypothesis we compared apparent depth in small discs to depth in long bars with equivalent width and disparity. We used three estimation techniques: a virtual ruler, a touch-sensor (for haptic estimates) and a disparity probe. We found that depth estimates were significantly larger for the bar stimuli than for the disc stimuli for all methods of estimation and different configurations. In a second experiment, we measured perceived depth as a function of the height of the bar and the radius of the disc. Perceived depth increased with increasing bar height and disc radius suggesting that disparity is integrated along the vertical edges. We discuss size-disparity correlation and inter-neural excitatory connections as potential mechanisms that could account for these results.