Perceiving binocular depth with reference to a common surface

A common surface is a spatial regularity of our terrestrial environment. Fo r instance, we walk on the common ground surface, lay a variety of objects on the table top, and display our favorite paintings on the wall. It has be en proposed that the visual system utilizes this regularity as a reference frame for coding objects' distances. Presumably, by treating the common sur face as such-ie an anticipated constant-the visual system can reduce its co ding redundancy, and divert its resources to representing other information . For intermediate-distance space perception, it has been found that absolu te distance judgment is most accurate when a common ground surface is avail able. Here we explored if the common surface also serves as the reference f rame for the processing of binocular-disparity information, which is a pred ominant cue for near-distance space perception. We capitalized on an establ ished observation where the perceived slant of a surface with linear binocu lar-disparity gradient is underestimated. Clearly, if the visual system uti lizes this incorrectly represented slant surface as a reference frame for c oding the objects' locations, the perceived depth separation between the ob jects will be adversely affected. Our results confirm this, by showing that the depth judgment of objects (two laterally separated Vertical lines) on, or in the vicinity of, the surface is underestimated. Furthermore, we show that the impact of the common surface on perceived depth separation most l ikely occurs at the surface-representation level where the visual surface h as been explicitly delineated, rather than at the earlier disparity-process ing level.