The contribution of vergence change to the measurement of relative disparity.

The relative disparity between two objects in a scene can in principle be measured directly from the retinal images, without knowledge of eye position. But relative disparity increment thresholds are lowest when the relative disparity is small and the objects are not widely separated in the visual field: thus, some relative disparities are easier for the visual system to measure than others. We consider, after others, a second method by which the visual system could measure relative disparity, based on change in vergence ("delta vergence" or DV). The DV mechanism could be more reliable than the retinal mechanism when visual targets are widely separated in visual direction or depth. We used a cue-conflict paradigm to measure the extent to which perceived depth depends on DV. As target separation increased, so did reliance on DV. As intertarget disparity increased, reliance on DV increased for one observer but not for two others.