Illusory differences which are stereoacuity test cues.

Some persons perceive an illusion of unequal blackness when viewing two equally black rods in a stereoacuity test. With binocular vision, the blacker rod is usually the nearer of the two. If blackness is used as a cue to nearness, a stereoacuity test can be performed successfully without using stereopsis. Nevertheless, stereoacuity obtained by this means is usually inferior to that obtained with distance judgments. For this reason it is advisable to warn subjects against using a blackness difference as a cue to the nearer rod. Those who saw this illusion binocularly also saw it monocularly. Therefore, it is more than just an artifact of stereopsis. One person observed an illusory difference in rod width as well as blackness.