Apparent image rotation in stereoscopic vision: The unbalance of the pupils

In binocular vision the apparent rotation of an object relative to the frontal plane seldom corresponds with the actual rotation. The reason for this is a difference in the brightness of the object in both eyes. It can be produced by a difference in the pupil size, adaptation, or sensitivity. One of the purposes of the pupils seems to be to compensate for this brightness unbalance and to make the apparent rotation come closer to the actual one. Unbalance was produced by placing a density filter before one eye, and the corresponding rotations were then measured. The rotations are affected by the irradiation in the eye, as a physical component, but also by the lateral inhibition in the visual nervous system. If the object and the background have a different color but there is no brightness difference on the edges of the object, there is no distorted rotation.