Parallactic movement beats binocularity in the presence of external visual noise

Binocular vision provides a considerable advantage over monocular vision when stationary particles partly obstruct the view. Such situations occur in real life, e.g., when drivers are trying to identify objects through a windshield dotted with snowflakes. In the process of driving, any bumpiness of the road will bring about a parallactic movement of particles on the windshield with respect to the visual object. We investigated whether this parallactic movement diminishes the advantage of binocular over monocular vision.

Using computer graphics, we simulated a driving situation with snowflakes represented by noise particles on the windshield. Ten observers tried to identify a Landolt ring (8 possible orintations, gap always 2.5 arcmin) presented for 2 s at a viewing distance of 2 m. The partly obstructing noise particles, either stationary or moving vertically at three sinusoidal velocities, were presented at a viewing distance of 0.8 m, corresponding to a stereodisparity well beyond Panum's fusional area. We compared the percentage of correct responses and the reaction time between binocular and monocular vision.

When the 'snowflakes' were stationary, binocular vision yielded more correct responses than monocular vision (52.2 ± 1.8% vs 39.7 ± 1.7%). When the 'snowflakes' were moving, the task was much easier and the binocular advantage less pronounced (95.8 ± 1.4% vs 85.3 ± 5.2%). The reaction time with stationary noise was 1.25 s for binocular and 1.31 s for monocular vision. With moving noise, averaged over all three velocities, the reaction time was 1.23 s for binocular and 1.36 s for monocular vision.

Parallactic movement of partly obstructing particles reduces the advantage of binocular over monocular vision to practically irrelevant values.