Advantage of binocularity in the presence of external visual noise

External visual noise plays a major role in real life, for instance, when a driver tries to identify an object through a snow flurry or through a dirty windshield. The goal of the present investigation was to quantify, under such a condition, the advantage of binocular over monocular vision or, rather, the disadvantage of having only one eye. Eight observers judged the orientation of a Landolt ring (gap always 10 arcmin), partly obscured by noise particles of different sizes (5, 10 or 20 arcmin). The noise particles were presented at a stereo disparity of 62 arcmin, i.e. beyond Panum's fusional area. We compared the percentage of correct responses and the reaction time between binocular and monocular vision. Control conditions: (1) binocular vision with noise particles located immediately in front of the Landolt ring (stereo disparity +/- 0), and (2) absence of noise particles. With regard to the percentage of correct responses, an advantage of binocular over monocular observation occurred only when the obscuring particles were presented at the stereo disparity of 62 arcmin. The advantage depended on the size of the noise particles. The factor was 1.24 for particles of 5 arcmin, 1.49 for 10 arcmin and 1.59 for 20 arcmin. With regard to the reaction time, there was no difference between binocular and monocular vision. Binocular vision provides a considerable advantage over monocular vision when particles partly obstruct the view. This advantage is due to the capability of the visual system to construct a coherent percept of an object of which different parts are imaged in the right and left eye.