Binocular single vision : psychophysical studies on underlying sensory and motor processes

The present thesis deals with binocular single vision. By closing one of the two eyes alternately (in particular in the presence of nearby objects), one can easily see that we are confronted with two different images of the outside world. Nevertheless, when we look at this world with both eyes we still perceive it for the greater part singly. This in spite of the fact that the two images often differ so much that the world would be perceived as clearly double if the same images did not enter the visual system through different eyes but, projected on top of each other, through the same eye. This singleness of binocular vision is accomplished by motor processes which use the eye muscles to direct the eyes towards the same point in space, and by sensory processes in the brain which avoid doubling of the binocular image. Interpretation of certain types of differences between the images in the two eyes results in stereopsis (depth perception) which disappears when one of the eyes is closed. As a result, the observer can assess the occurrence of these image differences not only by using the comparatively inconspicuous and inaccurate information provided by image doubling, which is the subject of the present thesis, but also indirectly by using the conspicuous and accurate depth cue. It was decided to avoid this complication by confining the study to those situations where these interfering depth effects did not occur. The processes underlying binocular single vision have been studied in psychophysical experiments. Subjects were asked to make judgments on the percepts evoked by visual stimuli. The procedures and stimuli chosen were such that they practically guaranteed that the judgmPnts were always based on the same information. In this way the mysteriously variable nature of a person's psyche, which is unavoidably involved in the judgments, was restricted.