Simultaneous binocular integration of the visual tilt effect in normal and stereoblind observers

Apparent displacement of an edge from the subjective visual vertical is typically induced by the simultaneous inspection of a large rotating textured background. In the present series of experiments we have tested the binocular integration of such an effect in normal and stereoblind observers in whom inspection of the rotating background and vision of the vertical edge are presented to different eyes. In normal subjects there was no difference between the binocular and the dichoptic viewing condition while in stereoblind subjects there was a clearcut decrease in the strength of the tilt effect under dichoptic conditions. A residual tilt effect, however, was still present in the latter group under dichoptic conditions and this shows that binocular interactions related to visual tilt can be dissociated from those required for stereopsis.