Abstract
We sought to determine if perceived depth can elicit vergence eye movements independent of binocular disparity. A flat surface in the frontal plane appears slanted about a vertical axis when the image in one eye is vertically compressed relative to the image in the other eye: the induced size effect (Ogle, 1938). We show that vergence eye movements accompany horizontal gaze shifts across such surfaces, consistent with the direction of the perceived slant, despite the absence of a horizontal disparity gradient. All images were extinguished during the gaze shifts so that eye movements were executed open-loop. We also used vertical compression of one eye's image to null the perceived slant resulting from prior horizontal compression of that image, and show that this reduces the vergence accompanying horizontal gaze shifts across the surface, even though the horizontal disparity is unchanged. When this last experiment was repeated using vertical expansions in place of the vertical compressions, the perceived slant was increased and so too was the vergence accompanying horizontal gaze shifts, although the horizontal disparity again remained unchanged. We estimate that the perceived depth accounted, on average, for 15-41% of the vergence in our experiments depending on the conditions.