Directional discrimination of motion in depth based on changing target vergence

Difference limens for the apparent direction of motion in depth were measured in eight observers. Two points of light were presented dichoptically in an otherwise dark field. Their lateral motion (changing target-vergence stimulus) mimicked the oscillation of an object in depth along different paths. All seven paths of 100 cm length had a common distant position in the median plane 150 cm from the observer. The directions of the paths were equidistant; the second and sixth paths were directed toward the left and right eye of the observer, respectively. Discrimination of direction was best for motion along the median plane and deteriorated toward the periphery. There was no indication of local minima of the difference limen for motions toward the eyes as has been observed with changing-disparity stimuli.