Slow corrections to arm movements for target perturbations in depth

Abstract

Reliable depth perception is essential for successfully guiding ones hand toward objects. Binocular disparities are often considered to be the primary source of metric information about depth. Moreover, as the hand approaches the object, errors in initial depth judgments give rise to relative disparities that could be used to correct the movement. In the present study we examine whether people can quickly adjust their movements on the basis of information from binocular disparities. It only takes 110-150 ms to correct a hand's movement if the object toward which the hand is moving is suddenly displaced. It takes about the same time to adjust the way one moves a computer mouse, and thereby correct the cursor's movement, if the target is displaced on the screen. If arm movements are to be adjusted on the basis of relative disparities it is vital that the response to such information is also this fast, because the information itself only becomes available when the hand comes near the object. We examined cursor movements, rather than hand movements, because this makes it easier to control the visual information that the subjects can use. Subjects sat 80 cm from a computer screen and quickly moved a cursor to a target by moving the mouse. The cursor moved in a horizontal plane at eye level, in the same direction as the mouse. Shutter spectacles were used to present the cursor and target at the desired depths. Once the cursor reached the target the latter disappeared and a new one appeared elsewhere. On some trials the target jumped 15 cm in depth while the cursor was moving toward it. It took subjects more than 200 ms to respond to this displacement. When the experiment was repeated with the targets 8cm below eye height, so that a movement in depth gave rise to a small vertical movement on the screen, the response was much faster. Thus binocular disparities are unlikely to be important for guiding the hand toward objects unless we move rather slowly.