Interactions between cues to motion in depth

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The motion of an object towards an observer is indicated by the following sources of information (cues): (1) Changes in the absolute disparity of the images in the two eyes and the vergence movements of the eyes that these create. (2) An increase in the relative binocular disparity of the near and far regions of the object (internal disparity). (3) Changing relative binocular disparity between the object and stationary objects. (4) Looming of the image of the object. (5) Motion parallax between the far and near regions of the object. (6) Changes in accommodation. I have designed and built a unique apparatus that allows me to independently control each of the above cues. Two identical image objects (textured surfaces or real 3-D objects) are placed in the two arms of a mirror stereoscope and moved to-and-fro along linear tracts within arm's reach. The image objects fuse to create a stereo object moving towards and away from the observer. Absolute disparity (vergence) is controlled by rotating each track. The internal disparity of the stereo object is controlled by rotating each image object about its mid-vertical axis. Looming is controlled by keeping the image objects at one distance while changing absolute disparity by moving the image objects laterally or relative disparity by rotating them. I can thus change any motion-in-depth cue or any combinations of cues using real objects really moving in depth. It is generally believed that changes in absolute disparity do not create sensations of motion in depth. I have already shown with the new apparatus that this assumption is incorrect. The crucial factor is whether or not there is image looming that contradicts disparity signals. Manipulations of internal disparity over a stationary textured frontal surface are known to affect the perceived distance and shape of the surface. I will use the new apparatus to investigate the distance scaling of internal disparity with real surfaces and 3-D objects moving in depth. Changes in internal disparity are a cue to motion in depth only when the object is assumed to be rigid. I will investigate the role of assumed rigidity by using familiar objects, abstract objects and objects that articulate.