Dichoptic motion perception limited to depth of fixation?

When counterphase spatio-temporal flicker is presented to the left and right eye continuous directional motion can be perceived. Here, we investigate whether this type of dichoptic motion can be observed at different depth planes. Four observers indicated direction of motion for dichoptic motion stimuli, presented in a context containing crossed and uncrossed disparity information in different conditions. Our results show that despite the presence of disparity cues in the stimulus, discrimination of motion direction remained maximal at interocular phase offsets that correspond to binocular motion perception at zero disparity. This constraint brings into question perception of dichoptic motion as the result of an early binocular motion system. We compared our results with predictions of a computational stereo-motion model [Qian, N. (1994). Computing stereo disparity and motion with known binocular cell properties. Neural Computations, 6, 390–404; Qian, N., & Andersen, R. A. (1997). A physiological model for motion-stereo integration and a unified explanation of Pulfrich-like phenomena. Vision Research, 37, 1683–1698]. In contrast to our empirical results, simulations of cell activation in this hybrid energy model predict maximal activation at non-zero disparities. It is concluded that perception of dichoptic motion is a by-product of early interocular combination at low contrasts rather than the result of a dedicated stereo-motion system.