Breakdown of Apparent Motion under Stereoscopic Viewing

To study the effect of binocular disparity on apparent motion, we measured the cumulative time of its breakdown during a 30 s fixation viewing period. Two light spots, both on the left side of the fixation point, were alternately presented one by one on a CRT display (unilateral condition). These spots were binocularly disparate and viewed through a stereoscope. While one spot near the fixation point was presented on a zero disparity plane, the other spot (more peripheral) was either on a zero, uncrossed, or crossed disparity plane, so that three-dimensional motion could be seen depending on disparity values. We found that the duration of the breakdown of apparent motion was longer when uncrossed and zero-disparity spots were paired to produce apparent motion, and it was shorter when crossed and zero-disparity spots were paired. However, such disparity-specific tendencies were not obtained when the two spots were presented on both sides of the fixation point (bilateral condition). The disparity-specific tendencies in the unilateral condition can be explained by assuming that three-dimensional apparent motion that is consistent with the motion perspective may be stable because we experience it more frequently. Thus, we assume that perception of motion, both apparent and real, may develop through everyday experiences of moving to and fro in the environment rather than seeing objects move.