The apparent displacement of a moving light spot

Summary

Apparent displacement of position was examined with a small light spot moving vertically downward. The apparent positions of the moving spot were measured by means of the comparison spot, presented to the right side of the motion-track at each of four time intervals, 80, 200, 360, and 720 msec, after (1) the start of, (2) the end of, or (3) 200 or 720 msec after the marker spot was flashed while the moving spot was in motion. Results from five Ss revealed the following tendencies. Any point on the motion-track always displaced its apparent position in the direction of the motion. The starting point showed the greatest displacement at the time interval of 720 msec. The end point displaced its apparent position considerably even at the time interval of 80 msec, indicating no significant increase of the displacement at longer intervals. Intermediate points showed greater displacement than the starting and the end points. Even a moving spot which was flashed for only a second and whose motion was hardly noticeable, also displaced its apparent position downward, and increased the displacement with time after the spot was turned off. These findings were considered in relation to the shifts of reference system in the perception of motion.