FIGURE 4. Results of experiments 2 and 3 showing the roles of motion direction and speed.

The stimuli consisted of bars and dots, respectively. The motion conditions are depicted along the abscissa of the histograms. The perceived depth ratios in the motion parallax and stereo condition were normalized to unity and all other conditions were scaled accordingly. Dir, direction. a, Both speed and motion differences enhance perceived depth. b, Replication of ref. 7. For small disparity differences (4 arcmin), there is no clear benefit of speed. c, Replication of experiment 2. When the disparity difference between the two depth planes is sufficiently large (30 arcmin), a substantial benefit of speed is observed. Error bars, s.e. across three observers.