Replicating and extending Bourdon's (1902) experiment on motion parallax

Gradients Account for the Thresholds for Perceiving Motion and Depth in Observer-Produced Parallax Displays

We examined whether the thresholds of motion and depth perception produced by motion parallax could be specified by the concept of a disparity gradient. We manipulated both the motion parallax amplitude and the angular separation of two dots and calculated the percentages of trials in which participants perceived motion or depth. The results showed that the amplitude of motion parallax for the threshold increased as the separation became larger with the gradients of 0.023, 0.072, and 0.430 for the lower depth, the lower motion, and the upper depth thresholds, respectively. These findings indicate that the gradient is a useful concept to specify the motion and depth thresholds together rather than parallax amplitude alone.

Psychophysical point: a disc tends to become a point when Weber's law fails

In three experiments, we showed that the visual system treats a dot somewhat like a geometrical point, which has a location but no area. We represented a "point" or "dot" with a small disc (diameter of 0.08º of visual angle), and a "disc" with a larger disc (diameter of 1.5º). The Weber fraction of the dot was larger than that of the disc. In Experiment 1, the relative retinal image size cues for depth for the dot and the disc were placed in conflict with the motion parallax cue. We found that the dot indicated the positions defined by the motion parallax cue better than the disc did. In Experiment 2, we placed a constant retinal image size in conflict with convergence eye movements. We found that a binocularly fused dot appeared to move in depth with convergence eye movement, whereas a fused disc appeared to move less. In Experiment 3, we examined the apparent sizes of the afterimages of a dot and a disc and found that Emmert's law failed for the dot afterimage; the apparent size of the dot afterimage changed very little for different distances-as though it had no area-whereas the apparent size of the disc afterimage changed by an extent predicted by Emmert's law. The differences in the dot and disc conditions could not be explained by the differences in the Weber fractions alone.

Simple 3-D stimulus for motion parallax and its simulation

Simulation of a given stimulus situation should produce the same perception as the original. Rogers et al (2009 Perception 38 907-911) simulated Wheeler's (1982, PhD thesis, Rutgers University, NJ) motion parallax stimulus and obtained quite different perceptions. Wheeler's observers were unable to reliably report the correct direction of depth, whereas Rogers's were. With three experiments we explored the possible reasons for the discrepancy. Our results suggest that Rogers was able to see depth from the simulation partly due to his experience seeing depth with random dot surfaces.