Direction of perceived motion-in-depth in ambiguous dynamic random element stereograms

We measured five subjects' thresholds for both stereo detection and change in axis of perceived motion-in-depth as a function of the bar width of square-wave reversing square-wave gratings-In-depth. Stereo detection varied curvilinearly with bar width, while change in the perceived direction of motion-in-depth varied linearly with bar width on logarithmic coordinates. The changes in perceived axis of motion-in-depth could not be explained by variations with bar width in disparity thresholds suggesting that subjects perceived bars move along an axis towards the nearer of two alternative positions in 3D space.