What causes stereoscopic tilt from spatial frequency disparity

A controversy still exists concerning whether the tilt created with interocular spatial frequency disparity arises from a computation of spatial frequency differences or from cumulative positional disparity. In a first experiment, we examined the influence of positional disparity on tilt created with frequency disparity, reasoning that if tilt were computed from spatial frequency differences, the perceived angle should remain unaltered since adding a positional disparity does not change the harmonic content of the stimulus. The results indicated that positional disparity weakened perceived tilt. In a second experiment, we tested the idea that tilt results from the calculation of increasing positional disparity across the display, arguing if local matches of features in the two eyes are made in computing tilt, then the solution to binocular correspondence may be less ambiguous if the same number of cycles was displayed for both spatial frequencies. Perceived tilt increased when the number of cycles was equal, although the angle of tilt still decreased with positional disparity. In Experiment 3, we further reduced potential sources of ambiguity for the binocular matching process by employing D 10s (the tenth derivative of a Gaussian) instead of grating patterns. Positional disparity exerted essentially no influence on the perceived angle of tilt of the D10s. Taken together, the results of these experiments suggest that tilt from frequency disparity can be explained solely on the basis of positional disparity.