Fine-to-coarse scale disambiguation in stereopsis.

Spatial frequency selectivity has been incorporated into various theories of stereo matching, along with spatial scale interactions operating from coarse-to-fine spatial scales. We concentrate here on the role of fine scale information in the stereo matching process and show that fine scale information is capable of disambiguating matches made at coarser scales. An ambiguous coarse scale stimulus was created by presenting a low frequency (2 c/deg) sine wave in anti-phase to the two eyes, whose endpoints betrayed no information about which way the sine waves should be matched. It could be seen with crossed or uncrossed disparity equally validly and at chance from trial to trial. To this was added a fine scale (8 c/deg) filtered random dot stimulus specifying unambiguously a certain disparity. Observers judged the apparent depth of the two stimuli as the disparity of the fine scale stimulus was varied. The sine wave was usually perceived to have the same sign disparity as the fine scale stimulus. Depth matching with the two superimposed stimuli confirmed that the coarse scale stimulus was actually disambiguated, and seen with disparities equal to half its spatial period. The results suggest the operation of a cross-spatial scale matching disambiguation process, which can operate in a fine-to-coarse fashion.