Coarse spatial scales constrain the range of binocular fusion on fine scales.

To examine interactions among spatial scales in disparity processing, we have measured the upper disparity limit for binocular single vision (the diplopia threshold) for high-spatial-frequency test stimuli in the presence of cosine gratings of lower spatial frequency that defined a surface in depth. When the frequency of this grating surface was 2.0 octaves below that of the test, the test fusion range was reduced by a factor of 3-4 relative to the condition in which no grating surface was present. However, gratings 4.0 octaves below the test frequency had no effect, and the test and grating were seen transparently at different depths. Further experiments indicate that the effect is orientation specific and that high-frequency gratings do not affect low-frequency tests. Finally, experiments using grating surfaces tilted in depth indicate that fusion at high spatial frequencies is constrained to a range centered on the local disparity of the surface defined by the lower frequency. These results are important for computational models for stereopsis that are based on coarse-to-fine matching strategies.