Why is spatial stereoresolution so low?

Spatial stereoresolution (the finest detectable modulation of binocular disparity) is much poorer than luminance resolution (finest detectable luminance variation). In a series of psychophysical experiments, we examined four factors that could cause low stereoresolution: (1) the sampling properties of the stimulus, (2) the disparity gradient limit, (3) low-pass spatial filtering by mechanisms early in the visual process, and (4) the method by which binocular matches are computed. Our experimental results reveal the contributions of the first three factors. A theoretical analysis of binocular matching by interocular correlation reveals the contribution of the fourth: the highest attainable stereoresolution may be limited by (1) the smallest useful correlation window in the visual system, and (2) a matching process that estimates the disparity of image patches and assumes that disparity is constant across the patch. Both properties are observed in disparity-selective neurons in area V1 of the primate (Nienborg et al., 2004).