Spatial frequency channels and size-disparity correlation of contrast sensitivity for stereopsis

Purpose. Contrast sensitivity for stereopsis shows size-disparity correlation (Smallman and Macleod, 1994). We investigated the relationship between the size-disparity correlation and spatial frequency channels by measuring the contrast sensitivity function for various disparity and comparing them to the spatial frequency channels for luminance pattern detection. Methods. Stimuli were random-dot stereograms (4.3° × 4.3°), in which two regions with opposite depth positioned vertically one above the other. The spatial frequency content of the pattern was limited to a 0.4 octave bandwidth with various central spatial frequencies between 0.375 and 11.5 cpd. The observer responded which of upper or lower region appeared to be closer than the other. Contrast threshold was obtained based on the response in a staircase procedure and contrast sensitivity function was measured for various disparities between 1 and 32 min. Results. The peak of contrast sensitivity function for stereopsis shifted to lower frequency with the increase of disparity size, agreeing with the results by Smallman and Macleod. However, the shift of the peak frequency cannot be explained by a simple phase disparity model as they suggested. The peak rather agrees with the peak of spatial frequency channels proposed by Wilson et al (1983). and all of the contrast sensitivity functions could be explained fairly well by probability summation of two or three spatial frequency channels. Interestingly, spatial frequency channels with a peak frequency higher than 8 cpd was not necessarily to explain our data. Conclusion. Our data suggest that the spatial frequency channels for luminance pattern are also responsible for stereopsis and that the disparity tuning of the spatial frequency channels is the cause of the frequency-disparity correlation. This is consistent with the similarity between stereopsis and luminance pattern in spatial frequecny tunings (Shioiri et al., 1994).