SIZE DISPARITY CORRELATION IN STEREOPSIS AT CONTRAST THRESHOLD

Contrast thresholds for 75% correct depth identification in narrow-ban d filtered random dot stereograms were determined for different center spatial frequencies and binocular disparities. Rigorous control over vergence was maintained during testing, and a forced-choice procedure was used. The resulting contrast sensitivity function for stereopsis r evealed sensitivity over a greater range of disparities at low than at high spatial frequencies. Sensitivity peaked for large disparities at low spatial frequencies and for small disparities at high spatial fre quencies. When disparities were converted to effective binocular phase differences, the variation of contrast sensitivity with phase followe d a consistent pattern across spatial frequencies, with peak sensitivi ty occurring mainly for binocular phases of between 90-degrees and 180 -degrees. These results have implications for the extent of spatial in tegration at the input Co the disparity sensing mechanism. A model pos tulating a spread of positional disparities independent of the spatial frequency selectivity of disparity-sensitive units cannot account for the results. But the size-disparity correlation strongly evident in o ur data is predicted by certain models of stereopsis, such as phase di sparity encoding. An ideal observer analysis is developed that demonst rates that our results were not forced by the nature of the stimulus e mployed; rather, the quantum efficiency for stereopsis at contrast thr eshold follows the size-disparity correlation.