PURPOSE. Bloch's law predicts reciprocity between stimulus intensity and viewing duration for psychophysical thresholds for viewing durations shorter than a critical duration. For stereoscopic discrimination thresholds, Ogle and Weil (1958) found reciprocity for disparity and viewing duration for extended durations, but the effects of stimulus parameters such as spatial frequency and contrast were not studied. The purpose of these experiments was to determine the effects of contrast and spatial frequency on the integration time for stereothresholds in normal and stereodeficient subjects.
METHOD. The subjects included monkeys and humans with either normal or subnormal binocular vision. Stereothresholds were determined by the method of constant stimuli for local (line stimuli or Gabor patches) and global (random dot) stimuli for durations of 16 to 1000 msec. The threshold vs. duration data were fitted by non-linear regression to determine the longest viewing duration for intensity-time reciprocity and the stereothreshold that was independent of duration.
RESULTS. Stereothresholds increased systematically for viewing durations shorter than a critical duration of 100-150 msec, independent of both the specific stimulus parameters investigated (contrast, spatial frequency, configuration--local vs. global) and the subject's duration-independent stereothreshold. The results for human and non-human subjects were remarkably similar.
CONCLUSIONS. The general properties of intensity-time functions for stereopsis follow Bloch's law, with the critical time for full reciprocity somewhat longer than for two-dimensional stimuli. The critical time is independent of other stimulus or physiological factors that affect stereoacuity.