Stereogram design for testing local stereopsis.

The basic ability to utilize disparity as a depth cue, local stereopsis, does not suffice to respond to random dot stereograms. They aslo demand complex perceptual processing to resolve the depth ambiguities which help mask their pattern from monocular recognition. This requirement may cause respnse failure in otherwise good stereo subjects and seems to account for the long durations of exposure required for target disparities which are still much larger than the best stereo thresholds. We have evaluated spatial stimuli that test local stereopsis and have found that targets need to be seth optimal separation of components and brief exposure duration (250 msec), thresholds fall in the range of 5 to 15 sec arc. Intertrial randomization of target lateral placement can effectively eliminate monocular cues and yet allow excellent stereoacuity. Stereoscopic acuity is less seriously affected by a small amount of optical defocus when target elements are widely separated than when they are crowded, though in either case the performance decrement is higher than for ordinary visual acuity.