Stimulus uncertainty does not impair stereopsis

The effect of stimulus uncertainty on the stereoscopic resolution of letters was examined for two classes of letters: (1) letters presented stereoscopically as random-element stereograms, and (2) letters presented as two-dimensional physical contours. The variables of stimulus discriminability (stereoscopic vs. physical contours) and stimulus uncertainty (number of alternative letter targets) were combined factorially. Stereoscopically presented letters were more difficult to resolve, but stimulus uncertainty had the same effect for both stereoscopic and physically defined letters. The additivity of these two variables suggests that the perception of stereoscopic forms is an automatic process not impaired by uncertainty about the form to be resolved.