A curious illusion suggests complex cue interactions in distance perception

Binocular perception of the distance to, and between, point light targets depends on vergence angle: Increasing vergence angle decreases apparent distance and vice versa. Placing a prism "base out" requires increased convergence for target fixation; "base in" requires decreased convergence: The triangulation account of distance perception predicts that apparent target distance should decrease and increase respectively. It was found that the results predicted from the triangulation account were not observed. Egocentric target distance was judged to be greater regardless of prism orientation or target distance. A heuristic model provided an explanation for this phenomenon and allowed for the prediction of modulations of the overestimate with simple manipulations of the viewing environment. Further experiments confirmed these predictions and demonstrated that the effects of the prism could be greatly attenuated by adding additional distance cues.