A curious illusion suggests complex cue interactions in distance perception

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