Judgments of apparent shape contaminated by knowledge of reality: Viewing circles obliquely

In two experiments, participants viewed a circular disc oriented at a slant (which therefore appeared elliptical). The disc was made of luminous material and situated in a darkened chamber to remove perspective cues. Both student artists and non-artists exaggerated the circularity when attempting to match it on a computer screen by contracting or expanding a presented ellipse along its vertical axis. Crucially, however, a control group who remained ignorant about the circle's true shape (and were allowed to believe it really was an ellipse) did not exaggerate its circularity. The results of the second experiment suggested that participants were unaware that they were exaggerating the circularity, since their estimations of their own performance were more similar to the projective shape than to the shape they judged the ellipse to be. It seems that knowledge of reality contaminates judgments of appearance over and above any similar effect that might arise from the presence of perspective cues (which were eliminated in these experiments).