Perceived distance, shape and size

If distance, shape and size are judged independently from the retinal and e xtra-retinal information at hand, different kinds of information can be exp ected to dominate each judgement, so that errors in one judgement need not be consistent with errors in other judgements. In order to evaluate how ind ependent these three judgements are, we examined how adding information tha t improves one judgement influences the others. Subjects adjusted the size and the global shape of a computer-simulated ellipsoid to match a tennis ba ll. They then indicated manually where they judged the simulated ball to be . Adding information about distance improved the three judgements in a cons istent manner, demonstrating that a considerable part of the errors in all three judgements were due to misestimating the distance. Adding information about shape that is independent of distance improved subjects' judgements of shape, but did not influence the set size or the manually indicated dist ance. Thus, subjects ignored conflicts between the cues when judging the sh ape, rather than using the conflicts to improve their estimate of the ellip soid's distance. We conclude that the judgements are quite independent, in the sense that no attempt is made to attain consistency, but that they do r ely on some common measures, such as that of distance. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.