The brightness-weight illusion

Bigger objects look heavier than smaller but otherwise identical objects. When hefted as well as seen, however, bigger objects feel lighter (the size-weight illusion), confirming that the association between visual size and weight has a perceptual component. Darker objects also look heavier than brighter but otherwise identical objects. It is uncertain, however, if this association also has a perceptual element, or if it simply reflects the fact that, in English at least, the same verbal label (light) is applied to both surface brightness and weight. To address this, we looked for a brightness equivalent of the size-weight illusion. Paired-comparison judgments of weight were obtained for balls differing only in color. Based on vision alone, darker objects were judged to be heavier. When the balls were hefted as well as seen, this association was reversed (i.e., a brightness-weight illusion), consistent with it having a perceptual component. To gauge the strength of the illusion (in grams), a white and a black ball (both 129 g) were each compared against a set of mid-gray balls varying in weight. When the balls were hefted as well as seen, the white ball felt approximately 8 g heavier than the black ball, a difference corresponding to 6.2% of their actual weight. Possible environmental origins of the association between surface lightness and weight are considered.