A reexamination of the size-weight illusion induced by visual size cues.

The size-weight illusion induced by visually perceived sizes was reexamined to investigate whether this illusion is a sensory based or cognitive-based phenomenon. A computer-augmented environment was utilized to manipulate visual size information of target objects independently of their haptic information. Two physical cubes of equal mass (30.0 g) and size (3.0 x 3.0 x 3.0 cm) were suspended in parallel by wires attached to small graspable rings, in order to keep haptically obtained information constant between lifts. Instead of directly seeing each physical cube, subjects viewed 3D graphics of a cube with a wire and a ring that were precisely superimposed onto each physical cube. Seventeen subjects vertically lifted these augmented cubes, one after the other, by grasping the attached rings, and then reported their perception of cube heaviness. The graphical size of a comparison cube pseudo randomly varied for every comparison from 1.0 x 1.0 x 1.0 to 9.0 x 9.0 x 9.0 cm, while that of a standard cube remained constant (5.0 x 5.0 x 5.0 cm). Results indicated that the size-weight illusion frequently and systematically occurred for all the subjects such that when the comparison cube was relatively smaller than the standard cube, it was perceived to be heavier, and vice versa. As the size difference increased between the standard cube and the comparison cube, more subjects experienced the illusion, and vice versa. Follow-up tests showed occurrence of the size-weight illusion was significantly correlated with subject's sensitivity to discriminate weight, but not with sensitivity to discriminate visual size. Results suggest that the size-weight illusion induced by only visual size cues in an augmented environment is sensory based, and depends on an individual's integrated perception based on multimodal sensory information.