Haptic illusion during finger-held disk rotation: revisiting a deceptively simple illusion

The "disk-rotating" haptic illusion reported by Cormack (1973), in which a disk or coin held in one hand and turned end over end between thumb and forefinger of the other hand is perceived as longer by the turning hand. This illusion may occur because of the interplay of the rotating and holding hands. To separate the contributions of each hand, two custom-made devices were used to allow independent rotation and holding of the disk. Twenty participants handled disks (30 and 39 mm in diameter), and rotated them for one minute under the following conditions: (a) ordinary use of both hands, (b) using both hands but turning and holding the disk using the two custom-made devices simultaneously, and (c) using only one hand for turning or holding. The magnitude of illusion increased to a level comparable to that reported by Cormack (1973) under conditions (a) and (b). However, under condition (c), the illusion for the rotating hand grew about 25% of those obtained under (a) and (b), with almost no illusion for the holding hand. Nevertheless, reconstruction of the estimated illusion using each magnitude of illusion obtained for the "one-hand" conditions of (c), this estimation showed a moderate illusion growing gradually. Rotation by the turning hand may mainly contribute to the illusion, while holding hand is involved with this illusion interactively. The results are discussed along with adaptation, aftereffect, and contrast effect, which may be involved in processing of bimanual signals underlying this illusion.