The influence of arm posture on the Uznadze haptic aftereffect

The role of arm posture in the Uznadze haptic aftereffect is investigated: two identical test stimuli (i.e., spheres, TS) clenched simultaneously appear haptically different in size after hands have been adapted to two spheres (adapting stimuli, AS) differing in size: the hand adapted to a small AS feels TS bigger than the hand adapted to a big AS. In two experiments, participants evaluated the haptic impressions of two TS after adaptation by finding their match on a visual scale. In Experiment 1, all tasks were carried out with arms either uncrossed or crossed. In Experiment 2, only the matching task was performed with arms either uncrossed or crossed while adaptation was conducted by continuously changing arm posture from uncrossed to crossed and vice versa. The illusion occurred irrespectively of arm posture; however, its magnitude was smaller when adaptation was carried out in the classical condition of uncrossed arms. Results are discussed in light of two functional mechanisms: low-level somatotopic mapping (i.e., stimuli conformation) and high-level level factors (i.e., arm posture) that could modulate the haptic perception.