A haptic illusion created by gravity
Haptic force estimation is a critical aspect of human dexterity. To manipulate objects or interact with a haptic interface successfully, the normal and tangential components of the contact forces produced by our fingertips must be carefully coordinated. The tight coupling between tangential and normal contact forces observed during voluntary movements indicates that the nervous system can estimate these forces with high accuracy. Here, we examined the influence of gravity on manual force production in an isometric task. We trained participants to produce isometric tangential forces with the thumb and index finger on a dynamometer held in precision grip. They were tasked with reproducing these forces in a normal gravitational environment (1 g) and during parabolic maneuvers creating phases of micro- (0 g) and hypergravity (1.6 g). The isometric task results showed that arm weight biases estimation of the forces exerted by the fingertips. Reproduced tangential forces were consistently larger downward than upward in 1 g. This asymmetry was reduced under microgravity and increased under hypergravity. Critically, the normal forces did not reflect this asymmetry, demonstrating that tangential forces were misestimated and pointing to a haptic illusion created by gravity. This gravitational effect on haptic force estimation may have implications for the design of prostheses incorporating force feedback, haptic devices for teleoperations, or haptic supports aimed at improving sensorimotor performance in space.