Does visually induced self-motion affect grip force when holding an object?

Accurate control of grip force during object manipulation is necessary to prevent the object from slipping, especially to compensate for the action of gravitational and inertial forces resulting from hand/object motion. The goal of the current study was to assess whether the control of grip force was influenced by visually induced self-motion (i.e., vection), which would normally be accompanied by changes in object load. The main task involved holding a 400-g object between the thumb and the index finger while being seated within a virtual immersive environment that simulated the vertical motion of an elevator across floors. Different visual motions were tested, including oscillatory (0.21 Hz) and constant-speed displacements of the virtual scene. Different arm-loading conditions were also tested: with or without the hand-held object and with or without oscillatory arm motion (0.9 Hz). At the perceptual level, ratings from participants showed that both oscillatory and constant-speed motion of the elevator rapidly induced a long-lasting sensation of self-motion. At the sensorimotor level, vection compellingness altered arm movement control. Spectral analyses revealed that arm motion was entrained by the oscillatory motion of the elevator. However, we found no evidence that grip force used to hold the object was visually affected. Specifically, spectral analyses revealed no component in grip force that would mirror the virtual change in object load associated with the oscillatory motion of the elevator, thereby allowing the grip-to-load force coupling to remain unaffected. Altogether, our findings show that the neural mechanisms underlying vection interfere with arm movement control but do not interfere with the delicate modulation of grip force. More generally, those results provide evidence that the strength of the coupling between the sensorimotor system and the perceptual level can be modulated depending on the effector.