Does visually - induced self - motion affect grip force when holding an object ? 1 2 3

24 Accurate control of grip force during object manipulation is necessary to prevent the 25 object from slipping, especially to compensate for the action of gravitational and inertial 26 forces resulting from hand/object motion. The goal of the current study was to assess whether 27 the control of grip force was influenced by visually-induced self-motion (i.e., vection) which 28 would normally be accompanied by changes in object load. The main task involved holding a 29 400g object between the thumb and the index finger while being seated within a virtual 30 immersive environment that simulated the vertical motion of an elevator across floors. 31 Different visual motions were tested, including oscillatory (0.21 Hz) and constant speed 32 displacements of the virtual scene. Different arm-loading conditions were also tested: with or 33 without the hand-held object, and with or without oscillatory arm motion (0.9 Hz). At the 34 perceptual level, ratings from participants showed that both oscillatory and constant speed 35 motion of the elevator rapidly induced a long lasting sensation of self-motion. At the 36 sensorimotor level, vection compellingness altered arm movement control. Spectral analyses 37 revealed that arm motion was entrained by the oscillatory motion of the elevator. However we 38 found no evidence that grip force used to hold the object was visually affected. Specifically, 39 spectral analyses revealed no component in grip force that would mirror the virtual change in 40 object load associated with the oscillatory motion of the elevator, thereby allowing the grip41 to-load force coupling to remain unaffected. Altogether, our findings show that the neural 42 mechanisms underlying vection interfere with arm movement control but do not interfere with 43 the delicate modulation of grip force. More generally those results provide evidence that the 44 strength of the coupling between the sensorimotor system and the perceptual level can be 45 modulated depending on the effector. 46