Vestibular feedback maintains reaching accuracy during body movement

When using our arms to interact with the world, unintended body motion can introduce movement error. A mechanism which could detect and compensate for such motion would be beneficial. Observations of arm movements evoked by vestibular stimulation provide some support for this mechanism. However, the physiological function underlying these artificially-evoked movements is unclear from previous research. For such a mechanism to be functional, it should only operate when the arm is being controlled in an earth-fixed rather than body-fixed reference frame. In the latter case, compensation would be unnecessary and even deleterious. To test this hypothesis, subjects were gently rotated in a chair while asked to maintain their outstretched arm pointing either towards earth-fixed (EF) or body-fixed (BF) memorised targets. Galvanic vestibular stimulation (GVS) was applied concurrently during rotation to isolate the influence of vestibular input, uncontaminated by inertial factors. During the EF task, GVS produced large polarity-dependent corrections in arm position. These corrections mimicked those evoked when chair velocity was altered without any GVS, indicating a compensatory arm response to a sensation of altered body motion. In stark contrast, corrections were completely absent during the BF task, despite the same chair movement profile and arm posture. These effects persisted when we controlled for differences in limb kinematics between the two tasks. Our results demonstrate that vestibular control of the upper-limb maintains reaching accuracy during unpredictable body motion. The observation that such responses only occurred when reaching within an EF reference frame confirms the functional nature of vestibular-evoked arm movement.