Posture, balance and movement: Role of the vestibular system in balance control during stance and movements

Abstract

Balance control and gait are controlled by many structures in the brain, but the vestibular system plays a very specific role in this control system. First of all, the vestibular labyrinths provides the detection of head orientation relative to the gravity vector with an accuracy better than 0.5 degrees, which is about 4–5× better than vision. This accurate detection of head orientation allows postural adaptation relative to gravity preventing imbalance and falls. Our experiments in healthy subjects (in complete darkness, with masked hearing) show that the vestibular-somatosensory threshold of the direction of movement (randomly supplied rotations–translations in 3D) and 2D tilt is remarkably different for the various movement planes. These thresholds increase markedly in bilateral vestibular deficit patients (BVD), despite the fact that somatosensory input is still available. This clearly showed that movement perception and spatial orientation strongly depend on vestibular input and that somatosensory substitution and central compensation are insufficient. However, it also appeared that perception of the gravity vector may fail in specific conditions when additional sensory input is completely absent, which did not hold for isolated visual or somatosensory input. Second, the vestibulo-spinal reflexes allow FAST corrections of balance in case of an unexpected balance perturbations. We could show, using the CAREN extended gait and balance analysis system, that in line with our expectation, gait and balance are severely impaired in BVD that were selected for vestibular implantation, as they were unable to produce adequate fast corrections. Increase of somatosensory feedback by a vibrotactile belt improved the intuitive gravity vector detection leading to a better balance and improved self-confidence, but did not restore the fast postural corrections needed for fast postural corrections.