Spatial localization investigated by continuous pointing during visual and gravitoinertial changes

In order to accurately localize an object, human observers must integrate multiple sensory cues related to the environment and/or to the body. Such multisensory integration must be repeated over time, so that spatial localization is constantly updated according to environmental changes. In the present experimental study, we examined the multisensory integration processes underlying spatial updating by investigating how gradual modifications of gravitoinertial cues (i.e., somatosensory and vestibular cues) and visual cues affect target localization skills. These were assessed by using a continuous pointing task toward a body-fixed visual target. The "single" rotation of the gravitoinertial vector (produced by off-axis centrifugation) resulted in downward pointing errors, which likely were related to a combination of oculogravic and somatogravic illusions. The "single" downward pitch rotation of the visual background produced an elevation of the arm relative to the visual target, suggesting that the rotation of the visual background caused an illusory target elevation (induced-motion phenomenon). Strikingly, the errors observed during the "combined" rotation of the visual background and of the gravitoinertial vector appeared as a linear combination of the errors independently observed during "single" rotations. In other words, the centrifugation effect on target localization was reduced by the visual background rotation. The observed linear combination indicates that the weights of visual and gravitoinertial cues were similar and remained constant throughout the stimulation.