Direction-dependent integration of vision and proprioception in reaching under the influence of the mirror illusion.

Recent models of multisensory integration predict differential weighting of information from different sensory modalities in different spatial directions. This direction-dependent weighting account suggests a heavier weighting for vision in the azimuthal (left-right) direction and a heavier weighting for proprioception in the radial (near-far) direction. Visually induced reaching errors, as demonstrated in previous 'mirror illusion' reaching experiments, should therefore be greater under visual-proprioceptive conflict in the azimuthal direction than in the radial direction. We report two experiments designed to investigate the influence of direction-dependent weighting on the visual bias of reaching movements under the influence of a mirror-illusion. In Experiment 1, participants made reaches straight forward, and showed terminal reaching errors that were biased by vision in both directions, but this bias was significantly greater in the azimuthal as compared to the radial direction. In Experiment 2, participants made reaches from right to left, and showed a significant bias only in the azimuthal direction. These results support the direction-dependent weighting of visual and proprioceptive information, with vision relatively more dominant in the azimuthal direction, and proprioception relatively stronger in the radial direction.