When mirrors lie: “Visual capture” of arm position impairs reaching performance

If we stand at a mirror’s edge, we can see one half of our body reflected in the mirror, as if it were the other half of our body, seen “through” the mirror. We used this mirror illusion to examine the effect of conflicts between visually and proprioceptively specified arm positions on subsequent reaching movements made with the unseen right arm. When participants viewed their static left arm in the mirror (i.e., as if it were their right arm), subsequent right-arm reaching movements were affected significantly more when there was conflict between the apparent visual and the proprioceptively specified right-arm positions than when there was no conflict. This result demonstrates that visual capture of arm position can occur when individual body parts are viewed in the mirror and that this capture has a measurable effect on subsequent reaching movements made with an unseen arm. The result has implications for how the brain represents the body across different sensory modalities.