Acuity of Proprioceptive Localization Varies with Body Region

We accurately sense locations of objects touching various points on the body and, if they are irritants, make accurate rapid movements to remove them. Such movements require accurate proprioception of orientation and motion of the reaching limb and of the target. However, it is unknown whether acuity of these sensations is similar for different points on the body. We investigated accuracy of comfortable speed reaching movements of the right index-tip by 10 subjects (five females) to touch 12 different body locations with and without vision with the body part stationary in different locations and moving in different directions. Reaching movements to points on the face/head and trunk had mean errors averaging less than 0.2 cm greater than under vision conditions. Mean errors for reaches to touch points on the left arm and digits were less accurate (p < 0.05), but average less than 1 cm relative to vision conditions. Mean errors for reaches to touch points on the left lower limb were least accurate (p < 0.05), with mean errors averaging 1.5-3.1 cm relative to movements made with vision. We conclude that there is high proprioceptive acuity for locations of points on axial structures and the left upper limb including the digits, which contrasts with previous reports of greatly distorted proprioceptive maps of the face/head and hand. Apparently low proprioceptive acuity for points on the leg may be task sensitive as many lower limb motor tasks can be performed accurately without vision.