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.

High proprioceptive acuity in slow and fast hand movements.

We can accurately reach to touch our index fingertip to various points on the body without vision. Awareness of location/motion of the index fingertip and other body parts through proprioception is required for such movements. Proprioception involves processing sensory information, but it is also debated whether internal model estimates of body state from motor commands improve proprioception. We tested the hypothesis that proprioceptive errors increase with increases in speed of hand movement and whether an internal model contributes to more accurate proprioception, especially in higher speed movements. Ten subjects made voluntary reaching movements with their dominant arm to touch its index-tip to the index-tip of the non-dominant arm that was moved passively or actively at three speeds (slow, comfortable, fast) in various directions. Four conditions required the experimenter to passively move the subject's target arm at slow, comfortable and fast speeds and in different directions. A fifth condition required the subject to actively move both arms to perform the task. Subjects performed these tasks with high accuracy during slow and comfortable speed movements of the target arm. Errors averaged 3.7 mm larger when the target was moved faster and were equivalent to errors for slower movements (p < 0.014). Errors in the active and passive target movement conditions were also equivalent (p < 0.001). These findings show that proprioception is accurate across many different speeds of passive and active target motion and that there was no evidence than an internal model contributes to improved accuracy of proprioception during active movements.