PERCEPTION OF ARM ORIENTATION IN 3-DIMENSIONAL SPACE

The purpose of this investigation was to determine the preferred coord inate system for perception of arm (humerus) orientation in three-dime nsional space. Perception of arm orientation relative to trunk-fixed v ersus earth-fixed axes were compared in seven human subjects. The expe rimenter first moved the subject's trunk and arm into a target configu ration (in which the arm's orientation relative to the trunk and/or ea rth was perceived and memorized by the subject) and then moved the tru nk and arm to a new configuration. The blindfolded subject then attemp ted to reproduce the target orientation of their arm relative to eithe r the trunk (i.e., reproduce shoulder angles - intrinsic kinesthetic c oordinate system) or earth-fixed axes (extrinsic kinesthetic coordinat e system). Perceptual errors were similar for both shoulder (arm relat ive to trunk) and extrinsic (arm relative to earth) angles. However, e levation angles were perceived with greater accuracy than yaw angles i n the two coordinate systems. Also, perceptual errors for arm yaw angl es in the extrinsic kinesthetic coordinate system task were better pre dicted from changes in trunk orientation than the errors for other ang les. Furthermore, four subjects matched arm yaw angle relative to the trunk-fixed axis more accurately than to the earth-fixed axis in the e xtrinsic coordinate system task. These results suggests a bias toward perception of yaw angles relative to trunk-fixed axes (i.e., in an int rinsic coordinate system). These data suggest that the preferred coord inate system for kinesthetic perception of arm orientation is probably fixed in the trunk. Sensory receptors in soft tissues surrounding the shoulder joint can provide sensations related directly to intrinsic ( shoulder) angles, but not to angles of the arm in relation to external axes. However, elevation angles of the arm are perceived with about e qual accuracy in relation to the trunk and the gravitational axis. Acc urate perceptions of the angle of the arm with respect to gravity may be important for computations of the shoulder joint torques needed whe n producing upper limb movements.