KINESTHETIC PERCEPTIONS OF INTRINSIC ANTERIOR-POSTERIOR AXES

The purpose of this research was to investigate whether kinesthetic an d proprioceptive perceptions of ''straight ahead'' were defined by a h ead-or trunk-fixed axis. Subjects were instructed to align the forearm with the head or trunk anterior-posterior (a/p) axis by elbow flexion or extension in the horizontal plane in five different conditions. Ln each condition the experimenter varied initial elbow and shoulder hor izontal flexion or extension angles and head and/or trunk orientation (by rotation about a vertical axis) on each trial before the subject m oved the forearm to align it with the head or trunk axis. The upper li mb motion was voluntarily constrained to the horizontal plane through the shoulder. Variable errors were significantly lower when subjects a ligned the forearm to the trunk-fixed a/p axis. Furthermore, the perce ptual errors showed a greater dependence on body segment orientations when the forearm was aligned to the head axis than to the trunk axis. We conclude that the trunk a/p axis is preferred to the head a/p axis for specifying upper limb segment orientations in the horizontal plane at the kinesthetic perceptual level.