Anticipatory postural adjustment in the absence of normal peripheral feedback.

The response to voluntary unloading (by the subject's contralateral arm) and imposed unloading (by the experimenter) of a 1 kg weight supported at the wrist were studied in normal volunteers (controls) and in a deafferented subject (patient). The patient had no touch, pressure or kinesthetic sensations in either of the arms, but the motor nerve fibers were unaffected. The reflex activity generated by imposed unloading in the controls was never observed in the patient. The displacement's amplitude of the unloaded forearm was 3 x smaller with voluntary than with imposed unloading. In both the controls and the patient, the displacement was of similar amplitude and preceded by an anticipatory postural adjustment. It is concluded that this postural adjustment is of central origin since it can be generated in the absence of peripheral feedback.