Directional sensibility of hairy skin and postural control.

People can feel and report the direction of very small movements which cause changes in the tension of the forearm's hairy skin. This subjective sensory function may perhaps reflect more fundamental sensorimotor tasks. The hypothesis was investigated by measuring body sway and movement of six female and male volunteers who were performing the tandem-stance Romberg test with open and closed eyes. The increase in sway and movement after eye closure was reduced significantly when the subjects were allowed to use one forearm to touch a spatially fixed object from below. Three objects were used, a solid Perspex rod, an easily rotating steel ball, and a pointed metal peg whose tip was attached to the skin with a droplet of contact glue. Possible mechanical support could be excluded on basis of the objects' technical properties and the magnitudes of forearm movements. Movement of the forearm relative to an object could provide spatial information about changes of the forearm's position in space. Likewise, changes of skin tension that were caused by such movements could be useful. The Perspex rod and the steel ball might provide both types of information. However, the glued peg only caused changes of skin tension, but reduced sway and movement equally effectively. Therefore, information from tension receptors of the forearm's hairy skin underlying the accurate subjective directional sensibility also appears to participate in basic motor control.