CONTRIBUTION OF TACTILE AFFERENT INFORMATION TO THE CONTROL OF ISOMETRIC FINGER FORCES

The ability to match the voluntary isometric force output of the right and left index fingers when the contact surfaces differ in shape was examined. Before the experiment, subjects were trained to produce both a ''low'' force level (50+/-25 g) and a ''high'' force level (200+/-5 0 g) with the right and left index finger, separately. Following the t raining session, subjects were instructed to match the forces of both fingers simultaneously within the required range (either low or high) so that the forces were perceived to be identical. One of the index fi ngers pushed against a conical contact pad, while the other pushed aga inst a flat contact pad. Midway through the experiment, the two contac t pads were reversed. Subjects consistently produced less force with t he finger pressing against the conical pad. This asymmetry could alrea dy be seen during the beginning of the ramp increase in force and cont inued throughout the trial, independent of the target force levels (lo w or high). These findings suggest tactile afferent information at the fingertip is important for determining the voluntary force exerted by the finger. It must be properly integrated with other peripheral info rmation as well as with the central motor command, otherwise the perce ption of force is distorted. Furthermore, the perception of the force produced seemingly is dependent on the extent to which the skin of the fingertip is indented rather than the local pressure exerted at the s kin.