MECHANISMS FOR FORCE ADJUSTMENTS TO UNPREDICTABLE FRICTIONAL CHANGES AT INDIVIDUAL DIGITS DURING 2-FINGERED MANIPULATION

Previous studies on adaptation of fingertip forces to local friction a t individual digit-object interfaces largely focused on static phases of manipulative tasks in which humans could rely on anticipatory contr ol based on the friction in previous trials. Here we instead analyze m echanisms underlying this adaptation after unpredictable changes in lo cal friction between consecutive trials. With the tips of the right in dex and middle fingers or the right and left index fingers, subjects r estrained a manipulandum whose horizontal contact surfaces were locate d side by side. At unpredictable moments a tangential force was applie d to the contact surfaces in the distal direction at 16 N/s to a plate au at 4 N. The subjects were free to use any combination of normal and tangential forces at the two fingers, but the sum of the tangential f orces had to counterbalance the imposed lend. The contact surface of t he right index finger was fine-grained sandpaper, whereas that of the cooperating finger was changed between sandpaper and the more slippery rayon. The load increase automatically triggered normal force respons es at both fingers. When a finger contacted rayon, subjects allowed sl ips to occur at this finger during the load force increase instead of elevating the normal force. These slips accounted for a partitioning o f the load force between the digits that resulted in an adequate adjus tment of the normal:tangential force ratios to the local friction at e ach digit. This mechanism required a fine control of the normal forces . Although the normal force at the more slippery surface had to be com paratively low to allow slippage, the normal fords applied by the nons lipping digit at the same time had to be high enough to prevent loss o f the manipulandum. The frictional changes influenced the normal force s applied before the load ramp as well as the size of the triggered no rmal force responses similarly at both fingers, that is, with rayon at one contact surface the normal forces increased at both fingers. Thus to independently adapt fingertip forces to the local friction the nor mal forces were controlled at an interdigital level by using sensory i nformation from both engaged digits. Furthermore, subjects used both s hort- and long term anticipatory mechanisms in a manner consistent wit h the notion that the central nervous system (CNS) entertains internal models of relevant object and task properties during manipulation.