Stabilization of the total force in multi-finger pressing tasks studied with the 'inverse piano' technique

Abstract

When one finger changes its force, other fingers of the hand can show unintended force changes in the same direction (enslaving) and in the opposite direction (error compensation). We tested a hypothesis that externally imposed changes in finger force predominantly lead to error compensation effects in other fingers thus stabilizing the total force. A novel device, the "inverse piano", was used to impose controlled displacements to one of the fingers over different magnitudes and at different rates. Subjects (n=10) pressed with four fingers at a constant force level and then one of the fingers was unexpectedly raised. The subjects were instructed not to interfere with possible changes in the finger forces. Raising a finger caused an increase in its force and a drop in the force of the other three fingers. Overall, total force showed a small increase. Larger force drops were seen in neighbors of the raised finger (proximity effect). The results showed that multi-finger force stabilizing synergies dominate during involuntary reactions to externally imposed finger force changes. Within the referent configuration hypothesis, the data suggest that the instruction "not to interfere" leads to adjustments of the referent coordinates of all the individual fingers.

Figure 1

A. Schematic of force sensors mounted on linear motors. Linear motors produced motion of the force sensor in the vertical direction. The letters I, M, R, and L refer to the keys corresponding to the index, middle, ring and little fingers, respectively. In the schematic the key under the ring finger is raised. B. Illustration of feedback given to subjects. A target range (±5.0 % MVC) was given. The target was shown for the first 5 seconds, disappeared for the next 5 seconds during the key raising/lowering, and then returned for the last 3 seconds of the trial. C. Example force data from a single trial. Change in force was computed as the forces after the key was raised minus the background force before the key was raised. The background forces were the average finger forces 250 ms before the key was raised. D. Example full wave rectified EMG data from a single trial. EMG amplitude was averaged for 500 ms before key was raised and averaged from start of motor movement to cessation of key movement. The averages in these two windows were compared to see if there was a difference in muscle activity during key raising.

Figure 1

A. Schematic of force sensors mounted on linear motors. Linear motors produced motion of the force sensor in the vertical direction. The letters I, M, R, and L refer to the keys corresponding to the index, middle, ring and little fingers, respectively. In the schematic the key under the ring finger is raised. B. Illustration of feedback given to subjects. A target range (±5.0 % MVC) was given. The target was shown for the first 5 seconds, disappeared for the next 5 seconds during the key raising/lowering, and then returned for the last 3 seconds of the trial. C. Example force data from a single trial. Change in force was computed as the forces after the key was raised minus the background force before the key was raised. The background forces were the average finger forces 250 ms before the key was raised. D. Example full wave rectified EMG data from a single trial. EMG amplitude was averaged for 500 ms before key was raised and averaged from start of motor movement to cessation of key movement. The averages in these two windows were compared to see if there was a difference in muscle activity during key raising.

Figure 2

The effect of a non-raised finger location with respect to the raised finger (‘proximity’) on the force change (‘compensation’). The proximity values of 1, 2, and 3 refer to the distance of non-raised finger in relation to the raised finger. Values were averaged across all trials. Error bars are standard error.