Generalization of object manipulation skills learned without limb motion

Abstract

Recent work suggests that human subjects may learn mappings between object motion and exerted torque during manipulation of freely pivoting or unstable objects. In the present work, we studied an object manipulation task involving no arm movement to determine how subjects internally represent the force-motion relationship of an object during a skilled manipulation task. Human subjects learned to balance a simulated inverted pendulum. The simulation was controlled by pressing on a fixed force sensor, and applied forces resulted in motion of the simulated pendulum on a computer screen according to its equation of motion. Each subject initially learned the task in one arm posture and was tested 1 d later in a new arm posture. In one test condition, the effects of arm torque were matched to the original task, and in the other test condition, the simulation was unchanged. The pattern of skill transfer to different arm postures suggested that subjects had learned joint torque responses rather than a general model of the object interface forces. A second experiment showed that the advantage of training with matched arm torques was object specific, because torque-matched training on a tracking task involving similar forces was not a substitute for training in the balancing task.

Figure 1.

Experimental setup. Subjects controlled a virtual cart—pole system by pressing on a force sensor in either the X or Y directions. Arm segments were at an angle of ∼0 or 90° with respect to the body. A leftward inclination of virtual pole from the vertical (dotted) corresponded to a positive angle θ. The force drove the entire cart-pole system to the left or right across the screen and caused corresponding changes in the angle of inclination of the pole as the pole pivoted about its base (dashed, solid transition).

Figure 2.

Experimental protocol. Arm configurations of experimental subjects (S1—S9) during training on day 1 (first column) and during testing on day 2 (second and third columns). The position of the hand is indicated by a circle. Arrows indicate the direction of exerted force that would move the cart in the cart—pole simulation to the right. T, Matched torque between training and test; F, matched force between training and test; LH, left-handed subjects. Subjects S1 and S2 received analogous test sequences with reversed test orders, as did subjects S5 and S6. Subject S9 had an inverted relationship between exerted force and screen movement at test 2. Subject S3 received the same sequence as S9 but without the inverted relationship seen by subject S9.

Figure 3.

Balancing performance for a single subject. Time course of the displayed pole angle during balancing trials for subject S7. Each panel shows performance on three individual trials. ○, First trial; ⋄, second trial; and ▿, third trial. The top two panels show the first (top left panel) and the last (top right panel) three trials on day 1. The first three trials in the matched torque condition on day 2 are shown in the bottom left panel, and the first three trials in the matched force condition on day 2 are shown in the bottom right panel. The angle is plotted every 0.5 sec over the trial. The displayed pole angle was 20 times the true angle. Trials ended when the angle exceeded ±360°, corresponding to a true angle of 18°. T, Time.

Figure 4.

Individual balancing performance in the test conditions. Individual differences between performance measures (S, SDX, and TTF) in the test conditions and in the naive condition. The S and SDX panels show naive minus test, and the TTF panel shows test minus naive. Left panels show results for experiment 1 and right panels show results for experiment 2. The top panels show the results for S, the middle panels show SDX, and the bottom panels show TTF. F, Matched force condition (experiment 1); T, matched torque condition; Bal, matched balancing condition (experiment 2); and Tr, matched tracking condition. Subjects whose test order was the reverse of that shown are indicated by a dotted line, and the rest are indicated by solid lines.