Kinematics and dynamics are not represented independently in motor working memory: evidence from an interference study

Abstract

Our capacity to learn multiple dynamic and visuomotor tasks is limited by the time between the presentations of the tasks. When subjects are required to adapt to equal and opposite position-dependent visuomotor rotations (Krakauer et al., 1999) or velocity-dependent force fields (Brashers-Krug et al., 1996) in quick succession, interference occurs that prevents the first task from being consolidated in memory. In contrast, such interference is not observed between learning a position-dependent visuomotor rotation and an acceleration-dependent force field. On the basis of this finding, it has been argued that internal models of kinematic and dynamic sensorimotor transformations are learned independently (Krakauer et al., 1999). However, these findings are also consistent with the perturbations interfering only if they depend on the same kinematic variable. We evaluated this hypothesis using kinematic and dynamic transformations matched in terms of the kinematic variable on which they depend. Subjects adapted to a position-dependent visuomotor rotation followed 5 min later by a position-dependent rotary force field either with or without visual feedback of arm position. The force field tended to rotate the hand in the direction opposite to the visuomotor rotation. To assess learning, all subjects were retested 24 hr later on the visuomotor rotation, and their performance was compared with a control group exposed only to the visuomotor rotation on both days. Adapting to the position-dependent force field, both with and without visual feedback, impaired learning of the visuomotor rotation. Thus, interference between our kinematic and dynamic transformations was observed, suggesting that the key determinant of interference is the kinematic variable on which the transformation depends.

Fig. 1.

Experimental setup. Subjects moved a force-reflecting manipulandum between targets in a horizontal plane. The targets were virtual spheres presented using a three-dimensional projection system with shutter glasses. The force exerted by the manipulandum was servo-controlled to create a position-dependent rotary force field, and visual feedback was altered to create a position-dependent visuomotor rotation.

Fig. 2.

Performance in the first and last cycles under the visuomotor rotation (a) and elastic force field with visual feedback (b). The gray curves show the individual paths of the cursor representing hand position for a single subject (different subjects are shown ina and b). The straight solid lines indicate the direction and extent of the corresponding targets. For the first cycle, two movement paths (directed to opposing targets) are shown in each plot. For the last cycle, all eight paths are shown in one plot. Because the force field was presented after adaptation to the visuomotor rotation, subjects initially directed their movements 30° clockwise to the targets. These rotated directions are indicated by the straight dashed lines inb. In the first cycle, large errors were observed under both transformations. However, after 30 cycles, these errors were greatly reduced.

Fig. 3.

Adaptation to the visuomotor rotation. Curves show mean angular error (between movement and target directions) as a function of cycle on day 1 (solid lines) and day 2 (dashed lines). The height of the gray area represents ±1 SE. a shows results for the control group, who adapted only to the visuomotor rotation on day 1.b and c show results from two groups, who on day 1 adapted to the visuomotor rotation and 5 min later adapted to the position-dependent rotary force field either with (a) or without (b) visual feedback. The two groups exposed to the force field on day 1 exhibited greater directional errors at the start of day 2 than did the control group.

Fig. 4.

Initial and final angular errors under the visuomotor rotation. The height of each white barrepresents the angular error averaged over the second and third cycles; the height of each gray bar represents the corresponding angular error averaged over the 29th and 30th cycles. The bars outlined with solid and dashed lines represent days 1 and 2, respectively. Separate means are reported for each of the three experimental groups. Vertical lines represent the SE.

Fig. 5.

Adaptation to the elastic rotary force field.a, Distance traveled by the hand as a function of cycle (normalized for maximum displacement of the hand). Thedashed and solid lines represent mean distances for the visual feedback and no-visual-feedback groups, respectively. The height of the gray area around thedashed curve and the gray linearound the solid curve represent ±1 SE.b, Corresponding mean angular errors for the two groups. For both groups, the magnitude of the angular error decreased as the visuomotor rotation previously adapted to was gradually unlearned during adaptation to the force field.