Is interlimb transfer of force-field adaptation a cognitive response to the sudden introduction of load?

Abstract

Recently, Criscimagna-Hemminger et al. (2003) reported a pattern of generalization of force-field adaptation between arms that differs from the pattern that occurs across different configurations of the same arm. Although the intralimb pattern of generalization points to an intrinsic encoding of dynamics, the interlimb transfer described by these authors indicates that information about force is represented in a frame of reference external to the body. In the present study, subjects adapted to a viscous curl-field in two experimental conditions. In one condition, the field was introduced suddenly and produced clear deviations in hand paths; in the second condition, the field was introduced gradually so that at no point during the adaptation process could subjects observe or did they have to correct for a substantial kinematic error. In the first case, a pattern of interlimb transfer consistent with Criscimagna-Hemminger et al. (2003) was observed, whereas no transfer of learning between limbs occurred in the second condition. The findings suggest that there is limited transfer of fine compensatory-force adjustment between limbs. Transfer, when it does occur, may be primarily the result of a cognitive strategy that arises as a result of the sudden introduction of load and associated kinematic error.

Figure 1.

A, Subjects were trained with their right hand to move in a force-field environment and were tested for transfer of learning to their left hand. B, Abrupt-training condition in which the load was introduced suddenly (after 15 null-field trials). C, Gradual-training condition in which the force field was smoothly introduced. B, C, The initial angular deviation (mean ± 1 SE) is shown over the course of training for the right hand. Representative hand paths and compensatory forces for single subjects are shown. The dots show positions of the hand during movement and the vectors represent forces applied by the hand of the subject. deg.,Degree; dev., deviation.

Figure 3.

A, Control 1. Trials performed in the null-field condition. From left to right, end of familiarization with the left hand (before), null-field catch trials over the course of learning with the right hand in the gradual-training condition (during training), and first trial to test for transfer of aftereffect to the left hand (after). One sees that aftereffects develop during training with the right hand that do not transfer to the left hand. The bar plot shows the mean initial angular deviation across subjects (±1 SE). B, Control 2. Intralimb transfer of learning is observed in the gradual-training condition. The figure shows limb configurations and movement directions that were used in the two intralimb transfer tests, along with hand paths for each of the four control subjects (c1 to c4). Dashed lines show hand paths that correspond to force-field catch trials during the familiarization phase; solid lines show the first trial of each intralimb transfer test.

Figure 2.

Transfer of learning to the left hand is not observed when the load is applied gradually. There is partial transfer of learning in the abrupt condition. A, Movements of the left hand for single subjects. Dashed lines show hand paths for force-field catch trials that were interspersed during the familiarization phase before training. Solid lines show hand paths in the first trial of the transfer tests. Subjects s1 to s4 were from groups 1 to 4 (Table 1). B, Initial angular deviation averaged (mean ± 1 SE) in each trial of the transfer tests. The bar plots show the mean over subjects of the two force-field catch trials that were introduced during the familiarization phase and the mean (±1 SE) of the first test trials. The plot gives the first trial of the transfer test for the experimental group and, for the group of naive subjects, the first test trial with the left hand in the force field. deg., Degree; dev., deviation.