Learning motor synergies makes use of information on muscular load

Abstract

Prism adaptation, a form of procedural learning, requires the integration of visual and motor information for its proper acquisition. Although the role of the visual feedback has begun to be understood, the nature of the motor information necessary for the development of the adaptation remains unknown. In this work we have tested the idea that modifying the arm load at different stages of the adaptation process, and the ensuing change of motor information perceived by the subjects, would modify the final properties of the adaptation. We trained a set of subjects to throw balls to a target while wearing prism glasses and varied the weight of their arms at different time points during the task. We observed that the acquisition of the adaptation was not affected by the change in load. However, its persistence (i.e., the aftereffect) was reduced when tested under a weight condition different from the training trials. Furthermore, when the training weight conditions were restored later during testing, a second, late aftereffect was unmasked, suggesting that the missing aftereffect did not disappear but had remained latent. Our results show that the internal representation of a motor memory incorporates information about load conditions and that the memory stored under a specific weight condition can be fully retrieved only when the original training condition is restored.

Figure 1

Experimental design. The thicker lines represent the period during which weights were used to change the muscular tension of the arm. Groups 2 and 4 wore 750-gram bracelets, whereas groups 3 and 5 wore 1500-gram bracelets. (PRE) Throws before donning 30-diopter lenses; (PRI) throws with the lenses on; (POS) throws after removing the lenses.

Figure 2

Prism adaptation and aftereffect in groups 1, 4, and 5. Plots of trial number vs. horizontal displacement from the target for throws made before, while, and after the subjects donned 30-diopter prism lenses. The addition of weight to the throwing arm (groups 4 and 5) during the PRE and PRI trials had no effect on the performance of the task or on the onset of the adaptation. However, the weight–no weight transition between PRI and POS was associated with a smaller aftereffect. Furthermore, the reinstatement of the load halfway through the POS trials led to the appearance of a second, late aftereffect in both experimental groups. Data represent means ±s.e.m.

Figure 3

Prism adaptation and aftereffect in groups 1, 2, and 3. Plots of trial number vs. horizontal displacement from the target for throws made before, while, and after the subjects donned 30-diopter prism lenses. The addition of weight to the throwing arm (groups 2 and 3) had no effect on the performance of the task, the onset of the adaptation, or the magnitude of the aftereffect. Similarly, the removal of the load halfway through the POS trials did not reveal any latent aftereffect in either group. Data represent means ±s.e.m.

Figure 4

(A) Magnitude of the aftereffect (distance from the target in centimeters) at the beginning of, and halfway through, the POS trials. Groups 4 and 5 exhibited a smaller aftereffect on the first POS throw (light bars) as a result of the load–no load shift. The reinstatement of the load at the fourteenth throw (dark bars) was accompanied by a second aftereffect. In contrast, load transitions in groups 2 and 3 had no effect. (B) Similar results were observed for the aftereffect persistence (accumulated distance from the target in centimeters). (*) P < 0.05 as compared with the control group. Data represent means±s.e.m.

Figure 4

(A) Magnitude of the aftereffect (distance from the target in centimeters) at the beginning of, and halfway through, the POS trials. Groups 4 and 5 exhibited a smaller aftereffect on the first POS throw (light bars) as a result of the load–no load shift. The reinstatement of the load at the fourteenth throw (dark bars) was accompanied by a second aftereffect. In contrast, load transitions in groups 2 and 3 had no effect. (B) Similar results were observed for the aftereffect persistence (accumulated distance from the target in centimeters). (*) P < 0.05 as compared with the control group. Data represent means±s.e.m.

Figure 5

Direct comparison between the horizontal displacements recorded for the two throws at the load transitions (throw 13, light bars vs. throw 14, dark bars) during the POS trials. Significant differences (i.e., a late aftereffect) were observed only for cases in groups 4 and 5 where the reintroduction of the weight re-established the conditions under which the initial adaptation occurred. (*) P < 0.05 vs. the corresponding thirteenth throw. Data represent means ±s.e.m.