A comparison of force adaptation in toddlers and adults during a drawer opening task

Abstract

Adapting movements to rapidly changing conditions is fundamental for interacting with our dynamic environment. This adaptability relies on internal models that predict and evaluate sensory outcomes to adjust motor commands. Even infants anticipate object properties for efficient grasping, suggesting the use of internal models. However, how internal models are adapted in early childhood remains largely unexplored. This study investigated a naturalistic force adaptation task in 1.5-, 3-year-olds, and young adults. Participants opened a drawer with temporarily increased resistance, creating sensory prediction errors between predicted and actual drawer dynamics. After perturbation, all age groups showed lower peak speed, longer movement time, and more movement units with trial-wise changes analyzed as adaptation process. Results revealed no age differences in adapting peak speed and movement units, but 1.5- and 3-year-olds exhibited higher trial-to-trial variability and were slower in adapting their movement time, although they also adapted their movement time more strongly. Upon removal of perturbation, we found significant aftereffects across all age groups, indicating effective internal model adaptation. These results suggest that even 1.5-year-olds form internal models of force parameters and adapt them to reduce sensory prediction errors, possibly through more exploration and with more variable movement dynamics compared to adults.

Keywords: Children; Cognition; Development; Motor adaptation; Motor learning.

Fig. 1

Hypothetical adaptation and deadaptation process for (a) peak speed, (b) movement time, and (c) movement units. The gray area represents adaptation trials (13–24) with increased drawer resistance. Initially, peak speed should decrease and movement time and movement units increase, signaling initial errors (II). Over time, participants should adapt, so increasing peak speed and decreasing movement time and movement units to baseline average, reflecting low residual errors (III). During the deadaptation block, we expect aftereffects characterized by overshooting peak speed and undershooting movement time (IV). No clear hypothesis for movement units was formulated because participants could either open the drawer fast and stop the drawer at the end of movement (IVa) or stop the movement directly after recognizing the low resistance and then accelerate the drawer again (IVb). By the end of deadaptation, low residual errors are anticipated if internal models were adapted (V).

Fig. 2

Experimental setup. (a) The fully opened drawer from the perspective of a participant. The slot was only accessible at the very end so the participants had to open the drawer completely to get cubes out of the drawer that were placed into the box cut out on the participant’s left. The red circle highlights the drawer knob that was used to open the drawer with the right hand. (b) The experimenter’s view with cubes to motivate participants and a weight to perturb the resistance of the drawer.

Fig. 3

Speed profiles of drawer movement in the adaptation block. Drawer speed for averaged baseline trials 3–12 (yellow), first adaptation trial (dashed pink), averaged last three adaptation trials (solid pink), and all remaining adaptation trials (from light gray to dark gray with increasing trial number) is depicted for (a) 1.5-year-olds, (b) 3-year-olds, and (c) adults. The first adaptation trial (dashed pink) showed the slowest drawer speed and the longest movement time. By the end of the adaptation block, the solid pink line (representing the average of the last three adaptation trials) was close to the baseline average (yellow line) in each age group. Descriptively, 1.5- and 3-year-olds showed longer movement time (later intersection with x-axis) and more movement units (more local maxima) compared to adults. On the top right of each panel speed profiles of one example participant of each age group are presented.

Fig. 4

Perturbation effects for (a) peak speed, (b) movement time, and (c) movement units for 1.5-years-old (orange), 3-years-old (green), and adults (blue). Averages are depicted in circles; error bars reflect the standard errors. The adaptation block in which the drawer resistance was increased is marked in gray. The initial error is shown as a difference between the first adaptation trial and the baseline average, the residual error (adaptation) shows the difference between the average of the last three adaptation trials and the baseline average. The aftereffect indicates the difference between the first deadaptation trial and the baseline average. The residual error (deadaptation) shows the difference between the average of the last three deadaptation trials and the baseline average. For successful perturbation of drawer resistance, initial error and aftereffect should differ from baseline average. Residual errors should be close to the baseline average to indicate that the dependent variable has been fully (de-)adapted to the baseline average.

Fig. 5

Trial-wise (de-)adaptation for (a) peak speed, (b) movement time, and (c) movement units averaged across 1.5-year-olds (orange), 3-year-olds (green) and adults (blue) for each trial with shaded error bars showing the standard errors. The adaptation block (trials 13–24) in which the drawer resistance was increased is marked in gray. On the top right of each panel, the theoretical behavior for each variable is illustrated.

Fig. 6

Speed profiles of drawer movement in the deadaptation block. Drawer speed for the averaged baseline trials (yellow), averaged last three adaptation trials (pink), first deadaptation trial (dotted purple) and averaged last three deadaptation trials (solid purple), and all remaining trials (from light gray to dark gray) for (a) 1.5-year-olds, (b) 3-year-olds, and (c) adults. At the beginning of the deadaptation block, the drawer speed overshot the last adaptation trial value (dotted purple line) in all three age groups. In tendency, this overshoot decreased with ongoing trials (solid purple line) in the direction to the baseline level (yellow line) and the last adaptation trials (pink line) in each age group. However, it seems that all age groups did not fully deadapt the drawer speed profiles to the level of the previous experimental blocks. Also, in the deadaptation block, descriptively, adults had fewer movement units and shorter movement time than children. On the top right of each panel speed profiles of one example participant of each age group are presented.