Bimanual Stick Manipulation Task Reveals How the Motor System Coordinates Redundant Body Movements to Adapt to Visual Perturbations

Core Concepts

The motor system coordinates redundant body movements to adapt to visual perturbations by leveraging the baseline relationship between the direction of the end-effector (stick tip) movement and the angle of stick tilt.

Abstract

The study investigated how the motor system coordinates redundant body movements to adapt to visual perturbations in a bimanual stick manipulation task. The key findings are:

Participants adopted a stereotypical strategy of flexibly changing the tilt angle of the stick depending on the direction of the stick tip movement, reflecting a baseline relationship between the two. This relationship likely emerged from an optimization process to minimize movement distance.

When an end-effector relevant perturbation (visual rotation of the stick tip) was introduced, participants adapted by changing the stick tilt angle as if they were voluntarily aiming at the adapted direction, while the tip movement direction remained at the baseline level. This suggests the motor system uses the baseline relationship as a scaffold to guide implicit adaptation.

Even when an end-effector irrelevant perturbation (stick tilt angle) was introduced, the motor system attempted to correct the stick tilt error, which led to undesirable errors in the tip movement direction. This indicates the motor system predicts the visual feedback of the end-effector irrelevant dimension and tries to eliminate any dissociation between the observed and predicted states.

The adaptation patterns were influenced by the combination of end-effector relevant and irrelevant perturbations, suggesting an interaction between the two adaptation processes constrained by the baseline relationship.

Overall, the findings provide a new understanding that the baseline relationship between the end-effector and redundant body movements plays a crucial role in how the motor system controls and implicitly adapts movement patterns in redundant systems.

Stats

The average distance between the initial and final positions of the hands was closer to the mathematically derived minimum value (Cohen's d = 1.914) than the distance when both hands moved in parallel (Cohen's d = 0.334).
The stick-tilt angle at the peak velocity and movement offset was significantly different from 0° when aiming at targets other than the 0° target (t-test, |t(71)| > 6.4, Bonferroni corrected P < 0.001).

Quotes

"The monotonically increasing curve, which will be referred to as the "baseline TMD–STA relationship", reflects a constraint between the direction of tip movement and the angle of stick tilt angle."
"Owing to the redundant nature of this task, participants could adapt to change the visual tip movement using different strategies."
"Unexpectedly, this stick-tilt correction was accompanied by undesirable tip-movement directional errors."

Key Insights Distilled From

Implicit motor adaptation patterns in a redundant motor task manipulating a stick with both hands

by Kobayashi,T.... at www.biorxiv.org 06-14-2023

https://www.biorxiv.org/content/10.1101/2023.06.13.544873v2

Deeper Inquiries

How might the motor system's adaptation patterns in this redundant task differ in populations with motor impairments, such as stroke or Parkinson's disease?

In populations with motor impairments like stroke or Parkinson's disease, the adaptation patterns in this redundant task may differ significantly. Individuals with stroke often experience motor deficits, such as weakness, spasticity, and coordination problems, which can impact their ability to perform complex motor tasks. In the context of this study, individuals with stroke may have difficulty in adapting to visual perturbations in the end-effector relevant and irrelevant dimensions. The motor system's ability to correct for errors and adjust movement patterns may be compromised, leading to challenges in achieving the desired task goals. Additionally, individuals with Parkinson's disease may exhibit bradykinesia, rigidity, and tremors, which can further affect their motor adaptation capabilities. The tight coupling between end-effector and redundant body movements observed in this study may pose additional challenges for individuals with motor impairments, as they may struggle to coordinate and adjust these movements effectively.

What other types of redundant motor tasks could be used to further investigate the principles underlying motor adaptation in complex, high-dimensional systems?

Several other types of redundant motor tasks could be used to explore the principles of motor adaptation in complex, high-dimensional systems. One example is a bimanual reaching task where participants have to reach for targets using both hands simultaneously. This task introduces redundancy in the motor system as both hands can contribute to achieving the task goal. Another task could involve manipulating a tool or instrument with multiple degrees of freedom, similar to the stick manipulation task in the study. Tasks that require precise coordination of multiple joints and muscles, such as playing a musical instrument or performing a complex dance routine, could also be used to investigate motor adaptation in high-dimensional systems. Additionally, tasks that involve virtual reality environments or robotic devices can provide controlled settings to study motor adaptation in complex tasks.

Given the tight coupling between the end-effector and redundant body movements observed in this study, how might the motor system's ability to learn and generalize novel movement patterns be influenced by this constraint?

The tight coupling between end-effector and redundant body movements observed in this study can have a significant impact on the motor system's ability to learn and generalize novel movement patterns. The constraint imposed by the baseline relationship between tip-movement direction and stick-tilt angle may limit the flexibility of the motor system in adapting to new movement patterns. While this constraint can provide stability and efficiency in performing familiar tasks, it may hinder the motor system's ability to quickly adapt to novel environments or tasks. The motor system may struggle to generalize learning from one task to another if the movement patterns are tightly coupled and specific to the task at hand. This constraint could potentially limit the motor system's adaptability and transferability of skills across different motor tasks, especially in complex, high-dimensional systems where multiple degrees of freedom are involved.