# Dynamic Thumb Proprioception: Quantification with a Novel Robotic Task

## TL;DR

Researchers developed a new robotic task to measure how well people can sense their thumb's position during movement, finding that the thumb is less accurate than the index finger.

## Contribution

A novel robotic task was introduced to quantify dynamic thumb proprioception and compare it with index finger performance.

## Key findings

- Thumb localization error was about 25° and did not change with speed or circle diameter.
- Reversing thumb rotation increased error, which adapted rapidly over 20 trials.
- Thumb localization error was larger than index finger error and correlated with a robotic proprioception assessment.

## Abstract

The thumb plays a crucial role in hand function, yet its proprioceptive abilities remain poorly understood. Here we quantified dynamic thumb localization ability in unimpaired participants, using a novel task in which a robot moved the thumb in a circle and participants pressed a button when they felt their thumb aligning with a target point on a screen. After pressing the button, they received visual error feedback in the form of a ball jumping toward the target. To characterize thumb localization ability, we varied thumb speed and rotation diameter, assessed the effect of a propriovisual rotational perturbation, and compared index finger performance. Following task familiarization, thumb localization error was ~25° and did not change significantly with speed or circle diameter. Reversing thumb rotation increased error followed by rapid error adaptation across the next 20 trials, as would be expected if individuals formed an internal model based on a body-centered (movement-aligned) frame of reference rather than a world-centered spatial frame. Localization error was larger for the thumb than the index finger error for the same task (p = 0.02) and was correlated with a different, robotic assessment of finger proprioception (ρ = 0.61, p = 0.001). These findings indicate that dynamic thumb localization is somewhat inaccurate, although it can leverage visual feedback within a body-centered reference frame, a form of passive, cross-sensory adaptation. Further, in unimpaired adults, the dynamic proprioceptive abilities of the thumb and index finger are related, with thumb proprioception ability being less accurate than the finger.