# Underestimation and Overestimation of Hand and Arm Length Coexist in Children

## TL;DR

Children have conflicting mental maps of their arm length, underestimating the hand but overestimating the full arm when tested.

## Contribution

The study reveals that structural and functional body representations of the arm are independent in children.

## Key findings

- Children underestimate the hand's length in structural localization tasks.
- Children overestimate their full arm length in functional reach tasks.
- Structural and functional estimates show no correlation in arm length perception.

## Abstract

The present study assessed the structural and functional representation of the upper limb in a large cohort (N = 84) of typically developing children aged 6 to 10. The first task aimed at obtaining a structural measure of the representation of the arm, specifically the two segments that compose it: the forearm and the hand. Participants were asked to localize three landmarks (elbow, wrist, and tip of the middle finger) while blindfolded and upon tactile stimulation of the three landmarks. The second task required a functional estimation of the represented length of the arm. Participants judged whether their arm fully outstretched would be long enough to touch an object presented at seven different distances without being allowed to perform the movement. The two tasks revealed opposite patterns of (mis‐)representation. At the structural level, the hand length was underestimated, while the forearm representation matched the actual size. This resulted in an underestimation of total arm length in the structural task. At the functional level, total arm length was overestimated across all age groups. Moreover, there was no relationship between estimates on the structural and functional tasks. These results support the coexistence of multiple, independent body representations in children.