Hand size underestimation grows during childhood

Abstract

Cortical body size representations are distorted in the adult, from low-level motor and sensory maps to higher levels multisensory and cognitive representations. Little is known about how such representations are built and evolve during infancy and childhood. Here we investigated how hand size is represented in typically developing children aged 6 to 10. Participants were asked to estimate their hand size using two different sensory modalities (visual or haptic). We found a distortion (underestimation) already present in the youngest children. Crucially, such distortion increases with age and regardless of the sensory modality used to access the representation. Finally, underestimation is specific for the body as no bias was found for object estimation. This study suggests that the brain does not keep up with the natural body growth. However, since motor behavior nor perception were impaired, the distortion seems functional and/or compensated for, for proper interaction with the external environment.

Figure 1

3D printed hands and objects used for Experiment 1 and 2 respectively. (A) 15 different 3D printed Hands used in Experiment 1. (B) 15 3D printed objects used in Experiment 2. (C) In Experiment 1, participants were presented with one hand at a time and asked to judge whether it was smaller or larger than their own. In Experiment 2 participant were presented with a reference object on the right space, either in front of them (in the Visual condition) or under their right hand (haptic condition). At each trial, a second target object was presented to the left and participant was asked to judge whether the target was smaller or larger than the reference.

Figure 2

Hand and Object size estimation across time for the Haptic and Visual Condition. (A) Children significantly underestimate their hand size while being accurate in estimating an object size. While object size estimation remains consistently accurate across time, hand underestimation increases during childhood with older children underestimating their hand size more than older children do. Bars represent group values. Dots represent single subjects’ values. (B) Children underestimate their hand in both visual and haptic task. Error bars indicate standard error.

Figure 3

(A) Correlation between Visual and Haptic task for Experiment 1. (B) Hand and Object size estimation variability. Children show higher variability when judging their hand compared to judging an object size.