Strong biomechanical constraints on young children's mental imagery of hands

Mental rotation (MR) of body parts is a useful paradigm to investigate how people manipulate mental imagery related to body schema. It has been documented that adult participants use 'motor imagery' for MR of hands: a behavioural indication is a biomechanical effect, that is, hand pictures in orientations to which imitative hand movement would be biomechanically difficult require longer response times to be visually identified as the left or right hand. However, little is known about the typical developmental trajectory of the biomechanical effect, which could offer clues to understanding how children acquire the ability to manipulate body schema. This study investigated developmental changes in the biomechanical effect in schoolchildren. Eighty-four children (from 6 to 11 years old, grouped into 1st, 2nd, 3rd, 4th and 5th graders) and fifteen adults made hand laterality judgements in an MR paradigm. The results indicated that the biomechanical effect is stronger for younger children, and that there is a transitional period (around 7-8 years) during which children shift from action execution to imagery in manipulating body schema. The results suggest that mental imagery of hands has a stronger motor aspect in the transitional period than later in childhood and adulthood.