This study examined at what age children can engage in the hand rotation task (as a measure of motor imagery); whether engagement changes across development and; the influence of age and motor skill on performance. Children were aged 5–12 years (N = 101; 52 girls), with no IQ or motor skill impairment. Less than 40% of 5–6 year olds completed the hand rotation with sufficient accuracy for further analysis, compared with 80% of 7–8 year olds, and 90% aged 9 and above. From age 7, either or both response time (RT) and accuracy conformed to the biomechanical constraints of corresponding physical movements. Although RT did not improve with age, accuracy did, with 11 year olds significantly more accurate than 7 and 8 year olds. Importantly, efficiency (RT/accuracy) improved with age and both age, in months, and motor skill level were significant predictors of efficiency, accounting for 35% and 8% of variability, respectively. Improvements in motor imagery ability during childhood are likely the result of increased neural efficiency, developing as the result of complex interactions between endogenous and exogenous factors. This highlights the need for a multidisciplinary approach to further our understanding of the emergence of motor imagery ability.
Motor imagery paradigms, which require individuals to produce a dynamic simulation of movement (without any overt accompanying movement), are increasingly being used to investigate the ability of an individual to mentally represent movement. Internal representations of movement are thought to play a critical role in a number of motor control processes, including mental rehearsal and observational learning (Jeannerod, 2001), motor planning (Wolpert & Ghahramani, 2000), and online movement control (Desmurget and Grafton, 2000 and Izawa and Shadmehr, 2011). These representations are dynamic, in that they are constantly updated as a result of an individual’s movement interactions with their environment and changes in body kinematics that occur during development (Choudhury et al., 2007, Miall and Wolpert, 1996 and Wolpert et al., 1995). It is currently unclear at what age these representations form or become consciously accessible, but it has been suggested that children must first gain some level of implicit knowledge of the relationship between the motor commands they generate, the environment and the effects on their moving body before they can accurately generate an internal representation of movement (Caeyenberghs, Wilson, Van Roon, Swinnen, & Smits-Engelsman, 2009). Understanding the emergence and development of these internal representations is crucial to our understanding of motor development and to enable us to better understand the atypical motor imagery performance of children with motor skill impairment (see below). To examine these representations, we need to be sure that the motor imagery tasks being utilized are age-appropriate.
Currently, the majority of studies examining motor imagery in children have utilized tasks that have been borrowed from adult studies and though for the most part these tasks are supported by neuroimaging data that indicates they can effectively engage participants in motor imagery (e.g., de Lange et al., 2006, Kosslyn et al., 1998, Parsons, 1987 and Parsons and Fox, 1998), their valid use in children is less well established. Therefore, the first aim of this experimental study was to explore whether one such task, the hand rotation task, engages children aged 5–12 years in motor imagery. To establish this, response times and accuracy to posturally congruent and posturally incongruent stimuli were compared in order to confirm the presence of biomechanically constrained movement simulations (indicating the use of motor, rather than visual, imagery: see below). Following this, if analyses were to indicate that children were effectively engaged in motor imagery while performing the hand rotation task, the study aimed to determine how performance changes across age and what influence motor skill level has on performance.
The current study has addressed methodological weaknesses present in previous work to allow us to better understand the development of motor imagery, and more specifically, how performance on the hand rotation task improves with age. First, it was demonstrated that although the task may be useful for some children under the age of 7 years, it should be used with caution and in conjunction with solid screening and data analysis techniques. Second, we found that above the age of 7 years, the majority of children were constrained by the biomechanical constraints of real movement when performing the hand rotation task, supporting the use of motor imagery. Third, age-related improvements were identified on the hand rotation task. Age was clearly the biggest predictor of performance efficiency, but motor skill level continued to contribute to the variability in performance after accounting for age. This is the first study to consider both age and standardized motor skill scores together. We argue that the findings that both chronological age and motor skill level contribute independently to motor imagery ability suggests that improved performance is likely the result of increasing levels of neural efficiency and that improvements in neural efficiency are not only the result of age-related changes, but the complex interaction of endogenous and exogenous factors throughout development. We suggest that multidisciplinary studies are needed to fully understand factors affecting the development of motor imagery ability in children.
