To throw or to place: does onward intention affect how a child reaches for an object?

Abstract

Picking up an object is a seemingly simple and isolated task; however, research has demonstrated that adults plan a reach-to-grasp movement on the basis of forthcoming actions. For example a greater deceleration period is seen in an initial reach movement which precedes a place movement compared to a throw movement. This task-specific or second-order motor planning is also seen in infants and toddlers; however, the developmental progression is unclear. Reach-to-grasp movements of 48 children, split into four age groups (4-5, 6-7, 8-9 and 10-11 years) were recorded. These movements preceded a tight place, a loose place or a throw action. All the children showed some degree of tailoring kinematics to the onward action. In the 4-5 year-old group, this was demonstrated by a longer movement duration in the place actions compared to the throw action. In the older children the proportion of time spent decelerating increased as the precision requirements of the task increased. These results demonstrate that all children are able to use second-order planning to integrate onward task demands into their movements. The capacity for this increases with age but is not fully mature at 11 years. These developmental effects may be explained by the relative weighting of costs involved in tailoring a reach action compared to the benefits of producing a more efficient onward action.