Emergence of anticipatory actions in a novel task

Humans normally adapt earlier segments of multistep motor actions in such a way that the execution of later segments is facilitated. For example, the kinematics of grasping movements are adapted to the requirements of the intended subsequent object manipulations. Here we studied which factors foster adaptation of earlier action segments to later ones in a novel task for which no prior experience existed. Participants executed a two-step isometric force production task, in which the force produced in the first segment determined the difficulty of the second segment. Adaptation of the first segment to the second one benefited from explicit knowledge of the dependency between both segments but not from extensive prior experience with the second segment. These observations show that adaptation of motor actions to subsequent actions demands the construction of a task representation that allows to plan the first action segment with respect to its successor. How specifically the first segment is tailored to the second one does not depend on prior experience with the second segment but depends on experience from performing the interdependent two-step action sequence.

Habit outweighs planning in grasp selection for object manipulation

Object-directed grasping movements are adapted to intended interactions with an object. We address whether adjusting the grasp for object manipulation is controlled habitually, based on past experiences, or by goal-directed planning, based on an evaluation of the expected action outcomes. Therefore, we asked participants to grasp and rotate a dial. In such tasks, participants typically grasp the dial with an excursed, uncomfortable arm posture, which then allows to complete the dial rotation in a comfortable end-state. We extended this task by manipulating the contingency between the orientation of the grasp and the resulting end-state of the arm. A one-step (control) group rotated the dial to a single target. A two-step group rotated the dial to an initial target and then in the opposite direction. A three-step group rotated the dial to the initial target, then in the opposite direction, and then back to the initial target. During practice, the two-step and three-step groups reduced the excursion of their grasps, thus avoiding overly excursed arm postures after the second rotation. When the two-step and three-step groups were asked to execute one-step rotations, their grasps resembled those that were acquired during the two-step and three-step rotations, respectively. However, the carry-over was not complete. This suggests that adjusting grasps for forthcoming object manipulations is controlled by a mixture of habitual and goal-directed processes. In the present experiment, the former contributed approximately twice as much to grasp selection than the latter.

Mild impairments of motor imagery skills in children with DCD

It has been hypothesized that the underlying mechanism of clumsy motor behaviour in children with Developmental Coordination Disorder (DCD) is caused by a deficit in the internal modelling for motor control. An internal modelling deficit can be shown on a behavioural level by a task that requires motor imagery. Motor imagery skills are suggested to be related to anticipatory action planning, but motor imagery and action planning have not been tested within the same child. In the present study, action planning and motor imagery skills were assessed in 82 children between 7 and 12 years of age. Twenty-one of these children met the criteria for DCD, which was assessed by the McCarron Assessment of Neuromuscular Development and 56 of these children were used in the control group. Motor imagery was tested by a mental rotation task of hands that were shown from a back and palm point of view. The results show that motor imagery is affected in children with DCD but only in conditions with complex task constraints (i.e., rotation of hand stimuli presented in palm view). These results provide partial support for the internal modelling deficit hypothesis. We were not able to elicit motor planning deficits in this group, however, and argue that more complex planning tasks may be needed to identify such deficits.

The contribution of cognitive, kinematic, and dynamic factors to anticipatory grasp selection

Object-directed grasping movements are usually adjusted in anticipation of the direction and extent of a subsequent object rotation. Such anticipatory grasp selections have been mostly explained in terms of the kinematics of the arm movement. However, object rotations of different directions and extents also differ in their dynamics and in how the tasks are represented. Here, we examined how the dynamics, the kinematics, and the cognitive representation of an object manipulation affect anticipatory grasp selections. We asked participants to grasp an object and rotate it by different angles and in different directions. To examine the influence of dynamic factors, we varied the object's weight. To examine the influence of the cognitive task representation, we instructed identical object rotations as either toward-top or away-from-top rotations. While instructed object rotation and cognitive task representation did affect grasp selection over the entire course of the experiment, a rather small effect of object weight only appeared late in the experiment. We suggest that grasp selections are determined on different levels. The representation of the kinematics of the object movement determines grasp selection on a trial-by-trial basis. The effect of object weight affects grasp selection by a slower adaptation process. This result implies that even simple motor acts, such as grasping, can only be understood when cognitive factors, such as the task representation, are taken into account.