The present reaction on the paper of Smeets and Brenner focuses on two premises of their proposed model The first is that grasping is nothing more than pointing with two fingers. It is argued that this assumption cannot be upheld in light of the differences between both actions with respect to neuromuscular structures, muscular innervation, use of visual feedback, and basic function The second premise of the model is that the velocity profile of the transport component is symmetrical and independent of intrinsic object properties. It is shown that the symmetrical velocity profile represents a boundary condition and is influenced by intrinsic object properties. Given these concerns, it is doubtful that the model in its present form will add much to our understanding of the control of grasping.
Bert Steenbergen and Wouter Hulstijn
Bert Steenbergen and Andrea Utley
Edwin Van Thiel and Bert Steenbergen
In this study, we examined the degree and timing of shoulder displacements during hitting, reaching, and grasping movements performed by young adults with hemiparetic cerebral palsy. The participants performed unimanual and bimanual arm movements towards targets and objects of different sizes. On the basis of the assumption that shoulder displacement due to trunk translation and rotation is a successful, adaptive reaction to reduced joint mobility in the affected arm, the fluency of hand displacements was expected to remain invariant under variations of shoulder displacement as is also the case in healthy participants. The results point in this direction. With respect to the timing of shoulder displacement, prior research suggested that hemiparetic movements can be characterized by inconsistent motion-timing patterns—that is, the timing of shoulder and hand-displacement onsets varied between trials. Therefore, the within-subject variability of the movement-onset asynchrony between hand and ipsilateral shoulder displacement was expected to be larger on the impaired side than on the unimpaired side. This prediction was not confirmed. which challenges these earlier conclusions. Additionally. we also examined the peak-velocity asynchrony of the hand and shoulder. Contrary to the onset asynchrony, the peak asynchrony varied between the hitting and reaching task and between the hitting and grasping task. For the reaching and grasping tasks, there were also significant differences between hands. Again, variability of the (peak-velocity) asynchrony was not significantly increased when comparing the impaired hand with the unimpaired hand. The results suggests that the hemiparetic participants were capable of flexibly recruiting and sequencing the various degrees of freedom of their impaired side required for successful task completion, albeit in different magnitudes and sequenced differently.
Marcel Mutsaarts, Bert Steenbergen and Harold Bekkering
Anticipatory planning was examined in detail for a complex object manipulation task by capitalizing on both the complexity and the number of elements in the movement sequences in seven individuals with hemiparetic cerebral palsy (HCP) and seven left-handed control participants. Participants had to grasp a hexagonal knob using one of five possible grasping patterns as quicklly as possible following a starting cue (condition I), and sometimes, they had to rotate it subsequently either 60˚ or 120˚ clockwise or counterclockwise (condition II). In the first condition, the HCP participants appear to anticipate the comfort of the different grasping patterns before movement onset, as controls did. However, when the task consisted of more than one movement part, HCP participants did not complete their planning processes before movement onset, which was contrary to controls. Instead, the results suggest that they use a step-by-step planning strategy, that is, they planned the latter parts of a movement sequence as the movement unfolds. The results are discussed in the light of possible capacity limitations of an internal model for grip selection, and a recent model on the planning and on-line control of movement performance.
Bert Steenbergen and John van der Kamp
We investigated attentional processes that support the performance of high-skilled soccer players with hemiparetic cerebral palsy. Participants (N = 10) dribbled a slalom course as quickly and accurately as possible under two attentional-focus manipulation conditions. In the task-relevant focus condition, they attended to the foot that was in contact with the ball, whereas in the task-irrelevant focus condition, they monitored a series of words played on a tape. The time taken to complete the slalom course was registered. Performances of individuals with left and right hemiparesis were compared to explore differential effects of hemispheric lesion. The high-skilled players with congenital hemiparesis showed similar attentional-focus effects as those previously reported in the literature for high-skilled players without neurological disorders (Beilock et al., 2002; Ford et al., 2005). Task-relevant focus increased dribbling time, whereas a task-irrelevant focus did not result in a significant change in dribbling time. These findings generalized to each of the five participants with left hemiparesis (i.e., damage to the right hemisphere). By contrast, the effects of a task-relevant focus were less consistent for participants with right hemiparesis (i.e., left-hemisphere damage). This corroborates suggestions that the reinvestment of procedural knowledge is a left-lateralized function. The implications for the training of individuals with congenital brain damage are discussed.
Loes Janssen, Céline Crajé, Matthias Weigelt and Bert Steenbergen
We examined anticipatory motor planning and the interaction among both hands in a discrete bimanual task. To this end, participants had to grasp and manipulate two cylindrical objects simultaneously under varying conditions in which (a) the grip selection requirements, i.e., orientation of the to-be-grasped objects, differed between the two hands and (b) the type of grip for one hand was preinstructed, while the grip for the other hand was free choice. Results showed that participants, when grasping for two bars with a free grip choice, prioritized planning for comfortable end postures over symmetry of movement execution. Furthermore, when participants were free to choose a grip for their left hand, but were instructed on how to grasp an object with their right hand, we found no interaction between the grip selections of both hands, suggesting that motor planning proceeds independently for both hands.
Dominique van Roon, Bert Steenbergen and Ruud G.J. Meulenbroek
People with cerebral palsy (CP) are known to rely heavily on visual guidance when making targeted upper-limb movements. In the present study, we examined whether being able to visually monitor the moving limb forms a precondition for people with CP to make accurate upper-limb movements. Eight participants with tetraparetic CP and eight controls were asked to produce large-amplitude, straight-line drawing movements on a digitizing tablet. In half the trials, vision of the moving limb was blocked. Accuracy constraints were manipulated by varying the width of the target and by imposing a maximum width of the movement path. Surprisingly, when vision was blocked movement accuracy was comparable in the two groups. Thus, people with tetraparetic CP do not strictly require constant vision of their moving limb to make accurate upper-limb movements. They compensated for the lack of visual information, however, by prolonging movement time. Using a high pen force proved a general strategic adaptation, possibly to filter out unwanted noise from the motor system or to enhance proprioceptive input.
Femke van Abswoude, John van der Kamp and Bert Steenbergen
Effective learning methods are essential for motor skill development and participation in children with low motor abilities. Current learning methods predominantly aim to increase declarative knowledge through explicit instructions that necessitate sufficient working memory capacity. This study investigated the roles of declarative knowledge and working memory capacity in explicit motor learning of children with low motor abilities. We studied both acquisition performance (i.e., performance during practice) and learning (i.e., the improvement in performance from pretest to posttest). After practice with explicit instructions, children with low motor abilities showed significant learning, albeit that improvement was relatively small. However, working memory capacity and declarative knowledge did not predict learning. By contrast, working memory capacity and declarative knowledge did predict performance during practice. These findings suggest that explicit instructions enhance motor performance during practice, but that motor learning per se is largely implicit in children with low motor abilities.