Nondominant-to-dominant hand interference in bimanual movements is facilitated by gradual visuomotor perturbation.

In simultaneous bimanual movements, interference between the hands is always a possibility, particularly when movements are spatially incongruent. In a previous study using bimanual target-directed movements and abrupt visual feedback perturbation of one hand, I showed asymmetric interference from the dominant to the nondominant hand. The signature of that interference reflected the directional control strength of the dominant hand, supporting a recent theory of functional lateralization of arm movements, and extending it to a bimanual context. Nondominant-to-dominant interference was not observed in this task. The current study in healthy young adults used a bimanual paradigm in which one hand had to adapt to a gradual visuomotor perturbation, while the other hand operated under kinesthetic control, without visual feedback. In this arrangement, the kinesthetically guided hand provides a canvas on which the visually guided, and perturbed hand can 'paint' its interference. Results of this study showed two patterns of interference: in some participants the nonvisible hand deviated in the same direction (isodirectional) relative to the perturbed hand, in others it mirrored the direction (anisodirectional) of the perturbed hand. In isodirectional participants, dominant-to-nondominant and nondominant-to-dominant hand interference was symmetrical and relatively weak, whereas there was strong nondominant-to-dominant hand interference in anisodirectional participants, suggesting interference in the form of endpoint accuracy control strength of the nondominant hand. Based on these findings, the study discusses potential mechanisms enabling the nondominant hand to exert this control strength onto the dominant hand.

Abrupt, but not gradual visuomotor distortion facilitates adaptation in children with developmental coordination disorder.

A previous experiment investigating visuomotor adaptation in typically developing children and children with Developmental Coordination Disorder (DCD) suggested poor adaptation to an abruptly induced visuomotor perturbation. In the current study, using a similar center-out drawing task, but administering either an abrupt or a gradual perturbation, and twice as many adaptation trials, we show that typically developing children are well able to successfully update an existing internal model in response to a 60 degrees rotation of the visual feedback, independent of the perturbation condition. Children with DCD, however, updated their internal map more effectively during exposure to an abrupt visuomotor perturbation than to a gradual one. This may suggest that the adaptation process in children with DCD responds differently to small vs. large steps of visuomotor discrepancies. Given the known role of the cerebellum in providing an error signal necessary for updating the internal model in response to a gradual visuomotor distortion, the results of our study add to the growing body of evidence implicating compromised cerebellar function in DCD.

Effects of increased complexity of visuo-motor transformations on children's arm movements.

The effects of increasing complexity of visuo-motor transformations on movement were examined in 4-, 6-, and 8-year-old children and adults. Participants performed a 'center-out' drawing task under three increasingly complex conditions: (1) Normal transformation: The target, line path and hand position were fully visible, in the horizontal plane, throughout the movement. (2) Aligned transformation: The target and line path were displayed horizontally above the workspace, with vision of the arm/hand occluded. (3) Vertical transformation: The target and line paths were presented on a vertical computer monitor with vision of the arm/hand occluded. Results showed that with increasing age, movements became faster, straighter, and smoother. The 4- and 6-year-old children were more variable in their specification of movement direction than the 8-year-old children and the adults, and were also more affected by the complexity of the transformation. This suggested that besides the complexity of the visual transformation, the familiarity/experienced environment might also play a role in 'sharpening' the transformation maps represented in movement planning.

Ataxia reflected in the simulated movements of patients with cerebellar lesions.

Previous studies demonstrated that the time required to simulate mentally a complex movement is highly correlated with the time required to execute the same task. The purpose of this experiment was to examine whether this relationship exists when execution times are prolonged as a consequence of the motor abnormalities exhibited by patients with substantial cerebellar pathology. The paradigm required subjects to alternate between moving a hand-held stylus horizontally on a digitizing tablet through a four-segment template and imagining the same movement through the same template. These two modes of performance were compared based on the times required to complete the two types of trials. Performance using both upper extremities was assessed using templates with two different levels of difficulty. Difficulty was varied by interposing gates that narrowed the path through the template. Using a MANOVA, measurements of actual and simulated movement times were compared between the group of cerebellar patients and a group of age- and sex-matched controls. The results showed that: (1) both movement times and mental-simulation times were greater for cerebellar patients than for control subjects under all experimental conditions, (2) both the movement times and the mental-simulation times of the patients were greater on the more-affected side than on the less-affected side, and (3) on the more-affected side, there was no significant difference between the patients' simulation and movement times for either the more difficult or less difficult condition. Thus, the consequence of cerebellar dysfunction on the time required to execute a volitional movement is reflected in the time needed to simulate the same behavior.

Adaptation to gradual as compared with sudden visuo-motor distortions.

If visual feedback is discordant with movement direction, the visuo-motor mapping is disrupted, but can be updated with practice. In this experiment subjects practiced discrete arm movements under conditions of visual feedback rotation. One group was exposed to 10 degree-step increments of visual feedback rotation up to a total of 90 degrees, a second group to a 90 degree visual feedback rotation throughout the experiment. After the first group reached the 90 degree visual feedback rotation, its subjects performed faster, with less spatial error, and showed larger aftereffects than the subjects who practiced constantly under the 90 degree visual feedback rotation condition. Results suggest that gradually increasing feedback distortion allows more complete adaptation than a large, sudden distortion onset.