Shared neural resources between left and right interlimb coordination skills: the neural substrate of abstract motor representations.

Functional magnetic resonance imaging was used to reveal the shared neural resources between movements performed with effectors of the left versus right body side. Prior to scanning, subjects extensively practiced a complex coordination pattern involving cyclical motions of the ipsilateral hand and foot according to a 90 degrees out-of-phase coordination mode. Brain activity associated with this (nonpreferred) coordination pattern was contrasted with pre-existing isodirectional (preferred) coordination to extract the learning-related brain networks. To identify the principal candidates for effector-independent movement encoding, the conjunction of training-related activity for left and right limb coordination was determined. A dominantly left-lateralized parietal-to-(pre)motor activation network was identified, with activation in inferior and superior parietal cortex extending into intraparietal sulcus and activation in the premotor areas, including inferior frontal gyrus (pars opercularis). Similar areas were previously identified during observation of complex coordination skills by expert performers. These parietal-premotor areas are principal candidates for abstract (effector-independent) movement encoding, promoting motor equivalence, and they form the highest level in the action representation hierarchy.

Coordination deficits on the ipsilesional side after unilateral stroke: the effect of practice on nonisodirectional ipsilateral coordination.

Previous studies have identified motor deficits on the ipsilesional side of patients recovering from a cerebro-vascular accident (CVA), including deficits in interlimb coordination. In the present study, unilateral stroke patients and a control group of healthy age-matched controls performed nonisodirectional coordination of the ipsilateral limbs across two days of practice with feedback. Findings revealed that control subjects were already quite successful at initiation of practice but further improved the coordination pattern across both days. The group of CVA patients also showed some improvement but problems with coordination of the ipsilateral limb segments persisted across practice. Variability in both timing and amplitude of both limb segments did improve with practice in both groups but these measures remained significantly higher in the CVA patients. Even though isodirectional and nonisodirectional coordination of the ipsilateral limb segments are normally considered to be part of the intrinsic motor repertoire, the present study suggests that nonisodirectional ipsilesional limb coordination poses considerable difficulties for CVA patients that are not easily overcome with feedback-assisted practice.

Conceptual binding: integrated visual cues reduce processing costs in bimanual movements.

In discrete reaction time (RT) tasks, it has been shown that nonsymmetric bimanual movements are initiated slower than symmetric movements in response to symbolic cues. By contrast, no such RT differences are found in response to direct cues ("direct cue effect"). Here, we report three experiments showing that the direct cue effect generalizes to rhythmical bimanual movements and that RT cost depends on different cue features: 1) symbolic versus direct or 2) integrated (i.e., action of both hands is indicated as one entity) versus dissociated (i.e., action of each hand is indicated separately). Our main finding was that dissociated symbolic cues were most likely processed serially, resulting in the longest RTs, which were substantially reduced with integrated symbolic cues. However, extra RT costs for switching to nonsymmetrical bimanual movements were overcome only when the integrated cues were direct. We conclude that computational resources might have been exceeded when the response needs to be determined for each hand separately, but not when a common response for both hands is selected. This supports the idea that bimanual control benefits from conceptual binding.

Unimanual and bimanual voluntary movement in Huntington's disease.

Unimanual and bimanual cyclical forearm movements were studied in 15 Huntington's disease (HD) patients and 15 healthy, gender- and age-matched controls. Whereas the unimanual task was only performed at maximal speed, the bimanual movements were performed according to the in-phase and anti-phase mode at different cycling frequencies. The HD patients also performed the tasks after 12 months of follow-up. Findings revealed that maximal cycling frequency during unimanual movement was significantly lower in HD patients as compared with controls. In addition, measures of relative phasing established that bimanual cyclical movements were performed with lower accuracy and higher variability in HD patients. The differential variability between both groups was magnified by increasing the cycling frequency and coordinative complexity whereas only coordinative complexity differentially affected the accuracy of relative phasing. The obtained performance measures were found to be significantly correlated with disease duration (unimanual) and with the score on the total motor scale, the Mini-Mental State Examination and the Stroop Interference Test (uni- and bimanual). After 12 months, maximal cycling frequency of unimanual elbow flexion-extension was significantly decreased in HD patients whereas the quality of the in-phase and anti-phase movement patterns remained stable.