Motor sequence complexity and performing hand produce differential patterns of hemispheric lateralization

Effects of interlimb and intralimb constraints on bimanual shoulder-elbow and shoulder-wrist coordination patterns

The present study addressed the interactions between interlimb and intralimb constraints during the control of bimanual multi-joint movements. Participants performed eight coordination tasks involving bilateral shoulder-elbow (expt I) and shoulder-wrist (expt II) movements. Three principal findings were obtained. First, the principle of muscle homology (in-phase coordination), giving rise to mirror symmetrical movements with respect to the midsagittal plane, had a powerful influence on the quality of interlimb coordination. In both experiments, the accuracy and stability of inter- and/or intralimb coordination deteriorated as soon as the antiphase mode was introduced in one or both joint pairs. However, the mutual influences between bilateral distal and proximal joint pairs varied across coordination tasks and effectors. Second, the impact of intralimb coordination modes on the quality of intralimb coordination was inconsistent between adjacent (expt I) and non-adjacent joint (expt II) combinations. Third, the mode of interlimb coordination affected the quality of intralimb coordination, whereas strong support for the converse effect was not obtained. Taken together, these observations point to a hierarchical control structure whereby interlimb coordination constraints have a stronger impact on the global coordination of the system than intralimb constraints, whose impact is substantially dependent on effector and task. The finding that intralimb coordination is subordinate to interlimb coordination during the production of bimanual multi-joint coordination patterns indicates that symmetry is a major organizational principle in the neural control of complex movement.

Functional implications of ipsilesional motor deficits after unilateral stroke

OBJECTIVE

To investigate the functional impact of ipsilesional motor deficits after unilateral stroke and the best predictors of those deficits.

DESIGN

Observational cohort.

SETTING

Primary care Veterans Affairs and private medical center.

PARTICIPANTS

Volunteer right-handed sample; stroke patients with left (LHD) or right hemisphere damage (RHD) a mean of 3.9 to 5.2 years poststroke and able-bodied participants who were tested using their left (LAB) or right hand.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURE

The Jebsen-Taylor Hand Function Test (JHFT).

RESULTS

Ipsilesional JHFT performance was impaired to the same extent in the LHD and RHD groups. LHD patients with apraxia had poorer scores on the JHFT than LHD patients without apraxia and the LAB group. Regression analyses showed that severity of apraxia was the best predictor of JHFT performance for the LHD group and that right (ipsilesional) motor performance (grip strength, finger tapping) was the best predictor of JHFT performance for the RHD group.

CONCLUSIONS

Ipsilesional deficits are present on simulated activities of daily living after LHD or RHD, suggesting that rehabilitation after stroke should include the ipsilesional arm and that ipsilesional limb apraxia is a better predictor of ipsilesional functional motor skills after LHD than aphasia or simple motor skills (grip strength, finger tapping). These findings suggest that limb apraxia should be assessed more routinely after stroke of the left hemisphere.

Ipsilateral Motor Cortex Activity During Unimanual Hand Movements Relates to Task Complexity

Functional imaging studies have revealed recruitment of ipsilateral motor areas during the production of sequential unimanual finger movements. This phenomenon is more prominent in the left hemisphere during left-hand movements than in the right hemisphere during right-hand movements. Here we investigate whether this lateralization pattern is related specifically to the sequential structure of the unimanual action or generalizes to other complex movements. Using event-related fMRI, we measured activation changes in the motor cortex during three types of unimanual movements: repetitions of a sequence of movements with multiple fingers, repetitive “chords” composed of three simultaneous key presses, and simple repetitive tapping movements with a single finger. During sequence and chord movements, strong ipsilateral activation was observed and was especially pronounced in the left hemisphere during left-hand movements. This pattern was evident for both right-handed and, to a lesser degree, left-handed individuals. Ipsilateral activation was less pronounced in the tapping condition. The site of ipsilateral activation was shifted laterally, ventrally, and anteriorly with respect to that observed during contralateral movements and the time course of activation implied a role in the execution rather than planning of the movement. A control experiment revealed that strong ipsilateral activity in left motor cortex is specific to complex movements and does not depend on the number of required muscles. These findings indicate a prominent role of left hemisphere in the execution of complex movements independent of the sequential nature of the task.

Distinct neural systems underlie learning visuomotor and spatial representations of motor skills

Motor skill learning depends upon acquiring knowledge about multiple features of sequential behaviors, including their visuomotor and spatial properties. To investigate the neural systems that distinguish these representations, we carried out functional magnetic resonance imaging (fMRI) as healthy adults learned to type sequences on a novel keyboard. On the initial training day, learning-related changes in brain activation were found in distributed cortical regions, only a subset of which correlated with improvements in movement time (MT), suggesting their preeminence in controlling movements online. Subjects received extended training on the sequences during the ensuing week, after which they returned to the scanner for another imaging session. Relative to performance at the end of the first training day, continued plasticity was most striking in the inferior parietal cortex and new areas of plasticity were uncovered in the caudate and cerebellum. Plasticity in these regions correlated with reaction time (RT), suggesting their role in planning sequences before movement onset. Two transfer conditions probed for "what" subjects learned. The probe for visuomotor learning produced increased activation in visual analysis (left inferior visual cortex) and advance planning (left caudate) systems. The probe for spatial learning produced increased activation in visuomotor-transformation (left dorsal visual pathway) and retrieval (left precuneus) systems. Increased activity in all of these regions correlated with increased RT, but not MT, indicating that both transfer conditions interfered with the neural representation of plans for the sequences, but not processes that controlled their implementation. These findings demonstrated that neuroanatomically dissociable systems support the acquisition of visuomotor and spatial representations of actions.

Left inferior parietal dominance in gesture imitation: an fMRI study

The inability to imitate gestures is an essential feature of apraxia. However, discrepancies exist between clinical studies in apraxic patients and neuroimaging findings on imitation. We therefore aimed to investigate: (1) which areas are recruited during imitation under conditions similar to clinical tests for apraxic deficits; (2) whether there are common lateralized areas subserving imitation irrespective of the acting limb side; and also (3) whether there are differences between hand and finger gestures. We used fMRI in 12 healthy, right handed subjects to investigate the imitation of four types of variable gestures that were presented by video clips (16 different finger and 16 different hand gestures with either the right or the left arm). The respective control conditions consisted of stereotyped gestures (only two gestures presented in pseudorandom order). Subtraction analysis of each type of gesture imitation (variable>stereotyped) revealed a bilateral activation pattern including the inferior parietal cortex Brodmann Area (BA 40), the superior parietal cortex, the inferior frontal cortex (opercular region), the prefrontal motor cortex, the lateral occipito-temporal junction, and the cerebellum. These results were supported by statistical conjunction of all four subtraction analyses and by the common analysis of all four types of gesture imitation. The direct comparison of the right and left hemispheric activation revealed a lateralization to the left only of the inferior parietal cortex. Comparisons between different types of gesture imitation yielded no significant results. In conclusion, gesture imitation recruits bilateral fronto-parietal regions, with significant lateralization of only one area, namely the left inferior parietal cortex. These in vivo data indicate left inferior parietal dominance for gesture imitation in right handers, confirming lesion-based theories of apraxia.