The effect of motor overflow on bimanual asymmetric force coordination

Facilitated adaptation via structural learning increases bimanual interference

Bimanual coordination is an essential feature of the motor system, yet interactions between the limbs during independent control remain poorly understood. Interference between the two hands, or the assimilation of movement characteristics between the two effectors, can be induced by perturbing one arm (e.g., via visuomotor rotation) and then measuring the effects in the contralateral limb. In this study, we sought to further determine the role adaptation plays in bimanual interference using a structural learning paradigm to alter feedback regulation in reaching. We trained healthy participants to counter 60 unique random rotations in right hand visual feedback over 240 reaches. Following this, we assessed feedforward and feedback measures of interference in a bimanual reaching task where the right hand was exposed to a fixed visual feedback rotation while the left hand reached without visual feedback. We found that participants who had been exposed to the structural training task in the right hand showed increased left hand interference during the first 20 trials of the test task. Moreover, interference was greater in feedback, rather than feedforward control parameters. The results further suggest that structural learning enhances bimanual interference via sensory feedback upregulation.

Dual-site TMS as a tool to probe effective interactions within the motor network: a review

Dual-site transcranial magnetic stimulation (ds-TMS) is well suited to investigate the causal effect of distant brain regions on the primary motor cortex, both at rest and during motor performance and learning. However, given the broad set of stimulation parameters, clarity about which parameters are most effective for identifying particular interactions is lacking. Here, evidence describing inter- and intra-hemispheric interactions during rest and in the context of motor tasks is reviewed. Our aims are threefold: (1) provide a detailed overview of ds-TMS literature regarding inter- and intra-hemispheric connectivity; (2) describe the applicability and contributions of these interactions to motor control, and; (3) discuss the practical implications and future directions. Of the 3659 studies screened, 109 were included and discussed. Overall, there is remarkable variability in the experimental context for assessing ds-TMS interactions, as well as in the use and reporting of stimulation parameters, hindering a quantitative comparison of results across studies. Further studies examining ds-TMS interactions in a systematic manner, and in which all critical parameters are carefully reported, are needed.

Bimanual Interference Increases with Force Demands and is Facilitated by Visuomotor Adaptation

When simultaneously performing asymmetrical movements with both hands, there is a tendency for the action of one limb to interfere with control of the other. Little is known about how sensory feedback influences interference. We conducted two experiments to determine how manipulating force feedback and visual feedback alter bimanual coordination during center-out reaching. In the adaptive experiment, asymmetrical reaching was induced by a visual feedback rotation for the right hand while the left hand operated under kinesthetic control (i.e., without visual feedback); in the non-adaptive experiment, asymmetrical reaching was induced by having participants move their right hand to rotated targets under veridical visual feedback, again with the left hand operating under kinesthetic control. In both experiments, we applied a spring resistive force to each hand, with different groups of participants experiencing 0 N/m, 30 N/m, or 60 N/m of resistance. In the adaptive experiment, interference increased with an increase in the force demands for movement in a dose-response fashion (i.e., the higher the resistive force, the larger the interference), but this result did not hold generally for the non-adaptive experiment. Our results indicate that adapting to a visuomotor perturbation may increase sensitivity to feedback gains, including to sensory information not present in the perturbation. Additionally, interference may reflect the application of an explicit strategy used for one limb to control the other, and the addition of an implicit adapting process may bolster this communication of motor information across motor cortices.

The modulation of short and long-latency interhemispheric inhibition during bimanually coordinated movements

Bimanual coordination is essential for the performance of many everyday tasks. There are several types of bimanually coordinated movements, classified according to whether the arms are acting to achieve a single goal (cooperative) or separate goals (independent), and whether the arms are moving symmetrically or asymmetrically. Symmetric bimanual movements are thought to facilitate corticomotor excitability (CME), while asymmetric bimanual movements are thought to recruit interhemispheric inhibition to reduce functional coupling between the motor cortices. The influences of movement symmetry and goal conceptualisation on interhemispheric interactions have not been studied together, and not during bimanually active dynamic tasks. The present study used transcranial magnetic stimulation (TMS) to investigate the modulation of CME and short- and long-latency interhemispheric inhibition (SIHI and LIHI, respectively) during bimanually active dynamic tasks requiring different types of bimanual coordination. Twenty healthy right-handed adults performed four bimanual tasks in which they held a dumbbell in each hand (independent) or a custom device between both hands (cooperative) while rhythmically flexing and extending their wrists symmetrically or asymmetrically. Motor-evoked potentials were recorded from the right extensor carpi ulnaris. We found CME was greater during asymmetric tasks than symmetric tasks, and movement symmetry did not modulate SIHI or LIHI. There was no effect of goal conceptualisation nor any interaction with movement symmetry for CME, SIHI or LIHI. Based on these results, movement symmetry and goal conceptualisation may not modulate interhemispheric inhibition during dynamic bimanual tasks. These findings contradict prevailing thinking about the roles of CME and interhemispheric inhibition in bimanual coordination.

Bilateral Contralaterally Controlled Functional Electrical Stimulation Reveals New Insights Into the Interhemispheric Competition Model in Chronic Stroke

Background. Upper-limb chronic stroke hemiplegia was once thought to persist because of disproportionate amounts of inhibition imposed from the contralesional on the ipsilesional hemisphere. Thus, one rehabilitation strategy involves discouraging engagement of the contralesional hemisphere by only engaging the impaired upper limb with intensive unilateral activities. However, this premise has recently been debated and has been shown to be task specific and/or apply only to a subset of the stroke population. Bilateral rehabilitation, conversely, engages both hemispheres and has been shown to benefit motor recovery. To determine what neurophysiological strategies bilateral therapies may engage, we compared the effects of a bilateral and unilateral based therapy using transcranial magnetic stimulation. Methods. We adopted a peripheral electrical stimulation paradigm where participants received 1 session of bilateral contralaterally controlled functional electrical stimulation (CCFES) and 1 session of unilateral cyclic neuromuscular electrical stimulation (cNMES) in a repeated-measures design. In all, 15 chronic stroke participants with a wide range of motor impairments (upper extremity Fugl-Meyer score: 15 [severe] to 63 [mild]) underwent single 1-hour sessions of CCFES and cNMES. We measured whether CCFES and cNMES produced different effects on interhemispheric inhibition (IHI) to the ipsilesional hemisphere, ipsilesional corticospinal output, and ipsilateral corticospinal output originating from the contralesional hemisphere. Results. CCFES reduced IHI and maintained ipsilesional output when compared with cNMES. We found no effect on ipsilateral output for either condition. Finally, the less-impaired participants demonstrated a greater increase in ipsilesional output following CCFES. Conclusions. Our results suggest that bilateral therapies are capable of alleviating inhibition on the ipsilesional hemisphere and enhancing output to the paretic limb.

Neural motor control differs between bimanual common-goal vs. bimanual dual-goal tasks

Coordinating bimanual movements is essential for everyday activities. Two common types of bimanual tasks are common goal, where two arms share a united goal, and dual goal, which involves independent goals for each arm. Here, we examine how the neural control mechanisms differ between these two types of bimanual tasks. Ten non-disabled individuals performed isometric force tasks of the elbow at 10% of their maximal voluntary force in both bimanual common and dual goals as well as unimanual conditions. Using transcranial magnetic stimulation, we concurrently examined the intracortical inhibitory modulation (short-interval intracortical inhibition, SICI) as well as the interlimb coordination strategies utilized between common- vs. dual-goal tasks. Results showed a reduction of SICI in both hemispheres during dual-goal compared to common-goal tasks (dominant hemisphere: P = 0.04, non-dominant hemisphere: P = 0.03) and unimanual tasks (dominant hemisphere: P = 0.001, non-dominant hemisphere: P = 0.001). For the common-goal task, a reduction of SICI was only seen in the dominant hemisphere compared to unimanual tasks (P = 0.03). Behaviorally, two interlimb coordination patterns were identified. For the common-goal task, both arms were organized into a cooperative "give and take" movement pattern. Control of the non-dominant arm affected stabilization of bimanual force (R² = 0.74, P = 0.001). In contrast, for the dual-goal task, both arms were coupled together in a positive fashion and neither arm affected stabilization of bimanual force (R² = 0.31, P = 0.1). The finding that intracortical inhibition and interlimb coordination patterns were different based on the goal conceptualization of bimanual tasks has implications for future research.