Search

Default in-phase coupling of hand movements needs to be suppressed when temporal coordination is required for out-of-phase bimanual movements. There is lack of knowledge on how the brain overrides these default in-phase movements to enable a required interval of activity between hands. We used a visually cued bimanual temporal coordination (vc-BTC) paradigm with a constant rhythmical time base of 1 s, to test the accuracy of in-phase and out-of-phase (0.1, 0.2,…,0.9) finger tapping. We hypothesized that (1) stronger anatomical and effective interhemispheric connectivity between the hand areas of the primary motor cortex (M1HAND) predict higher temporal offsets between hands in the out-of-phase conditions of the vc-BTC; (2) patients with relapsing-remitting multiple sclerosis (RRMS) and clinically isolated syndrome (CIS) have reduced interhemispheric connectivity and altered between-hand coupling. Anatomical connectivity was determined by fractional anisotropy of callosal hand motor fibers (FA-hCMF). Effective connectivity was probed by short interval interhemispheric inhibition (S-IHI) using paired-coil transcranial magnetic stimulation (TMS). In healthy subjects, higher FA-hCMF and S-IHI correlated with higher temporal offsets between hands in the out-of-phase conditions of the tapping test. FA-hCMF was reduced in patients with RRMS but not in CIS, while S-IHI was reduced in both patient groups. These abnormalities were associated with smaller temporal offsets between hands leading to less deviation from the required phasing in the out-of-phase tapping conditions. Findings provide multiple levels of evidence that callosal anatomical and effective connectivity between the hand areas of the motor cortices play important roles in visually cued bimanual temporal coordination performance.

Objective: Lithium has been widely used to treat bipolar affective disorder for over 60years. Still, its acute effects in human cerebral cortex are poorly understood. This study aimed at investigating the acute effects of lithium on motor cortex excitability as measured by transcranial magnetic stimulation (TMS)., Methods: Ten healthy young adults participated in a double-blind placebo-controlled randomized crossover study with four sessions, where a single oral dose of lithium carbonate (450mg, 900mg, or 1350mg) or placebo was tested. Focal TMS of the hand area of left motor cortex was used to test resting and active motor thresholds, motor evoked potential input-output curve (MEP IO-curve), slope of the MEP IO-curve and paired-pulse measures of intracortical inhibition and facilitation before, and two and four hours after drug administration., Results: Two hours post drug administration, 450mg of lithium carbonate increased the slope of the MEP IO-curve while 1350mg tended to decrease it. Lithium had no effect on motor thresholds, or intracortical inhibition or facilitation., Conclusions: The acute effects of lithium on MEP IO-curve, a marker of corticospinal excitability, are consistent with an inverted U-shaped dose-response relationship., Significance: Findings are important for our understanding of the therapeutic and toxic effects of lithium on the human central nervous system., (Copyright © 2014 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.)

Background: Recently, a link between resting motor threshold (RMT) and local tissue microstructure, as indexed by fractional anisotropy (FA), was demonstrated in large parts of white matter. However, regions showing such correlations were generally found outside of the corticospinal tract (CST). Therefore, the question arises whether other electrophysiologic measurements could be more locally related to microstructural properties of the CST. In this study, we explored the relationship between such measurements and regional FA in a group of healthy volunteers., Objective/hypothesis: We hypothesized that RMT might be more related to an overall susceptibility of white matter to TMS, whereas other electrophysiologic markers might be more specifically related to properties of the CST only., Methods: Thirty-seven subjects were included. We studied RMT, active motor threshold (AMT), intensity to evoke a motor-evoked potential (MEP) of 1 mV (S1mV), MEP input-output curve (IO-curve), and central motor conduction time (CMCT) using transcranial magnetic stimulation, and FA of the corticospinal tract using diffusion tensor magnetic resonance imaging. We performed voxel-wise and TBSS correlation analysis between these electrophysiologic measurements and FA. In addition, we tested for significant correlation between these parameters and mean diffusivity (MD)., Results: On voxel-wise analysis, we did not detect significant correlations between any electrophysiologic parameter (RMT, AMT, S1mV, IO curve slope, CMCT) and FA. With TBSS, we detected correlations between FA and bilateral AMT, as well as left-hemispheric S1mV, but these correlations were found in locations unlikely to contribute to motor pathways., Conclusions: Although a relationship between structure and function has been shown in many other regions of the brain, it seems to be much more challenging to demonstrate such a relationship in the CST of healthy subjects., (Copyright © 2012 Elsevier Inc. All rights reserved.)

In relapsing-remitting multiple sclerosis (RRMS) the corpus callosum (CC) is often and early affected by macroscopic lesions when investigated by conventional MRI. We sought to determine to which extent microstructural and effective disconnection of the CC are already present in RRMS patients at the earliest stages of the disease prior to evidence of macroscopic CC lesion. We compared 16 very early RRMS patients (median expanded disability status scale (EDSS), 1.5; range, 0-2.0) to an age-matched group of healthy controls and focused analysis to the motor CC, i.e. that part of the CC relaying interhemispheric motor information. A combined functional magnetic resonance imaging/diffusion tensor imaging fiber-tracking procedure was applied to identify the callosal motor fibers (CMFs) connecting the hand areas of the primary motor cortices of the two hemispheres. Fractional anisotropy (FA) within the motor CC (FA-CC) assessed the CMF microstructural integrity. Bifocal paired transcranial magnetic stimulation (TMS) tested short-interval interhemispheric inhibition (S-IHI), an established measure of CMF effective connectivity. FA-CC and S-IHI were significantly reduced in early RRMS compared to healthy controls. Furthermore, a significant linear correlation between microstructure (FA-CC) and function (S-IHI) in the controls was broken down in the patients. These abnormalities were obtained in the absence of macroscopic CMF lesion in conventional MRI, and whilst motor hand/arm function in the nine-hole-peg test and corticospinal conduction time were normal. Findings suggest that reductions in FA and S-IHI may serve as surrogate markers of motor callosal disconnection at the earliest stages of RRMS prior to development of macroscopic lesion., (Copyright © 2010 Wiley-Liss, Inc.)

Electromyographic mirror activity (MA) refers to involuntary activation of the non-active limb during intended strictly unilateral movements of the other limb. MA occurs in the majority of healthy adults but little is known about its neurophysiological foundation. Here we examined in healthy adults the hypothesis that transcallosal interhemispheric inhibition (IHI) between the primary motor cortices determines the extent to which MA occurs. IHI was tested by an established paired transcranial magnetic stimulation protocol [A. Ferbert et al. (1992) J. Physiol. (Lond.), 453, 525-546]. In a first experiment we found that the magnitudes of IHI and MA were inversely correlated. In a second experiment we sought to establish a more causative relation by exploring the changes induced by low-frequency repetitive transcranial magnetic stimulation of primary motor cortex on IHI and MA. These changes were also significantly inversely correlated, i.e. a repetitive transcranial magnetic stimulation-induced increase in IHI was associated with a decrease in MA, and a decrease in IHI was associated with an increase in MA. These results provide strong and consistent evidence that transcallosal inhibitory interhemispheric interaction provides a significant route by which unwanted MA during intended unimanual motor tasks can be controlled.