Mark van de Ruit

Rapid acquisition of the transcranial magnetic stimulation stimulus response curve.

BACKGROUND Transcranial magnetic stimulation is frequently used to construct stimulus response (SR) curves in studies of motor learning and rehabilitation. A drawback of the established method is the time required for data acquisition, which is frequently greater than a participants ability to maintain attention. The technique is therefore difficult to use in the clinical setting. OBJECTIVE To reduce the time of curve acquisition by determining the minimum acquisition time and number of stimuli required to acquire an SR curve. METHODS SR curves were acquired from first dorsal interosseous (FDI) and abductor digiti minimi (ADM) at 6 interstimulus intervals (ISI) between 1.4 and 4 s in 12 participants. To determine if low-frequency rTMS might affect the SR curve, MEP amplitudes were monitored before and after 3 min of 1 Hz rTMS delivered at 120% of resting motor threshold in 12 participants. Finally, SR curves were acquired from FDI, ADM and Biceps Brachii (BB) in 12 participants, and the minimum number of stimuli was calculated using a sequential MEP elimination process. RESULTS There were no significant differences between curves acquired with 1.4 s ISI and any other ISI. Low frequency rTMS did not significantly depress MEP amplitude (P = 0.87). On average, 61 ± 18 (FDI), 60 ± 16 (ADM) and 59 ± 16 (BB) MEPs were needed to construct a representative SR curve. CONCLUSIONS This study demonstrates that reliable SR curves may be acquired in less than 2 min. At this rate, SR curves become a clinically feasible method for assessing corticospinal excitability in research and rehabilitation settings.

Spinal inhibition of descending command to soleus motoneurons is removed prior to dorsiflexion

Non‐technical summary  The coordination of antagonistic muscle activity starts well in advance of the onset of voluntary movement. We recently demonstrated that antagonist muscle responses evoked by stimulation of the brain were increased prior to voluntary contraction at the ankle. Although our data indicated that this was explained by activation of a subcortical motor program, the neural pathways involved are unknown. Here we probe the transmission in the underlying neuronal networks by peripheral nerve stimulation in order to investigate the neural pathways responsible for this facilitation of antagonist muscle responses. We demonstrate that this stimulation produces a spinal inhibition of the antagonist muscle responses, which is removed prior to voluntary contraction. We propose that the removal of this inhibition might explain the increased antagonist muscle responses prior to voluntary contraction at the ankle. This mechanism might enable the direction of movement to be changed quickly during functional motor tasks such as dribbling.

Nonlinear coupling between cortical oscillations and muscle activity during isotonic wrist flexion

Coupling between cortical oscillations and muscle activity facilitates neuronal communication during motor control. The linear part of this coupling, known as corticomuscular coherence, has received substantial attention, even though neuronal communication underlying motor control has been demonstrated to be highly nonlinear. A full assessment of corticomuscular coupling, including the nonlinear part, is essential to understand the neuronal communication within the sensorimotor system. In this study, we applied the recently developed n:m coherence method to assess nonlinear corticomuscular coupling during isotonic wrist flexion. The n:m coherence is a generalized metric for quantifying nonlinear cross-frequency coupling as well as linear iso-frequency coupling. By using independent component analysis (ICA) and equivalent current dipole source localization, we identify four sensorimotor related brain areas based on the locations of the dipoles, i.e., the contralateral primary sensorimotor areas, supplementary motor area (SMA), prefrontal area (PFA) and posterior parietal cortex (PPC). For all these areas, linear coupling between electroencephalogram (EEG) and electromyogram (EMG) is present with peaks in the beta band (15–35 Hz), while nonlinear coupling is detected with both integer (1:2, 1:3, 1:4) and non-integer (2:3) harmonics. Significant differences between brain areas is shown in linear coupling with stronger coherence for the primary sensorimotor areas and motor association cortices (SMA, PFA) compared to the sensory association area (PPC); but not for the nonlinear coupling. Moreover, the detected nonlinear coupling is similar to previously reported nonlinear coupling of cortical activity to somatosensory stimuli. We suggest that the descending motor pathways mainly contribute to linear corticomuscular coupling, while nonlinear coupling likely originates from sensory feedback.

StimTrack: An open-source software for manual transcranial magnetic stimulation coil positioning

BACKGROUND During Transcranial Magnetic Stimulation (TMS) experiments researchers often use a neuronavigation system to precisely and accurately maintain coil position and orientation. NEW METHOD This study aimed to develop and validate an open-source software for TMS coil navigation. StimTrack uses an optical tracker and an intuitive user interface to facilitate the maintenance of position and orientation of any type of coil within and between sessions. Additionally, online access to navigation data is provided, hereby adding e.g. the ability to start or stop the magnetic stimulator depending on the distance to target or the variation of the orientation angles. RESULTS StimTrack allows repeatable repositioning of the coil within 0.7mm for translation and <1° for rotation. Stimulus-response (SR) curves obtained from 19 healthy volunteers were used to demonstrate that StimTrack can be effectively used in a typical experiment. An excellent intra and inter-session reliability (ICC >0.9) was obtained on all parameters computed on SR curves acquired using StimTrack. COMPARISON WITH EXISTING METHOD StimTrack showed a target accuracy similar to that of a commercial neuronavigation system (BrainSight, Rogue Research Inc.). Indeed, small differences both in position (∼0.2mm) and orientation (<1°) were found between the systems. These differences are negligible given the human error involved in landmarks registration. CONCLUSIONS StimTrack, available as supplementary material, is found to be a good alternative for commercial neuronavigation systems facilitating assessment changes in corticospinal excitability using TMS. StimTrack allows researchers to tailor its functionality to their specific needs, providing added value that benefits experimental procedures and improves data quality.

Interindividual Variability in Use-Dependent Plasticity Following Visuomotor Learning: The Effect of Handedness and Muscle Trained

ABSTRACT Motor learning has been linked with increases in corticospinal excitability (CSE). However, the robustness of this link is unclear. In this study, changes in CSE associated with learning a visuomotor tracking task were mapped using transcranial magnetic stimulation (TMS). TMS maps were obtained before and after training with the first dorsal interosseous (FDI) of the dominant and nondominant hand, and for a distal (FDI) and proximal (biceps brachii) muscle. Tracking performance improved following 20 min of visuomotor training, while map area was unaffected. Large individual differences were observed with 18%–36% of the participants revealing an increase in TMS map area. This result highlights the complex relationship between motor learning and use-dependent plasticity of the motor cortex.

Intra and inter-session reliability of rapid Transcranial Magnetic Stimulation stimulus-response curves of tibialis anterior muscle in healthy older adults

Objective The clinical use of Transcranial Magnetic Stimulation (TMS) as a technique to assess corticospinal excitability is limited by the time for data acquisition and the measurement variability. This study aimed at evaluating the reliability of Stimulus-Response (SR) curves acquired with a recently proposed rapid protocol on tibialis anterior muscle of healthy older adults. Methods Twenty-four neurologically-intact adults (age:55–75 years) were recruited for this test-retest study. During each session, six SR curves, 3 at rest and 3 during isometric muscle contractions at 5% of maximum voluntary contraction (MVC), were acquired. Motor Evoked Potentials (MEPs) were normalized to the maximum peripherally evoked response; the coil position and orientation were monitored with an optical tracking system. Intra- and inter-session reliability of motor threshold (MT), area under the curve (AURC), MEPmax, stimulation intensity at which the MEP is mid-way between MEPmax and MEPmin (I50), slope in I50, MEP latency, and silent period (SP) were assessed in terms of Standard Error of Measurement (SEM), relative SEM, Minimum Detectable Change (MDC), and Intraclass Correlation Coefficient (ICC). Results The relative SEM was ≤10% for MT, I50, latency and SP both at rest and 5%MVC, while it ranged between 11% and 37% for AURC, MEPmax, and slope. MDC values were overall quite large; e.g., MT required a change of 12%MSO at rest and 10%MSO at 5%MVC to be considered a real change. Inter-sessions ICC were >0.6 for all measures but slope at rest and MEPmax and latency at 5%MVC. Conclusions Measures derived from SR curves acquired in <4 minutes are affected by similar measurement errors to those found with long-lasting protocols, suggesting that the rapid method is at least as reliable as the traditional methods. As specifically designed to include older adults, this study provides normative data for future studies involving older neurological patients (e.g. stroke survivors).

The TMS Motor Map does not change following a single session of mirror training either with or without motor imagery

Both motor imagery and mirror training have been used in motor rehabilitation settings to promote skill learning and plasticity. As motor imagery and mirror training are suggested to be closely linked, it was hypothesized that mirror training augmented by motor imagery would increase corticospinal excitability (CSE) significantly compared to mirror training alone. Forty-four participants were split over two experimental groups. Each participant visited the laboratory once to receive either mirror training alone or mirror training augmented with layered stimulus response training (LSRT), a type of motor imagery training. Participants performed 16 min of mirror training, making repetitive grasping movements paced by a metronome. Transcranial magnetic stimulation (TMS) mapping was performed before and after the mirror training to test for changes in CSE of the untrained hand. Self-reports suggested that the imagery training was effective in helping the participant to perform the mirror training task as instructed. Nonetheless, neither training type resulted in a significant change of TMS map area, nor was there an interaction between the groups. The results from the study revealed no effect of a single session of 16 min of either mirror training or mirror training enhanced by imagery on TMS map area. Despite the negative result of the present experiment, this does not suggest that either motor imagery or mirror training might be ineffective as a rehabilitation therapy. Further study is required to allow disentangling the role of imagery and action observation in mirror training so that mirror training can be further tailored to the individual according to their abilities.

TMS assessed cortical representation scales with stimulation intensity and muscle activation

Transcranial magnetic stimulation (TMS) is routinely used to construct a map of corticospinal excitability (CSE). TMS elicited motor evoked potentials (MEPs) are known to increase both with stimulation intensity and muscle activation. Whilst a wide variety of stimulation intensities and levels of muscle activation are used to generate TMS maps, their effect on the cortical representation has yet to be systematically explored. Two experiments were performed to describe the effect of stimulation intensities (Experiment 1) and muscle activation (Experiment 2) on the map outcome measures: aspect ratio, centre of gravity (COG), map area and map volume. Twelve participants were recruited for each experiment. TMS maps were acquired from the first dorsal interosseous (FDI). Maps were acquired using 80 stimuli pseudorandomly across a 6x6 cm area with a 1.5 s interstimulus interval, allowing the maps to be acquired in two minutes. In Experiment 1 maps were compared at 5, 10, 20 and 40% of the maximum voluntary contraction. All maps were acquired with a stimulation intensity of 120% of the resting motor threshold (RMT). In Experiment 2 maps were compared at the stimulation intensities of 110, 120 and 130% of RMT, whilst the muscle was at rest. A significant increase in map area and map volume were observed with stimulation intensity and level of muscle activation as would be expected. Neither the COG nor the aspect ratio were changed with either increased stimulation intensity or muscle activation. This study demonstrates that the cortical representation scales with stimulation intensity and level of muscle activation, but the shape of the map does not change.