Continuous theta burst stimulation of the supplementary motor area: effect upon perception and somatosensory and motor evoked potentials

Effects of repetitive practice of motor tasks on somatosensory gating

Introduction

During voluntary muscle contraction, the amplitude of the somatosensory evoked potential (SEP) is reduced by inhibiting sensory information from a peripheral nerve supplying the contracted muscle. This phenomenon is called “gating.” We reported that participants with good motor skills indicated strong suppression of somatosensory information. The present study investigated the effects of motor performance improvement following repetitive practice on the SEP amplitude.

Methods

The ball rotation task (BR task) was practiced by 15 healthy participants repetitively. SEPs were recorded before (pre) and after (post) repetitive practice.

Results

The BR task performance was significantly improved and the required muscle activation to perform the task was significantly reduced after the repetitive practice. The degree of gating was not significant between pre and post- for the SEP amplitude. A significant correlation was found between changes in SEP amplitude from pre to post and performance improvement.

Discussion

After repetitive practice, the degree of gating did not change, but the performance of the BR task improved, and the muscle activity required for the BR task decreased. These results suggest that repetitive practice does not change the degree of gating but changes the mechanism of gating. Furthermore, they indicate that suppression of the somatosensory area may play a role in improving task performance.

Continuous theta burst TMS of area MT+ impairs attentive motion tracking

Attentive motion tracking deficits measured using multiple object tracking (MOT) tasks have been identified in a number of neurodevelopmental disorders such as amblyopia and autism. These deficits are often attributed to the abnormal development of high-level attentional networks. However, neuroimaging evidence from amblyopia suggests that reduced MOT performance can be explained by impaired function in motion-sensitive area MT+ alone. To test the hypothesis that a subtle disruption of MT+ function could cause MOT impairment, we assessed whether continuous theta burst stimulation (cTBS) of MT+ influenced MOT task accuracy in individuals with normal vision. The MOT stimulus consisted of four target and four distractor dots and was presented at ±10° eccentricity (right/left hemifield). fMRI-guided cTBS was applied to left MT+. Participants (n = 13, age: 27 ± 3) attended separate active and sham cTBS sessions where the MOT task was completed before, 5-min post- and 30-min post-cTBS. Active cTBS significantly impaired MOT task accuracy relative to baseline for the right (stimulated) hemifield 5-min (10 ± 2% reduction) and 30-min (14 ± 3% reduction) post-stimulation. No impairment occurred within the left (control) hemifield after active cTBS or for either hemifield after sham cTBS. These results highlight the importance of lower level motion processing for MOT, suggesting that a minor disruption of MT+ function alone is sufficient to cause a deficit in MOT performance.

Dexterous Object Manipulation Requires Context-Dependent Sensorimotor Cortical Interactions in Humans

Dexterous object manipulation is a hallmark of human evolution and a critical skill for everyday activities. A previous work has used a grasping context that predominantly elicits memory-based control of digit forces by constraining where the object should be grasped. For this "constrained" grasping context, the primary motor cortex (M1) is involved in storage and retrieval of digit forces used in previous manipulations. In contrast, when choice of digit contact points is allowed ("unconstrained" grasping), behavioral studies revealed that forces are adjusted, on a trial-to-trial basis, as a function of digit position. This suggests a role of online feedback of digit position for force control. However, despite the ubiquitous nature of unconstrained hand-object interactions in activities of daily living, the underlying neural mechanisms are unknown. Using noninvasive brain stimulation, we found the role of primary motor cortex (M1) and somatosensory cortex (S1) to be sensitive to grasping context. In constrained grasping, M1 but not S1 is involved in storing and retrieving learned digit forces and position. In contrast, in unconstrained grasping, M1 and S1 are involved in modulating digit forces to position. Our findings suggest that the relative contribution of memory and online feedback modulates sensorimotor cortical interactions for dexterous manipulation.

Reward-driven enhancements in motor control are robust to TMS manipulation

A wealth of evidence describes the strong positive impact that reward has on motor control at the behavioural level. However, surprisingly little is known regarding the neural mechanisms which underpin these effects, beyond a reliance on the dopaminergic system. In recent work, we developed a task that enabled the dissociation of the selection and execution components of an upper limb reaching movement. Our results demonstrated that both selection and execution are concommitently enhanced by immediate reward availability. Here, we investigate what the neural underpinnings of each component may be. To this end, we aimed to alter the cortical excitability of the ventromedial prefrontal cortex and supplementary motor area using continuous theta-burst transcranial magnetic stimulation (cTBS) in a within-participant design (N = 23). Both cortical areas are involved in determining an individual’s sensitivity to reward and physical effort, and we hypothesised that a change in excitability would result in the reward-driven effects on action selection and execution to be altered, respectively. To increase statistical power, participants were pre-selected based on their sensitivity to reward in the reaching task. While reward did lead to enhanced performance during the cTBS sessions and a control sham session, cTBS was ineffective in altering these effects. These results may provide evidence that other areas, such as the primary motor cortex or the premotor area, may drive the reward-based enhancements of motor performance.

Interrogating cortical function with transcranial magnetic stimulation: insights from neurodegenerative disease and stroke

Transcranial magnetic stimulation (TMS) is an accessible, non-invasive technique to study cortical function in vivo. TMS studies have provided important pathophysiological insights across a range of neurodegenerative disorders and enhanced our understanding of brain reorganisation after stroke. In neurodegenerative disease, TMS has provided novel insights into the function of cortical output cells and the related intracortical interneuronal networks. Characterisation of cortical hyperexcitability in amyotrophic lateral sclerosis and altered motor cortical function in frontotemporal dementia, demonstration of cholinergic deficits in Alzheimer's disease and Parkinson's disease are key examples where TMS has led to advances in understanding of disease pathophysiology and potential mechanisms of propagation, with the potential for diagnostic applications. In stroke, TMS methodology has facilitated the understanding of cortical reorganisation that underlie functional recovery. These insights are critical to the development of effective and targeted rehabilitation strategies in stroke. The present review will provide an overview of cortical function measures obtained using TMS and how such measures may provide insight into brain function. Through an improved understanding of cortical function across a range of neurodegenerative disorders, and identification of changes in neural structure and function associated with stroke that underlie clinical recovery, more targeted therapeutic approaches may now be developed in an evolving era of precision medicine.

Transcranial direct current stimulation enhances foot sole somatosensation when standing in older adults

Foot-sole somatosensation is critical for safe mobility in older adults. Somatosensation arises when afferent input activates a neural network that includes the primary somatosensory cortex. Transcranial direct current stimulation (tDCS), as a strategy to increase somatosensory cortical excitability, may, therefore, enhance foot-sole somatosensation. We hypothesized that a single session of tDCS would improve foot-sole somatosensation, and thus mobility, in older adults. Twenty healthy older adults completed this randomized, double-blinded, cross-over study consisting of two visits separated by one week. On each visit, standing vibratory threshold (SVT) of each foot and the timed-up-and-go test (TUG) of mobility were assessed immediately before and after a 20-min session of tDCS (2.0 mA) or sham stimulation with the anode placed over C3 (according to the 10/20 EEG placement system) and the cathode over the contralateral supraorbital margin. tDCS condition order was randomized. SVT was measured with a shoe insole system. This system automatically ramped up, or down, the amplitude of applied vibrations and the participant stated when they could or could no longer feel the vibration, such that lower SVT reflected better somatosensation. The SVTs of both foot soles were lower following tDCS as compared to sham and both pre-test conditions [F_(1,76) > 3.4, p < 0.03]. A trend towards better TUG performance following tDCS was also observed [F_(1,76) = 2.4, p = 0.07]. Greater improvement in SVT (averaged across feet) moderately correlated with greater improvement in TUG performance (r = 0.48, p = 0.03). These results suggest that tDCS may enhance lower-extremity somatosensory function, and potentially mobility, in healthy older adults.

Contribution of the supplementary motor area and the cerebellum to the anticipatory postural adjustments and execution phases of human gait initiation

Several brain structures including the brainstem, the cerebellum and the frontal cortico-basal ganglia network, with the primary and premotor areas have been shown to participate in the functional organization of gait initiation and postural control in humans, but their respective roles remain poorly understood. The aim of this study was to better understand the role of the supplementary motor area (SMA) and posterior cerebellum in the gait initiation process. Gait initiation parameters were recorded in 22 controls both before and after continuous theta burst transcranial stimulation (cTBS) of the SMA and cerebellum, and were compared to sham stimulation, using a randomized double-blind design study. The two phases of gait initiation process were analyzed: anticipatory postural adjustments (APAs) and execution, with recordings of soleus and tibialis anterior muscles. Functional inhibition of the SMA led to a shortened APA phase duration with advanced and increased muscle activity; during execution, it also advanced muscle co-activation and decreased the duration of stance soleus activity. Cerebellar functional inhibition did not influence the APA phase duration and amplitude but increased muscle co-activation, it decreased execution duration and showed a trend to increase velocity, with increased swing soleus muscle duration and activity. The results suggest that the SMA contributes to both the timing and amplitude of the APAs with no influence on step execution and the posterior cerebellum in the coupling between the APAs and execution phases and leg muscle activity pattern during gait initiation.

N30 Somatosensory Evoked Potential Is Negatively Correlated with Motor Function in Parkinson's Disease

Objective

The aim of this study was to investigate frontal N30 status in Parkinson’s disease (PD) and to examine the correlation between the amplitude of frontal N30 and the severity of motor deficits.

Methods

The frontal N30 was compared between 17 PD patients and 18 healthy volunteers. Correlations between the amplitude of frontal N30 and the Unified Parkinson’s Disease Rating Scale (UPDRS) motor score of the more severely affected side was examined.

Results

The mean latency of the N30 was not significantly different between patients and healthy volunteers (p = 0.981), but the mean amplitude was lower in PD patients (p < 0.025). There was a significant negative correlation between the amplitude of N30 and the UPDRS motor score (r = -0.715, p = 0.013).

Conclusions

The frontal N30 status indicates the motor severity of PD. It can be a useful biomarker reflecting dopaminergic deficits and an objective measurement for monitoring the clinical severity of PD.

A Review of Transcranial Magnetic Stimulation and Multimodal Neuroimaging to Characterize Post-Stroke Neuroplasticity

Following stroke, the brain undergoes various stages of recovery where the central nervous system can reorganize neural circuitry (neuroplasticity) both spontaneously and with the aid of behavioral rehabilitation and non-invasive brain stimulation. Multiple neuroimaging techniques can characterize common structural and functional stroke-related deficits, and importantly, help predict recovery of function. Diffusion tensor imaging (DTI) typically reveals increased overall diffusivity throughout the brain following stroke, and is capable of indexing the extent of white matter damage. Magnetic resonance spectroscopy (MRS) provides an index of metabolic changes in surviving neural tissue after stroke, serving as a marker of brain function. The neural correlates of altered brain activity after stroke have been demonstrated by abnormal activation of sensorimotor cortices during task performance, and at rest, using functional magnetic resonance imaging (fMRI). Electroencephalography (EEG) has been used to characterize motor dysfunction in terms of increased cortical amplitude in the sensorimotor regions when performing upper limb movement, indicating abnormally increased cognitive effort and planning in individuals with stroke. Transcranial magnetic stimulation (TMS) work reveals changes in ipsilesional and contralesional cortical excitability in the sensorimotor cortices. The severity of motor deficits indexed using TMS has been linked to the magnitude of activity imbalance between the sensorimotor cortices. In this paper, we will provide a narrative review of data from studies utilizing DTI, MRS, fMRI, EEG, and brain stimulation techniques focusing on TMS and its combination with uni- and multimodal neuroimaging methods to assess recovery after stroke. Approaches that delineate the best measures with which to predict or positively alter outcomes will be highlighted.

Use of transcranial magnetic stimulation in the treatment of selected movement disorders

Transcranial magnetic stimulation (TMS) is a valuable technique for assessing the underlying neurophysiology associated with various neuropathologies, and is a unique tool for establishing potential neural mechanisms responsible for disease progression. Recently, repetitive TMS (rTMS) has been advanced as a potential therapeutic technique to treat selected neurologic disorders. In healthy individuals, rTMS can induce changes in cortical excitability. Therefore, targeting specific cortical areas affected by movement disorders theoretically may alter symptomology. This review discusses the evidence for the efficacy of rTMS in Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and multiple sclerosis. It is hoped that gaining a more thorough understanding of the timing and parameters of rTMS in individuals with neurodegenerative disorders may advance both clinical care and research into the most effective uses of this technology.

Theta burst stimulation over the supplementary motor area in Parkinson's disease

To investigate whether a period of continuous theta burst stimulation (cTBS) over the supplementary motor area (SMA) induces cortical plasticity and thus improves bradykinesia in Parkinson's disease (PD) in the medication ON and OFF state. In total, 26 patients with Parkinson's disease were tested with both real and sham stimulation. The group was divided into an OFF-medication (4 females, mean age 65 years, disease duration 6 years) and an ON-medication group (7 females, mean age 61 years, disease duration 7 years) with each containing 13 individuals. Both groups were evaluated in terms of electrophysiological (motor-evoked potentials) and behavioural [Purdue Pegboard test (PPT), UPDRS motor subscore] parameters before (baseline condition) and after a 40-second period of real or sham continuous theta burst stimulation over the SMA ON and OFF dopaminergic drugs. Patients in the OFF group demonstrated an improved UPDRS III score (p < 0.05) and a better performance in the PPT for the less affected side (p < 0.025) compared to baseline after real stimulation. However, electrophysiological parameters did not change in either the ON or the OFF state. cTBS over the SMA has a mild effect on motor symptoms of the upper limb in the OFF state of PD patients. In contrast, stimulation did not change cortico-spinal excitability. A lack of change (i.e. no plasticity) to brain stimulation protocols is a known finding in PD. A clinical improvement in the OFF state, however, contrasts with this and the mechanism of these induced changes is worth further exploration.

The influence of an acute bout of aerobic exercise on cortical contributions to motor preparation and execution

Increasing evidence supports the use of physical activity for modifying brain activity and overall neurological health. Specifically, aerobic exercise appears to have a positive effect on cognitive function, which some have suggested to be a result of increasing levels of arousal. However, the role of aerobic exercise on movement-related cortical activity is less clear. We tested the hypothesis that (1) an acute bout of exercise modulates excitability within motor areas and (2) transient effects would be sustained as long as sympathetic drive remained elevated (indicated by heart rate). In experiment 1, participants performed unimanual self-paced wrist extension movements before and after a 20-min, moderate intensity aerobic exercise intervention on a recumbent cycle ergometer. After the cessation of exercise, Bereitschaftspotentials (BP), representative cortical markers for motor preparation, were recorded immediately postexercise (Post) and following a return to baseline heart rate (Post[Rest]). Electroencephalography (EEG) was used to measure the BP time-locked to onset of muscle activity and separated into three main components: early, late and reafferent potentials. In experiment 2, two additional time points postexercise were added to the original protocol following the Post[Rest] condition. Early BP but not late BP was influenced by aerobic exercise, evidenced by an earlier onset, indicative of a regionally selective effect across BP generators. Moreover, this effect was sustained for up to an hour following exercise cessation and this effect was following a return to baseline heart rate. These data demonstrate that acute aerobic exercise may alter and possibly enhance the cortical substrates required for the preparation of movement.

Transcranial Magnetic Stimulation Studies of Human Time Perception: A Primer

The study of the neural basis of time perception has seen a resurgence of interest within the past decade. A variety of these studies have included the use of transcranial magnetic stimulation (TMS), a noninvasive technique for stimulating discrete regions of the surface of the brain. Here, the results of these studies are reviewed and their conclusions are interpreted within a context-dependent framework. However, the use of TMS as an investigatory technique has much unexplored potential that may be particularly beneficial to the study of time perception. As such, considerations are made regarding the design of TMS studies of time perception and future directions are outlined that may be utilized to further elucidate the neural basis of timing in the human brain.