An initial transient-state and reliable measures of corticospinal excitability in TMS studies

A transcranial magnetic stimulation study of the effect of visual orientation on the putative human mirror neuron system

Mirror neurons are a class of motor neuron that are active during both the performance and observation of behavior, and have been implicated in interpersonal understanding. There is evidence to suggest that the mirror response is modulated by the perspective from which an action is presented (e.g., egocentric or allocentric). Most human research, however, has only examined this when presenting intransitive actions. Twenty-three healthy adult participants completed a transcranial magnetic stimulation experiment that assessed corticospinal excitability whilst viewing transitive hand gestures from both egocentric (i.e., self) and allocentric (i.e., other) viewpoints. Although action observation was associated with increases in corticospinal excitability (reflecting putative human mirror neuron activity), there was no effect of visual perspective. These findings are discussed in the context of contemporary theories of mirror neuron ontogeny, including models concerning associative learning and evolutionary adaptation.

Long-Term Effects of Serial Anodal tDCS on Motion Perception in Subjects with Occipital Stroke Measured in the Unaffected Visual Hemifield

Transcranial direct current stimulation (tDCS) is a novel neuromodulatory tool that has seen early transition to clinical trials, although the high variability of these findings necessitates further studies in clinically relevant populations. The majority of evidence into effects of repeated tDCS is based on research in the human motor system, but it is unclear whether the long-term effects of serial tDCS are motor-specific or transferable to other brain areas. This study aimed to examine whether serial anodal tDCS over the visual cortex can exogenously induce long-term neuroplastic changes in the visual cortex. However, when the visual cortex is affected by a cortical lesion, up-regulated endogenous neuroplastic adaptation processes may alter the susceptibility to tDCS. To this end, motion perception was investigated in the unaffected hemifield of subjects with unilateral visual cortex lesions. Twelve subjects with occipital ischemic lesions participated in a within-subject, sham-controlled, double-blind study. MRI-registered sham or anodal tDCS (1.5 mA, 20 min) was applied on five consecutive days over the visual cortex. Motion perception was tested before and after stimulation sessions and at 14- and 28-day follow-up. After a 16-day interval an identical study block with the other stimulation condition (anodal or sham tDCS) followed. Serial anodal tDCS over the visual cortex resulted in an improvement in motion perception, a function attributed to MT/V5. This effect was still measurable at 14- and 28-day follow-up measurements. Thus, this may represent evidence for long-term tDCS-induced plasticity and has implications for the design of studies examining the time course of tDCS effects in both the visual and motor systems.

Evolution of premotor cortical excitability after cathodal inhibition of the primary motor cortex: a sham-controlled serial navigated TMS study

Background

Premotor cortical regions (PMC) play an important role in the orchestration of motor function, yet their role in compensatory mechanisms in a disturbed motor system is largely unclear. Previous studies are consistent in describing pronounced anatomical and functional connectivity between the PMC and the primary motor cortex (M1). Lesion studies consistently show compensatory adaptive changes in PMC neural activity following an M1 lesion. Non-invasive brain modification of PMC neural activity has shown compensatory neurophysiological aftereffects in M1. These studies have contributed to our understanding of how M1 responds to changes in PMC neural activity. Yet, the way in which the PMC responds to artificial inhibition of M1 neural activity is unclear. Here we investigate the neurophysiological consequences in the PMC and the behavioral consequences for motor performance of stimulation mediated M1 inhibition by cathodal transcranial direct current stimulation (tDCS).

Purpose

The primary goal was to determine how electrophysiological measures of PMC excitability change in order to compensate for inhibited M1 neural excitability and attenuated motor performance.

Hypothesis

Cathodal inhibition of M1 excitability leads to a compensatory increase of ipsilateral PMC excitability.

Methods

We enrolled 16 healthy participants in this randomized, double-blind, sham-controlled, crossover design study. All participants underwent navigated transcranial magnetic stimulation (nTMS) to identify PMC and M1 corticospinal projections as well as to evaluate electrophysiological measures of cortical, intracortical and interhemispheric excitability. Cortical M1 excitability was inhibited using cathodal tDCS. Finger-tapping speeds were used to examine motor function.

Results

Cathodal tDCS successfully reduced M1 excitability and motor performance speed. PMC excitability was increased for longer and was the only significant predictor of motor performance.

Conclusion

The PMC compensates for attenuated M1 excitability and contributes to motor performance maintenance.

Ipsilateral corticospinal responses to ballistic training are similar for various intensities and timings of TMS

AIM

In previous studies, unilateral ballistic training either increased or decreased corticospinal excitability for the untrained opposite limb. The objective here was to investigate whether these discrepancies can be explained by methodological differences such as the intensity of stimulation assessing excitability or the timing of excitability testing after training.

METHODS

Motor evoked potentials (MEP) were elicited by stimulating the ipsilateral cortex at high intensity (70% MEPmax) and low intensity (20% MEPmax) at specific time-points after performance of 300 ballistic movements of the index finger.

RESULTS

Ballistic practice significantly facilitated MEP size for high-intensity stimuli, whereas responses to low-intensity stimulation were variable. MEP sizes at individual time-points were not significantly facilitated until 4 min after training, although there was no difference between early and late responses when grouped over multiple time-points.

CONCLUSIONS

The data indicate that previous discrepancies in ipsilateral responses to ballistic training cannot be attributed to specific procedures used to assess corticospinal excitability as there was no tendency towards depression of MEP amplitude at any point post-exercise for both testing intensities. This suggests that other experimental factors such as locus of attention or availability of visual feedback are more likely to account for the discrepancies.

Shaping reversibility? Long-term deep brain stimulation in dystonia: the relationship between effects on electrophysiology and clinical symptoms

Long-term results show that benefits from chronic deep brain stimulation in dystonia are maintained for many years. Despite this, the neurophysiological long-term consequences of treatment and their relationship to clinical effects are not well understood. Previous studies have shown that transcranial magnetic stimulation measures of abnormal long-term potentiation-like plasticity (paired associative stimulation) and GABAa-ergic inhibition (short-interval intracortical inhibition), which are seen in dystonia, normalize after several months of deep brain stimulation. In the present study, we examine the same measures in a homogenous group of 10 DYT1 gene-positive patients after long-term deep brain stimulation treatment for at least 4.5 years. Recordings were made 'on' deep brain stimulation and after stopping deep brain stimulation for 2 days. The results show that: (i) on average, prior to discontinuing deep brain stimulation, the paired associative stimulation response was almost absent and short-interval intracortical inhibition was reduced compared with normal. This pattern differs from that in both healthy volunteers and from the typical pattern of enhanced plasticity and reduced inhibition seen in deep brain stimulation-naïve dystonia. It is similar to that seen in untreated Parkinson's disease and may relate to thus far unexplained clinical phenomena like parkinsonian symptoms that have sometimes been observed in patients treated with deep brain stimulation. (ii) Overall, there was no change in average physiological or clinical status when deep brain stimulation was turned off for 2 days, suggesting that deep brain stimulation had produced long-term neural reorganization in the motor system. (iii) However, there was considerable variation between patients. Those who had higher levels of plasticity when deep brain stimulation was 'on', had the best retention of clinical benefit when deep brain stimulation was stopped and vice versa. This may indicate that better plasticity is required for longer term retention of normal movement when deep brain stimulation is off. (iv) Patients with the highest plasticity 'on' deep brain stimulation were those who had been receiving stimulation with the least current drain. This suggests that it might be possible to 'shape' deep brain stimulation of an individual patient to maximize beneficial neurophysiological patterns that have an impact on clinical status. The results are relevant for understanding long-term consequences and management of deep brain stimulation in dystonia.

Deep brain stimulation effects in dystonia: time course of electrophysiological changes in early treatment

Deep brain stimulation to the internal globus pallidus is an effective treatment for primary dystonia. The optimal clinical effect often occurs only weeks to months after starting stimulation. To better understand the underlying electrophysiological changes in this period, we assessed longitudinally 2 pathophysiological markers of dystonia in patients prior to and in the early treatment period (1, 3, 6 months) after deep brain stimulation surgery. Transcranial magnetic stimulation was used to track changes in short-latency intracortical inhibition, a measure of excitability of GABA(A) -ergic corticocortical connections and long-term potentiation-like synaptic plasticity (as a response to paired associative stimulation). Deep brain stimulation remained on for the duration of the study. Prior to surgery, inhibition was reduced and plasticity increased in patients compared with healthy controls. Following surgery and commencement of deep brain stimulation, short-latency intracortical inhibition increased toward normal levels over the following months with the same monotonic time course as the patients' clinical benefit. In contrast, synaptic plasticity changed rapidly, following a nonmonotonic time course: it was absent early (1 month) after surgery, and then over the following months increased toward levels observed in healthy individuals. We postulate that before surgery preexisting high levels of plasticity form strong memories of dystonic movement patterns. When deep brain stimulation is turned on, it disrupts abnormal basal ganglia signals, resulting in the absent response to paired associative stimulation at 1 month. Clinical benefit is delayed because engrams of abnormal movement persist and take time to normalize. Our observations suggest that plasticity may be a driver of long-term therapeutic effects of deep brain stimulation in dystonia.

Assessment and modulation of neural plasticity in rehabilitation with transcranial magnetic stimulation

Despite intensive efforts to improve outcomes after acquired brain injury, functional recovery is often limited. One reason for this limitation is the challenge in assessing and guiding plasticity after brain injury. In this context, transcranial magnetic stimulation (TMS), a noninvasive tool of brain stimulation, could play a major role. TMS has been shown to be a reliable tool for measuring plastic changes in the motor cortex associated with interventions in the motor system, such as motor training and motor cortex stimulation. In addition, as illustrated by the experience in promoting recovery from stroke, TMS is a promising therapeutic tool to minimize motor, speech, cognitive, and mood deficits. In this review, we will focus on stroke to discuss how TMS can provide insights into the mechanisms of neurologic recovery and how it can be used for measurement and modulation of plasticity after an acquired brain insult.

A near infra-red spectroscopy study of the effects of pre-frontal single and paired pulse transcranial magnetic stimulation

OBJECTIVE

Concentration changes in hemoglobin following single and paired pulse (2 and 15 ms inter-stimulus interval) transcranial magnetic stimulation (TMS) was investigated using near infra-red spectroscopy (NIRS).

METHODS

TMS was delivered to left pre-frontal cortex at typical intensities used in neuroscience research and concentrations of deoxyhemoglobin (Hb), oxyhemoglobin (HbO) and total hemoglobin (HbT) were measured.

RESULTS

Significant drops in concentration of HbO and HbT were observed and while there was no effect of the different pulse types on amplitude, there was a difference in the time taken to return to baseline.

CONCLUSIONS

The changes observed imply that in pre-frontal cortex, the different TMS pulse types result in differential effects on oxygen consumption.

SIGNIFICANCE

This study aids our understanding of the physiological effects of single and paired pulse TMS.

Goal-directed visuomotor skill learning: off-line enhancement and the importance of the primary motor cortex

PURPOSE

The time course and neural substrates of motor skill learning are not well-understood in healthy or neurologic patient populations. Certain motor skills undergo off-line skill enhancement following training and the primary motor cortex (M1) may be involved. It is unknown if goal-directed visuomotor skill undergoes off-line enhancement or if M1 is associated with that enhancement.

METHODS

32 right-handed, healthy subjects were randomly assigned to two groups: real repetitive transcranial magnetic stimulation (rTMS) or sham rTMS applied to the contralateral M1 immediately following one 20-minute finger tracking training session. Tracking performance and cortical excitability were assessed before and after training, following rTMS and 24 hours post-training.

RESULTS

Results demonstrate that skill performance continues to develop for at least 30 minutes after training completion, is maintained for 24 hours post-training, and is not affected by inhibitory rTMS applied to M1. Level of skill improvement was associated with the degree of intracortical inhibition increase.

CONCLUSIONS

These results suggest dispersed information processing for goal-directed visuomotor skill learning following training and a relationship between cortical excitability and skill development in healthy individuals. These findings invite further investigation of the neural mechanisms underlying motor skill learning and may have rehabilitation implications for patients with neurologic injury.

Neuronavigation increases the physiologic and behavioral effects of low-frequency rTMS of primary motor cortex in healthy subjects

Low-frequency repetitive transcranial magnetic stimulation (rTMS) can exert local and inter-hemispheric neuromodulatory effects on cortical excitability. These physiologic effects can translate into changes in motor behavior, and may offer valuable therapeutic interventions in recovery from stroke. Neuronavigated TMS can maximize accurate and consistent targeting of a given cortical region, but is a lot more involved that conventional TMS. We aimed to assess whether neuronavigation enhances the physiologic and behavioral effects of low-frequency rTMS. Ten healthy subjects underwent two experimental sessions during which they received 1600 pulses of either navigated or non-navigated 1 Hz rTMS at 90% of the resting motor threshold (RMT) intensity over the motor cortical representation for left first dorsal interosseous (FDI) muscle. We compared the effects of navigated and non-navigated rTMS on motor-evoked potentials (MEPs) to single-pulse TMS, intracortical inhibition (ICI) and intracortical facilitation (ICF) by paired-pulse TMS, and performance in various behavioral tasks (index finger tapping, simple reaction time and grip strength tasks). Following navigated rTMS, the amplitude of MEPs elicited from the contralateral (unstimulated) motor cortex was significantly increased, and was associated with an increase in ICF and a trend to decrease in ICI. In contrast, non-navigated rTMS elicited nonsignificant changes, most prominently ipsilateral to rTMS. Behaviorally, navigated rTMS significantly improved reaction time RT and pinch force with the hand ipsilateral to stimulation. Non-navigated rTMS lead to similar behavioral trends, although the effects did not reach significance. In summary, navigated rTMS leads to more robust modulation of the contralateral (unstimulated) hemisphere resulting in physiologic and behavioral effects. Our findings highlight the spatial specificity of inter-hemispheric TMS effects, illustrate the superiority of navigated rTMS for certain applications, and have implications for therapeutic applications of rTMS.

Tactile-dependant corticomotor facilitation is influenced by discrimination performance in seniors

Background

Active contraction leads to facilitation of motor responses evoked by transcranial magnetic stimulation (TMS). In small hand muscles, motor facilitation is known to be also influenced by the nature of the task. Recently, we showed that corticomotor facilitation was selectively enhanced when young participants actively discriminated tactile symbols with the tip of their index or little finger. This tactile-dependant motor facilitation reflected, for the large part, attentional influences associated with performing tactile discrimination, since execution of a concomitant distraction task abolished facilitation. In the present report, we extend these observations to examine the influence of age on the ability to produce extra motor facilitation when the hand is used for sensory exploration.

Methods

Corticomotor excitability was tested in 16 healthy seniors (58-83 years) while they actively moved their right index finger over a surface under two task conditions. In the tactile discrimination (TD) condition, participants attended to the spatial location of two tactile symbols on the explored surface, while in the non discrimination (ND) condition, participants simply moved their finger over a blank surface. Changes in amplitude, in latency and in the silent period (SP) duration were measured from recordings of motor evoked potentials (MEP) in the right first dorsal interosseous muscle in response to TMS of the left motor cortex.

Results

Healthy seniors exhibited widely varying levels of performance with the TD task, older age being associated with lower accuracy and vice-versa. Large inter-individual variations were also observed in terms of tactile-specific corticomotor facilitation. Regrouping seniors into higher (n = 6) and lower performance groups (n = 10) revealed a significant task by performance interaction. This latter interaction reflected differences between higher and lower performance groups; tactile-related facilitation being observed mainly in the former group. Latency measurements and SP durations were not affected by task conditions.

Conclusions

The present findings provide further insights into the factors influencing task-dependant changes in corticomotor excitability in the context of aging. Our results, in particular, highlight the importance of adjusting task demands and controlling for attention when attempting to elicit task-specific motor facilitation in older persons engaged in fine manual actions. Such information could be critical in the future for planning interventions to re-educate or maintain hand function in the presence of neurological impairments.

Navigated transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is a unique method for non-invasive brain imaging. The fundamental difference between TMS and other available non-invasive brain imaging techniques is that when a physiological response is evoked by stimulation of a cortical area, that specific cortical area is causally related to the response. With other imaging methods, it is only possible to detect and map a brain area that participates in a given task or reaction. TMS has been shown to be clinically accurate and effective in mapping cortical motor areas and applicable to the functional assessment of motor tracts following stroke, for example. Many hundreds of studies have been published indicating that repetitive TMS (rTMS) may also have multiple therapeutic applications. Techniques and protocols for individually targeting and dosing rTMS urgently need to be developed in order to ascertain the accuracy, repeatability and reproducibility required of TMS in clinical applications. We review the basic concepts behind navigated TMS and evaluate the currently accepted physical and physiological factors contributing to the accuracy and reproducibility of navigated TMS. The advantages of navigated TMS over functional MRI in motor cortex mapping are briefly discussed. Illustrative cases utilizing navigated TMS are shown in presurgical mapping of the motor cortex, in therapy for depression, and in the follow-up of recovery from stroke.