Differences In Motor Threshold Research Articles

Agreement and Consistency of Absolute and Relative Corticospinal Stimulus-Response Curves for Upper, Lower, and Axial Musculature in Healthy Adults.

To assess the agreement and consistency of absolute and relative stimulus-response curve (SRC) parameter estimates for upper extremity, lower extremity, and axial muscles. Thirty (15 W, age: 27.0 ± 6.3 y, height: 171.9 ± 8.9 cm, weight: 80.2 ± 19.3 kg) healthy adults completed absolute (5% to 100% stimulator output) and relative (65% to 160% motor threshold) SRCs of the first dorsal interosseous, vastus lateralis, and rectus abdominis during submaximal isometric contractions. Mean motor-evoked potential amplitudes were fit with nonlinear regression to derive MEPmax, V50, and slope. Absolute agreement and consistency were assessed with ICCs, Cronbachs alphas, and Bland-Altman plots. Independent t-tests were used to examine differences in motor threshold, physical activity, strength, and muscle activity among participants with valid and invalid SRC parameters. Absolute and relative SRCs displayed good agreement and consistency for MEPmax and V50 but not slope. Motor thresholds were lower among participants with valid absolute SRCs for the rectus abdominis and vastus lateralis. Motor threshold, physical activity, strength, and muscle activity did not differ among those with valid and invalid parameters for all relative SRCs and absolute SRCs for the first dorsal interosseous. Absolute and relative SRCs produce similar MEPmax and V50 estimates in the first dorsal interosseous, vastus lateralis, and rectus abdominis. The validity of absolute and relative SRC results may differ depending on individual characteristics and tested muscles.

Focal transcranial magnetic stimulation in awake rats: Enhanced glucose uptake in deep cortical layers

BackgroundTranscranial magnetic stimulation (TMS) is an emerging neuromodulation tool. However, preclinical models of TMS are limited. ObjectiveTo develop a method for performing TMS in awake rats and to characterize neuronal response to TMS by mapping glucose uptake following TMS administration. MethodsA headpost was implanted into rat skull serving as a refence to guide TMS target. Motor threshold measurement was used as the metric to assess the consistency in TMS delivery across animals and across sessions. Using a fluorescent glucose analogue (2-NBDG) as a marker of neuronal activity, we mapped glucose uptake in response to TMS of the rat motor cortex. ResultsThe average motor threshold (n = 41) was 34.6 ± 6.3 % of maximum stimulator output (MSO). The variability of motor threshold across animals was similar to what has been reported in human studies. Furthermore, there was no significant difference in motor threshold measured across 3 separate days. Enhancement in fluorescent signals were TMS dose (power)-dependent, which centered around the motor cortex, covering an area medial-laterally 2 mm, rostral-caudally 4 mm at 55 % MSO, and 3 mm at 35 % MSO. The count of total cells with significant fluorescent signal was: 107 ± 23 (55 % MSO), 73 ± 11 (35 % MSO) and 42 ± 11 (sham, 5% MSO). ConclusionsOur method allows for consistent motor threshold assessment for longitudinal studies. Notably, cells with fluorescent signal enhancement were consistently aggregated in deep cortical layers, with minimal enhancement in superficial layers Comparisons with existing method(s)To our knowledge, this is the first study of focal TMS in awake rodents.

Machine Learning Methods Predict Individual Upper-Limb Motor Impairment Following Therapy in Chronic Stroke

Background. Accurate prediction of clinical impairment in upper-extremity motor function following therapy in chronic stroke patients is a difficult task for clinicians but is key in prescribing appropriate therapeutic strategies. Machine learning is a highly promising avenue with which to improve prediction accuracy in clinical practice. Objectives. The objective was to evaluate the performance of 5 machine learning methods in predicting postintervention upper-extremity motor impairment in chronic stroke patients using demographic, clinical, neurophysiological, and imaging input variables. Methods. A total of 102 patients (female: 31%, age 61 ± 11 years) were included. The upper-extremity Fugl-Meyer Assessment (UE-FMA) was used to assess motor impairment of the upper limb before and after intervention. Elastic net (EN), support vector machines, artificial neural networks, classification and regression trees, and random forest were used to predict postintervention UE-FMA. The performances of methods were compared using cross-validated R2. Results. EN performed significantly better than other methods in predicting postintervention UE-FMA using demographic and baseline clinical data (median P < .05). Preintervention UE-FMA and the difference in motor threshold (MT) between the affected and unaffected hemispheres were the strongest predictors. The difference in MT had greater importance than the absence or presence of a motor-evoked potential (MEP) in the affected hemisphere. Conclusion. Machine learning methods may enable clinicians to accurately predict a chronic stroke patient’s postintervention UE-FMA. Interhemispheric difference in the MT is an important predictor of chronic stroke patients’ response to therapy and, therefore, could be included in prospective studies.

1. A meta-analysis of the cortical silent period in epilepsies

The cortical silent period (CSP) following transcranial magnetic stimulation reflects GABAB-mediated inhibition in the primary motor cortex (M1), and could contribute to understand the pathophysiological substrates of epilepsies. Increased CSP duration has been reported in idiopathic generalized epilepsy (IGE) and in partial epilepsy (PE) involving the M1, although other studies yielded discordant findings. In this meta-analysis we systematically assessed the consistency of CSP changes in untreated epileptic patients. We searched for controlled studies evaluating the CSP in drug-naive or drug-free patients with IGE or PE. The mean difference between CSP duration obtained in patients and controls was calculated. Fourteen studies (267 patients and 234 controls) were included. A significant mean difference was found, with longer CSP in patients than in controls. The difference was still greater if IGE patients only were analyzed. CSP modifications did not depend from differences in motor threshold between patients and controls. Meta-analysis confirms CSP modifications in epilepsies, with enhancement of this cortical inhibitory measure in most IGE patients. This provides a rationale for investigations aiming to verify the hypotheses that increased CSP reflects compensatory neural phenomena counteracting transition from the interictal to ictal state, and that CSP variability reflects the pathophysiological heterogeneity of epileptic syndromes.

Evidence for motor cortex dedifferentiation in older adults

Older adults (OA) show more diffuse brain activity than young adults (YA) during the performance of cognitive, motor, and perceptual tasks. It is unclear whether this overactivation reflects compensation or dedifferentiation. Typically, these investigations have not evaluated the organization of the resting brain, which can help to determine whether more diffuse representations reflect physiological or task-dependent effects. In the present study we used transcranial magnetic stimulation (TMS) to determine whether there are differences in motor cortex organization of both brain hemispheres in young and older adults. We measured resting motor threshold, motor evoked potential (MEP) latency and amplitude, and extent of first dorsal interosseous representations, in addition to a computerized measure of reaction time. There was no significant age difference in motor threshold, but we did find that OA had larger contralateral MEP amplitudes and a longer contralateral MEP latency. Furthermore, the spatial extent of motor representations in OA was larger. We found that larger dominant hemisphere motor representations in OA were associated with higher reaction times, suggesting dedifferentiation rather than compensation effects.

On-line navigation with electric field modeling in TMS reduces stimulator-dependent differences in motor threshold

On-line navigation with electric field modeling in TMS reduces stimulator-dependent differences in motor threshold

Absence of Postexercise and Delayed Facilitation of Motor Cortex Excitability in Restless Legs Syndrome: Evidence of Altered Cortical Plasticity?

Restless legs syndrome (RLS) is a neurologic disorder with well-defined clinical and diagnostic criteria but the pathophysiology of which is unclear. Previous studies have suggested alterations in motor cortex function in RLS. We aimed to compare motor cortex plasticity in subjects with RLS versus healthy controls. Biphasic single-pulse transcranial magnetic stimulation (TMS) of the nondominant hemisphere was used to define motor evoked potential (MEP) amplitude, motor threshold, and silent period. Subjects also performed 3 blocks (30-s, 60-s, and 90-s duration) of a bimanual motor task (exercise condition). Amplitude of MEPs elicited immediately after each block, and then after a 15-minute rest period were compared with baseline. The time course of intracortical inhibition was also tested using paired-pulse TMS at 1- to 6-millisecond interstimulus intervals. Clinical neurophysiology laboratory in a General Hospital. For the single-pulse TMS procedures, the RLS group included 11 patients affected by primary RLS and the control group included 11 age- and sex-matched normal subjects. For the paired-pulse TMS procedures, there were 9 patients and 6 controls. None. There were no group differences in motor threshold and MEP amplitudes, but the silent period was significantly shorter in subjects with RLS. Compared with baseline, control subjects had larger MEP amplitudes after 30 and 60 seconds of exercise and also after the rest period. In contrast, MEP amplitudes in patients with RLS were not significantly different from baseline after any of the exercise conditions or following the rest period. Patients with RLS also had decreased short-latency paired-pulse inhibition. Findings show abnormal motor cortex inhibition and cortical excitability differences in RLS. We suggest the possibility of alterations in movement-related cortical plasticity in RLS.

How does error correction differ from error signaling? An event-related potential study

It has been a question of debate whether immediate error corrections in speeded forced-choice reaction time tasks require an error detection signal from the performance monitoring system or whether they reflect delayed correct responses that are executed after the premature error like in a horserace. In contrast, signaling the error by pressing a response button that is unrelated to the primary task is based on error detection. The present study investigates the similarities and differences between immediate error corrections and signaling responses by means of behavioral and event-related potential data. In a within-subject design, participants performed two sessions of the flanker task. In one session, errors had to be corrected by immediately pressing the correct response, in the other session, errors had to be signaled by pressing an error signaling button. Compared to the signaling session, in the correction session, more errors and error corrections were made, reaction times were shorter, and the amplitude of the error-related negativity (ERN) was reduced. Whereas the error significance did not seem to differ across session, participants have most likely reduced the motor threshold in the correction session to enable efficient immediate corrections. This interpretation is supported by the lateralized readiness potentials and is consistent with the response conflict monitoring hypothesis of the ERN. The present study demonstrates that differences in error corrections may be attributable to differences in motor threshold. We conclude that the error signaling procedure is a more direct and reliable way to behaviorally test the functional integrity of the performance monitoring system than the instruction to correct errors. The consequences for studies in patients and with pharmacological challenges are discussed.

Different TMS patterns of intracortical inhibition in early onset Alzheimer dementia and frontotemporal dementia

Objective: To investigate putative changes in cortical excitability of patients affected by early-onset mild dementia by means of transcranial magnetic stimulation (TMS) and to verify whether a peculiar neurophysiological profile may contribute to characterise Alzheimer's disease (AD) vs frontotemporal dementia (FTD). Methods: Motor threshold and intracortical inhibition (ICI) and facilitation (ICF) after paired-pulse TMS (inter-stimulus intervals from 1 to 20 ms) were studied in two groups of early-onset demented patients with a neuropsychological profile suggestive of AD ( n=12) and FTD ( n=8). Twelve age-matched healthy subjects were considered as control group. In both patient groups, recordings were performed before and after a single oral dose of 4 mg galantamine. Results: No significant difference in motor threshold was observed among the three studied groups. On the contrary, early-onset AD showed a significant reduction of ICI compared to control group, no changes were detected in FTD patients. No significant changes in ICF were found between both patient groups and healthy subjects. The acute administration of galantamine reversed the modified ICI in AD group. Conclusions: The differential pattern of ICI exhibited by early-onset AD vs FTD in the early stage of disease may represent a non-invasive, reproducible electrophysiological tool, which may contribute to early differential diagnosis and, possibly, to monitor therapeutic effectiveness. Significance: The present results support the possibility that subtle, early modifications in intracortical circuitry features AD, but not FTD patients.

Plasticity of the human motor system following muscle reconstruction: a magnetic stimulation and functional magnetic resonance imaging study.

Although motor system plasticity in response to neuromuscular injury has been documented, few studies have examined recovered and functioning muscles in the human. We examined brain changes in a group of patients who had a muscle transfer. Transcranial magnetic stimulation (TMS) was used to study a unique group of 9 patients who had upper extremity motor function restored using microneurovascular transfer of the gracilis muscle. The findings from the reconstructed muscle were compared to the homologous muscle of the intact arm. One patient was also studied with functional magnetic resonance imaging (fMRI). TMS showed that the motor threshold and short interval intracortical inhibition was reduced on the transplanted side while at rest but not during muscle activation. The difference in motor threshold decreased with the time since surgery. TMS mapping showed no significant difference in the location and size of the representation of the reconstructed muscle in the motor cortex compared to the intact side. In one patient with reconstructed biceps muscle innervated by the intercostal nerves, both TMS mapping and fMRI showed that the upper limb area rather than the trunk area of the motor cortex controlled the reconstructed muscle. Plasticity occurs in cortical areas projecting to functionally relevant muscles. Changes in the neuronal level are not necessarily accompanied by changes in motor representation. Brain reorganization may involve multiple processes mediated by different mechanisms and continues to evolve long after the initial injury. Central nervous system plasticity following neuromuscular injury may have functional relevance.

Interhemispheric asymmetry of motor cortical excitability in major depression as measured by transcranial magnetic stimulation.

Neuroimaging studies of major depressive disorder (MDD) indicate interhemispheric differences in prefrontal cortical activity (right greater than left). To investigate whether there are any interhemispheric differences of motor cortical excitability in MDD. Eight patients with treatment-refractory MDD off medication were assessed for the severity of their depression, and transcranial magnetic stimulation studies (bilateral motor threshold and paired-pulse studies) were conducted. Eight normal controls were also studied. MDD patients showed significant interhemispheric differences in motor threshold and paired-pulse curves, both of which showed lower excitability on the left hemisphere. Such differences were absent in controls. Our findings may aid the further understanding of the neurophysiology underlying MDD.

Mechanisms of Cortical Reorganization in Lower-Limb Amputees

The human motor system undergoes reorganization after amputation, but the site of motor reorganization and the mechanisms involved are unknown. We studied the site and mechanisms of motor reorganization in 16 subjects with traumatic lower-limb amputation. Stimulation at different levels in the CNS was used to determine the site of reorganization. The mechanisms involved were evaluated by measuring the thresholds for transcranial magnetic stimulation (TMS) and by testing intracortical inhibition and facilitation. With TMS, the threshold for muscle activation on the amputated side was lower than that of the intact side, but with transcranial electrical stimulation there was no difference in motor threshold between the two sides. TMS at the maximal output of the stimulator activated a higher percentage of the motor neuron pool (%MNP) on the amputated side than on the intact side. The %MNP activated by spinal electrical stimulation was similar on the two sides. Paired TMS study showed significantly less intracortical inhibition on the amputated side. Our findings suggest that motor reorganization after lower-limb amputation occurs predominately at the cortical level. The mechanisms involved are likely to include reduction of GABAergic inhibition.