Short- and long-latency afferent inhibition; uses, mechanisms and influencing factors

Motor cortex excitability in restless legs syndrome: A systematic review and insights into pathophysiology via transcranial magnetic stimulation

Restless Legs Syndrome (RLS) is a neurological disorder characterized by a complex and only partially understood pathophysiology. Numerous studies have investigated motor cortex excitability in RLS using transcranial magnetic stimulation (TMS), but a comprehensive systematic review assessing their methodological quality and synthesizing their findings is lacking. A systematic search of PubMed focused on various TMS parameters, including resting and active motor thresholds, motor evoked potential characteristics, contralateral and ipsilateral silent periods, and multiple measures of intracortical inhibition and facilitation. The quality of each study was evaluated using custom checklists and the modified Newcastle-Ottawa Quality Assessment Scale. Out of the studies screened, 21 (comprising 319 RLS patients and 278 healthy controls) met the inclusion criteria. Despite considerable variability in TMS methodologies-such as differences in the muscles tested, interstimulus intervals, testing timing, and the potential confounding effects of medications-a consistent finding was a reduction in short-interval intracortical inhibition, a key indicator of central inhibitory GABAergic activity. Additionally, altered post-exercise facilitation and delayed facilitation suggest abnormal motor plasticity in RLS patients. These findings suggest that despite the methodological heterogeneity, TMS abnormalities may play a crucial role in the pathophysiology of RLS. In this context, RLS symptoms are likely to originate at different levels within a complex neural network, ultimately leading to altered, possibly transient and circadian, excitability in a multifaceted neurophysiological system. While further rigorous research and reproducible evidence are needed, these insights may contribute to the identification of new diagnostic markers and the development of innovative therapeutic strategies for RLS.

Somatosensory cortex and body representation: Updating the motor system during a visuo-proprioceptive cue conflict

The brain's representation of hand position is critical for voluntary movement. Representation is multisensory, combining visual and proprioceptive cues. When these cues conflict, the brain recalibrates its unimodal estimates, shifting them closer together to compensate. Converging evidence from research in perception, behavior, and neurophysiology suggest that such updates to body representation are communicated to the motor system to keep hand movements accurate. We hypothesized that primary somatosensory cortex (SI) is crucial in this updating process due to its role in proprioception and connections with primary motor cortex. We tested this hypothesis in two experiments. We predicted that proprioceptive, but not visual, recalibration would be associated with change in short latency afferent inhibition (SAI), a measure of sensorimotor integration (influence of sensory input on motor output) (Expt. 1). We further predicted that modulating SI activity with repetitive transcranial magnetic stimulation (TMS) should affect recalibration of the proprioceptive estimate of hand position, but have no effect on the visual estimate or on the normal inverse relationship between proprioceptive and visual recalibration (Expt. 2). Our results are consistent with these predictions, supporting the idea that (1) SI is indeed a key region in facilitating motor system updates based on changes in body representation, and (2) this function is mediated by unisensory (proprioceptive) processing, separate from multisensory visuo-proprioceptive computations. Other aspects of the body representation (visual and multisensory) may be conveyed to the motor system via separate pathways, e.g. from posterior parietal regions to motor cortex.

Changes in the cortical GABAergic inhibitory system with ageing and ageing-related neurodegenerative diseases

The human cortical inhibitory system is known to play a vital role for normal brain development, function, and plasticity. GABA is the most prominent inhibitory neurotransmitter in the CNS and is a key regulator not only for motor control and motor learning, but also for cognitive processes. With ageing and many neurodegenerative pathologies, a decline in GABAergic function in several cortical regions together with a reduced ability to task-specifically modulate and increase inhibition in the primary motor cortex has been observed. This decline in intracortical inhibition is associated with impaired motor control but also with diminished motor-cognitive (i.e. dual-tasking) and cognitive performance (e.g. executive functions). Furthermore, more general well-being such as sleep quality, stress resistance or non-specific pain perception are also associated with reduced GABA functioning. The current review highlights the interplay between changes in GABAergic function and changes in motor control, motor-cognitive and cognitive performance associated with healthy ageing, as well as in seniors with neurodegenerative diseases such as mild cognitive impairment. Furthermore, recent evidence highlighting the ability to up- or downregulate cortical inhibition by means of physical exercise programs is presented and discussed.

Corticospinal and Clinical Effects of Muscle Tendon Vibration in Neurologically Impaired Individuals. A Scoping Review

This review verified the extent, variety, quality and main findings of studies that have tested the neurophysiological and clinical effects of muscle tendon vibration (VIB) in individuals with sensorimotor impairments. The search was conducted on PubMed, CINAHL, and SportDiscuss up to April 2024. Studies were selected if they included humans with neurological impairments, applied VIB and used at least one measure of corticospinal excitability using transcranial magnetic stimulation (TMS). Two investigators assessed the studies' quality using critical appraisal checklists and extracted relevant data. The 10 articles included were diverse in populations and methods, generally rated as 'average' to 'good' quality. All studies reported an increased corticospinal excitability in the vibrated muscle, but the effects of VIB on non-vibrated muscles remain unclear. Positive clinical changes in response to VIB were reported in a few studies, such as a decreased spasticity and improved sensorimotor function. These changes were sometimes correlated with corticospinal effects, suggesting a link between VIB-induced plasticity and clinical improvements. Despite the limited and heterogeneous literature, this review supports the facilitatory influence of VIB on motor outputs controlling vibrated muscles, even with altered sensorimotor functions. It highlights knowledge gaps and suggests future research directions on VIB mechanisms and clinical implications.

Attenuated afferent inhibition correlated with impaired gait performance in Parkinson's disease patients with freezing of gait

Background

The neural mechanisms underlying freezing of gait (FOG) in Parkinson's disease (PD) have not been completely comprehended. Sensory-motor integration dysfunction was proposed as one of the contributing factors. Here, we investigated short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI), and analyzed their association with gait performance in FOG PD patients, to further validate the role of sensorimotor integration in the occurrence of FOG in PD.

Methods

Twenty-five levodopa responsive-FOG PD patients (LR-FOG), fifteen levodopa unresponsive-FOG PD patients (LUR-FOG), twenty-eight PD patients without FOG (NO-FOG PD) and twenty-two healthy controls (HC) were included in the study. Clinical features such as PD motor symptoms, FOG severity and cognitive abilities were evaluated using clinical scales in subjects with PD. All participants underwent paired associative stimulation (PAS) to evaluate SAI and LAI in addition to regular input-output curve by transcranial magnetic stimulation. The performances of gait were assessed using a portable gait analyzing system in 10-meter timed Up and Go task. The correlations between the gait spatiotemporal parameters or the scores of FOG scale and the magnitudes of SAI or LAI were analyzed.

Results

Compared to HC and NO-FOG PD patients, SAI was decreased in FOG PD subgroups. LAI was also reduced in both LR-FOG PD and LUR-FOG PD in relative to HC; however, only LUR-FOG PD showed significant reduction of LAI in comparison to NO-FOG PD group. FOG PD patients showed poorer gait performance compared to HC and NO-FOG PD group. The reduction of SAI and LAI were correlated with the impaired gait spatiotemporal parameters or scores of FOG scale in PD with FOG.

Conclusion

The SAI and LAI were attenuated in PD patients with FOG, and the reduction of SAI or LAI were correlated to disturbed gait performance, indicating that sensory-motor integration dysfunction played a role in the development of FOG in PD.

Short-latency afferent inhibition and its relationship to covert sensory and motor hand impairment in multiple sclerosis

OBJECTIVE

To investigate sensorimotor integration by quantifying short-latency afferent inhibition (SAI) in people with MS who experience manual dexterity problems compared to controls.

METHODS

22 people with MS with self-reported manual dexterity problems and 10 sex and age-matched controls were assessed using various upper extremity clinical tests. SAI was assessed by a transcranial magnetic stimulation pulse over the primary motor cortex preceded by peripheral nerve stimulation to the median nerve at 6 interstimulus intervals 2 - 8 ms longer than individualized N20 latencies.

RESULTS

Although within normal limits, persons with MS exhibited significantly slower Nine Hole Peg Test performance and pinch strength in the dominant hand. They also exhibited greater sensory impairment (monofilament test) in the dominant hand. Persons with MS showed significantly greater disinhibition of SAI in the dominant hand compared to controls, which was significantly correlated with weaker pinch strength.

CONCLUSION

Reduced SAI in people with MS, particularly in the dominant hand, signifies disruptions in cortical cholinergic inhibitory activity and is associated with lower pinch strength.

SIGNIFICANCE

Evaluating changes in SAI may offer insight into the disrupted cortical cholinergic inhibitory activity that contributes to sensorimotor disintegration, potentially advancing disease management in persons with MS.

Mechanisms of tremor-modulating effects of primidone and propranolol in essential tremor

INTRODUCTION

Primidone and propranolol are primary treatments for essential tremor, however the exact mechanisms underlying their efficacy are not fully elucidated. Understanding how these medications alleviate tremor may guide the development of additional pharmacologic treatments. Our prospective observational study employed transcranial magnetic stimulation (TMS) to explore mechanisms of primidone and propranolol effects in essential tremor. Eyeblink classical conditioning (EBCC) was tested as a potential predictor of treatment response.

METHODS

Patients with essential tremor underwent two evaluations: prior to commencing primidone or propranolol and following a minimum of three months of treatment. Tremor severity was assessed using accelerometry and clinically. TMS was employed to study changes in corticospinal excitability - resting and active motor thresholds, resting and active input/output curves and intracortical excitability - cortical silent period (CSP), short interval intracortical inhibition intensity curve (SICI), long interval intracortical inhibition (LICI), intracortical facilitation (ICF), and short afferent inhibition (SAI). EBCC, a marker of cerebellar function, was studied at baseline.

RESULTS

Of the 54 enrolled patients (28 primidone, 26 propranolol), 35 completed both visits. Primidone effect on decreasing hand tremor was associated with decreased corticospinal excitability, prolongation of CSP, increased LICI, increased SAI and decreased SICI. Propranolol effect on hand tremor was associated with decreased corticospinal excitability and increased SAI. Better EBCC at baseline predicted better response to primidone.

CONCLUSIONS

Primidone exerts its therapeutic effects by blocking voltage-gated sodium channels and by modulating GABA-A and GABA-B intracortical circuits. Propranolol's central effects are likely mediated via noradrenergic modulation of GABA outflow.

Short-latency afferent inhibition is reduced with cold-water immersion of a limb and remains reduced after removal from the cold stimulus

The experience of pain that is induced by extremely cold temperatures can exert a modulatory effect on motor cortex circuitry. Although it is known that immersion of a single limb in very cold water can increase corticomotor excitability it is unknown how afferent input to the cortex shapes excitatory and inhibitory processes. Therefore, the purpose of this study was to examine motor-evoked potentials (MEP), short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI) in response to immersion of a single hand in cold water. Transcranial magnetic stimulation (TMS) was used to assess MEPs, and peripheral nerve stimulation of the median nerve paired with TMS was used to measure SAI and LAI in motor circuits of the ipsilateral hemisphere. Measurements were obtained from electromyography (EMG) of the first dorsal interosseous (FDI) at baseline, during cold-water immersion, and during recovery from cold-water immersion. The intervention caused unconditioned MEPs to increase during exposure to the cold stimulus (P = 0.008) which then returned to baseline levels once the hand was removed from the cold water. MEP responses were decoupled from SAI responses, where SAI was reduced during exposure to the cold stimulus (P = 0.005) and remained reduced compared to baseline when the hand was removed from the cold water (P = 0.002). The intervention had no effect on LAI. The uncoupling of SAI from MEPs during the recovery period suggests that the mechanisms underlying the modulation of corticospinal excitability by sensory input may be distinct from those affecting intracortical inhibitory circuits. HIGHLIGHTS: What is the central question of this study? Does immersion of a limb in very cold water influence corticospinal excitability and the level of afferent inhibition exerted on motor cortical circuits? What is the main finding and its importance? In additional to perception of temperature, immersion in 6°C water also induced perceptions of pain. Motor evoked potential (MEP) amplitude increased during immersion, and short-latency afferent inhibition (SAI) of the motor cortex was reduced during immersion; however, these responses differed after the limb was removed from the cold stimulus, as MEPs returned to normal levels while SAI remained suppressed.

Supraspinal Activation Induced by Visual Kinesthetic Illusion Modulates Spinal Excitability

Repetitive passive movement (RPM) enhances reciprocal inhibition. RPM is more effective when performed rapidly and at wide joint angles. However, patients with limited joint range of motion may not receive the most effective RPM. Therefore, having an alternative method for performing RPM in patients who cannot perform actual exercise due to limited joint motion is necessary. This study investigated the effects of RPM on spinal excitability using a visual kinesthetic illusion. Participants included 17 healthy adults (7 women). Measurements were taken before, during, and immediately after the intervention. We established two intervention conditions: the control condition, in which participants focused their attention forward, and the illusion condition, in which participants watched a video about RPM. F-waves from the tibialis anterior and soleus muscles were measured, and F-wave persistence and F/M amplitude ratios were analyzed. Under the illusion condition, compared with the preintervention condition, the F/M amplitude ratio of the tibialis anterior increased by approximately 44% during the intervention (p < 0.05), whereas the F-wave persistence of the soleus decreased by approximately 23% from the immediate start of the intervention (p < 0.05). This study suggests that a visual kinesthetic illusion can increase the spinal excitability of the tibialis anterior, whereas reciprocal inhibition can decrease the spinal excitability of the soleus.

Sensorimotor Integration in Chronic Low Back Pain

OBJECTIVE

Chronic low back pain (CLBP) impacts on spine movement. Altered sensorimotor integration can be involved. Afferents from the lumbo-pelvic area might be processed differently in CLBP and impact on descending motor control. This study aimed to determine whether afferents influence the corticomotor control of paravertebral muscles in CLBP. Fourteen individuals with CLBP (11 females) and 13 pain-free controls (8 females) were tested with transcranial magnetic stimulation (TMS) to measure the motor-evoked potential [MEP] amplitude of paravertebral muscles. Noxious and non-noxious electrical stimulation, and magnetic stimulation in the lumbo-sacral area were used as afferent stimuli and triggered 20 to 200 ms prior to TMS. EMG modulation elicited by afferent stimulation alone was measured to control net motoneuron excitability. MEP/EMG ratio was used as a measure of corticospinal excitability with control of net motoneuron excitability. MEP/EMG ratio was larger at 60, 80 and 100-ms intervals in CLBP compared to controls, and afferent stimulations alone reduced EMG amplitude greater in CLBP than controls at 100 ms. Our results suggest alteration in sensorimotor integration in CLBP highlighted by a greater facilitation of the descending corticospinal input to paravertebral muscles. Our results can help to optimise interventions by better targeting mechanisms.

TMS-derived short afferent inhibition discriminates cognitive status in older adults without dementia

Aging is a complex and diverse biological process characterized by progressive molecular, cellular, and tissue damage, resulting in a loss of physiological integrity and heightened vulnerability to pathology. This biological diversity corresponds with highly variable cognitive trajectories, which are further confounded by genetic and environmental factors that influence the resilience of the aging brain. Given this complexity, there is a need for neurophysiological indicators that not only discern physiologic and pathologic aging but also closely align with cognitive trajectories. Transcranial Magnetic Stimulation (TMS) may have utility in this regard as a non-invasive brain stimulation tool that can characterize features of cortical excitability. Particularly, as a proxy for central cholinergic function, short-afferent inhibition (SAI) dysfunction is robustly associated with cognitive deficits in the latter stages of Alzheimer's Disease and Related Dementia (ADRD). In this study, we evaluated SAI in healthy young adults and older adults who, though absent clinical diagnoses, were algorithmically classified as cognitively normal (CN) or cognitively impaired (CI) according to the Jak/Bondi actuarial criteria. We report that SAI is preserved in the Old-CN cohort relative to the young adults, and SAI is significantly diminished in the Old-CI cohort relative to both young and CN older adults. Additionally, diminished SAI was significantly associated with impaired sustained attention and working memory. As a proxy measure for central cholinergic deficits, we discuss the potential value of SAI for discerning physiological and pathological aging.

Hebbian priming of human motor learning

Motor learning relies on experience-dependent plasticity in relevant neural circuits. In four experiments, we provide initial evidence and a double-blinded, sham-controlled replication (Experiment I-II) demonstrating that motor learning involving ballistic index finger movements is improved by preceding paired corticospinal-motoneuronal stimulation (PCMS), a human model for exogenous induction of spike-timing-dependent plasticity. Behavioral effects of PCMS targeting corticomotoneuronal (CM) synapses are order- and timing-specific and partially bidirectional (Experiment III). PCMS with a 2 ms inter-arrival interval at CM-synapses enhances learning and increases corticospinal excitability compared to control protocols. Unpaired stimulations did not increase corticospinal excitability (Experiment IV). Our findings demonstrate that non-invasively induced plasticity interacts positively with experience-dependent plasticity to promote motor learning. The effects of PCMS on motor learning approximate Hebbian learning rules, while the effects on corticospinal excitability demonstrate timing-specificity but not bidirectionality. These findings offer a mechanistic rationale to enhance motor practice effects by priming sensorimotor training with individualized PCMS.

A Novel Paired Somatosensory-Cerebellar Stimulation Induces Plasticity on Cerebellar-Brain Connectivity

The cerebellum receives and integrates a large amount of sensory information that is important for motor coordination and learning. The aim of the present work was to investigate whether peripheral nerve and cerebellum paired associative stimulation (cPAS) could induce plasticity in both the cerebellum and the cortex. In a cross-over design, we delivered right median nerve electrical stimulation 25 or 10 ms before applying transcranial magnetic stimulation over the cerebellum. We assessed changes in motor evoked potentials (MEP), somatosensory evoked potentials (SEP), short-afferent inhibition (SAI), and cerebellum-brain inhibition (CBI) immediately, and 30 min after cPAS. Our results showed a significant reduction in CBI 30 minutes after cPAS, with no discernible changes in MEP, SEP, and SAI. Notably, cPAS10 did not produce any modulatory effects on these parameters. In summary, cPAS25 demonstrated the capacity to induce plasticity effects in the cerebellar cortex, leading to a reduction in CBI. This novel intervention may be used to modulate plasticity mechanisms and motor learning in healthy individuals and patients with neurological conditions.

Transcutaneous auricular vagus nerve stimulation enhances short-latency afferent inhibition via central cholinergic system activation

The present study examined the effects of transcutaneous auricular vagus nerve stimulation (taVNS) on short-latency afferent inhibition (SAI), as indirect biomarker of cholinergic system activation. 24 healthy adults underwent intermittent taVNS (30 s on/30 s off, 30 min) or continuous taVNS at a frequency of 25 Hz (15 min) along with earlobe temporary stimulation (15 min or 30 min) were performed in random order. The efficiency with which the motor evoked potential from the abductor pollicis brevis muscle by transcranial magnetic stimulation was attenuated by the preceding median nerve conditioning stimulus was compared before taVNS, immediately after taVNS, and 15 min after taVNS. Continuous taVNS significantly increased SAI at 15 min post-stimulation compared to baseline. A positive correlation (Pearson coefficient = 0.563, p = 0.004) was observed between baseline SAI and changes after continuous taVNS. These results suggest that 15 min of continuous taVNS increases the activity of the cholinergic nervous system, as evidenced by the increase in SAI. In particular, the increase after taVNS was more pronounced in those with lower initial SAI. This study provides fundamental insight into the clinical potential of taVNS for cholinergic dysfunction.

The effects of verbal and spatial working memory on short- and long-latency sensorimotor circuits in the motor cortex

Multiple sensorimotor loops converge in the motor cortex to create an adaptable system capable of context-specific sensorimotor control. Afferent inhibition provides a non-invasive tool to investigate the substrates by which procedural and cognitive control processes interact to shape motor corticospinal projections. Varying the transcranial magnetic stimulation properties during afferent inhibition can probe specific sensorimotor circuits that contribute to short- and long-latency periods of inhibition in response to the peripheral stimulation. The current study used short- (SAI) and long-latency (LAI) afferent inhibition to probe the influence of verbal and spatial working memory load on the specific sensorimotor circuits recruited by posterior-anterior (PA) and anterior-posterior (AP) TMS-induced current. Participants completed two sessions where SAI and LAI were assessed during the short-term maintenance of two- or six-item sets of letters (verbal) or stimulus locations (spatial). The only difference between the sessions was the direction of the induced current. PA SAI decreased as the verbal working memory load increased. In contrast, AP SAI was not modulated by verbal working memory load. Visuospatial working memory load did not affect PA or AP SAI. Neither PA LAI nor AP LAI were sensitive to verbal or spatial working memory load. The dissociation of short-latency PA and AP sensorimotor circuits and short- and long-latency PA sensorimotor circuits with increasing verbal working memory load support multiple convergent sensorimotor loops that provide distinct functional information to facilitate context-specific supraspinal control.

The Effectiveness of Paired Associative Stimulation on Motor Recovery after Stroke: A Scoping Review

Paired associative stimulation (PAS) is a non-invasive brain stimulation technique combining transcranial magnetic stimulation and peripheral nerve stimulation. PAS allows connections between cortical areas and peripheral nerves (C/P PAS) or between cortical regions (C/C PAS) to be strengthened or weakened by spike-timing-dependent neural plasticity mechanisms. Since PAS modulates both neurophysiological features and motor performance, there is growing interest in its application in neurorehabilitation. We aimed to synthesize evidence on the motor rehabilitation role of PAS in stroke patients. We performed a literature search following the PRISMA Extension for Scoping Reviews Framework. Eight studies were included: one investigated C/C PAS between the cerebellum and the affected primary motor area (M1), seven applied C/P PAS over the lesional, contralesional, or both M1. Seven studies evaluated the outcome on upper limb and one on lower limb motor recovery. Although several studies omit crucial methodological details, PAS highlighted effects mainly on corticospinal excitability, and, more rarely, an improvement in motor performance. However, most studies failed to prove a correlation between neurophysiological changes and motor improvement. Although current studies seem to suggest a role of PAS in post-stroke rehabilitation, their heterogeneity and limited number do not yet allow definitive conclusions to be drawn.

Investigating the effects of dopamine on short- and long-latency afferent inhibition

Short- and long-latency afferent inhibition (SAI and LAI respectively) are phenomenon whereby the motor evoked potential induced by transcranial magnetic stimulation (TMS) is inhibited by a sensory afferent volley consequent to nerve stimulation. It remains unclear whether dopamine participates in the genesis or modulation of SAI and LAI. The present study aimed to determine if SAI and LAI are modulated by levodopa (l-DOPA). In this placebo-controlled, double-anonymized study Apo-Levocarb (100 mg l-DOPA in combination with 25 mg carbidopa) and a placebo were administered to 32 adult males (mean age 24 ± 3 years) in two separate sessions. SAI and LAI were evoked by stimulating the median nerve and delivering single-pulse TMS over the motor hotspot corresponding to the first dorsal interosseous muscle of the right hand. SAI and LAI were quantified before and 1 h following ingestion of drug or placebo corresponding to the peak plasma concentration of Apo-Levocarb. The results indicate that Apo-Levocarb increases SAI and does not significantly alter LAI. These findings support literature demonstrating increased SAI following exogenous dopamine administration in neurodegenerative disorders. KEY POINTS: Short- and long-latency afferent inhibition (SAI and LAI respectively) are measures of corticospinal excitability evoked using transcranial magnetic stimulation. SAI and LAI are reduced in conditions such as Parkinson's disease which suggests dopamine may be involved in the mechanism of afferent inhibition. 125 mg of Apo-Levocarb (100 mg dopamine) increases SAI but not LAI. This study increases our understanding of the pharmacological mechanism of SAI and LAI.

Short-latency afferent inhibition as a biomarker of cholinergic degeneration compared to PET imaging in Parkinson's disease

INTRODUCTION

Short-latency afferent inhibition (SAI) is a relatively cheap and non-invasive method that has been proposed as a cholinergic marker in Parkinson's disease (PD). We aim to verify the clinical feasibility of SAI as a cholinergic marker in PD using positron emission tomography (PET) with the tracer (2R,3R)-5-(2-[18F]fluoroethoxy)benzovesamicol ([18F]FEOBV) as a reference.

METHODS

We examined relations between SAI and [18F]FEOBV PET using linear regression analysis, with the primary motor cortex (M1) as primary region of interest. Additionally, we examined relations of both measures with clinical features.

RESULTS

30 PD patients with varying degrees of cognitive dysfunction and 10 healthy controls (HC) were included in the analysis. SAI was not related to tracer uptake in M1 in the PD group (p = .291) or the HC group (p = .206). We could not replicate the previously published relations between SAI and cholinergic symptoms, such as cognition, psychotic experiences and olfactory function.

CONCLUSION

SAI was not related to [18F]FEOBV imaging parameters, nor to clinical measures of cholinergic dysfunction. Therefore, SAI may not be feasible as a clinically applied cholinergic marker in PD.

Short-latency afferent inhibition is reduced in people with multiple sclerosis during fatiguing muscle contractions

Understanding how inhibitory pathways influence motor cortical activity during fatiguing contractions may provide valuable insight into mechanisms associated with multiple sclerosis (MS) muscle activation. Short-latency afferent inhibition (SAI) reflects inhibitory interactions between the somatosensory cortex and the motor cortex, and although SAI is typically reduced with MS, it is unknown how SAI is regulated during exercise-induced fatigue. The current study examined how SAI modulates motor evoked potentials (MEPs) during fatiguing contractions. Fourteen people with relapsing-remitting MS (39 ± 6 years, nine female) and 10 healthy individuals (36 ± 6 years, six female) participated. SAI was induced by stimulation of the median nerve that was paired with TMS over the motor representation of the abductor pollicis brevis. A contraction protocol was employed that depressed force generating capacity using a sustained 3-min 15% MVC, immediately followed by a low-intensity (15% MVC) intermittent contraction protocol so that MEP and SAI could be measured during the rest phases of each duty cycle. Similar force, electromyography and MEP responses were observed between groups. However, the MS group had significantly reduced SAI during the contraction protocol compared to the healthy control group (p < .001). Despite the MS group reporting greater scores on the Fatigue Severity Scale and Modified Fatigue Impact Scale, these scales did not correlate with inhibitory measures. As there were no between-group differences in SSEPs, MS-related SAI differences during the fatiguing contractions were most likely associated with disease-related changes in central integration.

Cortical Sensorimotor Integration as a Neural Origin of Impaired Grip Force Direction Control following Stroke

BACKGROUND

Stroke is a major cause of disability worldwide. Upper limb impairment is prevalent after stroke. One of the post-stroke manifestations is impaired grip force directional control contributing to diminished abilities to grip and manipulate objects necessary for activities of daily living. The objective of this study was to investigate the neural origin of the impaired grip force direction control following stroke. Due to the importance of online adjustment of motor output based on sensory feedback, it was hypothesized that grip force direction control would be associated with cortical sensorimotor integration in stroke survivors.

METHODS

Ten chronic stroke survivors participated in this study. Cortical sensorimotor integration was quantified by short latency afferent inhibition (SAI), which represents the responsiveness of the primary motor cortex to somatosensory input. Grip force direction control was assessed during paretic grip.

RESULTS

Grip force direction control was significantly associated with SAI. This relationship was independent of sensory impairment level.

CONCLUSIONS

Cortical sensorimotor integration may play a significant role in the grip force direction control important for gripping and manipulating objects with the affected hand following stroke. This knowledge may be used to inform personalized rehabilitation treatment. For example, for patients with impaired grip force direction control, behavioral therapy focusing on feedback motor control, augmented by use of brain stimulation to reinforce cortical sensorimotor integration such as paired associative stimulation, may be applied.

Modulation of Motor Cortical Inhibition and Facilitation by Touch Sensation from the Glabrous Skin of the Human Hand

Touch sensation from the glabrous skin of the hand is essential for precisely controlling dexterous movements, yet the neural mechanisms by which tactile inputs influence motor circuits remain largely unexplored. By pairing air-puff tactile stimulation on the hand's glabrous skin with transcranial magnetic stimulation (TMS) over the primary motor cortex (M1), we examined the effects of tactile stimuli from single or multiple fingers on corticospinal excitability and M1's intracortical circuits. Our results showed that when we targeted the hand's first dorsal interosseous (FDI) muscle with TMS, homotopic (index finger) tactile stimulation, regardless of its point (fingertip or base), reduced corticospinal excitability. Conversely, heterotopic (ring finger) tactile stimulation had no such effect. Notably, stimulating all five fingers simultaneously led to a more pronounced decrease in corticospinal excitability than stimulating individual fingers. Furthermore, tactile stimulation significantly increased intracortical facilitation (ICF) and decreased long-interval intracortical inhibition (LICI) but did not affect short-interval intracortical inhibition (SICI). Considering the significant role of the primary somatosensory cortex (S1) in tactile processing, we also examined the effects of downregulating S1 excitability via continuous theta burst stimulation (cTBS) on tactile-motor interactions. Following cTBS, the inhibitory influence of tactile inputs on corticospinal excitability was diminished. Our findings highlight the spatial specificity of tactile inputs in influencing corticospinal excitability. Moreover, we suggest that tactile inputs distinctly modulate M1's excitatory and inhibitory pathways, with S1 being crucial in facilitating tactile-motor integration.

Age-related differences in long-term potentiation-like plasticity and short-latency afferent inhibition and their association with cognitive function

Background

The neurophysiological differences in cortical plasticity and cholinergic system function due to ageing and their correlation with cognitive function remain poorly understood.

Aims

To reveal the differences in long-term potentiation (LTP)-like plasticity and short-latency afferent inhibition (SAI) between older and younger individuals, alongside their correlation with cognitive function using transcranial magnetic stimulation (TMS).

Methods

The cross-sectional study involved 31 younger adults aged 18-30 and 46 older adults aged 60-80. All participants underwent comprehensive cognitive assessments and a neurophysiological evaluation based on TMS. Cognitive function assessments included evaluations of global cognitive function, language, memory and executive function. The neurophysiological assessment included LTP-like plasticity and SAI.

Results

The findings of this study revealed a decline in LTP among the older adults compared with the younger adults (wald χ2=3.98, p=0.046). Subgroup analysis further demonstrated a significant reduction in SAI level among individuals aged 70-80 years in comparison to both the younger adults (SAI(N20): (t=-3.37, p=0.018); SAI(N20+4): (t=-3.13, p=0.038)) and those aged 60-70 (SAI(N20): (t=-3.26, p=0.025); SAI(N20+4): (t=-3.69, p=0.006)). Conversely, there was no notable difference in SAI level between those aged 60-70 years and the younger group. Furthermore, after employing the Bonferroni correction, the correlation analysis revealed that only the positive correlation between LTP-like plasticity and language function (r=0.61, p<0.001) in the younger group remained statistically significant.

Conclusions

During the normal ageing process, a decline in synaptic plasticity may precede cholinergic system dysfunction. In individuals over 60 years of age, there is a reduction in LTP-like plasticity, while a decline in cholinergic system function is observed in those over 70. Thus, the cholinergic system may play a vital role in preventing cognitive decline during normal ageing. In younger individuals, LTP-like plasticity might represent a potential neurophysiological marker for language function.

Early modulations of neural oscillations during the processing of emotional body language

The processing of threat-related emotional body language (EBL) has been shown to engage sensorimotor cortical areas early on and induce freezing in the observers' motor system, particularly when observing fearful EBL. To provide insights into the interplay between somatosensory and motor areas during observation of EBL, here, we used high-density electroencephalography (hd-EEG) in healthy humans while they observed EBL stimuli involving fearful and neutral expressions. To capture early sensorimotor brain response, we focused on P100 fronto-central event-related potentials (ERPs) and event-related desynchronization/synchronization (ERD/ERS) in the mu-alpha (8-13 Hz) and lower beta (13-20 Hz) bands over the primary motor (M1) and somatosensory (S1) cortices. Source-level ERP and ERD/ERS analyses were conducted using eLORETA. Results revealed higher P100 amplitudes in motor and premotor channels for 'Neutral' compared with 'Fear'. Additionally, analysis of ERD/ERS showed increased beta band desynchronization in M1 for 'Neutral', and the opposite pattern in S1. Source-level estimation showed significant differences between conditions mainly observed in the beta band over sensorimotor areas. These findings provide high-temporal resolution evidence suggesting that seeing fearful EBL induces early activation of somatosensory areas, which in turn could suppress M1 activity. These findings highlight early dynamics within the observer's sensorimotor system and hint at a sensorimotor mechanism supporting freezing during the processing of EBL.

Sensorimotor integration in cranial muscles tested by short- and long-latency afferent inhibition

OBJECTIVE

To compressively investigate sensorimotor integration in the cranial-cervical muscles in healthy adults.

METHODS

Short- (SAI) and long-latency afferent (LAI) inhibition were probed in the anterior digastric (AD), the depressor anguli oris (DAO) and upper trapezius (UT) muscles. A transcranial magnetic stimulation pulse over primary motor cortex was preceded by peripheral stimulation delivered to the trigeminal, facial and accessory nerves using interstimulus intervals of 15-25 ms and 100-200 ms for SAI and LAI respectively.

RESULTS

In the AD, both SAI and LAI were detected following trigeminal nerve stimulation, but not following facial nerve stimulation. In the DAO, SAI was observed only following trigeminal nerve stimulation, while LAI depended only on facial nerve stimulation, only at an intensity suprathreshold for the compound motor action potential (cMAP). In the UT we could only detect LAI following accessory nerve stimulation at an intensity suprathreshold for a cMAP.

CONCLUSIONS

The results suggest that integration of sensory inputs with motor output is profoundly influenced by the type of sensory afferent involved and by the functional role played by the target muscle.

SIGNIFICANCE

Data indicate the importance of taking into account the sensory receptors involved as well as the function of the target muscle when studying sensorimotor integration, both in physiological and neurological conditions.

From rubber hands to neuroprosthetics: Neural correlates of embodiment

Our interaction with the world rests on the knowledge that we are a body in space and time, which can interact with the environment. This awareness is usually referred to as sense of embodiment. For the good part of the past 30 years, the rubber hand illusion (RHI) has been a prime tool to study embodiment in healthy and people with a variety of clinical conditions. In this paper, we provide a critical overview of this research with a focus on the RHI paradigm as a tool to study prothesis embodiment in individuals with amputation. The RHI relies on well-documented multisensory integration mechanisms based on sensory precision, where parietal areas are involved in resolving the visuo-tactile conflict, and premotor areas in updating the conscious bodily representation. This mechanism may be transferable to prosthesis ownership in amputees. We discuss how these results might transfer to technological development of sensorised prostheses, which in turn might progress the acceptability by users.

Cortical Contributions to Imagined Power Grip Task: An EEG-Triggered TMS Study

Previous studies have demonstrated that motor imagery leads to desynchronization in the alpha rhythm within the contralateral primary motor cortex. However, the underlying electrophysiological mechanisms responsible for this desynchronization during motor imagery remain unclear. To examine this question, we conducted an investigation using EEG in combination with noninvasive transcranial magnetic stimulation (TMS) during index finger abduction (ABD) and power grip imaginations. The TMS was administered employing diverse coil orientations to selectively stimulate corticospinal axons, aiming to target both early and late synaptic inputs to corticospinal neurons. TMS was triggered based on the alpha power levels, categorized in 20th percentile bins, derived from the individual alpha power distribution during the imagined tasks of ABD and power grip. Our analysis revealed negative correlations between alpha power and motor evoked potential (MEP) amplitude, as well as positive correlations with MEP latency across all coil orientations for each imagined task. Furthermore, we conducted functional network analysis in the alpha band to explore network connectivity during imagined index finger abduction and power grip tasks. Our findings indicate that network connections were denser in the fronto-parietal area during imagined ABD compared to power grip conditions. Moreover, the functional network properties demonstrated potential for effectively classifying between these two imagined tasks. These results provide functional evidence supporting the hypothesis that alpha oscillations may play a role in suppressing MEP amplitude and latency during imagined power grip. We propose that imagined ABD and power grip tasks may activate different populations and densities of axons at the cortical level.

Advancements in Transcranial Magnetic Stimulation Research and the Path to Precision

Transcranial magnetic stimulation (TMS) has become increasingly popular in clinical practice in recent years, and there have been significant advances in the principles and stimulation modes of TMS. With the development of multi-mode and precise stimulation technology, it is crucial to have a comprehensive understanding of TMS. The neuroregulatory effects of TMS can vary depending on the specific mode of stimulation, highlighting the importance of exploring these effects through multimodal application. Additionally, the use of precise TMS therapy can help enhance our understanding of the neural mechanisms underlying these effects, providing us with a more comprehensive perspective. This article aims to review the mechanism of action, stimulation mode, multimodal application, and precision of TMS.

Plasticity of face-hand sensorimotor circuits after a traumatic brachial plexus injury

Background

Interactions between the somatosensory and motor cortices are of fundamental importance for motor control. Although physically distant, face and hand representations are side by side in the sensorimotor cortex and interact functionally. Traumatic brachial plexus injury (TBPI) interferes with upper limb sensorimotor function, causes bilateral cortical reorganization, and is associated with chronic pain. Thus, TBPI may affect sensorimotor interactions between face and hand representations.

Objective

The aim of this study was to investigate changes in hand-hand and face-hand sensorimotor integration in TBPI patients using an afferent inhibition (AI) paradigm.

Method

The experimental design consisted of electrical stimulation (ES) applied to the hand or face followed by transcranial magnetic stimulation (TMS) to the primary motor cortex to activate a hand muscle representation. In the AI paradigm, the motor evoked potential (MEP) in a target muscle is significantly reduced when preceded by an ES at short-latency (SAI) or long-latency (LAI) interstimulus intervals. We tested 18 healthy adults (control group, CG), evaluated on the dominant upper limb, and nine TBPI patients, evaluated on the injured or the uninjured limb. A detailed clinical evaluation complemented the physiological investigation.

Results

Although hand-hand SAI was present in both the CG and the TBPI groups, hand-hand LAI was present in the CG only. Moreover, less AI was observed in TBPI patients than the CG both for face-hand SAI and LAI.

Conclusion

Our results indicate that sensorimotor integration involving both hand and face sensorimotor representations is affected by TBPI.

Abnormalities in the face primary motor cortex in oromandibular dystonia

OBJECTIVE

To comprehensively investigate excitability in face and hand M1 and sensorimotor integration in oromandibular dystonia (OMD) patients.

METHODS

Short-interval intracortical inhibition (SICI), intracortical facilitation (ICF), short (SAI) and long (LAI) afferent inhibition were investigated in face and hand M1 using transcranial magnetic stimulation protocols in 10 OMD patients. Data were compared with those obtained in 10 patients with focal hand dystonia (FHD), in 10 patients with blepharospasm (BSP), and 10 matched healthy subjects (HS).

RESULTS

Results demonstrated that in OMD patients SICI was reduced in face M1 (p < 0.001), but not in hand M1, compared to HS. In FHD, SICI was significantly impaired in hand M1 (p = 0.029), but not in face M1. In BSP, SICI was normal in both face and hand M1 while ICF and LAI were normal in all patient groups and cortical area tested. SAI was significantly reduced (p = 0.003) only in the face M1 of OMD patients.

CONCLUSIONS

In OMD, SICI and SAI were significantly reduced. These abnormalities are specific to the motor cortical area innervating the muscular district involved in focal dystonia.

SIGNIFICANCE

In OMD, the integration between sensory inflow and motor output seem to be disrupted at cortical level with topographic specificity.

Clinical diagnostic utility of transcranial magnetic stimulation in neurological disorders. Updated report of an IFCN committee

The review provides a comprehensive update (previous report: Chen R, Cros D, Curra A, Di Lazzaro V, Lefaucheur JP, Magistris MR, et al. The clinical diagnostic utility of transcranial magnetic stimulation: report of an IFCN committee. Clin Neurophysiol 2008;119(3):504-32) on clinical diagnostic utility of transcranial magnetic stimulation (TMS) in neurological diseases. Most TMS measures rely on stimulation of motor cortex and recording of motor evoked potentials. Paired-pulse TMS techniques, incorporating conventional amplitude-based and threshold tracking, have established clinical utility in neurodegenerative, movement, episodic (epilepsy, migraines), chronic pain and functional diseases. Cortical hyperexcitability has emerged as a diagnostic aid in amyotrophic lateral sclerosis. Single-pulse TMS measures are of utility in stroke, and myelopathy even in the absence of radiological changes. Short-latency afferent inhibition, related to central cholinergic transmission, is reduced in Alzheimer's disease. The triple stimulation technique (TST) may enhance diagnostic utility of conventional TMS measures to detect upper motor neuron involvement. The recording of motor evoked potentials can be used to perform functional mapping of the motor cortex or in preoperative assessment of eloquent brain regions before surgical resection of brain tumors. TMS exhibits utility in assessing lumbosacral/cervical nerve root function, especially in demyelinating neuropathies, and may be of utility in localizing the site of facial nerve palsies. TMS measures also have high sensitivity in detecting subclinical corticospinal lesions in multiple sclerosis. Abnormalities in central motor conduction time or TST correlate with motor impairment and disability in MS. Cerebellar stimulation may detect lesions in the cerebellum or cerebello-dentato-thalamo-motor cortical pathways. Combining TMS with electroencephalography, provides a novel method to measure parameters altered in neurological disorders, including cortical excitability, effective connectivity, and response complexity.

Multimodal integration and modulation of visual and somatosensory inputs on the corticospinal excitability

OBJECTIVE

Corticospinal excitability may be affected by various sensory inputs under physiological conditions. In this study, we aimed to investigate the corticospinal excitability by using multimodal conditioning paradigms of combined somatosensory electrical and visual stimulation to understand the sensory-motor integration.

METHODS

We examined motor evoked potentials (MEP) obtained by using transcranial magnetic stimulation (TMS) that were conditioned by using a single goggle-light-emitting diode (LED) stimulation, peripheral nerve electrical stimulation (short latency afferent inhibition protocol), or a combination of both (goggle-LED+electrical stimulation) at different interstimulus intervals (ISIs) in 14 healthy volunteers.

RESULTS

We found MEP inhibition at ISIs of 50-60 ms using the conditioned goggle-LED stimulation. The combined goggle-LED stimulation at a 60 ms ISI resulted in an additional inhibition to the electrical stimulation.

CONCLUSIONS

Visual inputs cause significant modulatory effects on the corticospinal excitability. Combined visual and somatosensory stimuli integrate probably via different neural circuits and/or interneuron populations. To our knowledge, multimodal integration of visual and somatosensory inputs by using TMS-short latency inhibition protocol have been evaluated via electrophysiological methods for the first time in this study.

Contribution of neural circuits tested by transcranial magnetic stimulation in corticomotor control of low back muscle: a systematic review

Introduction

Transcranial magnetic stimulation (TMS) is widely used to investigate central nervous system mechanisms underlying motor control. Despite thousands of TMS studies on neurophysiological underpinnings of corticomotor control, a large majority of studies have focused on distal muscles, and little is known about axial muscles (e.g., low back muscles). Yet, differences between corticomotor control of low back and distal muscles (e.g., gross vs. fine motor control) suggest differences in the neural circuits involved. This systematic review of the literature aims at detailing the organisation and neural circuitry underlying corticomotor control of low back muscles tested with TMS in healthy humans.

Methods

The literature search was performed in four databases (CINAHL, Embase, Medline (Ovid) and Web of science) up to May 2022. Included studies had to use TMS in combination with EMG recording of paraspinal muscles (between T12 and L5) in healthy participants. Weighted average was used to synthesise quantitative study results.

Results

Forty-four articles met the selection criteria. TMS studies of low back muscles provided consistent evidence of contralateral and ipsilateral motor evoked potentials (with longer ipsilateral latencies) as well as of short intracortical inhibition/facilitation. However, few or no studies using other paired pulse protocols were found (e.g., long intracortical inhibition, interhemispheric inhibition). In addition, no study explored the interaction between different cortical areas using dual TMS coil protocol (e.g., between primary motor cortex and supplementary motor area).

Discussion

Corticomotor control of low back muscles are distinct from hand muscles. Our main findings suggest: (i) bilateral projections from each single primary motor cortex, for which contralateral and ipsilateral tracts are probably of different nature (contra: monosynaptic; ipsi: oligo/polysynaptic) and (ii) the presence of intracortical inhibitory and excitatory circuits in M1 influencing the excitability of the contralateral corticospinal cells projecting to low back muscles. Understanding of these mechanisms are important for improving the understanding of neuromuscular function of low back muscles and to improve the management of clinical populations (e.g., low back pain, stroke).

Smoking status ameliorates cholinergic impairments in cortical inhibition in patients with schizophrenia

Rationale Modulation of cortical excitability, in particular inhibition, is impaired in patients with schizophrenia. Chronic nicotine consumption, which is prevalent in this group, has been shown to alter cortical excitability in healthy individuals and to increase inhibitory activity. Thus, beneficial effects of smoking on impaired cortical excitability in patients with schizophrenia have been proposed, though direct experimental evidence is still lacking.

OBJECTIVES

We aimed to explore the effect of chronic smoking on cortical excitability by comparing smoking and non-smoking patients with schizophrenia.

METHOD

Twenty-six smoking and 19 non-smoking patients diagnosed with schizophrenia were included. Transcranial magnetic stimulation (TMS) applied to the primary motor cortex served as experimental paradigm for measuring corticospinal and intracortical excitability as follows: Resting motor threshold (RMT) and the input/output curve (I/O curve) were obtained to assess corticospinal excitability. Intracortical excitability was explored using paired-pulse TMS techniques (intracortical facilitation (ICF), short-latency intracortical inhibition (SICI) and short-latency afferent inhibition (SAI)).

RESULTS

A significantly stronger inhibition in the cholinergically driven SAI protocol was observed in smokers compared to non-smokers. All other measures did not show significant differences between groups.

CONCLUSION

Our results suggest an increased inhibition within cholinergic circuits due to chronic nicotine consumption in schizophrenia. This increase may compensate impaired cholinergic neurotransmission and could explain the high rate of smokers in schizophrenia.

Insufficient sleep may alter cortical excitability near the migraine attack: A blinded TMS crossover study

BACKGROUND

Migraine is a brain disorder with a multifaceted and unexplained association to sleep. Brain excitability likely changes periodically throughout the migraine cycle. In this study we examine the effect of insufficient sleep on neuronal excitability during the course of the migraine cycle.

METHODS

We examined 54 migraine patients after two nights of eight-hour habitual sleep and two nights of four-hour restricted sleep in a randomised, blinded crossover study. We performed transcranial magnetic stimulation and measured cortical silent period, short- and long-interval intracortical inhibition, intracortical facilitation and short-latency afferent inhibition. We analysed how responses changed before and after attacks with linear mixed models.

RESULTS

Short- interval intracortical inhibition was more reduced after sleep restriction compared to habitual sleep the shorter the time that had elapsed since the attack (p = 0.041), and specifically in the postictal phase (p = 0.013). Long-interval intracortical inhibition was more increased after sleep restriction with time closer before the attack (p = 0.006), and specifically in the preictal phase (p = 0.034). Short-latency afferent inhibition was more decreased after sleep restriction with time closer to the start of the attack (p = 0.026).

CONCLUSION

Insufficient sleep in the period leading up to a migraine attack may cause dysfunction in cortical GABAergic inhibition. The results also suggest that migraine patients may have increased need for sufficient sleep during a migraine attack to maintain normal neurological function after the attack.

Morphology, Connectivity, and Encoding Features of Tactile and Motor Representations of the Fingers in the Human Precentral and Postcentral Gyrus

Despite the tight coupling between sensory and motor processing for fine manipulation in humans, it is not yet totally clear which specific properties of the fingers are mapped in the precentral and postcentral gyrus. We used fMRI to compare the morphology, connectivity, and encoding of the motor and tactile finger representations (FRs) in the precentral and postcentral gyrus of 25 5-fingered participants (8 females). Multivoxel pattern and structural and functional connectivity analyses demonstrated the existence of distinct motor and tactile FRs within both the precentral and postcentral gyrus, integrating finger-specific motor and tactile information. Using representational similarity analysis, we found that the motor and tactile FRs in the sensorimotor cortex were described by the perceived structure of the hand better than by the actual hand anatomy or other functional models (finger kinematics, muscles synergies). We then studied a polydactyly individual (i.e., with a congenital 6-fingered hand) showing superior manipulation abilities and divergent anatomic-functional hand properties. The perceived hand model was still the best model for tactile representations in the precentral and postcentral gyrus, while finger kinematics better described motor representations in the precentral gyrus. We suggest that, under normal conditions (i.e., in subjects with a standard hand anatomy), the sensorimotor representations of the 5 fingers in humans converge toward a model of perceived hand anatomy, deviating from the real hand structure, as the best synthesis between functional and structural features of the hand.SIGNIFICANCE STATEMENT Distinct motor and tactile finger representations exist in both the precentral and postcentral gyrus, supported by a finger-specific pattern of anatomic and functional connectivity across modalities. At the representational level, finger representations reflect the perceived structure of the hand, which might result from an adapting process harmonizing (i.e., uniformizing) the encoding of hand function and structure in the precentral and postcentral gyrus. The same analyses performed in an extremely rare polydactyly subject showed that the emergence of such representational geometry is also found in neuromechanical variants with different hand anatomy and function. However, the harmonization process across the precentral and postcentral gyrus might not be possible because of divergent functional-structural properties of the hand and associated superior manipulation abilities.

Experimental environment improves the reliability of short-latency afferent inhibition

Evidence indicates attention can alter afferent inhibition, a Transcranial Magnetic Stimulation (TMS) evoked measure of cortical inhibition following somatosensory input. When peripheral nerve stimulation is delivered prior to TMS, a phenomenon known as afferent inhibition occurs. The latency between the peripheral nerve stimulation dictates the subtype of afferent inhibition evoked, either short latency afferent inhibition (SAI) or long latency afferent inhibition (LAI). While afferent inhibition is emerging as a valuable tool for clinical assessment of sensorimotor function, the reliability of the measure remains relatively low. Therefore, to improve the translation of afferent inhibition within and beyond the research lab, the reliability of the measure must be improved. Previous literature suggests that the focus of attention can modify the magnitude of afferent inhibition. As such, controlling the focus of attention may be one method to improve the reliability of afferent inhibition. In the present study, the magnitude and reliability of SAI and LAI was assessed under four conditions with varying attentional demands focused on the somatosensory input that evokes SAI and LAI circuits. Thirty individuals participated in four conditions; three conditions were identical in their physical parameters and varied only in the focus of directed attention (visual attend, tactile attend, non- directed attend) and one condition consisted of no external physical parameters (no stimulation). Reliability was measured by repeating conditions at three time points to assess intrasession and intersession reliability. Results indicate that the magnitude of SAI and LAI were not modulated by attention. However, the reliability of SAI demonstrated increased intrasession and intersession reliability compared to the no stimulation condition. The reliability of LAI was unaffected by the attention conditions. This research demonstrates the impact of attention/arousal on the reliability of afferent inhibition and has identified new parameters to inform the design of TMS research to improve reliability.

Combining Transcranial Magnetic Stimulation and Deep Brain Stimulation: Current Knowledge, Relevance and Future Perspectives

Deep brain stimulation (DBS) has emerged as an invasive neuromodulation technique for the treatment of several neurological disorders, but the mechanisms underlying its effects remain partially elusive. In this context, the application of Transcranial Magnetic Stimulation (TMS) in patients treated with DBS represents an intriguing approach to investigate the neurophysiology of cortico-basal networks. Experimental studies combining TMS and DBS that have been performed so far have mainly aimed to evaluate the effects of DBS on the cerebral cortex and thus to provide insights into DBS's mechanisms of action. The modulation of cortical excitability and plasticity by DBS is emerging as a potential contributor to its therapeutic effects. Moreover, pairing DBS and TMS stimuli could represent a method to induce cortical synaptic plasticity, the therapeutic potential of which is still unexplored. Furthermore, the advent of new DBS technologies and novel treatment targets will present new research opportunities and prospects to investigate brain networks. However, the application of the combined TMS-DBS approach is currently limited by safety concerns. In this review, we sought to present an overview of studies performed by combining TMS and DBS in neurological disorders, as well as available evidence and recommendations on the safety of their combination. Additionally, we outline perspectives for future research by highlighting knowledge gaps and possible novel applications of this approach.

Short-and long-latency afferent inhibition of the human leg motor cortex by H-reflex subthreshold electrical stimulation at the popliteal fossa

In humans, peripheral sensory stimulation inhibits subsequent motor evoked potentials (MEPs) induced by transcranial magnetic stimulation; this process is referred to as short- or long-latency afferent inhibition (SAI or LAI, respectively), depending on the inter-stimulus interval (ISI) length. Although upper limb SAI and LAI have been well studied, lower limb SAI and LAI remain under-investigated. Here, we examined the time course of the soleus (SOL) muscle MEP following electrical tibial nerve (TN) stimulation at the popliteal fossa at ISIs of 20-220 ms. When the conditioning stimulus intensity was three-fold the perceptual threshold, MEP amplitudes were inhibited at an ISI of 220 ms, but not at shorter ISIs. TN stimulation just below the Hoffman (H)-reflex threshold intensity inhibited MEP amplitudes at ISIs of 30, 35, 100, 180 and 200 ms. However, the relationship between MEP inhibition and the P30 latency of somatosensory evoked potentials (SEPs) did not show corresponding ISIs at the SEP P30 latency that maximizes MEP inhibition. To clarify whether the site of afferent-induced MEP inhibition occurs at the cortical or spinal level, we examined the time course of SOL H-reflex following TN stimulation. H-reflex amplitudes were not significantly inhibited at ISIs where MEP inhibition occurred but at an ISI of 120 ms. Our findings indicate that stronger peripheral sensory stimulation is required for lower limb than for upper limb SAI and LAI and that lower limb SAI and LAI are of cortical origin. Moreover, the direct pathway from the periphery to the primary motor cortex may contribute to lower limb SAI.

Reduced Short-Latency Afferent Inhibition in Parkinson's Disease Patients with L-dopa-Unresponsive Freezing of Gait

BACKGROUND

Freezing of gait (FOG) in Parkinson's disease (PD), especially the "L-dopa-unresponsive" subtype, is associated with the dysfunction of non-dopaminergic circuits.

OBJECTIVE

We sought to determine whether cortical sensorimotor inhibition evaluated by short-latency afferent inhibition (SAI) related to cholinergic and gamma-aminobutyric acid (GABA)-ergic activities is impaired in PD patients with L-dopa-unresponsive FOG (ONOFF-FOG).

METHODS

SAI protocol was performed in 28 PD patients with ONOFF-FOG, 15 PD patients with "off" FOG (OFF-FOG), and 25 PD patients without FOG during medication "on" state. Additionally, 10 ONOFF-FOG patients underwent SAI testing during both "off" and "on" states. Twenty healthy controls participated in this study. Gait was measured objectively using a portable Inertial Measurement Unit system, and participants performed 5-meter Timed Up and Go single- and dual-task conditions. Spatiotemporal gait characteristics and their variability were determined. FOG manifestations and cognition were assessed with clinical scales.

RESULTS

Compared to controls, PD patients without FOG and with OFF-FOG, ONOFF-FOG PD patients showed significantly reduced SAI. Further, dopaminergic therapy had no remarkable effect on this SAI alterations in ONOFF-FOG. Meanwhile, OFF-FOG patients presented decreased SAI only relative to controls. PD patients with ONOFF-FOG exhibited decreased gait speed, stride length, and increased gait variability relative to PD patients without FOG and controls under both walking conditions. For ONOFF-FOG patients, significant associations were found between SAI and FOG severity, gait characteristics and variability.

CONCLUSION

Reduced SAI was associated with severe FOG manifestations, impaired gait characteristics and variability in PD patients with ONOFF-FOG, suggesting the impaired thalamocortical cholinergic-GABAergic SAI pathways underlying ONOFF-FOG.

Face emotional expressions influence interhemispheric inhibition

The processing of face expressions is a key ability to perform social interactions. Recently, it has been demonstrated that the excitability of the hand primary motor cortex (M1) increases following the view of negative faces expressions. Interhemispheric interactions and sensory-motor integration are cortical processes involving M1, which are known to be modulated by emotional and social behaviors. Whether these processes may mediate the effects of face emotional expressions on M1 excitability is unknown. Therefore, the aim of this study was to investigate the influence of the passive viewing of face emotional expressions on M1 interhemispheric connections and sensory-motor integration using standardized transcranial magnetic stimulation (TMS) protocols. Nineteen healthy subjects participated in the study. Interhemispheric inhibition (IHI) and short-afferent inhibition (SAI) were probed in the right first dorsal interosseous (FDI) muscle 300 ms after the randomized presentation of seven different face expressions (neutral, sadness, fear, disgust, surprise and happiness). Results showed a significantly reduced IHI following the passive viewing of fearful faces compared to neutral (p = .001) and happy (p = .035) faces and following the view of sad faces compared to neutral faces (p = .008). No effect of emotional faces was detected on SAI. Data suggest that sensory-motor integration process does not mediate the increased excitability of M1 induced by the view of negative face expressions. By contrast, it may be underpinned by a depression of IHI, which from a functional point of view may promote symmetrical avoiding movements of the hands in response to aversive stimuli.

High-frequency oscillations-based precise temporal resolution of short latency afferent inhibition in the human brain

OBJECTIVE

Sensorimotor integration is a crucial process for adaptive behaviour and can be explored non-invasively with a conditioned transcranial magnetic stimulation (TMS) paradigm - i.e. short-latency afferent inhibition (SAI). To gain insight into the sensorimotor integration phenomenon, we used two different approaches to combine peripheral and cortical stimulation in the SAI paradigm, measuring not only the latency of low frequency somatosensory evoked potentials (SEPs) but also the peaks of high frequency oscillations (HFOs) underlying SEPs.

METHODS

The interstimulus intervals (ISIs) between the electrical stimulation of the median nerve and the motor cortex magnetic stimulation were determined relative to the latency of the earliest SEPs cortical potential (N20) or the HFOs peaks. In particular, the first and last negative and positive peaks of HFOs were extracted through a custom-made MATLAB script.

RESULTS

Thirty-three healthy subjects participated in this study. We found out that muscle responses after TMS were suppressed when ISIs were comprised between -1 to +3 ms relative to the N20 peak and at all ISIs relative to HFOs peaks, except for the first negative peak.

CONCLUSIONS

Coupling peripheral and cortical stimulation at early interstimulus intervals - before the SEPs N20 peak - may modulate muscle response.

SIGNIFICANCE

Our findings confirm that afferent inhibition is produced both through a direct (thalamus-motor cortex) and indirect (thalamus-somatosensory-motor cortex) pathway.

Older adults' episodic memory is related to a neurophysiological marker of brain cholinergic activity

Episodic memory is vulnerable to aging and may be influenced by age-related decline in the neurotransmitter acetylcholine. We probed this relation using a novel, minimally invasive transcranial magnetic stimulation marker of brain acetylcholine: short-latency afferent inhibition (SAI). We used neuropsychological testing to construct a composite score of episodic memory in N = 19 community-dwelling older adults, and stratified older adults into Higher- (N = 9) versus Lower-memory (N = 10) groups before SAI. The Higher-memory group showed significantly stronger SAI than the Lower-memory group, indicating an association between higher brain acetylcholine levels and better episodic memory. The two memory groups were equivalent in the potential confounds of age, education, mood, subjective sleep quality, and executive function. These data converge with others to suggest that episodic memory is related to acetylcholine in older adults. This relation should be further investigated, especially with pharmacology and neuroimaging.

Evidence of Neuroplastic Changes after Transcranial Magnetic, Electric, and Deep Brain Stimulation

Electric and magnetic stimulation of the human brain can be used to excite or inhibit neurons. Numerous methods have been designed over the years for this purpose with various advantages and disadvantages that are the topic of this review. Deep brain stimulation (DBS) is the most direct and focal application of electric impulses to brain tissue. Electrodes are placed in the brain in order to modulate neural activity and to correct parameters of pathological oscillation in brain circuits such as their amplitude or frequency. Transcranial magnetic stimulation (TMS) is a non-invasive alternative with the stimulator generating a magnetic field in a coil over the scalp that induces an electric field in the brain which, in turn, interacts with ongoing brain activity. Depending upon stimulation parameters, excitation and inhibition can be achieved. Transcranial electric stimulation (tES) applies electric fields to the scalp that spread along the skull in order to reach the brain, thus, limiting current strength to avoid skin sensations and cranial muscle pain. Therefore, tES can only modulate brain activity and is considered subthreshold, i.e., it does not directly elicit neuronal action potentials. In this review, we collect hints for neuroplastic changes such as modulation of behavior, the electric activity of the brain, or the evolution of clinical signs and symptoms in response to stimulation. Possible mechanisms are discussed, and future paradigms are suggested.

Citicoline Treatment in Acute Ischemic Stroke: A Randomized, Single-Blind TMS Study

Background

Recent research on animal models of ischemic stroke supports the idea that pharmacological treatment potentially enhancing intrinsic brain plasticity could modulate acute brain damage, with improved functional recovery. One of these new drugs is citicoline, which could provide neurovascular protection and repair effects.

Objectives

The objective of this randomized, single-blind experimental study was to evaluate whether the treatment with Rischiaril^® Forte was able to restore intracortical excitability measures, evaluated through transcranial magnetic stimulation (TMS) protocols, in patients with acute ischemic stroke.

Methods

Patients with acute ischemic stroke were recruited and assigned to an eight-week therapy of standard treatment (control group - CG) or CDP-choline (Rischiaril^® Forte, containing 1,000 mg of citicoline sodium salt) added to conventional treatment (treatment group - TG). Each subject underwent a clinical evaluation and neurophysiological assessment using TMS, pretretament and posttreatment.

Results

A total of thirty participants (mean [SD] age, 68.1 [9.6] years; 11 women [37%]) completed the study. We did not observe significant changes in clinical scores after CDP-choline treatment (all p > 0.05), but we observed a significant improvement in short-interval intracortical inhibition (SAI) (p = 0.003) in the TG group compared to the CG group.

Conclusions

The eight-week treatment with citicoline after acute ischemic stroke may restore intracortical excitability measures, which partially depends on cholinergic transmission. This study extends current knowledge of the application of citicoline in acute ischemic stroke.

The Patho-Neurophysiological Basis and Treatment of Focal Laryngeal Dystonia: A Narrative Review and Two Case Reports Applying TMS over the Laryngeal Motor Cortex

Focal laryngeal dystonia (LD) is a rare, idiopathic disease affecting the laryngeal musculature with an unknown cause and clinically presented as adductor LD or rarely as abductor LD. The most effective treatment options include the injection of botulinum toxin (BoNT) into the affected laryngeal muscle. The aim of this narrative review is to summarize the patho-neuro-physiological and genetic background of LD, as well as the standard recommended therapy (BoNT) and pharmacological treatment options, and to discuss possible treatment perspectives using neuro-modulation techniques such as repetitive transcranial magnetic stimulation (rTMS) and vibrotactile stimulation. The review will present two LD cases, patients with adductor and abductor LD, standard diagnostic procedure, treatments and achievement, and the results of cortical excitability mapping the primary motor cortex for the representation of the laryngeal muscles in the assessment of corticospinal and corticobulbar excitability.

The recent history of afferent stimulation modulates corticospinal excitability

BACKGROUND

Transcranial magnetic stimulation (TMS) is widely used to probe corticospinal excitability and fast sensorimotor integration in the primary motor hand area (M1-HAND). A conditioning electrical stimulus, applied to the contralateral hand, can suppress the motor evoked potential (MEP) elicited by TMS of M1-HAND when the afferent stimulus arrives in M1-HAND at the time of TMS. The magnitude of this short-latency afferent inhibition (SAI) is expressed as the ratio between the conditioned and unconditioned MEP amplitude.

OBJECTIVE/HYPOTHESIS

We hypothesized that corticospinal excitability and SAI are influenced by the recent history of peripheral electrical stimulation.

METHODS

In twenty healthy participants, we recorded MEPs from the right first dorsal interosseus muscle. MEPs were evoked by single-pulse TMS of the left M1-HAND alone (unconditioned TMS) or by TMS preceded by electrical stimulation of the right index finger ("homotopic" conditioning) or little finger ("heterotopic" conditioning). The three conditions were either pseudo-randomly intermixed or delivered in blocks in which a single condition was repeated five or ten times. MEP amplitudes and SAI magnitudes were compared using linear mixed-effect models and one-way ANOVAs.

RESULTS

All stimulation protocols consistently produced SAI, which was stronger after homotopic stimulation. Randomly intermingling the three stimulation conditions reduced the relative magnitude of homotopic and heterotopic SAI as opposed to blocked stimulation. The apparent attenuation of SAI was caused by a suppression of the unconditioned but not the conditioned MEP amplitude during the randomly intermixed pattern.

CONCLUSION(S)

The recent history of afferent stimulation modulates corticospinal excitability. This "history effect" impacts on the relative magnitude of SAI depending on how conditioned and unconditioned responses are intermixed and needs to be taken into consideration when probing afferent inhibition and corticospinal excitability.

Cortical excitability and plasticity in Alzheimer's disease and mild cognitive impairment: A systematic review and meta-analysis of transcranial magnetic stimulation studies

BACKGROUND

Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation technique. When stimulation is applied over the primary motor cortex and coupled with electromyography measures, TMS can probe functions of cortical excitability and plasticity in vivo. The purpose of this meta-analysis is to evaluate the utility of TMS-derived measures for differentiating patients with Alzheimer's disease (AD) and mild cognitive impairment (MCI) from cognitively normal older adults (CN).

METHODS

Databases searched included PubMed, Embase, APA PsycInfo, Medline, and CINAHL Plus from inception to July 2021.

RESULTS

Sixty-one studies with a total of 2728 participants (1454 patients with AD, 163 patients with MCI, and 1111 CN) were included. Patients with AD showed significantly higher cortical excitability, lower cortical inhibition, and impaired cortical plasticity compared to the CN cohorts. Patients with MCI exhibited increased cortical excitability and reduced plasticity compared to the CN cohort. Additionally, lower cognitive performance was significantly associated with higher cortical excitability and lower inhibition. No seizure events due to TMS were reported, and the mild adverse response rate is approximately 3/1000 (i.e., 9/2728).

CONCLUSIONS

Findings of our meta-analysis demonstrate the potential of using TMS-derived cortical excitability and plasticity measures as diagnostic biomarkers and therapeutic targets for AD and MCI.