The influence of sensory afferent input on local motor cortical excitatory circuitry in humans

Effect of Intermittent Theta Burst Stimulation Dual-Target Stimulation on Lower Limb Function in Patients with Incomplete Spinal Cord Injury: A Randomized, Single-Blind, Sham-Controlled Study

OBJECTIVE

To explore the influence of intermittent theta burst stimulation (iTBS) dual-target stimulation on lower limb function in patients with incomplete spinal cord injury (iSCI).

METHODS

A randomized, single -blind, sham-controlled trial was used in this study. Thirty iSCI patients with lower limb dysfunction meeting the inclusion criteria were randomly divided into a sham group and an iTBS group, with 15 cases in each group. The iTBS group received conventional rehabilitation therapy combined with iTBS dual-target stimulation on the central cerebral sulcus and the nerve root of the spinal cord injury segment. The sham group was treated with conventional rehabilitation therapy combined with iTBS dual-target sham stimulation therapy. Comprehensive functional assessment was performed on all patients before treatment, on the day 3 and day 21 of treatment. The main evaluation indicators were as follows: amplitude and latency of motor-evoked potential (MEP) in the anterior tibial muscles of both lower limbs, latency of sensory-evoked potential (SEP) of both lower limbs, knee flexor strength and knee extensor strength, lower extremity motor score (LEMS), lower extremity sensory score, spinal cord independence measure (SCIM) score, and gait parameters (stride speed, stride frequency, stride length, and ground reaction force).

RESULTS

On day 21 of treatment, in the iTBS group, the MEP amplitude of the anterior tibial muscles increased, the latency of MEP shortened, knee flexor strength and knee extensor strength increased, and the LEMS and SCIM score of both lower limbs increased. In addition, there were statistically significant differences in the muscle strength of the knee flexion muscle, knee extensor muscle, MEP amplitude, LEMS, and SCIM between the 2 groups (P < 0.05). Among the 10 patients who could walk with an assisted walker, the step length and step frequency of the iTBS group were increased compared with the sham group after treatment (P < 0.01), and the ground reaction force was increased (P < 0.05). There was no significant difference in the lower extremity sensory score of the lower limbs between the 2 groups (P > 0.05).

CONCLUSIONS

ITBS dual-target stimulation can significantly improve the motor function of both lower limbs in patients with iSCI but does not significantly improve the sensory function of both lower limbs. Therefore, this treatment mode may participate in the reconstruction and repair of some nerve circuits in patients with iSCI. In addition, iTBS dual-target stimulation can improve the ability of iSCI patients to perform daily living.

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, relying on both visual and proprioceptive cues. When these cues conflict, the brain recalibrates its unimodal estimates, shifting them closer together to compensate. Converging lines of evidence from research in perception, behavior, and neurophysiology suggest that such updates to body representation must be communicated to the motor system to keep hand movements accurate. We hypothesized that primary somatosensory cortex (S1) plays a crucial role in conveying the proprioceptive aspects of the updated body representation to the motor system. 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 S1 activity with repetitive transcranial magnetic stimulation (TMS) should affect variance and recalibration associated with the proprioceptive estimate of hand position, but have no effect on the visual estimate (Expt. 2). Our results are consistent with these predictions, supporting the idea that (1) S1 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, upstream of 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.

High-frequency magnetic paired associated stimulation promotes motor function recovery in ischemic stroke patients: a study protocol for single-center, sham stimulation randomized controlled trials (H2MPAS)

BACKGROUND

Numerous studies have validated the clinical effectiveness of electromagnetic pairing-associated stimulation. Building upon this foundation, we have developed a novel approach involving high-frequency magnetic paired-associated stimulation, aiming to enhance clinical applicability and potentially improve efficacy. However, the clinical effectiveness of this approach remains unclear. Our objective is to demonstrate the therapeutic efficacy of this novel approach by employing high-frequency pairing to intervene in patients experiencing motor dysfunction following a stroke.

METHODS

This is a single-center, single-blind, sham stimulation controlled clinical trial involving patients with upper limb motor dysfunction post-stroke. The intervention utilizes paired magnetic stimulation, combining peripheral and central magnetic stimulation, in patients with Brunnstrom stage III-V stroke lasting from 3 months to 1 year. Evaluation of patients' upper limb motor function occurred before the intervention and after 3 weeks of intervention. Follow-up visits will be conducted after 5 weeks and 3 months of intervention. The primary outcome measure is the Action Research Arm Test, with secondary measures including the Fugl-Meyer Assessment-upper, Modified Barthel Index, modified Tardieu scale, functional near-infrared spectroscopy, and neuroelectrophysiology.

DISCUSSION

The high-frequency magnetic paired associative stimulation used in this study combined high-frequency magnetic stimulation with paired stimulation, potentially facilitating both cortical excitation through high-frequency stimulation and specific circuit enhancement through paired stimulation. As dual-coil magnetic stimulation equipment becomes increasingly popular, magnetic-magnetic paired associated stimulation may offer patients improved clinical outcomes at reduced costs.

TRIAL REGISTRATION

Chinese Clinical Trial Registry,ChiCTR2400083363. Registered on 23 April 2024.

Decreased short-latency afferent inhibition in individuals with mild cognitive impairment: A TMS-EEG study

TMS combined with EEG (TMS-EEG) is a tool to characterize the neurophysiological dynamics of the cortex. Among the TMS paradigms, short-latency afferent inhibition (SAI) allows the investigation of inhibitory effects mediated by the cholinergic system. The aim of this study was to compare cholinergic function in the DLPFC between individuals with mild cognitive impairment (MCI) and healthy controls (HC) using TMS-EEG with the SAI paradigm. In this study, 30 MCI and 30 HC subjects were included. The SAI paradigm consisted of 80 single pulse TMS and 80 SAI stimulations applied to the left DLPFC. N100 components, global mean field power (GMFP) and total power were calculated. As a result, individuals with MCI showed reduced inhibitory effects on N100 components and GMFP at approximately 100 ms post-stimulation and on β-band activity at 200 ms post-stimulation compared to HC. Individuals with MCI showed reduced SAI, suggesting impaired cholinergic function in the DLPFC compared to the HC group. We conclude that these findings underscore the clinical applicability of the TMS-EEG method as a powerful tool for assessing cholinergic function in individuals with MCI.

Effects of sensory afferent input on motor cortex excitability of agonist and antagonist muscles

In this study, we aimed to analyze control mechanisms of short-latency afferent inhibition (SAI) during motor output exertion from an agonist or antagonist muscle. The motor task involved index finger abduction (agonist) and adduction (antagonist). In Experiment 1, motor-evoked potentials (MEPs) were recorded from the first dorsal interosseous (FDI) muscle with and without SAI at three output force levels. In Experiment 2, MEPs were recorded with and without SAI at various time points immediately before the muscle output. Experiment 1 showed that inhibition decreased with an increase in muscle output in the agonist muscle but increased in the antagonist muscle. Experiment 2 showed a decreasing trend of inhibition in the agonist muscle immediately before contraction but showed no significant change in the antagonist muscle. MEPs without electrical stimulation during the reaction time increased in both directions of movement as compared to those in the resting state. These results suggest that SAI modulation strongly influences smooth motor output. Analyzing the inhibitory or enhanced mechanisms during the performance of motor output by SAI in patients with motor impairment and comparing them with the mechanisms seen in healthy participants will improve our understanding of the neurophysiological mechanisms relevant to various situations (e.g., rehabilitation and sports).

Prevalence of primary headaches in multiple sclerosis patients

BACKGROUND

Multiple sclerosis (MS) is the most common immune-mediated inflammatory disease of the central nervous system. It is characterized by symptoms such as visual disturbances, paresis with spasticity, paresthesia, numbness, and fatigue. However, several studies have shown a high prevalence of headaches in individuals with MS. Migraine and tension-type headaches are the most frequent types of headaches experienced by those with MS. Additionally, the role of MS disease-modifying agents must be considered. These agents have different modes of action and side effect profiles, and their use may sometimes trigger headaches in patients with MS.

OBJECTIVES

This study aimed to explore the prevalence and clinical characteristics of primary headaches in MS patients. The relationship between headache and clinical features of MS (Course of MS, duration, EDSS, brain imaging and DMD) are also investigated.

SUBJECTS AND METHODS

Two hundred and eighty-one MS patients diagnosed according to according to the 2017 revisions to the McDonald Criteria were included in the study. Data was collected from the MS unit medical records and from the interview with the patients. Patients with reported headaches are asked to recall their headache characteristics and patterns using an interviewer administered Arabic language-structured validated questionnaire.

RESULTS

The median age of patients was 33 years old, with a range of 22-55. Tension-type headache (TTH) was more common in males, patients with more severe disability (EDSS ≥ 3), and those with SPMS and PPMS phenotypes. Additionally, patients on rituximab or cyclophosphamide therapy were more likely to have TTH. On the other hand, females, patients with milder disability (EDSS < 3), and those with RRMS phenotype were more likely to have migraine. This was also true for patients with MRI lesions involving the periaqueductal gray, and those receiving INF or fingolimod (P < 0.05). Periaqueductal gray matter lesions were found in the MRI of 48 patients (40 %) who experienced headaches on more than 10 days per month. Sensorimotor lesions in the brain were found in 55 patients (53.4 %) with severe headaches (p-value < 0.001). Interferons were associated with an increased risk of worsening preexisting headaches and the appearance of de novo headaches related to its intake (odds ratio: 2.84, 3.72; relative risk: 1.63, 2.04; p-value = 0.03, < 0.001, respectively). On the other hand, rituximab was associated with a decreased risk of worsening preexisting headaches and the appearance of de novo headaches related to its intake (odds ratio: 0.04, 0.09; relative risk: 0.11, 0.18; p-value = < 0.001, < 0.001, respectively).

CONCLUSION

Primary headaches are a common occurrence in patients with MS. Migraines and tension-type headaches (TTH) are among the most prevalent types. It has been observed that interferon can exacerbate preexisting headaches and even cause new ones. Additionally, the location of MS plaques may play a role in the frequency and severity of headaches.

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.

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.

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.

Case report: High-frequency repetitive transcranial magnetic stimulation for treatment of hereditary spastic paraplegia type 11

Hereditary spastic paraplegia (HSP) is a heterogeneous group of inherited neurodegenerative disorders that currently have no cure. HSP type 11 (SPG11-HSP) is a complex form carrying mutations in the SPG11 gene. Neuropathological studies demonstrate that motor deficits in these patients are mainly attributed to axonal degeneration of the corticospinal tract (CST). Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive technique that can induce central nervous system plasticity and promote neurological recovery by modulating the excitability of cortical neuronal cells. Although rTMS is expected to be a therapeutic tool for neurodegenerative diseases, no previous studies have applied rTMS to treat motor symptoms in SPG11-HSP. Here, we report a case of SPG11-HSP with lower extremity spasticity and gait instability, which were improved by applying high-frequency rTMS (HF-rTMS) at the primary motor cortex (M1). Clinical and physiological features were measured throughout the treatment, including the Modified Ashworth Scale (MAS), Berg Balance Scale (BBS), the timed up and go (TUG) test and the 10-meter walk test time (10 MWT). The structure and excitability of the CST were assessed by diffusion tensor imaging (DTI) and transcranial magnetic stimulation (TMS), respectively. After treatment, the patient gained 17 points of BBS, along with a gradual decrease in MAS scores of the bilateral lower extremity. In addition, the TUG test and 10 MWT improved to varying degrees. TMS assessment showed increased motor evoked potential (MEP) amplitude, decreased resting motor threshold (RMT), decreased central motor conduction time (CMCT), and decreased difference in the cortical silent period (CSP) between bilateral hemispheres. Using the DTI technique, we observed increased fractional anisotropy (FA) values and decreased mean diffusivity (MD) and radial diffusivity (RD) values in the CST. It suggests that applying HF-rTMS over the bilateral leg area of M1 (M1-LEG) is beneficial for SPG11-HSP. In this study, we demonstrate the potential of rTMS to promote neurological recovery from both functional and structural perspectives. It may provide a clinical rationale for using rTMS in the rehabilitation of HSP patients.

Sensorimotor control of object manipulation following middle cerebral artery (MCA) stroke

Methods for assessing the loss of hand function post-stroke examine limited aspects of motor performance and are not sensitive to subtle changes that can cause deficits in everyday object manipulation tasks. Efficiently lifting an object entails a prediction of required forces based on intrinsic features of the object (sensorimotor integration), short-term updates in the forces required to lift objects that are poorly predicted (sensorimotor memory), as well as the ability to modulate distal fingertip forces, which are not measured by existing assessment tools used in clinics for both diagnostic and rehabilitative purposes. The presented research examined these three components of skilled object manipulation in 60 chronic, unilateral middle cerebral artery stroke participants. Performance was compared to age-matched control participants, and linear regressions were used to predict performance based on clinical scores. Most post-stroke participants performed below control levels in at least one of the tasks. Post-stroke participants presented with combinations of deficits in each of the tasks performed, regardless of the hemisphere damaged by the stroke. Surprisingly, the ability to modulate distal forces was impaired in those patients with damage ipsilateral (right hemisphere) to the hand being used. Sensorimotor integration was also impaired in patients with right hemisphere damage, though they performed at control levels in later lifts, whereas left-hemisphere-damaged patients did not. Lastly, during a task requiring sensorimotor memory, neither patient group performed outside of control ranges on initial lifts, with patients with right hemisphere damage showing impaired performance in later lifts suggesting they were unable to learn the mapping novel mapping of color and mass of the objects. The presented research demonstrates unilateral MCA stroke patients can have deficits in one or more components required for the successful manipulation of hand-held objects and that skillful object lifting requires intact bilateral systems. Further, this information may be used in future studies to aid efforts that target rehabilitation regimens to a stroke survivor's specific pattern of deficits.

Transcranial magnetic stimulation of the brain: What is stimulated? - A consensus and critical position paper

Transcranial (electro)magnetic stimulation (TMS) is currently the method of choice to non-invasively induce neural activity in the human brain. A single transcranial stimulus induces a time-varying electric field in the brain that may evoke action potentials in cortical neurons. The spatial relationship between the locally induced electric field and the stimulated neurons determines axonal depolarization. The induced electric field is influenced by the conductive properties of the tissue compartments and is strongest in the superficial parts of the targeted cortical gyri and underlying white matter. TMS likely targets axons of both excitatory and inhibitory neurons. The propensity of individual axons to fire an action potential in response to TMS depends on their geometry, myelination and spatial relation to the imposed electric field and the physiological state of the neuron. The latter is determined by its transsynaptic dendritic and somatic inputs, intrinsic membrane potential and firing rate. Modeling work suggests that the primary target of TMS is axonal terminals in the crown top and lip regions of cortical gyri. The induced electric field may additionally excite bends of myelinated axons in the juxtacortical white matter below the gyral crown. Neuronal excitation spreads ortho- and antidromically along the stimulated axons and causes secondary excitation of connected neuronal populations within local intracortical microcircuits in the target area. Axonal and transsynaptic spread of excitation also occurs along cortico-cortical and cortico-subcortical connections, impacting on neuronal activity in the targeted network. Both local and remote neural excitation depend critically on the functional state of the stimulated target area and network. TMS also causes substantial direct co-stimulation of the peripheral nervous system. Peripheral co-excitation propagates centrally in auditory and somatosensory networks, but also produces brain responses in other networks subserving multisensory integration, orienting or arousal. The complexity of the response to TMS warrants cautious interpretation of its physiological and behavioural consequences, and a deeper understanding of the mechanistic underpinnings of TMS will be critical for advancing it as a scientific and therapeutic tool.

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.

Influence of BDNF Val66Met polymorphism on excitatory-inhibitory balance and plasticity in human motor cortex

OBJECTIVE

While previous studies showed that the single nucleotide polymorphism (Val66Met) of brain-derived neurotrophic factor (BDNF) can impact neuroplasticity, the influence of BDNF genotype on cortical circuitry and relationship to neuroplasticity remain relatively unexplored in human.

METHODS

Using individualised transcranial magnetic stimulation (TMS) parameters, we explored the influence of the BDNF Val66Met polymorphism on excitatory and inhibitory neural circuitry, its relation to I-wave TMS (ITMS) plasticity and effect on the excitatory/inhibitory (E/I) balance in 18 healthy individuals.

RESULTS

Excitatory and inhibitory indexes of neurotransmission were reduced in Met allele carriers. An E/I balance was evident, which was influenced by BDNF with higher E/I ratios in Val/Val homozygotes. Both long-term potentiation (LTP-) and depression (LTD-) like ITMS plasticity were greater in Val/Val homozygotes. LTP- but not LTD-like effects were restored in Met allele carriers by increasing stimulus intensity to compensate for reduced excitatory transmission.

CONCLUSIONS

The influence of BDNF genotype may extend beyond neuroplasticity to neurotransmission. The E/I balance was evident in human motor cortex, modulated by BDNF and measurable using TMS. Given the limited sample, these preliminary findings warrant further investigation.

SIGNIFICANCE

These novel findings suggest a broader role of BDNF genotype on neurocircuitry in human motor cortex.

Association of short- and long-latency afferent inhibition with human behavior

Transcranial magnetic stimulation (TMS) paired with nerve stimulation evokes short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI), which are non-invasive assessments of the excitability of the sensorimotor system. SAI and LAI are abnormally reduced in various special populations in comparison to healthy controls. However, the relationship between afferent inhibition and human behavior remains unclear. The purpose of this review is to survey the current literature and synthesize observations and patterns that affect the interpretation of SAI and LAI in the context of human behavior. We discuss human behaviour across the motor and cognitive domains, and in special and control populations. Further, we discuss future considerations for research in this field and the potential for clinical applications. By understanding how human behavior is mediated by changes in SAI and LAI, this can allow us to better understand the neurophysiological underpinnings of human motor control.

Influence of Brain-Derived Neurotrophic Factor Genotype on Short-Latency Afferent Inhibition and Motor Cortex Metabolites

The Met allele of the brain-derived neurotrophic factor (BDNF) gene confers reduced cortical BDNF expression and associated neurobehavioral changes. BDNF signaling influences the survival, development, and synaptic function of cortical networks. Here, we compared gamma-aminobutyric acid (GABA)ergic network activity in the human primary motor cortex (M1) between the Met (Val/Met and Met/Met) and non-Met (Val/Val) genotype groups. Short- and long-interval intracortical inhibition, short-latency afferent inhibition (SAI), and long-latency afferent inhibition were measured using transcranial magnetic stimulation (TMS) as indices of GABAergic activity. Furthermore, the considerable inter-individual variability in inhibitory network activity typically measured by TMS may be affected not only by GABA but also by other pathways, including glutamatergic and cholinergic activities; therefore, we used 3-T magnetic resonance spectroscopy (MRS) to measure the dynamics of glutamate plus glutamine (Glx) and choline concentrations in the left M1, left somatosensory cortex, and right cerebellum. All inhibitory TMS conditions produced significantly smaller motor-evoked potentials than single-pulses. SAI was significantly stronger in the Met group than in the Val/Val group. Only the M1 Glx concentration was significantly lower in the Met group, while the BDNF genotype did not affect choline concentration in any region. Further, a positive correlation was observed between SAI and Glx concentrations only in M1. Our findings provide evidence that the BDNF genotype regulates both the inhibitory and excitatory circuits in human M1. In addition, lower Glx concentration in the M1 of Met carriers may alter specific inhibitory network on M1, thereby influencing the cortical signal processing required for neurobehavioral functions.

Preferential Activation of Unique Motor Cortical Networks With Transcranial Magnetic Stimulation: A Review of the Physiological, Functional, and Clinical Evidence

OBJECTIVES

The corticospinal volley produced by application of transcranial magnetic stimulation (TMS) over primary motor cortex consists of a number of waves generated by trans-synaptic input from interneuronal circuits. These indirect (I)-waves mediate the sensitivity of TMS to cortical plasticity and intracortical excitability and can be assessed by altering the direction of cortical current induced by TMS. While this methodological approach has been conventionally viewed as preferentially recruiting early or late I-wave inputs from a given populations of neurons, growing evidence suggests recruitment of different neuronal populations, and this would strongly influence interpretation and application of these measures. The aim of this review is therefore to consider the physiological, functional, and clinical evidence for the independence of the neuronal circuits activated by different current directions.

MATERIALS AND METHODS

To provide the relevant context, we begin with an overview of TMS methodology, focusing on the different techniques used to quantify I-waves. We then comprehensively review the literature that has used variations in coil orientation to investigate the I-wave circuits, grouping studies based on the neurophysiological, functional, and clinical relevance of their outcomes.

RESULTS

Review of the existing literature reveals significant evidence supporting the idea that varying current direction can recruit different neuronal populations having unique functionally and clinically relevant characteristics.

CONCLUSIONS

Further research providing greater characterization of the I-wave circuits activated with different current directions is required. This will facilitate the development of interventions that are able to modulate specific intracortical circuits, which will be an important application of TMS.

Role of cutaneous and proprioceptive inputs in sensorimotor integration and plasticity occurring in the facial primary motor cortex

KEY POINTS

Previous studies investigating the effects of somatosensory afferent inputs on cortical excitability and neural plasticity often used transcranial magnetic stimulation (TMS) of hand motor cortex (M1) as a model, but in this model it is difficult to separate out the relative contribution of cutaneous and muscle afferent input to each effect. In the face, cutaneous and muscle afferents are segregated in the trigeminal and facial nerves, respectively. We studied their relative contribution to corticobulbar excitability and neural plasticity in the depressor anguli oris M1. Stimulation of trigeminal afferents induced short-latency (SAI) but not long-latency (LAI) afferent inhibition of face M1, while facial nerve stimulation evoked LAI but not SAI. Plasticity induction was observed only after a paired associative stimulation protocol using the facial nerve. Physiological differences in effects of cutaneous and muscle afferent inputs on face M1 excitability suggest they play separate functional roles in behaviour.

ABSTRACT

The lack of conventional muscle spindles in face muscles raises the question of how sensory input from the face is used to control muscle activation. In 16 healthy volunteers, we probed sensorimotor interactions in face motor cortex (fM1) using short-afferent inhibition (SAI), long-afferent inhibition (LAI) and LTP-like plasticity following paired associative stimulation (PAS) in the depressor anguli oris muscle (DAO). Stimulation of low threshold afferents in the trigeminal nerve produced a clear SAI (P < 0.05) when the interval between trigeminal stimulation and transcranial magnetic stimulation (TMS) of fM1 was 15-30 ms. However, there was no evidence for LAI at longer intervals of 100-200 ms, nor was there any effect of PAS. In contrast, facial nerve stimulation produced significant LAI (P < 0.05) as well as significant facilitation 10-30 minutes after PAS (P < 0.05). Given that the facial nerve is a pure motor nerve, we presume that the afferent fibres responsible were those activated by the evoked muscle twitch. The F-wave in DAO was unaffected during both LAI and SAI, consistent with their presumed cortical origin. We hypothesize that, in fM1, SAI is evoked by activity in low threshold, presumably cutaneous afferents, whereas LAI and PAS require activity in (higher threshold) afferents activated by the muscle twitch evoked by electrical stimulation of the facial nerve. Cutaneous inputs may exert a paucisynaptic inhibitory effect on fM1, while proprioceptive information is likely to target inhibitory and excitatory polysynaptic circuits involved in LAI and PAS. Such information may be relevant to the physiopathology of several disorders involving the cranio-facial system.

Region-dependent bidirectional plasticity in M1 following quadripulse transcranial magnetic stimulation in the inferior parietal cortex

BACKGROUND

The ability to manipulate the excitability of the network between the inferior parietal lobule (IPL) and primary motor cortex (M1) may have clinical value.

OBJECTIVE

To investigate the possibility of inducing long-lasting changes in M1 excitability by applying quadripulse transcranial magnetic stimulation (QPS) to the IPL, and to ascertain stimulus condition- and site-dependent differences in the effects.

METHODS

QPS was applied to M1, the primary somatosensory cortex (S1), the supramarginal gyrus (SMG) and angular gyrus (AG) IPL areas, with the inter-stimulus interval (ISI) in the train of pulses set to either 5 ms (QPS-5) or 50 ms (QPS-50). QPS was repeated at 0.2 Hz for 30 min, or not presented (sham condition). Excitability changes in the target site were examined by means of single-pulse transcranial magnetic stimulation (TMS).

RESULTS

QPS-5 and QPS-50 at M1 increased and decreased M1 excitability, respectively. QPS at S1 induced no obvious change in M1 excitability. However, QPS at the SMG induced mainly suppressive effects in M1 for at least 30 min, regardless of the ISI length. Both QPS ISIs at the AG yielded significantly different MEP compared to those at the SMG. Thus, the direction of the plastic effect of QPS differed depending on the site, even under the same stimulation conditions.

CONCLUSIONS

QPS at the IPL produced long-lasting changes in M1 excitability, which differed depending on the precise stimulation site within the IPL. These results raise the possibility of noninvasive induction of functional plasticity in M1 via input from the IPL.

Putting the "Sensory" Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke

Integration of sensory and motor information is one-step, among others, that underlies the successful production of goal-directed hand movements necessary for interacting with our environment. Disruption of sensorimotor integration is prevalent in many neurologic disorders, including stroke. In most stroke survivors, persistent paresis of the hand reduces function and overall quality of life. Current rehabilitative methods are based on neuroplastic principles to promote motor learning that focuses on regaining motor function lost due to paresis, but the sensory contributions to motor control and learning are often overlooked and currently understudied. There is a need to evaluate and understand the contribution of both sensory and motor function in the rehabilitation of skilled hand movements after stroke. Here, we will highlight the importance of integration of sensory and motor information to produce skilled hand movements in healthy individuals and individuals after stroke. We will then discuss how compromised sensorimotor integration influences relearning of skilled hand movements after stroke. Finally, we will propose an approach to target sensorimotor integration through manipulation of sensory input and motor output that may have therapeutic implications.

Effects of bilateral priming on motor cortex function in healthy adults

Bilateral priming is a rehabilitation adjuvant that can improve upper limb motor recovery poststroke. It uses a table-top device to couple the upper limbs together such that active flexion and extension of one wrist leads to passive movement of the opposite wrist in a mirror symmetric pattern. Bilateral priming increases corticomotor excitability (CME) in the primary motor cortex (M1) of the passively driven wrist; however, the neurophysiological mechanisms underlying this increase remain unclear. This study explored these mechanisms by using transcranial magnetic stimulation over the right M1 and recording motor-evoked potentials from the passively driven left extensor carpi radialis of healthy adults. Intracortical measures were recorded before and 5 and 35 min after a single 15-min session of priming. One-millisecond short-interval intracortical inhibition, long-interval intracortical inhibition, late cortical disinhibition (LCD), and intracortical facilitation were recorded with a posterior-anterior (PA) intracortical current, whereas CME and short-interval intracortical facilitation (SICF) were recorded with both PA and anterior-posterior (AP) currents. CME with PA stimulation was also recorded ~1 h postpriming. PA CME was elevated 35 min postpriming and remained elevated ~1 h postpriming. LCD decreased, and AP SICF increased at both 5 and 35 min postpriming. However, these changes in LCD and AP SICF are unlikely to be the cause of the increased PA CME because of the differing timelines of their effects and AP and PA currents activating separate interneuron circuits. These results suggest that bilateral priming does not increase CME through alterations of the intracortical circuits investigated here. NEW & NOTEWORTHY This is the first study to measure how bilateral priming modulates corticomotor excitability with posterior-anterior and anterior-posterior intracortical currents, 1-ms short-interval intracortical inhibition, late cortical disinhibition, intracortical facilitation, and short-interval intracortical facilitation. We found corticomotor excitability with a posterior-anterior current increased by 35 min until ~1 h postpriming. Short-interval intracortical facilitation with an anterior-posterior current was greater for at least 35 min postpriming. This provides further insight into the neurophysiological mechanisms underlying bilateral priming.

Verbal working memory modulates afferent circuits in motor cortex

Verbal instruction and strategies informed by declarative memory are key to performance and acquisition of skilled actions. We previously demonstrated that anatomically distinct sensory-motor inputs converging on the corticospinal neurons of motor cortex are differentially sensitive to visual attention load. However, how loading of working memory shapes afferent input to motor cortex is unknown. This study used short-latency afferent inhibition (SAI) to probe the effect of verbal working memory upon anatomically distinct afferent circuits converging on corticospinal neurons in the motor cortex. SAI was elicited by preceding a suprathreshold transcranial magnetic stimulus (TMS) with electrical stimulation of the median nerve at the wrist while participants mentally rehearsed a two- or six-digit numeric memory set. To isolate different afferent intracortical circuits in motor cortex SAI was elicited, using TMS involving posterior-anterior (PA) or anterior-posterior (AP) monophasic current. Both PA and AP SAI were significantly reduced during maintenance of the six-digit compared to two-digit memory set. The generalized effect of working memory across anatomically distinct circuits converging upon corticospinal neurons in motor cortex is in contrast to the specific sensitivity of AP SAI to increased attention load. The common response across the PA and AP SAI circuits to increased working memory load may reflect an indiscriminate perisomatic mechanism involved in the voluntary facilitation of desired and/or suppression of unwanted actions during action selection or response conflict.

Somatosensory Inputs Induced by Passive Movement Facilitate Primary Motor Cortex Excitability Depending on the Interstimulus Interval, Movement Velocity, and Joint Angle

Somatosensory inputs affect primary motor cortex (M1) excitability; however, the effect of movement-induced somatosensory inputs on M1 excitability is unknown. This study examined whether M1 excitability is modulated by somatosensory inputs with passive movement in 29 healthy subjects. Motor-evoked potentials (MEPs), elicited by transcranial magnetic stimulation (TMS) were recorded from the first dorsal interosseous (FDI) muscle (Experiment 1). M- and F-waves were measured from the FDI muscle (Experiment 2). Passive movements of the index finger were performed in the adduction direction. TMS pulses were preceded by starting passive movements with interstimulus intervals (ISIs) of 30, 60, 90, 120, 150, 180, and 210 ms. TMS or electrical stimulation was performed in the midrange of the metacarpophalangeal joint during passive movements. MEPs were significantly facilitated at 90, 120, and 150 ms (p < 0.05). No M- or F-wave changes were observed for any ISI. In addition, we investigated whether MEP changes were dependent on passive movement velocity and joint angle. Passive movement was performed at two movement velocities (Experiment 3) or joint angles (Experiment 4). MEP facilitation was observed depending on the movement velocities or joint angles. These experiments demonstrated that somatosensory inputs induced by passive movements facilitated M1 excitability depending on the ISIs, passive movement velocity, and joint angle.

Technical development of transcutaneous electrical nerve inhibition using medium-frequency alternating current

Background

Innovative technical approaches to controlling undesired sensory and motor activity, such as hyperalgesia or spasticity, may contribute to rehabilitation techniques for improving neural plasticity in patients with neurologic disorders. To date, transcutaneous electrical stimulation has used low frequency pulsed currents for sensory inhibition and muscle activation. Yet, few studies have attempted to achieve motor nerve inhibition using transcutaneous electrical stimulation. This study aimed to develop a technique for transcutaneous electrical nerve inhibition (TENI) using medium-frequency alternating current (MFAC) to suppress both sensory and motor nerve activity in humans.

Methods

Surface electrodes were affixed to the skin of eight young adults to stimulate the median nerve. Stimulation intensity was increased up to 50% and 100% of the pain threshold. To identify changes in sensory perception by transcutaneous MFAC (tMFAC) stimulation, we examined tactile and pressure pain thresholds in the index and middle fingers before and after stimulation at 10 kHz. To demonstrate the effect of tMFAC stimulation on motor inhibition, stimulation was applied while participants produced flexion forces with the index and middle fingers at target forces (50% and 90% of MVC, maximum voluntary contraction).

Results

tMFAC stimulation intensity significantly increased tactile and pressure pain thresholds, indicating decreased sensory perception. During the force production task, tMFAC stimulation with the maximum intensity immediately reduced finger forces by ~ 40%. Finger forces recovered immediately after stimulation cessation. The effect on motor inhibition was greater with the higher target force (90% MVC) than with the lower target (50% MVC). Also, higher tMFAC stimulation intensity provided a greater inhibition effect on both sensory and motor nerve activity.

Conclusion

We found that tMFAC stimulation immediately inhibits sensory and motor activity. This pre-clinical study demonstrates a novel technique for TENI using MFAC stimulation and showed that it can effectively inhibit both sensory perception and motor activity. The proposed technique can be combined with existing rehabilitation devices (e.g., a robotic exoskeleton) to inhibit undesired sensorimotor activities and to accelerate recovery after neurologic injury.

Reduced Short-Latency Afferent Inhibition in Prefrontal but not Motor Cortex and Its Association With Executive Function in Schizophrenia: A Combined TMS-EEG Study

BACKGROUND

Cholinergic dysfunction is increasingly assumed to be involved in the pathophysiology of schizophrenia. Short-latency afferent inhibition (SAI) is a transcranial magnetic stimulation (TMS) paradigm that has been shown to assay central cholinergic activity from the motor cortex (M1). Recently, we established a method to index SAI from the dorsolateral prefrontal cortex (DLPFC), an area implicated in the pathophysiology of schizophrenia. We investigated SAI in M1 and DLPFC in schizophrenia. We hypothesized that modulation of N100 on TMS-evoked potentials (TEPs) from the DLPFC would be attenuated in patients with schizophrenia compared to healthy controls.

METHODS

SAI was examined in 12 patients, whose age was matched to controls, using TMS combined with electroencephalography (EEG). SAI was recorded with TMS applied to left M1 (M1-SAI) and DLPFC (DLPFC-SAI). For group comparison, we used the SAI data of healthy participants in our previous study.

RESULTS

In patients, N100 TEP was significantly attenuated with DLPFC-SAI, whereas P180 TEP was significantly increased with M1-SAI. Between patients and controls, there were significant differences in modulation of P180 TEP by M1-SAI (t22 = -2.748, P = .012; patients > controls) and N100 TEP by DLPFC-SAI (t22 = 5.456, P < .0001; patients < controls). Further, modulation of N100 TEP by DLPFC-SAI significantly correlated with executive function (r = -.740, P = .006, N = 12).

CONCLUSION

Our findings suggest that DLPFC-SAI but not M1-SAI were reduced in patients with schizophrenia and this was linked to deficits in cognition. This may reflect prefrontal cholinergic deficits and represent a biomarker for cholinergic and executive dysfunction in patients with schizophrenia.

Changes in cortical excitability during paired associative stimulation in Parkinson's disease patients and healthy subjects

Paired associative stimulation (PAS) combines repetitive peripheral nerve stimulation with motor cortex (M1) transcranial magnetic stimulation (TMS), to induce plastic-like changes of cortical excitability. While much attention has been dedicated to post-PAS effects little is known about processes during PAS. We compared the time-course of changes in M1 excitability during standard facilitatory PAS intervention among patients with Parkinson's disease (PD), known to have diminished post-PAS response, and healthy subjects. Compared to baseline pre-PAS MEPs, conditioned MEPs during PAS decreased significantly in both groups. The decrease was significantly larger in healthy subjects than in PD patients, regardless whether patients were drug-naïve or not. Although post-PAS excitability increase was also larger in healthy subjects than in PD patients, there was no significant correlation between the two phenomena, i.e. the extent of MEP decrease during PAS and the extent of the post-PAS excitability increase. The results highlight an apparent physiological paradox that repetitive application of an inhibitory stimulation pattern leads to subsequent prolonged facilitation, thus broadening the understanding of the phenomenology of PAS response. Results also suggest that in PD cortical circuits involved in conveying inhibition during PAS, are impaired at the clinical onset of the disease and are not influenced by subsequent PD treatment.

Reduced Prefrontal Short-Latency Afferent Inhibition in Older Adults and Its Relation to Executive Function: A TMS-EEG Study

Combining transcranial magnetic stimulation (TMS) with electroencephalography (EEG) allows for the assessment of various neurophysiological processes in the human cortex. One of these paradigms, short-latency afferent inhibition (SAI), is thought to be a sensitive measure of cholinergic activity. In a previous study, we demonstrated the temporal pattern of this paradigm from both the motor (M1) and dorsolateral prefrontal cortex (DLPFC) using simultaneous TMS-EEG recording. The SAI paradigm led to marked modulations at N100. In this study, we aimed to investigate the age-related effects on TMS-evoked potentials (TEPs) with the SAI from M1 and the DLPFC in younger (18-59 years old) and older (≥60 years old) participants. Older participants showed significantly lower N100 modulation in M1-SAI as well as DLPFC-SAI compared to the younger participants. Furthermore, the modulation of N100 by DLPFC-SAI in the older participants correlated with executive function as measured with the Trail making test. This paradigm has the potential to non-invasively identify cholinergic changes in cortical regions related to cognition in older participants.

Modulation of quadriceps corticospinal excitability by femoral nerve stimulation

INTRODUCTION

We explored the conditioning effect of a percutaneous electrical pulse of the femoral nerve on cortical motor evoked responses in the rectus femoris muscle.

METHODS

Corticospinal excitability of rectus femoris muscle was measured in sixteen healthy subjects, when a single transcranial magnetic pulse was preceded by an electrical femoral nerve stimulus, using twelve inter-stimulus intervals (from 10 to 275ms). We also evaluated the effects of the intensities of the transcranial magnetic and of the electrical pulses.

RESULTS

Quadriceps motor evoked potentials were inhibited and facilitated when a single femoral nerve electrical stimulus was delivered at inter-stimulus intervals of 25ms and 150ms, respectively. The facilitation was reduced when low electrical intensity was used, while the inhibition decreased with high intensity transcranial magnetic pulse.

CONCLUSION

Afferent inputs of a femoral stimulation modulate the responses elicited by transcranial magnetic pulses of the contralateral quadriceps motor cortex. This modulation indicates a sensorimotor integration of proximal lower limb muscles that may be mediated via different types of afferents. This could be of relevance for studies that explore the role of lower limb muscles in postural control and balance.

Paired Stimulation to Promote Lasting Augmentation of Corticospinal Circuits

After injury, electrical stimulation of the nervous system can augment plasticity of spared or latent circuits through focal modulation. Pairing stimulation of two parts of a spared circuit can target modulation more specifically to the intended circuit. We discuss 3 kinds of paired stimulation in the context of the corticospinal system, because of its importance in clinical neurorehabilitation. The first uses principles of Hebbian plasticity: by altering the stimulation timing of presynaptic neurons and their postsynaptic targets, synapse function can be modulated up or down. The second form uses synchronized presynaptic inputs onto a common synaptic target. We dub this a “convergent” mechanism, because stimuli have to converge on a common target with coordinated timing. The third form induces focal modulation by tonic excitation of one region (e.g., the spinal cord) during phasic stimulation of another (e.g., motor cortex). Additionally, endogenous neural activity may be paired with exogenous electrical stimulation. This review addresses what is known about paired stimulation of the corticospinal system of both humans and animal models, emphasizes how it qualitatively differs from single-site stimulation, and discusses the gaps in knowledge that must be addressed to maximize its use and efficacy in neurorehabilitation.

Pulse Duration as Well as Current Direction Determines the Specificity of Transcranial Magnetic Stimulation of Motor Cortex during Contraction

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Selective stimulation of inputs to corticospinal neurons may be achieved by manipulating current direction and pulse duration.

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Neural populations recruited by brief (30 μs) anterior–posterior currents exhibited the greatest sensitivity to somatosensory input.

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Pulse duration is an important determinant of what is activated with TMS in human motor cortex.

Background

Previous research suggested that anterior–posterior (AP) directed currents induced by TMS in motor cortex (M1) activate interneuron circuits different from those activated by posterior–anterior currents (PA). The present experiments provide evidence that pulse duration also determines the activation of specific interneuron circuits.

Objective

To use single motor unit (SMU) recordings to confirm the difference in onset latencies of motor-evoked potentials (MEPs) evoked by different current directions and pulse durations: AP₃₀, AP₁₂₀, PA₃₀ and PA₁₂₀. To test whether the amplitude of the MEPs is differentially influenced by somatosensory inputs from the hand (short-latency afferent inhibition, SAI), and examine the sensitivity of SAI to changes in cerebellar excitability produced by direct current stimulation (tDCS_Cb).

Methods

Surface electromyograms and SMUs were recorded from the first dorsal interosseous muscle. SAI was tested with an electrical stimulus to median or digital nerves ~20–25 ms prior to TMS delivered over the M1 hand area via a controllable pulse parameter TMS (cTMS) device. SAI was also tested during the application of anodal or sham tDCS_Cb. Because TMS pulse specificity is greatest at low stimulus intensities, most experiments were conducted with weak voluntary contraction to reduce stimulus threshold.

Results

AP₃₀ currents recruited the longest latency SMU and surface MEP responses. During contraction SAI was greater for AP₃₀ responses versus all other pulses. Online anodal tDCS_Cb reduced SAI for the AP₃₀ currents only.

Conclusions

AP₃₀ currents activate an interneuron circuit with functional properties different from those activated by other pulse types. Pulse duration and current direction determine what is activated in M1 with TMS.

A combined TMS-EEG study of short-latency afferent inhibition in the motor and dorsolateral prefrontal cortex

Combined transcranial magnetic stimulation and electroencephalography (TMS-EEG) enables noninvasive neurophysiological investigation of the human cortex. A TMS paradigm of short-latency afferent inhibition (SAI) is characterized by attenuation of the motor-evoked potential (MEP) and modulation of N100 of the TMS-evoked potential (TEP) when TMS is delivered to motor cortex (M1) following median nerve stimulation. SAI is a marker of cholinergic activity in the motor cortex; however, the SAI has not been tested from the prefrontal cortex. We aimed to explore the effect of SAI in dorsolateral prefrontal cortex (DLPFC). SAI was examined in 12 healthy subjects with median nerve stimulation and TMS delivered to M1 and DLPFC at interstimulus intervals (ISIs) relative to the individual N20 latency. SAI in M1 was tested at the optimal ISI of N20 + 2 ms. SAI in DLPFC was investigated at a range of ISI from N20 + 2 to N20 + 20 ms to explore its temporal profile. For SAI in M1, the attenuation of MEP amplitude was correlated with an increase of TEP N100 from the left central area. A similar spatiotemporal neural signature of SAI in DLPFC was observed with a marked increase of N100 amplitude. SAI in DLPFC was maximal at ISI N20 + 4 ms at the left frontal area. These findings establish the neural signature of SAI in DLPFC. Future studies could explore whether DLPFC-SAI is neurophysiological marker of cholinergic dysfunction in cognitive disorders.