Area 5 influences excitability within the primary motor cortex in humans

The behavioral and neural effects of parietal theta burst stimulation on the grasp network are stronger during a grasping task than at rest

Repetitive transcranial magnetic stimulation (TMS) is widely used in neuroscience and clinical settings to modulate human cortical activity. The effects of TMS on neural activity depend on the excitability of specific neural populations at the time of stimulation. Accordingly, the brain state at the time of stimulation may influence the persistent effects of repetitive TMS on distal brain activity and associated behaviors. We applied intermittent theta burst stimulation (iTBS) to a region in the posterior parietal cortex (PPC) associated with grasp control to evaluate the interaction between stimulation and brain state. Across two experiments, we demonstrate the immediate responses of motor cortex activity and motor performance to state-dependent parietal stimulation. We randomly assigned 72 healthy adult participants to one of three TMS intervention groups, followed by electrophysiological measures with TMS and behavioral measures. Participants in the first group received iTBS to PPC while performing a grasping task concurrently. Participants in the second group received iTBS to PPC while in a task-free, resting state. A third group of participants received iTBS to a parietal region outside the cortical grasping network while performing a grasping task concurrently. We compared changes in motor cortical excitability and motor performance in the three stimulation groups within an hour of each intervention. We found that parietal stimulation during a behavioral manipulation that activates the cortical grasping network increased downstream motor cortical excitability and improved motor performance relative to stimulation during rest. We conclude that constraining the brain state with a behavioral task during brain stimulation has the potential to optimize plasticity induction in cortical circuit mechanisms that mediate movement processes.

The Effects of Exercise on Synaptic Plasticity in Individuals With Mild Cognitive Impairment: Protocol for a Pilot Intervention Study

BACKGROUND

Mild cognitive impairment (MCI) is a syndrome preceding more severe impairment characterized by dementia. MCI affects an estimated 15% to 20% of people older than 65 years. Nonpharmacological interventions including exercise are recommended as part of overall MCI management based on the positive effects of exercise on cognitive performance. Interval training involves brief intermittent bouts of exercise interspersed with short recovery periods. This type of exercise promotes cognitive improvement and can be performed in individuals with MCI. Synaptic plasticity can be assessed in vivo by the neurophysiological response to repetitive transcranial magnetic stimulation (rTMS). A method to assess synaptic plasticity uses an intermittent theta burst stimulation (iTBS), which is a patterned form of rTMS. Individuals with MCI have decreased responses to iTBS, reflecting reduced synaptic plasticity. It is unknown whether interval training causes changes in synaptic plasticity in individuals living with MCI.

OBJECTIVE

This research will determine whether interval training performed using a cycle ergometer enhances synaptic plasticity in individuals with MCI. The three aims are to (1) quantify synaptic plasticity after interval training performed at a self-determined intensity in individuals with MCI; (2) determine whether changes in synaptic plasticity correlate with changes in serum brain-derived neurotrophic factor, osteocalcin, and cognition; and (3) assess participant compliance to the exercise schedule.

METHODS

24 individuals diagnosed with MCI will be recruited for assignment to 1 of the 2 equally sized groups: exercise and no exercise. The exercise group will perform exercise 3 times per week for 4 weeks. Synaptic plasticity will be measured before and following the 4-week intervention. At these time points, synaptic plasticity will be measured as the response to single-pulse TMS, reflected as the percent change in the average amplitude of 20 motor-evoked potentials before and after an iTBS rTMS protocol, which is used to induce synaptic plasticity. In addition, individuals will complete a battery of cognitive assessments and provide a blood sample from the antecubital vein to determine serum brain-derived neurotrophic factor and osteocalcin.

RESULTS

The study began in September 2023.

CONCLUSIONS

The proposed research is the first to assess whether synaptic plasticity is enhanced after exercise training in individuals with MCI. If exercise does indeed modify synaptic plasticity, this will create a new avenue by which we can study and manipulate neural plasticity in these individuals.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05663918; https://clinicaltrials.gov/study/NCT05663918.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

PRR1-10.2196/50030.

Effect of Object Texture and Weight on Ipsilateral Corticospinal Influences During Bimanual Holding in Humans

We tested the hypothesis that the ipsilateral corticospinal system, like the contralateral corticospinal system, controls the threshold muscle length at which wrist muscles and the stretch reflex begin to act during holding tasks. Transcranial magnetic stimulation was applied over the right primary motor cortex in 21 healthy subjects holding a smooth or coarse block between the hands. Regardless of the lifting force, motor evoked potentials in right wrist flexors were larger for the smooth block. This result was explained based on experimental evidence that motor actions are controlled by shifting spatial stretch reflex thresholds. Thus, the ipsilateral corticospinal system is involved in threshold position control by modulating facilitatory influences of hand skin afferents on motoneurons of wrist muscles during bimanual object manipulation.

Investigating the sub-regions of the superior parietal cortex using functional magnetic resonance imaging connectivity

Objectives

Traditionally, the superior parietal lobule (SPL) is usually investigated as one region of interest, particularly in functional magnetic resonance imaging (fMRI) studies. However, cytoarchitectonic analysis has shown that the SPL has a complex, heterogeneous topology that comprises more than seven sub-regions. Since previous studies have shown how the SPL is significantly involved in different neurological functions—such as visuomotor, cognitive, sensory, higher order, working memory and attention—this study aims to investigate whether these cytoarchitecturally different sub-regions have different functional connectivity to different functional brain networks.

Methods

This study examined 198 healthy subjects using resting-state fMRI and investigated the functional connectivity of seven sub-regions of the SPL to eight regional functional networks.

Results

The findings showed that most of the seven sub-regions were functionally connected to these targeted networks and that there are differences between these sub-regions and their functional connectivity patterns. The most consistent functional connectivity was observed with the visual and attention networks. There were also clear functional differences between Brodmann area (BA) 5 and BA7. BA5, with its three sub-regions, had strong functional connectivity to both the sensorimotor and salience networks.

Conclusion

These findings have enhanced our understanding of the functional organisations of the complexity of the SPL and its varied topology and also provide clear evidence of the functional patterns and involvements of the SPL in major brain functions.

Neurological Functional Connectivity in Unilateral Coronal Synostosis: A Side-Based Comparison

PURPOSE

Unicoronal synostosis (UCS) has been associated with reading, language, and social dysfunction. Limited brain function connectivity studies exist for UCS with none devoted to comparing outcomes by side of synostosis (left versus right-sided UCS).

METHODS

Twelve patients with surgically treated UCS, 7 right-sided and 5 left-sided, were age matched to healthy controls. Resting state functional MRI was acquired in a 3T Siemens TIM Trio scanner (Erlangen, Germany). Data was collected with intrinsic connectivity distribution and seed-connectivity analysis using BioImage Suite (Yale School of Medicine). Region of interest analysis was performed based on Brodmann areas related to emotional, executive, language, motor, and visuo-spatial function. Significance was set at P < 0.05.

RESULTS

Compared to controls, all UCS patients demonstrated decreased connectivity in areas of the parietal and temporal cortices responsible for visuo-motor coordination and language function. Right UCS patients demonstrated decreased intrinsic connectivity in regions related to complex motor movement and proprioception relative to control subjects. Left UCS patients demonstrated decreased seed connectivity between regions of the parietal lobe and occipital lobe related to motor coordination, visual function, and language compared to right UCS patients.

CONCLUSION

Unicoronal synostosis had decreased functional connectivity in regions associated with memory, visual information processing, and motor function. Moreover, left-sided UCS had decreased connectivity in circuits for motor coordination and language when compared to right-sided UCS. This study provides data suggestive of long-term sequelae of UCS that varies by sidedness, which may be responsible for neurocognitive impairments found in previous cognitive analyses.

Role of human prefrontal cortex in the modulation of conditioned eyeblink responses

Classical conditioning of the eyeblink reflex (EBC) is a simple form of associative motor learning. EBC is heavily dependent on cerebellar function, but experimental studies also suggest that the prefrontal cortex (PFC) orchestrates a neuronal network which interacts with the cerebellum to mediate the conditioned eyeblink responses (CR). To further investigate the role of PFC for EBC in humans, we aimed in this study at assessing whether acquisition of CR can be modulated by focal repetitive transcranial magnetic stimulation (rTMS) given as theta burst stimulation (TBS) over the dorsolateral PFC (DLPFC). A standard delay conditioning paradigm with a 540 ms tone as conditioned stimulus (CS) coterminating with a 100 ms air puff as unconditioned stimulus (US) was used in a total of 60 healthy subjects (35 female, 25 male, mean age 28.4 ± 2.4 years). One hundred paired CS-US trials and 30 extinction CS alone trials were given. TBS was applied over the DLPFC ipsilaterally to the US during the acquisition phase. Subjects were randomly assigned to three groups (n = 20) using excitatory intermittent TBS (iTBS), inhibitory continuous TBS (cTBS) or sham stimulation. CR acquisition was significantly enhanced by iTBS (mean total CR incidence 63.1 ± 6.5%) and significantly reduced by cTBS (13 ± 2%) compared to sham stimulation (25.1 ± 6.7%). We provide thus physiological evidence that the acquisition of this type of associative learning is critically modulated by PFC activity in humans.

Neural correlates of visuomotor adjustments during scaling of human finger movements

Visually guided finger movements include online feedback of current effector position to guide target approach. This visual feedback may be scaled or otherwise distorted by unpredictable perturbations. Although adjustments to visual feedback scaling have been studied before, the underlying brain activation differences between upscaling (visual feedback larger than real movement) and downscaling (feedback smaller than real movement) are currently unknown. Brain activation differences between upscaling and downscaling might be expected because within-trial adjustments during upscaling require corrective backwards accelerations, whereas correcting for downscaling requires forward accelerations. In this behavioural and fMRI study we investigated adjustments during up- and downscaling in a target-directed finger flexion-extension task with real-time visual feedback. We found that subjects made longer and more complete within-trial corrections for downscaling perturbations than for upscaling perturbations. The finger task activated primary motor (M1) and somatosensory (S1) areas, premotor and parietal regions, basal ganglia, and cerebellum. General scaling effects were seen in the right pre-supplementary motor area, dorsal anterior cingulate cortex, inferior parietal lobule, and dorsolateral prefrontal cortex. Stronger activations for down- than for upscaling were observed in M1, supplementary motor area (SMA), S1 and anterior cingulate cortex. We argue that these activation differences may reflect differing online correction for upscaling vs. downscaling during finger flexion-extension.

Sensorimotor Integration During Motor Learning: Transcranial Magnetic Stimulation Studies

The effect of sensory signals coming from skin and muscle afferents on the sensorimotor cortical networks is entitled as sensory-motor integration (SMI). SMI can be studied electrophysiologically by the motor cortex excitability changes in response to peripheral sensory stimulation. These changes include the periods of short afferent inhibition (SAI), afferent facilitation (AF), and late afferent inhibition (LAI). During the early period of motor skill acquisition, motor cortex excitability increases and changes occur in the area covered by the relevant zone of the motor cortex. In the late period, these give place to the morphological changes, such as synaptogenesis. SAI decreases during learning the motor skills, while LAI increases during motor activity. In this review, the role of SMI in the process of motor learning and transcranial magnetic stimulation techniques performed for studying SMI is summarized.

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

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

Neural effects of short-term training on working memory

Working memory training has been the focus of intense research interest. Despite accumulating behavioral work, knowledge about the neural mechanisms underlying training effects is scarce. Here, we show that 7 days of training on an n-back task led to substantial performance improvements in the trained task; furthermore, the experimental group showed cross-modal transfer, as compared with an active control group. In addition, there were two neural effects that emerged as a function of training: first, increased perfusion during task performance in selected regions, reflecting a neural response to cope with high task demand; second, increased blood flow at rest in regions where training effects were apparent. We also found that perfusion at rest was correlated with task proficiency, probably reflecting an improved neural readiness to perform. Our findings are discussed within the context of the available neuroimaging literature on n-back training.

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

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

Promoting plasticity in the somatosensory cortex to alter motor physiology

Somatosensory pathways and cortices contribute to the control of human movement. In humans, non-invasive transcranial magnetic stimulation techniques to promote plasticity within somatosensory pathways and cortices have revealed potent effects on the neurophysiology within motor cortices. In this mini-review, we present evidence to indicate that somatosensory cortex is positioned to influence motor cortical circuits and as such, is an ideal target for plasticity approaches that aim to alter motor physiology and behavior in clinical populations.

30 Hz theta-burst stimulation over primary somatosensory cortex modulates corticospinal output to the hand

BACKGROUND

The primary somatosensory cortex (SI) is important for hand function and has direct connectivity with the primary motor cortex (M1). Much of our present knowledge of this connectivity and its relevance to hand function is based on animal research. In humans, less is known about the neural mechanisms by which SI influences motor circuitry that outputs to the muscles controlling the hand.

OBJECTIVE

The present study investigated the influence of SI on corticospinal excitability, and inhibitory and excitatory intracortical neural circuitry within M1 before and after continuous theta-burst stimulation (cTBS). Motor-evoked potentials (MEPs), short-latency intracortical inhibition (SICI) and intracortical facilitation (ICF) were recorded from the first dorsal interosseous (RFDI) muscle of the right hand following 30 Hz cTBS over left-hemisphere SI and M1 delivered in separate sessions.

RESULTS

cTBS over SI facilitated MEPs and did not alter ICF or SICI. cTBS delivered over M1 suppressed MEPs and ICF and did not alter SICI.

CONCLUSIONS

These findings indicate that SI influences corticospinal output to the hand, possibly via corticocortical projections, and may be one mechanism by which somatosensory information influences hand control.

Current direction specificity of continuous θ-burst stimulation in modulating human motor cortex excitability when applied to somatosensory cortex

The present study examines the influence of primary somatosensory cortex (SI) on corticospinal excitability within primary motor cortex (M1) using repetitive transcranial magnetic stimulation. Two groups of subjects participated and both received continuous theta-burst stimulation (cTBS) over SI. One group received cTBS oriented to induce anterior-to-posterior (AP) followed by posterior-to-anterior (PA) current flow in the cortex and the other group received cTBS in the opposite direction (PA-AP). Motor evoked potentials (MEPs) were measured from the first dorsal interosseous muscle of the left and right hand before and at three time points (5, 25, 45 min) following cTBS over left-hemisphere SI. CTBS over SI in the AP-PA direction increased contralateral MEPs at 5 and 45 min with a near significant increase at 25 min. In contrast, PA-AP cTBS decreased contralateral MEPs at 25 min. We conclude that cTBS over SI modulates neural output directed to the hand with effects that depend on the direction of induced current.

Theta burst repetitive transcranial magnetic stimulation attenuates somatosensory evoked potentials from the lower limb

Background

Continuous theta burst stimulation (cTBS) is a form of repetitive transcranial magnetic stimulation which has been shown to alter cortical excitability in the upper limb representation of primary somatosensory cortex (SI). However, it is unknown whether cTBS modulates cortical excitability within the lower limb representation in SI. The present study investigates the effects of cTBS over the SI lower limb representation on cortical somatosensory evoked potentials (SEPs) and Hoffmann reflex (H-reflex) following tibial nerve stimulation at the knee. SEPs and H-reflex were recorded before and in four time blocks up to 30 minutes following cTBS targeting the lower limb representation within SI.

Results

Following cTBS, the P1-N1 first cortical potential was significantly decreased at 12–16 minutes. CTBS also suppressed the P2-N2 second cortical potential for up to 30 minutes following stimulation. The H-reflex remained statistically unchanged following cTBS although there was a modest suppression observed.

Conclusion

We conclude that cTBS decreases cortical excitability of the lower limb representation of SI as evidenced by suppressed SEP amplitude. The duration and magnitude of the cTBS after effects are similar to those observed in upper limb studies.

Plasticity-inducing TMS protocols to investigate somatosensory control of hand function

Hand function depends on sensory feedback to direct an appropriate motor response. There is clear evidence that somatosensory cortices modulate motor behaviour and physiology within primary motor cortex. However, this information is mainly from research in animals and the bridge to human hand control is needed. Emerging evidence in humans supports the notion that somatosensory cortices modulate motor behaviour, physiology and sensory perception. Transcranial magnetic stimulation (TMS) allows for the investigation of primary and higher-order somatosensory cortices and their role in control of hand movement in humans. This review provides a summary of several TMS protocols in the investigation of hand control via the somatosensory cortices. TMS plasticity inducing protocols reviewed include paired associative stimulation, repetitive TMS, theta-burst stimulation as well as other techniques that aim to modulate cortical excitability in sensorimotor cortices. Although the discussed techniques may modulate cortical excitability, careful consideration of experimental design is needed to isolate factors that may interfere with desired results of the plasticity-inducing protocol, specifically events that may lead to metaplasticity within the targeted cortex.