Changes in beta and high-gamma power in resting-state electrocorticogram induced by repetitive transcranial magnetic stimulation of primary motor cortex in unanesthetized macaque monkeys

A semi-automated pipeline for finite element modeling of electric field induced in nonhuman primates by transcranial magnetic stimulation

BACKGROUND

Transcranial magnetic stimulation (TMS) is used to treat a range of brain disorders by inducing an electric field (E-field) in the brain. However, the precise neural effects of TMS are not well understood. Nonhuman primates (NHPs) are used to model the impact of TMS on neural activity, but a systematic method of quantifying the induced E-field in the cortex of NHPs has not been developed.

NEW METHOD

The pipeline uses statistical parametric mapping (SPM) to automatically segment a structural MRI image of a rhesus macaque into five tissue compartments. Manual corrections are necessary around implants. The segmented tissues are tessellated into 3D meshes used in finite element method (FEM) software to compute the TMS induced E-field in the brain. The gray matter can be further segmented into cortical laminae using a volume preserving method for defining layers.

RESULTS

Models of three NHPs were generated with TMS coils placed over the precentral gyrus. Two coil configurations, active and sham, were simulated and compared. The results demonstrated a large difference in E-fields at the target. Additionally, the simulations were calculated using two different E-field solvers and were found to not significantly differ.

COMPARISON WITH EXISTING METHODS

Current methods segment NHP tissues manually or use automated methods for only the brain tissue. Existing methods also do not stratify the gray matter into layers.

CONCLUSION

The pipeline calculates the induced E-field in NHP models by TMS and can be used to plan implant surgeries and determine approximate E-field values around neuron recording sites.

The impact of physical therapy on dysphagia in neurological diseases: a review

A neurogenic dysphagia is dysphagia caused by problems with the central and peripheral nervous systems, is particularly prevalent in conditions such as Parkinson's disease and stroke. It significantly impacts the quality of life for affected individuals and causes additional burdens, such as malnutrition, aspiration pneumonia, asphyxia, or even death from choking due to improper eating. Physical therapy offers a non-invasive treatment with high efficacy and low cost. Evidence supporting the use of physical therapy in dysphagia treatment is increasing, including techniques such as neuromuscular electrical stimulation, sensory stimulation, transcranial direct current stimulation, and repetitive transcranial magnetic stimulation. While initial studies have shown promising results, the effectiveness of specific treatment regimens still requires further validation. At present, there is a lack of scientific evidence to guide patient selection, develop appropriate treatment regimens, and accurately evaluate treatment outcomes. Therefore, the primary objectives of this review are to review the results of existing research, summarize the application of physical therapy in dysphagia management, we also discussed the mechanisms and treatments of physical therapy for neurogenic dysphagia.

Transcranial magnetic stimulation in non-human primates: A systematic review

Transcranial magnetic stimulation (TMS) is widely employed as a tool to investigate and treat brain diseases. However, little is known about the direct effects of TMS on the brain. Non-human primates (NHPs) are a valuable translational model to investigate how TMS affects brain circuits given their neurophysiological similarity with humans and their capacity to perform complex tasks that approach human behavior. This systematic review aimed to identify studies using TMS in NHPs as well as to assess their methodological quality through a modified reference checklist. The results show high heterogeneity and superficiality in the studies regarding the report of the TMS parameters, which have not improved over the years. This checklist can be used for future TMS studies with NHPs to ensure transparency and critical appraisal. The use of the checklist would improve methodological soundness and interpretation of the studies, facilitating the translation of the findings to humans. The review also discusses how advancements in the field can elucidate the effects of TMS in the brain.

Different effects of I-wave periodicity repetitive TMS on motor cortex interhemispheric interaction

Background

Activity of the neural circuits in the human motor cortex can be probed using transcranial magnetic stimulation (TMS). Changing TMS-induced current direction recruits different cortical neural circuits. I-wave periodicity repetitive TMS (iTMS) substantially modulates motor cortex excitability through neural plasticity, yet its effect on interhemispheric interaction remains unclear.

Objective

To explore the modulation of interhemispheric interaction by iTMS applied in different current directions.

Materials and Methods

Twenty right-handed healthy young volunteers (aged 27.5 ± 5.0 years) participated in this study with three visits. On each visit, iTMS in posterior-anterior/anterior-posterior direction (PA-/AP-iTMS) or sham-iTMS was applied to the right hemisphere, with corticospinal excitability and intracortical facilitation of the non-stimulated left hemisphere evaluated at four timepoints. Ipsilateral silent period was also measured at each timepoint probing interhemispheric inhibition (IHI).

Results

PA- and AP-iTMS potentiated cortical excitability concurrently in the stimulated right hemisphere. Corticospinal excitability of the non-stimulated left hemisphere increased 10 min after both PA- and AP-iTMS intervention, with a decrease in short-interval intracortical facilitation (SICF) observed in AP-iTMS only. Immediately after the intervention, PA-iTMS tilted the IHI balance toward inhibiting the non-stimulated hemisphere, while AP-iTMS shifted the balance toward the opposite direction.

Conclusions

Our findings provide systematic evidence on the plastic modulation of interhemispheric interaction by PA- and AP-iTMS. We show that iTMS induces an interhemispheric facilitatory effect, and that PA- and AP-iTMS differs in modulating interhemispheric inhibition.

Monophasic-quadri-burst stimulation robustly activates bilateral swallowing motor cortices

A stable, reliable, non-invasive, quantitative assessment of swallowing function remains to be established. Transcranial magnetic stimulation (TMS) is commonly used to aid in the diagnosis of dysphagia. Most diagnostic applications involve single-pulse TMS and motor evoked potential (MEP) recordings, the use of which is not clinically suitable in patients with severe dysphagia given the large variability in MEPs measured from the muscles involved in swallowing. Previously, we developed a TMS device that can deliver quadripulse theta-burst stimulation in 16 monophasic magnetic pulses through a single coil, enabling the measurement of MEPs related to hand function. We applied a system for MEP conditioning that relies on a 5 ms interval-monophasic quadripulse magnetic stimulation (QPS5) paradigm to produce 5 ms interval-four sets of four burst trains; quadri-burst stimulation (QBS5), which is expected to induce long-term potentiation (LTP) in the stroke patient motor cortex. Our analysis indicated that QBS5 conditioned left motor cortex induced robust facilitation in the bilateral mylohyoid MEPs. Swallowing dysfunction scores after intracerebral hemorrhage were significantly correlated with QBS5 conditioned-MEP parameters, including resting motor threshold and amplitude. The degree of bilateral mylohyoid MEP facilitation after left side motor cortical QBS5 conditioning and the grade of severity of swallowing dysfunction exhibited a significant linear correlation (r = -0.48/-0.46 and 0.83/0.83; R² = 0.23/0.21 and 0.68/0.68, P < 0.001; Rt./Lt. side MEP-RMT and amplitudes, respectively). The present results indicate that RMT and amplitude of bilateral mylohyoid-MEPs after left motor cortical QBS5 conditioning as surrogate quantitative biomarkers for swallowing dysfunction after ICH. Thus, the safety and limitations of QBS5 conditioned-MEPs in this population should be further explored.

Depression induced by low-frequency repetitive transcranial magnetic stimulation to ventral medial frontal cortex in monkeys

The medial frontal cortex (MFC), especially its ventral part, has long been of great interest with respect to the pathology of mood disorders. A number of human brain imaging studies have demonstrated the abnormalities of this brain region in patients with mood disorders, however, whether it is critically and causally involved in the pathogenesis of such disorders remains to be fully elucidated. In this study, we examined how the suppression of neural activity in the ventral region of the MFC (vMFC) affects the behavioral and physiological states of monkeys by using repetitive transcranial magnetic stimulation (rTMS). By using low-frequency rTMS (LF-rTMS) as an inhibitory intervention, we found that LF-rTMS targeting the vMFC temporarily induced a depression-like state in monkeys, which was characterized by a reduced movement activity level, impaired sociability, and decreased motivation level, as well as increased plasma cortisol level. On the other hand, no such significant changes in behavioral and physiological states were observed when targeting the other MFC regions, dorsal or posterior. We further found that the administration of an antidepressant agent, ketamine, ameliorated the abnormal behavioral and physiological states induced by the LF-rTMS intervention. These findings causally indicate the involvement of the vMFC in the regulation of mood and the validity of the LF-rTMS-induced dysfunction of the vMFC as a nonhuman primate model of depression.

Dissecting the Prefrontal Network With Pathway-Selective Manipulation in the Macaque Brain—A Review

Macaque monkeys are prime animal models for studying the neural mechanisms of decision-making because of their close kinship with humans. Manipulation of neural activity during decision-making tasks is essential for approaching the causal relationship between the brain and its functions. Conventional manipulation methods used in macaque studies are coarse-grained, and have worked indiscriminately on mutually intertwined neural pathways. To systematically dissect neural circuits responsible for a variety of functions, it is essential to analyze changes in behavior and neural activity through interventions in specific neural pathways. In recent years, an increasing number of studies have applied optogenetics and chemogenetics to achieve fine-grained pathway-selective manipulation in the macaque brain. Here, we review the developments in macaque studies involving pathway-selective operations, with a particular focus on applications to the prefrontal network. Pathway selectivity can be achieved using single viral vector transduction combined with local light stimulation or ligand administration directly into the brain or double-viral vector transduction combined with systemic drug administration. We discuss the advantages and disadvantages of these methods. We also highlight recent technological developments in viral vectors that can effectively infect the macaque brain, as well as the development of methods to deliver photostimulation or ligand drugs to a wide area to effectively manipulate behavior. The development and dissemination of such pathway-selective manipulations of macaque prefrontal networks will enable us to efficiently dissect the neural mechanisms of decision-making and innovate novel treatments for decision-related psychiatric disorders.

Monophasic-Quadripulse Theta Burst Magnetic Stimulation for Motor Palsy Functional Evaluation After Intracerebral Hemorrhage

Transcranial magnetic stimulation (TMS) is commonly employed for diagnostic and therapeutic purposes to enhance recovery following brain injury, such as stroke or intracerebral hemorrhage (ICH). Single-pulse TMS, most commonly used for diagnostic purposes and with motor evoked potential (MEP) recordings, is not suitable for clinical use in patients with severe motor paresis. To overcome this problem, we developed a quadripulse theta burst transcranial magnetic stimulation (QTS) device that combines the output from 16 stimulators to deliver a train of 16 monophasic magnetic pulses through a single coil. High-frequency theta rhythm magnetic bursts (bursts of four monophasic pulses, at 500 Hz, i.e., with a 2-ms interpulse interval, repeated at 5 Hz) were generated via a set of 16 separate magnetic stimulators connected to a specially designed combination module. No adverse effects or electroencephalogram (EEGs) abnormalities were identified during or after the recordings. MEP amplification in the QTS during four-burst theta rhythm stimulations produced four independent MEPs 20 ms after each burst onset maximizing the final third or fourth burst, which exhibited significantly greater amplitude than those resulting from a single burst or pulse. Motor functional palsy grades after ICH and QTS-MEP parameters and resting motor threshold (RMT) and amplitudes were significantly correlated (r = −0.83/−0.81 and 0.89/0.87; R2 = 0.69/0.66 and 0.79/0.76, p &lt; 0.001; anterior/posterior-stimulus polarity, respectively). In conclusion, QTS-MEPs enabled a linear functional evaluation in patients with various degrees of motor paresis. However, the benefits, safety, and limitations of this device should be further explored in future studies.

Multimodal Autoencoder Predicts fNIRS Resting State From EEG Signals

In this work, we introduce a deep learning architecture for evaluation on multimodal electroencephalographic (EEG) and functional near-infrared spectroscopy (fNIRS) recordings from 40 epileptic patients. Long short-term memory units and convolutional neural networks are integrated within a multimodal sequence-to-sequence autoencoder. The trained neural network predicts fNIRS signals from EEG, sans a priori, by hierarchically extracting deep features from EEG full spectra and specific EEG frequency bands. Results show that higher frequency EEG ranges are predictive of fNIRS signals with the gamma band inputs dominating fNIRS prediction as compared to other frequency envelopes. Seed based functional connectivity validates similar patterns between experimental fNIRS and our model's fNIRS reconstructions. This is the first study that shows it is possible to predict brain hemodynamics (fNIRS) from encoded neural data (EEG) in the resting human epileptic brain based on power spectrum amplitude modulation of frequency oscillations in the context of specific hypotheses about how EEG frequency bands decode fNIRS signals.

Multi-scale neural decoding and analysis

Objective.

Complex spatiotemporal neural activity encodes rich information related to behavior and cognition. Conventional research has focused on neural activity acquired using one of many different measurement modalities, each of which provides useful but incomplete assessment of the neural code. Multi-modal techniques can overcome tradeoffs in the spatial and temporal resolution of a single modality to reveal deeper and more comprehensive understanding of system-level neural mechanisms. Uncovering multi-scale dynamics is essential for a mechanistic understanding of brain function and for harnessing neuroscientific insights to develop more effective clinical treatment.

Approach.

We discuss conventional methodologies used for characterizing neural activity at different scales and review contemporary examples of how these approaches have been combined. Then we present our case for integrating activity across multiple scales to benefit from the combined strengths of each approach and elucidate a more holistic understanding of neural processes.

Main results.

We examine various combinations of neural activity at different scales and analytical techniques that can be used to integrate or illuminate information across scales, as well the technologies that enable such exciting studies. We conclude with challenges facing future multi-scale studies, and a discussion of the power and potential of these approaches.

Significance.

This roadmap will lead the readers toward a broad range of multi-scale neural decoding techniques and their benefits over single-modality analyses. This Review article highlights the importance of multi-scale analyses for systematically interrogating complex spatiotemporal mechanisms underlying cognition and behavior.