TMS Transcranial Magnetic Stimulation Compatible EEG Electrode Cap

Category: Neuromodulation and Neuromonitoring

Time：2021-09-26

Introduction to TMS

Transcranial magnetic stimulation (TMS) is a cortical stimulation technique first developed by Barker et al. in 1985. It boasts advantages such as being painless, non-invasive, easy to perform, and safe and reliable, making it highly suitable for both clinical and laboratory studies of brain function.

Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation technique. It uses pulsed magnetic fields generated by an electrically charged coil placed on the scalp to penetrate the skull and reach targeted brain regions, where they induce an electromagnetic field. This induced electric field depolarizes neurons, triggering action potentials that modulate neuronal activity and influence cognitive functions of the brain. Since the 1980s, TMS technology has been widely applied in neuroscience and psychological research.

Introduction to Transcranial Magnetic Stimulation Combined with Electroencephalography (TMS-EEG) Technology

In recent years, with advances in technology, magnetically compatible EEG electrodes and amplifiers have been developed, making it possible to simultaneously record undisturbed EEG signals during TMS stimulation. To evaluate the regulatory effects of TMS on neuronal activity in the brain, researchers have combined TMS with EEG technology, giving rise to the technique known as transcranial magnetic stimulation-EEG (TMS-EEG). The TMS-EEG approach has gained widespread acclaim among researchers due to its simplicity and high temporal resolution.

TMS-EEG has been shown to be a highly promising technique that can enhance our ability to non-invasively probe brain function in both healthy and diseased states, providing reliable, objective, and quantifiable information related to excitatory and inhibitory neuronal activity, oscillatory neural dynamics, connectivity, and plasticity. To date, clinical studies have demonstrated that TMS-EEG can be applied to a wide range of clinical populations in both neurology and psychiatry. Future research should also focus on using TMS-EEG to monitor the efficacy of therapeutic interventions—such as pharmacological treatments and psychotherapy—in clinical populations. Developing personalized treatments through TMS-EEG represents another highly intriguing application. Furthermore, combining TMS-EEG with other neuroimaging techniques holds tremendous clinical potential.

Figure 1: A series of events triggered by TMS pulses. (Image from the reference.)

(1-2) A current pulse flows through the TMS coil (maximum I ~ 5 kA), generating a brief (~100 ms) but strong magnetic field (maximum 1–3 T).
(3) A changing magnetic field induces an electric field (~50-100 V/m) in the brain, which in turn generates another electric field (~50-100 V/m).
(4) A current is generated within the tissue (~0.1 mA/mm²).
(5) The flow of current (i.e., ions). This results in local membrane depolarization (>~10 mV).
(6) Voltage-gated ion channels open.
(7) An action potential is generated in the axon when depolarization reaches the threshold for firing.
(8) Neurotransmitters are released into the synaptic cleft.
(9) The generation of postsynaptic currents gives rise to postsynaptic excitatory (and inhibitory) potentials. If these potentials exceed the firing threshold, they in turn trigger the generation of action potentials. This cross-synaptic activation represents network activation. The potential differences (electric fields) generated by postsynaptic currents drive volumetric currents within the head and scalp.
(10) TMS-induced activation can be recorded using EEG. EEG signals can be described using linear models.

The combination of TMS and EEG facilitates the study of fundamental neuroscience questions in novel ways. The causal information provided by TMS overcomes the limitations of mere correlation inherent in EEG data. One of the key advantages of using TMS-EEG is that the responses of the EEG to TMS—known as evoked potentials or brain oscillations—can serve as neurophysiological markers of cortical excitability or connectivity. TMS-EEG data can be analyzed in both the time domain and the frequency domain; to date, most studies have focused on the former, namely TMS-evoked potentials (TEPs). TEPs are EEG responses that are time-locked to the TMS pulse. To analyze TEPs, it is necessary to average the signals across trials. The initial TMS-induced response may arise from the activation of neurons localized within the targeted area, followed subsequently by activation in regions connected via axonal pathways. TEPs consist of positive (P) and negative (N) deflections, reflecting the spatiotemporal summation of excitatory and inhibitory postsynaptic potentials. Although the neurophysiological basis of TEPs remains to be fully elucidated, they are considered a genuine and reproducible measure of cortical responsiveness. TMS applied to the primary motor cortex (M1) elicits several distinct peaks, including N15, P30, N45, P60, N100, and P180.

In the stimulated region as well as in distally interconnected brain areas, the detectable duration of TEPs reaches 400–500 milliseconds. Consequently, for certain TEP components, the maximum amplitude is recorded by electrodes located close to the stimulation site, whereas other components may be more prominent at more distant electrodes—for example, in the contralateral hemisphere. There is evidence suggesting that TEPs exhibit varying degrees of correlation with different neurotransmitters. The peak amplitude and temporal dynamics of TEPs depend on the stimulated region, the orientation of the coil, and the functional state of the underlying cortex; the latter may be influenced by factors such as behavior, level of consciousness, and neuropsychiatric disorders. Furthermore, the amplitude of TEPs is also affected by the intensity of the TMS pulse applied.

The effects of TMS on brain activity can also be further investigated in the frequency domain. When cortical regions are disrupted by TMS, the neuronal responses measured by electroencephalography (EEG) tend to oscillate at specific natural frequencies. Part of the explanation for this response may lie in the phase alignment of ongoing local brain oscillations, which is induced by the effect of TMS pulses on the targeted cortex. Thus, TMS-EEG allows researchers to manipulate and study brain rhythms by measuring the impact of TMS pulses on EEG signals as well as the associated behavioral effects. The same methodologies used to study EEG oscillations can also be applied to TMS-triggered oscillations. However, researchers should carefully distinguish between TMS-induced responses that are phase-locked—that is, signals that remain after averaging across trials—and TMS-induced responses that are not phase-locked—that is, signals that are canceled out during averaging. The latter require time-frequency representations (TFRs) computed at the single-trial level and subsequently averaged to preserve oscillatory activity associated with TMS pulses but not phase-locked. Such measurements may also involve certain baseline normalization procedures, sometimes referred to as TMS-related spectral perturbations (TRSP), which reveal a mixture of phase-locked and non-phase-locked responses that are difficult to separate.

What are the advantages of combining TMS with EEG?

It can measure the TMS stimulation site and stimulus propagation in real time, allowing you to understand the effects of TMS stimulation.

It is possible to observe the brain’s electroencephalographic (EEG) responses to TMS stimulation in real time and, at the same time, adjust the TMS treatment parameters based on these EEG responses, thereby achieving a combination of assessment and therapeutic intervention and laying the foundation for future targeted and precision treatments.

We can explore the mechanism of TMS treatment.

Technical Challenges of the TTMS-EEG Combined Approach

The combination of TMS and EEG is not simply a matter of stacking technologies; their integration requires overcoming numerous technical challenges.

Handling special types of noise is a key challenge. For example, within a few hundred milliseconds after a TMS pulse, large current noises will be generated at the electrode site. In addition, numerous other types of noise—such as electrooculographic noise, electromyographic noise, and electrode noise—also exist. All these factors can introduce artifacts.

TMS stimulation combined with simultaneous EEG recording places high demands on the hardware and software of the equipment, requiring efforts to minimize artifacts as much as possible.

Electrodes for EEG require specially designed, magnetically compatible electrodes. For high-density EEG with precise localization, EEG caps with 64 or more channels are typically used.

If standard EEG electrodes are used, the large current loops induced in the electrodes by the changing magnetic field when the TMS coil comes into contact with the electrodes can damage the electrodes. Moreover, these current loops generate heat, and the rapid heating of the electrodes can cause skin burns. Therefore, EEG signal acquisition under TMS requires specially designed EEG systems and electrodes. Electrodes used under TMS must meet the following conditions: They must have a sufficiently small diameter to ensure that the electrodes do not overheat and burn the skin; they must be coated with an appropriate surface material to prevent the electrodes from being affected by the currents induced by the changing magnetic field. Currently, there are two main approaches to reduce eddy-current heating in the electrodes: ① Reducing the area of the current loop or lowering the conductivity of the electrode; ② Using very small, pellet-shaped electrodes to minimize the electrode area and thereby reduce heat generation.

Glintech TMS-EEG Integrated Electrode Cap Solution

The Grintech TMS-EEG integrated electrode cap solution offers the following advantages:

The TMS-EEG-compatible electrodes are lightweight and thin, with a thickness of less than 5 millimeters, ensuring that their height does not interfere with the effectiveness of TMS stimulation.

It ensures that the coil is positioned as close as possible to the scalp, minimizing the stimulation energy required and enabling the elicitation of a response from the subject even at lower stimulation intensities.

The electrode can rotate, allowing you to organize the electrode leads before stimulation and ensuring minimal stimulation interference.

If you have any requirements for TMS-EEG combined electrode caps, please contact Greentech’s technical staff in the research and development field. Contact person: Li Mingzhe; phone: 15926282558.

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