Why TMS Plus EEG Is Such a Big Deal
Transcranial magnetic stimulation (TMS) can nudge brain circuits with a brief magnetic pulse. Electroencephalography (EEG) can record brain activity in milliseconds. When you combine them, you get something uniquely powerful: the ability to deliberately perturb the brain and immediately measure how that brain region, and its connected networks, respond.
That matters because many tools in neuroscience are observational. Resting EEG and fMRI can show correlations, but they cannot establish cause and effect. TMS-EEG can, at least in principle, map causal input-output relationships. In clinical terms, this creates a possible pathway to objective markers of cortical excitability, inhibition, and effective connectivity that are directly tied to a controlled stimulation event.
What Counts as a TMS-EEG Biomarker
In practice, researchers often focus on TMS-evoked potentials (TEPs), the sequence of EEG deflections that follow a TMS pulse. These waveforms reflect a combination of direct cortical activation and downstream synaptic activity. If a specific TEP component reliably changes with disease state, medication exposure, or treatment response, it may qualify as a biomarker.
The new international expert group review emphasizes that clinical utility depends on both reliability and interpretability. A biomarker is not simply a pattern that looks different between groups. It must be reproducible across sessions, research sites, and equipment setups, and it must be linked to a plausible neurophysiologic mechanism.
The Hidden Problem: Artifacts That Can Fool You
Here is the catch. TMS does not only stimulate the cortex. It also introduces several sources of noise that can masquerade as brain signals.
Some artifacts are electrical or mechanical. The magnetic pulse can overwhelm EEG amplifiers, capacitor recharge can introduce signal jumps, and electrode lead geometry can behave like an antenna. Other artifacts are physiological. Scalp muscles twitch, eyes blink, and the loud coil click triggers auditory responses. Even the physical sensation on the scalp can evoke sensory activity that overlaps with later TEP components.
The review argues that managing these confounds is not optional. If artifact profiles differ between patient groups, researchers risk false positives that appear to be disease biomarkers but are actually differences in muscle activation, attention, or sensory processing.
What Better Hardware and Setup Can Fix
One reason TMS-EEG is advancing now is improved instrumentation. Modern TMS-compatible EEG amplifiers with high dynamic range and fast sampling rates reduce amplifier saturation and shorten the pulse artifact window. Systems that allow control over capacitor recharge timing can shift recharge artifacts away from time windows that matter most clinically.
Neuronavigation is also increasingly treated as a standard requirement rather than a luxury. It helps maintain consistent coil position, orientation, and angle within and across sessions, which is critical when measuring subtle changes in TEPs over time, such as across a treatment course.
Many labs also implement practical steps that sound simple but have a meaningful impact. These include careful skin preparation to reduce electrode impedance, thoughtful cable routing to minimize electromagnetic induction, and noise masking to reduce auditory responses to the coil click.
Where This Is Heading: Toward Closed-Loop Precision TMS
The long-term goal is not just cleaner recordings. It is adaptive stimulation. If clinicians can measure the brain’s response in real time, stimulation parameters can be adjusted based on individual physiology rather than population averages.
The review highlights emerging approaches such as multi-locus TMS, which can electronically shift stimulation across targets without physically moving the coil. This enables faster adjustments and opens the door to more individualized protocols.
For clinicians and patients, the promise is clear. Fewer trial-and-error treatment courses, earlier signals indicating whether a protocol is working, and a deeper mechanistic understanding of why one person responds while another does not.
Citations
Ziemann U, Bai Y, Baumer FM, et al. Clinical utility and prospects of TMS-EEG: Updated review from an international expert group. Clinical Neurophysiology. 2026 (online January 9, 2026).
https://doi.org/10.1016/j.clinph.2025.2111487
Tremblay S, Rogasch NC, Premoli I, et al. Clinical utility and prospects of TMS-EEG. Clinical Neurophysiology. 2019;130(5).
https://doi.org/10.1016/j.clinph.2019.01.001