The Core Challenge in TMS-EEG Recording: Artefacts

The Fundamentals of TMS-EEG Recording

TMS-EEG integrates two modalities:

TMS

Delivers brief, high-intensity magnetic pulses to cortical regions

EEG

Records the electrical potentials of the neural populations responding to the pulse

This dual structure lets TMS-EEG recordings measure cortical excitability with high temporal resolution.

Challenges of Simultaneous Stimulation and Recording

During a TMS pulse the coil produces:

• Rapidly changing magnetic fields
• Large induced voltages in nearby conductors
• Mechanical vibrations
• Saturation effects in nearby electronics

These factors create artefacts that must be minimised by design, not by filtering alone. Electrode geometry, materials and integrated electronics directly determine how quickly the EEG signal recovers after stimulation.

Why Electrode Geometry Matters in TMS-EEG

Electrode thickness directly affects coil placement and artefact management.

How Electrode Thickness Affects Artefact Formation

A thick electrode increases the coil–scalp distance, and this:

• Weakens the magnetic field reaching the cortex
• Forces researchers to raise the stimulation intensity
• Leads to larger EEG artefacts
• Reduces the focality of stimulation

Ultra-Flat Electrode Technology: the 3 mm g.Ladybird

The introduction of 3 mm ultra-flat electrodes represents a major advance in TMS-compatible EEG technology. As the thinnest TMS electrode on the market, this design allows the coil to be positioned extremely close to the scalp. This improves both stimulation precision and EEG quality.

Benefits for TMS Pulse Strength and Crosstalk

By reducing the coil–scalp distance:

• Less stimulation power is needed
• Pulse artefacts shrink
• The electric field distribution becomes more homogeneous
• Coil stability improves in repetitive TMS

Active and Passive Electrode Systems

Active Electrodes

• Strong EEG signals even at higher impedances
• Fast recovery after TMS (<10 ms)
• Reduced sensitivity to environmental noise

Passive Electrodes

• Extremely fast post-pulse recovery
• Minimal risk of amplifier saturation
• Clean recording of early TMS-evoked potentials

Engineering Properties of Sintered Ag/AgCl Electrodes

Sintered silver/silver chloride provides a low noise floor, a stable half-cell potential, resistance to polarisation and excellent performance across 0–10 kHz. Thanks to their dense porous structure, sintered Ag/AgCl electrodes keep performing even under TMS-induced transients and high dynamic range conditions.

Cap Design and Electrode Layout for TMS-EEG

High-density EEG caps such as the g.GAMMAcap³ offer:

• 74 extended 10–20 positions
• 86 optional intermediate positions
• Flexible yet durable fabric
• Easy integration with ultra-flat electrodes

Applications in Clinical and Cognitive Neuroscience

TMS-EEG offers insights into:

• Cortical excitability
• Inhibitory/excitatory balance
• Network connectivity
• Plasticity mechanisms
• Neuromodulation effects
• Depression, stroke, epilepsy

Conclusion

The field of TMS-EEG recording continues to advance through innovations in electrode design, amplifier technology and closed-loop systems. 3 mm ultra-flat electrodes represent a major leap, delivering cleaner data, reduced artefacts and more precise stimulation control. Combined with high-performance amplifiers and dedicated caps, these tools give researchers the stability and accuracy that cutting-edge neurophysiology studies require.