Cortical modulation of short-latency TMS-evoked potentials

Area of Science:

• Neuroscience
• Cognitive Neuroscience
• Electrophysiology

Background:

• Transcranial magnetic stimulation-electroencephalogram (TMS-EEG) co-registration enables testing brain area reactivity via TMS-evoked potentials (TEPs).
• The functional significance of several described TEPs remains unclear.
• Short-latency potentials (P5, N8) have been recently identified, but their cortical origin is debated due to large amplitudes.

Purpose of the Study:

• To investigate the origin of short-latency TMS-evoked potentials (P5-N8).
• To determine if P5-N8 components reflect cortical reactivity by modulating primary motor cortex (MI) excitability using premotor cortex (PMC) stimulation.

Main Methods:

• TMS-EEG was recorded from 70 electrodes during simultaneous TMS application.
• TEPs were evoked by single-pulse TMS over MI at 110% of resting motor threshold (rMT).
• The effect of low-frequency (1 Hz) conditioning TMS on PMC was assessed on TEP amplitude before and after conditioning, alongside motor-evoked potentials (MEPs).

Main Results:

• PMC conditioning reduced P5-N8 amplitude in some participants and increased it in others.
• No significant changes were observed in corticospinal excitability (MEPs).
• An inverse correlation was found between P5-N8 amplitude modulation and MEP amplitude.

Conclusions:

• The observed modulation of P5-N8 amplitude, despite variability, suggests a potential cortical origin, linked to interactions between PMC and MI.
• Early latency TEP components, with careful experimental design, can serve as indicators of stimulated cortical reactivity.