Does second-scale intertrial interval affect motor evoked potentials induced by single-pulse transcranial magnetic...

Area of Science:

• Neuroscience
• Motor Control
• Electrophysiology

Background:

• Transcranial magnetic stimulation (TMS) is widely used to assess corticospinal excitability via motor-evoked potentials (MEPs).
• A common assumption in TMS research is that MEP amplitudes remain constant over time (time invariance).
• This assumption underpins the reliability of using characteristic MEP amplitudes to represent individual corticospinal excitability.

Purpose of the Study:

• To investigate whether varying intertrial intervals (ITIs) of single-pulse TMS influence the measured amplitude of motor-evoked potentials (MEPs).
• To challenge the prevailing assumption of time invariance in MEP amplitudes.
• To determine the impact of ITI on the estimation of individual corticospinal excitability.

Main Methods:

• Utilized navigated TMS to identify the optimal scalp location for stimulating the thenar motor cortex in healthy subjects.
• Determined resting motor threshold (MT) and delivered TMS at 120% MT.
• Analyzed single-trial MEP amplitudes from trains of 30 responses across various constant and randomized ITIs (1-10 seconds) and assessed effects of stimulus blocks.

Main Results:

• Individual single-trial MEP amplitudes were significantly affected by ITI (in 8/8 subjects) and block number (in 5/8 subjects).
• A significant interaction between ITI and block number was observed (in 6/8 subjects).
• These findings indicate that MEP amplitudes are time variant, impacting the accuracy of characteristic MEP amplitude estimates.

Conclusions:

• Individual motor-evoked potential amplitudes exhibit time-variant properties, contradicting the common assumption of time invariance.
• Estimates of individual characteristic MEP amplitudes should be interpreted with caution.
• Assuming time invariance may lead to erroneous conclusions regarding corticospinal excitability.