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Updated: Mar 16, 2026
Intracortical Inhibition Within the Primary Motor Cortex Can Be Modulated by Changing the Focus of Attention
Published on: September 11, 2017
Threshold tracking primary motor cortex inhibition: the influence of current direction
John Cirillo1, Winston D Byblow2
1Movement Neuroscience Laboratory, Department of Exercise Sciences and Centre for Brain Research, University of Auckland, Private Bag 92019, Auckland, 1142, New Zealand.
Anterior-posterior (AP) transcranial magnetic stimulation (TMS) offers a more sensitive method for measuring intracortical inhibition in the primary motor cortex (M1) compared to posterior-anterior (PA) stimulation. This technique enhances the assessment of inhibitory processes by preferentially recruiting late indirect waves.
Area of Science:
- Neuroscience
- Motor Control
- Electrophysiology
Background:
- Paired-pulse transcranial magnetic stimulation (TMS) is utilized to investigate inhibitory activity within the primary motor cortex (M1).
- The direction of induced current during TMS influences the recruitment of descending volleys, with anterior-posterior (AP) stimulation favoring late indirect (I-) waves, which are more sensitive to paired-pulse protocols.
- Previous threshold tracking TMS studies assessing intracortical inhibition primarily employed posterior-anterior (PA) stimulation, which preferentially recruits early I-waves.
Purpose of the Study:
- To compare the efficacy of threshold tracking TMS in assessing intracortical inhibition using AP stimulation (recruiting late I-waves) versus PA stimulation (recruiting early I-waves).
- To determine if AP stimulation provides a more sensitive measure of short-interval intracortical inhibition (SICI) and long-interval intracortical inhibition (LICI) in M1.
Main Methods:
- Electromyographic (EMG) recordings were taken from the first dorsal interosseous muscle in 15 participants.
- Motor evoked potentials (MEPs) were elicited over the left M1 using PA, AP, and lateromedial (LM) induced currents.
- I-wave recruitment was quantified by the onset latency difference between PA-LM and AP-LM.
- SICI and LICI were assessed using threshold tracking TMS with varying conditioning stimulus intensities and interstimulus intervals for both PA and AP stimulation.
Main Results:
- Threshold tracking TMS with an AP-induced current demonstrated greater SICI and LICI compared to PA-induced current.
- The degree of SICI for AP stimulation was positively associated with the efficacy of late I-wave recruitment, an association not observed with PA stimulation.
- LICI was not significantly associated with I-wave recruitment efficacy for either stimulation direction.
Conclusions:
- Threshold tracking TMS with an AP-induced current is a more robust and sensitive method for evaluating M1 intracortical inhibition than PA stimulation.
- The findings suggest that AP stimulation, by preferentially recruiting late I-waves, enhances the sensitivity of TMS for probing inhibitory circuits in the motor cortex.
