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Updated: Feb 5, 2026

Standardized Induction and Assessment of Long-term Potentiation-like Cortical Plasticity Using Transcranial Magnetic Stimulation
Published on: November 7, 2025
Involvement of different neuronal components in the induction of cortical plasticity with associative stimulation
Zhen Ni1, Robin F H Cash2, Carolyn Gunraj3
1Division of Neurology, Department of Medicine, University of Toronto and Krembil Research Institute, University Health Network, Toronto, Ontario, Canada; Human motor Control Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, USA.
Background:
Paired associative stimulation (PAS), with stimulus interval of 21.5 or 25 ms, using transcranial magnetic stimulation in the posterior-anterior (PA) current direction, produces a long-term-potentiation-like effect. Stimulation with PA directed current generates both early and late indirect (I)-waves while that in anterior-posterior (AP) current predominantly elicits late I-waves. Short interval intracortical inhibition (SICI) inhibits late I-waves but not early I-waves.
Objective:
To investigate how cortical inhibition modulates the effects of PAS.
Methods:
PAS at stimulus interval of 21.5 ms conditioned by SICI (SICI-PAS) was compared to PAS alone with both PA and AP directed currents.
Results:
PAS with both current directions increased cortical excitability. SICI-PAS increased cortical excitability in the PA but not the AP current direction.
Conclusions:
Both early and late I-waves circuits can mediate cortical PAS plasticity under different conditions. Plasticity induction with the late but not the early I-wave circuits is blocked by SICI.
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