Paired TMS pulses can be used to investigate the physiological basis of excitability of the motor cortex and its modulation by local facilitating and inhibiting corticocortical networks.
ppTMS consists of two consecutive TMS pulses: the first stimulus is called the conditioning stimulus (S1), the second pulse is called the test stimulus (S2).
Appropriate interstimulus intervals (ISI) within ppTMS elicits inhibitory or facilitatory effects on the motor cortex.
Depending on the ppTMS paradigm (ISI and intensity), the corresponding target muscle may either exhibit increased MEPs (facilitation) or reduces MEPs (inhibition):
Intracortical Facilitation (ICF) To induce ICF, a subthreshold S1, induced in a posterior-anterior direction, is followed by a suprathreshold S2 with an ISI between 10-15 ms. The effect observed is that the amplitude of I-waves is increased relative to cases where only S2 is delivered. Facilitation becomes stronger with increasing S1 intensity, but tends to be weaker with increasing S2 intensity.
Long-interval Intracortical Facilitation (LICF) When a suprathreshold S1 is followed by suprathreshold S2 with an ISI between 10-30 ms, the second MEP is enhanced relative to the first, showing the facilitating effect of the stimulation.
Long-interval Intracortical Inhibition (LICI) If both S1 and S2 are suprathreshold with an ISI between 50-200 ms, the second MEP is reduced relative to the first, which is a sign of intracortical inhibition.
Short-interval Intracortical Facilitation (SICF) SICF occurs when a suprathreshold S1 is followed by subthreshold S2 with an ISI between 1.1-1.5 ms. The effect observed is that the amplitude of the second I-wave is enhanced. SICF is suppressed if the subject had first experienced sensory stimulation of the relevant peripheral nerves.
Short-interval Intracortical Inhibition (SICI) When a subthreshold S1 is followed by suprathreshold S2 with an ISI of 3 ms, the I-waves is suppressed. SICI increases with S2 intensity.