State-Dependent Temporal Dynamics of Short-Latency Afferent Inhibition Across Current Direction and Pulse Duration Using Controllable Pulse-Parameter TMS.

BACKGROUND: Short-latency afferent inhibition (SAI) is widely used as a non-invasive measure of how somatosensory input modulates TMS-evoked motor output. However, SAI is modulated by motor state and stimulation parameters, suggesting that SAI may depend on the neural elements engaged by the stimulation protocol. This study examined how muscle activation state, transcranial magnetic stimulation (TMS) current direction, and pulse duration influence the temporal dynamics of SAI. METHODS: Eighteen healthy adults completed two sessions in which SAI was assessed at rest and during a 10% maximum voluntary contraction of the first dorsal interosseous muscle. Within each session, controllable pulse-parameter TMS was used to profile SAI across interstimulus intervals (10-36 ms), current directions (posterior-anterior [PA], anterior-posterior [AP]), and pulse durations (30, 70, 120 µs) in a factorial design. Gaussian functions were used to estimate the peak magnitude, timing, and temporal width of inhibition in each condition. Effects were analyzed using linear mixed-effects models. RESULTS: Voluntary contraction shifted peak inhibition to later ISIs and broadened the inhibitory window. AP stimulation yielded a broader inhibitory window than PA stimulation, whereas the peak magnitude of inhibition was increased during contraction for AP- but not PA-evoked responses. CONCLUSIONS: These findings indicate that motor state and stimulation parameters shape different features of the SAI temporal profile. Voluntary contraction altered the timing and width of inhibition, whereas current direction influenced the width and modulated the effect of muscle state on inhibition magnitude. SAI measured only at rest may not fully capture the state dependence of early somatosensory influences on motor cortical excitability; incorporating measurements across motor states and time courses may provide a more complete characterization.