Long-interval afferent inhibition measurement using two different methods: Normative values, repeatability and reliability

Background: The mechanism of Short-Latency Afferent Inhibition (SAI) is relatively well understood. In contrast, Long-Latency Afferent Inhibition (LAI) has not been as extensively studied as SAI, and its underlying mechanism remains unclear. Objective/Hypothesis: This study had two primary objectives: first, to determine the optimal ISIs for LAI measured by amplitude changes (A -LAI) using high -resolution ISI ranges; and second, to compare measurements of LAI by threshold-tracking (T -LAI). Methods: Twenty-eight healthy volunteers (12 males aged 24- 45 years) participated in the study. Paired peripheral electrical and transcranial magnetic stimulation (TMS) stimuli (TS1mv) were applied at varying (ISIs)- 100, 200, 250, 300, 350, 400, 450, 500, 550, 600, 700, 800, 900, 1000 ms. Results: Both A -LAI and T -LAI showed that LAI decreased progressively from a peak at 200 or 250 ms to 1000 ms. Using the A -LAI method, pronounced inhibition was observed at three specific ISIs: 100 ms, 250 ms and 450 ms. When A -LAI values were converted to equivalent threshold changes, they did not differ significantly from T -LAI. Reliability at distinguishing individuals, as indicated by intraclass correlation coefficient (ICC) was greater for A -LAI, with a peak value of 0.82 at 250 ms. Conclusion(s): The study demonstrates that ISIs of 100 ms and 250 ms can be reliably used in amplitude measurement LAI. The study demonstrates that both LAI measurements record a similar decline of inhibition with increasing ISI.