Reliability of transcranial magnetic stimulation measures of afferent inhibition.

Short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI) are well-known transcranial magnetic stimulation (TMS) paradigms used to probe the sensorimotor system. To date, there is a paucity of research examining the reliability of these neurophysiological measures. This information is required to validate the utility of afferent inhibition as a biomarker of neural function. The goal of this study was to quantify the absolute reliability, relative reliability, and smallest detectable change (SDC) of SAI and LAI using a test-retest paradigm. 30 healthy individuals (20.9 ± 2.5 years) participated in two sessions (intersession interval of ~7 days). Reliability was assessed with intraclass correlation coefficients (ICC), standard error of measurement (SEM), and SDC. The results show that LAI and SAI had poor-to-moderate relative reliability as determined by the ICC, with digital nerve LAI displaying the highest relative reliability (highest ICC with smallest confidence interval). The %SEM indicated a large amount of measurement error in all measures of afferent inhibition, with LAI exhibiting more measurement error than SAI. The SDC was large at the individual level (SDC), but analyses showed that the SDC is significantly reduced at the group-level (SDC). Our results indicate that digital nerve LAI is the most reliable outcome to differentiate between individuals within a sample. Further, results suggest that SAI and LAI are not appropriate indicators of individual neurophysiological change across time but can reliably detect changes in group-averaged data providing sample sizes are sufficient.