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_eas), 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_eas 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_indiv), but analyses showed that the SDC is significantly reduced at the group-level (SDC_group). 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.