Interhemispheric inhibition (IHI) reflects transcallosal interactions between bilateral primary motor cortices and is commonly assessed using a paired-pulse, transcranial magnetic stimulation (TMS) paradigm, in which a conditioning TMS pulse is delivered to M1 prior to a test TMS pulse delivered to the contralateral M1. Despite its widespread use as a marker of interhemispheric connectivity and intervention-related plasticity, its reliability remains unclear. The present study assessed the intersession reliability of short- (SIHI: 8, 10 ms) and long-latency IHI (LIHI: 40, 50 ms) recorded from the right and left first dorsal interosseous muscles in right-hand dominant healthy controls. Relative reliability was quantified using intraclass correlation coefficients (ICC), and absolute reliability was quantified using the standard error of measurement (SEM) and smallest detectable change (SDC). SIHI and LIHI demonstrated moderate-to-strong reliability when measured from the right, dominant hand with the greatest reliability observed at 8 ms (SIHI) and 50 ms (LIHI). Reliability of IHI was consistently lower in the left hand. Across conditions, measurement error was high, resulting in large SDC values at the individual level, indicating limited sensitivity for detecting individual change. In contrast, group-level SDC estimates were substantially lower, supporting the use of IHI for detecting group level effects with appropriate sample sizes (∼23 for SIHI; ∼18 for LIHI in the dominant hand of right-handed individuals). These findings indicate that while IHI can provide reliable group-level measures of interhemispheric function, its application for tracking individual changes across sessions may be limited.
