Test Re-test Reliability of Dual-site TMS Measures of SMA-M1 Connectivity Differs Across Inter-stimulus Intervals in Younger and Older Adults.

Dual-site transcranial magnetic stimulation (TMS) is a promising tool to measure supplementary motor area and primary motor cortex (SMA-M1) connectivity in younger and older adults, and could be used to understand the pathophysiology of movement disorders. However, test re-test reliability of dual-site TMS measures of SMA-M1 connectivity has not been established. We examined the reliability of SMA-M1 connectivity using dual-site TMS in two sessions in 30 younger and 30 older adults. For dual-site TMS, a conditioning pulse delivered to SMA (140% of active motor threshold) preceded a test pulse delivered to M1 (intensity that elicited MEPs of ~1 mV) by inter-stimulus intervals (ISI) of 6 ms, 7 ms, and 8 ms. Moderate intraclass correlation coefficients (ICC) were found for SMA-M1 connectivity at an ISI of 7 ms in younger (ICC: 0.69) and older adults (ICC: 0.68). Poor ICCs were found for SMA-M1 connectivity at ISIs of 6 ms and 8 ms in both age groups (ICC range: 0.01-0.40). We report evidence for stable measures of SMA-M1 connectivity at an ISI of 7 ms in both age groups. These findings are foundational for future research developing evidence-based interventions to strengthen SMA-M1 connectivity to improve bilateral motor control in older adults and populations with movement disorders.