Ann-Maree Vallence

Aging is associated with decline in voluntary motor control and decline in the quantity and quality of white matter, which results in impaired functional connectivity. The supplementary motor area (SMA) is densely connected with the primary motor cortex (M1) and, together, these two regions are important for selection, updating, execution of appropriate motor plans. Transcranial magnetic stimulation (TMS) protocols can be used to measure functionally relevant excitatory connections between SMA—M1: when a conditioning TMS pulse to SMA precedes a test TMS pulse to M1 at appropriate intervals, the MEP elicited by the test TMS pulse is facilitated due to activation of excitatory networks acting between SMA and M1. We aimed to determine whether SMA-M1 connectivity is reduced in older adults compared to younger adults. Purdue Pegboard was used to measure manual dexterity, the four square step test was used to measure dynamic balance, and dual-coil TMS was used to measure the excitability of connectivity between SMA—M1 in the hand motor region (and preSMA-M1 as a control). Younger adults performed better on the motor tasks than older adults. In younger adults, the interaction between SMA—M1 (but not PreSMA—M1) was facilitatory, replicating previous research; here, we extend this finding to show SMA—M1 facilitation is reduced in older compared to younger adults. Furthermore, a significant positive correlation was found between SMA—M1 facilitation and performance on the Purdue Pegboard task: greater SMA—M1 facilitation is associated with greater number of pegs placed. Together, these findings suggest that SMA—M1 connectivity is functionally relevant, contributing to manual dexterity performance, and that SMA—M1 connectivity is decreased with age. These data provide a neurophysiological basis on which to test whether strengthening SMA—M1 connectivity can improve voluntary motor control in older adults. Funding NHMRC (1088295), Western Australian Department of Health.