Modulation of dorsal premotor cortex differentially influences I-wave excitability in primary motor cortex of young and older adults

Previous research using transcranial magnetic stimulation (TMS) has demonstrated weakened connectivity between dorsal premotor cortex (PMd) and motor cortex (M1) with age. While this alteration is probably mediated by changes in the communication between the two regions, the effect of age on the influence of PMd on specific indirect (I) wave circuits within M1 remains unclear. The present study therefore investigated the influence of PMd on early and late I-wave excitability in M1 of young and older adults. Twenty-two young (mean +/- SD, 22.9 +/- 2.9 years) and 20 older (66.6 +/- 4.2 years) adults participated in two experimental sessions involving either intermittent theta burst stimulation (iTBS) or sham stimulation over PMd. Changes within M1 following the intervention were assessed with motor-evoked potentials (MEPs) recorded from the right first dorsal interosseous muscle. We applied posterior-anterior (PA) and anterior-posterior (AP) current single-pulse TMS to assess corticospinal excitability (PA(1mV); AP(1mV); PA(0.5mV), early; AP(0.5mV), late), and paired-pulse TMS short intracortical facilitation for I-wave excitability (PA SICF, early; AP SICF, late). Although PMd iTBS potentiated PA(1mV) and AP(1mV) MEPs in both age groups (both P < 0.05), the time course of this effect was delayed for AP(1mV) in older adults (P = 0.001). Furthermore, while AP(0.5mV), PA SICF and AP SICF were potentiated in both groups (all P < 0.05), potentiation of PA(0.5mV) was only apparent in young adults (P < 0.0001). While PMd influences early and late I-wave excitability in young adults, direct PMd modulation of the early circuits is specifically reduced in older adults.

Automated summary

Previous research has shown that the connectivity between dorsal premotor cortex (PMd) and motor cortex (M1) weakens with age. However, the influence of PMd on specific indirect wave circuits within M1 and its effect on age are unclear. This study investigates the influence of PMd on early and late I-wave excitability in M1 of young and older adults.