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) demonstrates weakened connectivity between dorsal premotor cortex (PMd) and motor cortex (M1) with age. While this alteration is likely 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 PMd influence on early and late I-wave excitability in M1 of young and older adults. 22 young (mean ± standard deviation, 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 (MEP) recorded from 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. KEY POINTS: Interneuronal circuits responsible for late I-waves within primary motor cortex (M1) mediate projections from dorsal premotor cortex (PMd), but this communication likely changes with advancing age. We investigated the effects of intermittent theta burst stimulation (iTBS) to PMd on transcranial magnetic stimulation (TMS) measures of M1 excitability in young and older adults. We found that PMd iTBS facilitated M1 excitability assessed with posterior-anterior (PA, early I-waves) and anterior-posterior (AP, late I-waves) current TMS in young adults, with a stronger effect for AP TMS. M1 excitability assessed with AP TMS also increased in older adults following PMd iTBS, but there was no facilitation for PA TMS responses. We conclude that changes in M1 excitability following PMd iTBS are specifically reduced for the early I-waves in older adults, which could be a potential target for interventions that enhance cortical excitability in older adults. Abstract figure legend We investigated the effects of ageing on the influence of dorsal premotor cortex (PMd) on indirect (I) wave circuits within motor cortex (M1) by applying intermittent theta burst stimulation (iTBS) to PMd in young and older adults. Transcranial magnetic stimulation (TMS) with posterior-anterior (PA) current to M1 was used to assess early I-wave circuits, whereas anterior-posterior (AP) current TMS was used to assess late I-wave circuits. Although we found that PMd iTBS increased M1 excitability for PA and AP TMS measures in young adults, facilitation in PA TMS was absent in older adults, suggesting that the influence of PMd on early I-waves is weakened with advancing age. This article is protected by copyright. All rights reserved.