Transcranial direct current stimulation might decrease lower leg glucose uptake and asymmetries in people with multiple sclerosis

s Brain Stimulation 14 (2021) 1589e1707 (IHi) during the processing phase of a fast motor response, as well as how both properties influence RT, are currently not fully understood. In this study, data were collected from 25 young [22.1 ± 4.4 years (mean ± SD)] and 28 older [(67.3 ± 4.2 years (mean ± SD)] healthy adults. We adopted a bimodal approach combining edited magnetic resonance spectroscopy (MRS) and dual-site transcranial magnetic stimulation (dsTMS) for probing GABA+ levels in bilateral SM1 and task-related neurophysiological modulations in corticospinal excitability (CSE), and primary motor cortex (M1)-M1 and dorsal premotor cortex (PMd)-M1 IHi, respectively. Whereas GABA+ levels were assessed during rest, TMS metrics were collected in the preparatory and premotor period of a choice RT task. Our findings revealed that older as compared to younger adults exhibited a reduced bilateral CSE suppression. Furthermore, irrespective of the direction of the IHi, a reduced magnitude of long latency M1-M1 and PMdM1 disinhibition during the preparatory period was observed in older adults as compared to their younger counterparts. Importantly, in older adults, the GABA+ levels in bilateral SM1 were related to individual differences in RT. In contrast, in young adults, this relationship was absent. Furthermore, there were no associations between neither task-related neurophysiological modulations and SM1 GABA+ levels. In conclusion, this work contributes to a comprehensive understanding of how age-related changes in neurochemical and neurophysiological processes may underlie increases in RT.