Despite holding significant promise for counteracting the deleterious effects of ageing on cognitive and motor function, little is known of the effects of facilitatory non-invasive brain stimulation (NBS) techniques on corticospinal excitability (CSE) in older adults. Thirty-three older adults (≥60 years) participated in four NBS sessions on separate days, receiving 10- and 20-min anodal transcranial direct current stimulation (atDCS), and 300 and 600 pulses of intermittent theta burst stimulation (iTBS) over the left M1. Motor-evoked potentials measured in the contralateral hand served as a measure of CSE before and for 30 min following each NBS intervention. At the group level, generalized post-stimulation CSE increases were observed (p < 0.001) with no significant differences between the two durations of each stimulation type (atDCS: p = 0.5; iTBS: p = 0.9). For individuals exhibiting overall facilitatory change to atDCS (‘responders’, n = 10), 20-min atDCS resulted in longer lasting CSE facilitation than 10 min. No such difference was observed between the two iTBS protocols. Considerable variability was observed inter-individually, where 52–58 % of the cohort exhibited the expected facilitation after each of the NBS protocols—as well as intra-individually, where 45–48 % of the cohort maintained consistent post-stimulation responses across the varying durations and types of stimulation. In conclusion, as shown previously in young adults, older adults demonstrate substantial variability in response to different facilitatory NBS protocols. Studies to assess the intra-individual reliability of these protocols are critical to progress towards translation of appropriate protocols (i.e. those that elicit the greatest response for each individual) into clinical practice.
Transcranial magnetic stimulation Theta burst stimulation Transcranial direct current stimulation Older adults Non-invasive brain stimulation Plasticity
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The research was supported by a National Health and Medical Research Council Project Grant (APP1050261; JJS, AJC, and MRH) and an Australian Research Council Discovery Project (DP130104317; JJS and MRH). The authors would like to thank Emily L. Goss for assistance with data collection and Michael I. Garry for helpful conversations regarding data analyses. Lastly, the authors would like to sincerely acknowledge all participants for volunteering their valuable time.
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