Abstract
BackgroundNeuromodulatory effects of transcranial direct current stimulation (tDCS) in older humans have shown heterogeneous results, possibly due to sub-optimal stimulation protocols associated with limited knowledge about optimized stimulation parameters in this age group. We systematically explored the association between the stimulation dosage of cathodal tDCS and induced after-effects on motor cortex excitability in the elderly. MethodThirty-nine healthy volunteers in two age groups, namely Pre-Elderly (50–65 years) and Elderly (66–80 years), participated in the study. Ten sessions of cathodal tDCS, with a combination of four intensities (1, 2, 3 mA and sham) and three durations (15, 20, 30 min) were conducted over the M1 in each participant. Cortical excitability changes were monitored with TMS-induced motor evoked potentials (MEPs) for up to 2 h after stimulation. ResultsMotor cortex excitability was reduced by cathodal stimulation intensities of 1 and 3 mA in both age groups, in accordance with results observed in the younger age groups of previous studies. For the 2 mA stimulation condition, an age-dependent conversion of plasticity into a stimulation duration-dependent excitability enhancement was observed in the Pre-Elderly group, whereas in the Elderly group, LTD-like plasticity was preserved, or abolished, depending on stimulation duration. ConclusionThe LTD-like plasticity effects induced by cathodal tDCS originally described in young adults are also observable in older humans, but non-linearities of the resulting plasticity were partially preserved only in the Pre-Elderly, but not the Elderly group. These results aid in understanding age-dependent plasticity dynamics in humans, and to define more efficient tDCS protocols in the aging brain.
Highlights
Aging in otherwise healthy humans is associated with a decline in cognitive and motor performance, which negatively affects the quality of life [1e3]
The results showed no age-dependent differences for the excitability-diminishing effects of cathodal transcranial direct current stimulation (tDCS), while the excitatory effect of anodal tDCS declined depending on age [37] for this specific tDCS protocol
The main results of this study show that cathodal tDCS over the motor cortex in healthy humans with advanced age reduce cortical excitability with 1 and 3 mA stimulation intensity, with longer lasting effects in the Pre-Elderly group
Summary
Aging in otherwise healthy humans is associated with a decline in cognitive and motor performance, which negatively affects the quality of life [1e3] These age-dependent alterations are caused by structural and neurophysiological alterations of the brain at the cellular, circuit, and systems level [4e6]. Conclusion: The LTD-like plasticity effects induced by cathodal tDCS originally described in young adults are observable in older humans, but non-linearities of the resulting plasticity were partially preserved only in the Pre-Elderly, but not the Elderly group. These results aid in understanding agedependent plasticity dynamics in humans, and to define more efficient tDCS protocols in the aging brain
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