Transcranial direct current stimulation (tDCS) modulates human behavior, neuronal patterns and metabolite concentrations, with exciting potential for neurorehabilitation. However, the understanding of tDCS-induced alterations on the neuronal level is incomplete and conclusions from young adults, in whom the majority of studies have been conducted, cannot be easily transferred to older populations. Here, we investigated tDCS-induced effects in older adults (N=48, age range 50-79 years) using magnetic resonance spectroscopy to quantify gamma-aminobutyric acid (GABA) levels as well as resting-state functional magnetic resonance imaging to assess sensorimotor network strength and inter-hemispheric connectivity. In a randomized, counterbalanced, cross-over design, we applied anodal (atDCS), cathodal (ctDCS) and sham (stDCS) stimulation over the left sensorimotor region. We observed a significant reduction of GABA levels after atDCS compared to stDCS, reflecting preserved neuromodulatory effect of atDCS in older adults. Moreover, resting-state functional coupling was decreased during atDCS compared to stDCS, most likely indicating augmented efficiency in brain network functioning. Increased levels of inter-hemispheric connectivity with age were diminished by atDCS, suggesting stimulation-induced functional decoupling. Further, the magnitude of atDCS-induced local plasticity was related to baseline functional network strength. Our findings provide novel insight into the neuronal correlates underlying tDCS-induced neuronal plasticity in older adults, and thus might help to develop tDCS interventions tailored to the aging brain.
Transcranial direct current stimulation (tDCS) modulates human behavior, neuronal patterns and metabolite concentrations, with exciting potential for neurorehabilitation. However, the understanding of tDCS-induced alterations on the neuronal level is incomplete and conclusions from young adults cannot be easily transferred to older populations. We used a systematic multimodal imaging approach in order to investigate the neurophysiological effects of tDCS in older adults and found stimulation-induced effects on gamma-aminobutyric acid levels, reflecting augmented local plasticity, and functional connectivity, suggesting modulation of network efficiency. Our findings may help to reconcile some of the recent reports on the variability of tDCS-induced effects, not only implicating age as a crucial modulating factor, but detailing its specific impact on functionality of neural networks.
This work was supported by the Bundesministerium für Bildung und Forschung (01GQ1424A) and the German-Israeli Foundation for Scientific Research and Development (I-1299-105.4/2015). The authors thank two anonymous reviewers for their helpful comments on an earlier version of the manuscript. The authors declare no competing financial interests.