The after effects of anodal tDCS in cortical excitability and intracortical inhibition during resting and active states in elderly

Abstract

Transcranial direct current stimulation (tDCS) has great potential for the neurorehabilition of stroke patients since it was shown to improve functional motor recovery if applied to the primary motor cortex (M1) (Hummel 2005; Lindenberg 2012). Transcranial magnetic stimulation (TMS) can be used to measure the after effects like modulations in the cortical excitability and intracortical inhibition (SICI). So far most studies tested the after effects in the hand muscles in rest and in relatively young subjects (Nitsche 2001; Antal 2007). However, among severe stroke patients motor evoked potentials (MEPs) are hard to induce in distal muscles. In the present study, we tested, first, whether the after effect of anodal tDCS induces similar effects in a more proximal muscle. Secondly, we tested whether the after effects of anodal tDCS can also be seen in a slightly contracted muscle. Finally, we tested if tDCS-effects typically seen in young people can also be observed in an elderly population (i.e. with an age better matched to stroke patients). In a crossover experiment, sixteen healthy right-handed elderly (age:65±4.8) received either anodal tDCS (1mA) or sham tTDCS to measure the after effects in the right extensor carpi radialis (ECR). TMS was used to measure the MEPs and short intracortical inhibition (SICI) before and directly after 20 min of tDCS. TMS measurements were perfromed while the ECR was in a resting or active motor state. Subjects were completely relaxed during the rest sessions, while during the active sessions the subjects contracted the ECR (10% of the maximum voluntary contraction). In elderly, the cortical excitability (MEP amplitude) was increased after applying anodal tDCS compared to sham tDCS. However, this increase was only significant and more clearly seen for the rest state (28%±13; p<0.05) and not for the active state (10%±21; p=0.65). SICI did not change significantly when compared between anodal and sham tDCS in rest (p=0.13) or in the active sessions (p=0.71). As expected, inhibition was generally released during the active sessions. Our results show that anodal tDCS modulates the cortical excitability of a more proximal muscle (ECR) in the same way as in hand muscles. We found no evidence that these effects are mediated by changes of SICI. Moreover, the after effects of anodal tDCS were more pronounced in a resting state than in an active state. Furthermore, we show that tDCS also has an effect in elderly. Overall, our results suggest that the ECR could also be a good targeting muscle for a stroke-neurorehabilitation-protocol but that the after effects tested by TMS might be more consistent when the target muscle is relaxed.