Anodal transcranial direct current stimulation: the effects on corticospinal excitability and motor performance
2017-02-27T03:46:53Z (GMT) by
Transcranial direct current stimulation (tDCS) is a noninvasive neuromodulatory technique extensively used as a method in neuroscience research and in treatment of various neurological and psychiatric disorders. Application of anode over the target cortical area is called anodal tDCS (a-tDCS), increases corticospinal excitability (CSE). Although a-tDCS is a promising technique for brain modulation, optimal parameters of stimulation are still not entirely set. The broad aim of this thesis was to provide optimal parameters of a-tDCS for enhancement of CSE and to establish a-tDCS protocol for induction of larger CSE changes and motor performance improvement, which lasts longer. Prior to the main studies, a systematic review and meta-analysis was conducted to verify the effects of a-tDCS with different stimulation parameters on CSE and motor performance in both healthy individuals and subjects with stroke. From the findings of the meta-analysis, it was concluded that longer application of a-tDCS and larger current densities are associated with longer and larger lasting effects. The trend of changes was in favour of motor performance improvement in both healthy individuals and patients with stroke. Healthy right-handed participants were recruited for all studies. Sample size was calculated based on the power and effect size (Appendix 1) of a pilot data analysis. Surface electrodes recorded electromyography activity of extensor carpi radialis (ECR) (Studies 3-5 and 7) and first dorsal interossei (FDI) muscles (Study 6) at rest. Single-pulse transcranial magnetic stimulation (TMS) was used to assess primary motor cortex (M1) excitability changes by recording peak-to-peak amplitude of motor evoked potentials (MEPs) of the target muscle(s). The first two experimental studies were conducted to test the intra- and inter-session reliability of the elicited MEPs (Study 2) and to fine-tune the set-up for application of TMS as an assessment tool and a-tDCS as the intervention (Study 3). Once, the set-up has been developed and tested, 4 other studies were conducted. Study 4 investigated the optimal current density for application of a-tDCS using four different current intensities (0.3, 0.7, 1.4 and 2 mA) with a constant electrode size of 6×4 cm2. The findings showed that the smallest current intensity (0. 3 mA) produces significantly larger CSE changes than the next two higher current densities (0.7 and 1.4 mA) with considerably less total charge to the cortical area. In study 5 the role of active electrode sizes on the induced M1 CSE changes was assessed. It was found that reducing stimulation electrode size (12 cm2) to one third of the conventional one (35cm2) increases the efficacy of a-tDCS for induction of larger M1 CSE. This increase could be due to spatially more focused stimulation. Study 6 investigated the optimal within-session repetition rate of a-tDCS applications (once, twice or three 10 minutes) and interval (5 or 25 minutes) between these stimulations for induction of longest lasting effects on M1 CSE and motor performance. The results showed that compared to a single 10 minutes stimulation, both twice and three times repetition of a-tDCS, induced excitability enhancements which lasted up to 24 hours. It should be noted that, significant improvement was only seen in motor performance following three times repetition with 25 minutes inter stimulus intervals. Study 7 investigated the effects of a novel noninvasive neuromodulatory paradigm named transcranial pulsed current stimulation (tPCS) on M1 CSE. Anodal-tPCS (a-tPCS) with short (a-tPCSSIPI) and long inter-pulse interval (a-tPCSLIPI) was compared to a-tDCS and sham a-tPCSSIPI. a-tPCSSIPI, but not a-tPCSLIPI and sham a-tPCSSIPI induced larger excitability changes in the human cortex compared to the conventional a-tDCS. Furthermore, a-tPCSSIPI induced larger CSE changes compared to a-tPCSLIPI. This suggests the importance of IPI in induction of CSE changes. This thesis demonstrated optimal parameters for a-tDCS application on healthy individuals. Establishing optimal parameters of stimulation is of particular importance to increase the a-tDCS lasting effects.