1. Reis J, Schambra HM, Cohen LG, et al. Noninvasive cortical stimulation enhances motor skill acquisition over multiple days through an effect on consolidation. Proceedings of the National Academy of Sciences of the United States of America. 2009;106(5):1590-5
In this paper, Reis and colleagues demonstrate that anodal tDCS over the primary motor cortex improves motor skill learning by increasing offline improvements (from one session to the next one) but not online improvements (within a session). Using tDCS over the cerebellum, Galea and colleagues demonstrated that cerebellar stimulation speeds up learning in a visuomotor rotation task. Those two studies highlight a dissocation between the role of the cerebellum (online learning) and the role of the primary motor cortex (retention/ offline learning).
2. Vines BW, Cerruti C, Schlaug G. Dual-hemisphere tDCS facilitates greater improvements for healthy subjectsʼ non-dominant hand compared to uni-hemisphere stimulation. BMC neuroscience. 2008;9:103.
In this study, Vines demonstrate that dual-hemisphere tDCS is more efficient than uni-hemispheric tDCS. This important for stroke rehabilitation where people either increase excitability of the lesioned hemisphere with anodal tDCS or decrease excitability of the contralateral hemisphere with cathodal tDCS to improve recovery (see #5). This study shows that the combination of both could be interesting to study.
3. Stagg CJ, Best JG, Stephenson MC, et al. Polarity-sensitive modulation of cortical neurotransmitters by transcranial stimulation. The Journal of neuroscience. 2009;29(16):5202-6.
In this study, Stagg and colleagues the impact of tDCS on the concentration of GABA and glutamate neurotransmitters. Using MRS spectroscopy, they show that both anodal and cathodal tDCS decrease GABA concentration below the stimulating electrode (primary motor cortex). In addition, cathodal stimulation also decreased glutamate concentration. Their follow-up study (about which I wrote here) demonstrates that the responsiveness of the motor cortex to tDCS is correlated with the speed of learning in a given skill learning task. Finally, this study might also explain some of the effect of tDCS on motor learning that I found in my paper.
4. Fritsch B, Reis J, Martinowich K, et al. Direct Current Stimulation Promotes BDNF-Dependent Synaptic Plasticity: Potential Implications for Motor Learning. Neuron. 2010;66(2):198-204.
This is the first in vitro model of tDCS that shed light on the neural basis of tDCS. It highlights the importance of the genotype for the response to tDCS and to motor skill learning in general.
5. Hummel FC, Celnik P a, Giraux P, et al. Effects of non-invasive cortical stimulation on skilled motor function in chronic stroke. Brain. 2005;128(Pt 3):490-9.
In this study, the authors demonstrate that anodal tDCS used on the lesioned hemisphere increases motor cortex excitability of chronic stroke patients as assessed by TMS measurements and their motor function as assessed by the Jebsen-Taylor Hand Function Test. This same test was used by Fregni and colleagues to study the influence of cathodal tDCS on the contralesional hemisphere. They found similar improvements than in the Hummel study. Together, those studies show that decreasing the excitability of the contralesional hemisphere or increasing the excitability of the ipsilesional hemisphere improve recovery of hand function of chronic stroke patients. Modulating the excitability of contralesional or ipsilesional hemisphere can also be performed with repetitive TMS protocols (Grefkes and Fink 2011).
6. Benninger DH, Lomarev MP, Lopez G, et al. Transcranial direct current stimulation for the treatment of Parkinsonʼs disease. Journal of Neurology, Neurosurgery & Psychiatry. 2010;81(10):1105-11.
This is one of the first clinical trial involving tDCS and Parkinson Disease patients. The results of the trial show some prolonged improvements of tDCS on motor funciton of PD patients. However, we need to better understand what are the optimal tDCS paremeters in order to optimize the influence of tDCS on improving motor function in patients (montage, intensity, during or before practice, etc...)
There are plenty of reviews discussing transcranial direct current stimulation. I picked this one for its historical perspective... Here is another one that I like...
7. Priori A. Brain polarization in humans: a reappraisal of an old tool for prolonged non-invasive modulation of brain excitability. Clinical Neurophysiology. 2003;114(4):589-595. This is a really old tool...