Motor learning has been associated with changes in GABA concentration in the motor cortex (M1) (Floyer-Lea et al. 2006) and GABAergic medication can disrupt the learning process (Donchin et al. 2002; Bütefisch et al. 2000). Indeed, a decrease in GABA concentration appears to be crucial for motor cortex reorganization elicited by motor learning (Jacobs & Donoghue 1991).
In the paper that I'll write about in this post, Charlotte Stagg and her colleagues from Oxford tested the hypothesis that the responsiveness of the GABA system correlated with the amount of motor learning across individuals (Stagg et al. 2011). In other words, if your brain can easily drive your GABA concentration down, can you learn faster or better a new motor task?
In a previous study, Stagg and colleagues demonstrated that anodal tDCS applied during 10 minutes over the left M1 (which control the right hand) leads to a decrease in GABA concentration as measured by MRS spectroscopy (Stagg et al. 2009). Based on those results, they wonder whether the decrease in GABA concentration elicited by anodal tDCS would correlate with the amount of motor learning.
To probe motor learning, they used a serial reaction time task (SRTT) where subjects have to press keys with their right hand in response to visual stimuli on the screen. In this task, the reaction time (the time between the appearance of the stimulus on the screen and the button press) is used to quantify the amount of learning. Importantly, the visual stimuli are not presented at random but there is a pattern in their presentation order. Over the course of the learning, the subjects, who are aware of this pattern, react more and more quickly to the appearance of the visual stimuli on the screen. Therefore, their reaction time decreases with learning and the lower the reaction time is, the best the learning is.
In their study, Stagg and colleagues demonstrated that people with a more responsive GABA system learned better. In other words, the amount of decrease of GABA concentration due to tDCS was positively correlated with the amount of decrease in reaction time during the SRTT.
Interestingly, the subjects performed the SRTT in a fMRI scanner. Again, the responsiveness of the GABA system was correlated with the decreased activation in the left M1 (the one that controls the right hand) during the task.
In summary, this study highlights the importance of the responsiveness of the GABA system for motor learning. What I like in this paper is the way they used tDCS. It was not used to influence the learning but it was couple to MRS spectroscopy to characterize the responsiveness of the GABA system.
I think this is a really elegant study but I would have loved to get the data with cathodal stimulation. Indeed, in their original paper (Stagg et al. 2009), these authors demonstrate that both anodal and cathodal tDCS stimulation decreased GABA concentration. It naturally leads to the question of the use of cathodal tDCS to probe the responsiveness of the GABA system.
It would be really nice if the next step could involve the study of the responsiveness of the GABA system of stroke patients. Indeed, GABA concentration appears to play a big role for recovery from stroke (Clarkson et al. 2010).
2. Donchin O, Sawaki L, Madupu G, Cohen LG, Shadmehr R. Mechanisms influencing acquisition and recall of motor memories. Journal of neurophysiology. 2002;88(4):2114-23.
3. Bütefisch CM, Davis BC, Wise SP, et al. Mechanisms of use-dependent plasticity in the human motor cortex. Proceedings of the National Academy of Sciences of the United States of America. 2000;97(7):3661-5.
4. Jacobs K, Donoghue JP. Reshaping the cortical motor map by unmasking latent intracortical connections. Science. 1991.
5. Stagg CJ, Bachtiar V, Johansen-Berg H. The Role of GABA in Human Motor Learning. Current biology : CB. 2011;21(6):480-484.
6. 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.
7. Clarkson AN, Huang BS, MacIsaac SE, Mody I, Carmichael ST. Reducing excessive GABA-mediated tonic inhibition promotes functional recovery after stroke. Nature. 2010;468(7321):305-309.