Samuel Pleasure, MD, Neurology, 05:37PM Mar 22, 2013
As we age we are all aware that our ability to learn a new motor skill is impaired compared to when we were younger. Normal aged individuals have clearly measurable decrements in their ability to learn certain kinds of motor tasks. A wide range of recent studies have indicated a role for motor cortex in learning new motor skills and a recent study in Annals of Neurology (Zimerman et al., January 2013) has provided some interesting new information along these lines. The authors compared normal young (22-31yo) vs older (58-88yo) men in their success in retaining a new motor task based on a finger tapping task (based on sequences of pressing using a 4 button keyboard). Unsurprisingly the younger participants were significantly better at learning this new task (ie they retained their skill for 24 hours better after the training sessions). The striking finding was that if the older individuals received a regimen of transcranial direct current stimulation to their contralateral (ie the side controlling the hand performing the task) motor cortex concurrent with the training sessions that the older individuals performed signficantly better on retention of the task. The slope of improvement at the task in the older subjects was essentially restored to that of the young subjects in this paradigm. Young subjects got no benefit for this treatment. The investigators used a sham controlled, blinded, cross-over design to avoid bias.
Thinking about what this means in the context of other studies on the role of motor cortex in motor task learning led the authors to propose that the tDCS promotes a transient state of enhanced excitatory plasticity in the cortex of the older subjects that helped them to learn this task but that the baseline level of plasticity in younger subjects was already sufficient that no further augmentaton occurred. Since we don't know the specific cellular effects of tDCS, certainly other interpretations are possible - eg tDCS could affect inhibitory networks that are normally inhibiting new learning in older individuals. However, despite the "black box" like effect being seen here, I still find it rather interesting that such a simple and seemingly crude manipulation could have measurable and significant effects. Certainly there are a multitude of potentially useful interventions that one could try with this approach, and no doubt these will follow soon - and perhaps become the subject of late night TV ads sometime soon!