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Abstract:

Introduction: Transcranial direct current stimulation (tDCS) (1) is a non-invasive, painless cortical stimulation technique (2) that is well tolerated by healthy subjects and patients. Recent studies have demonstrated that non-invasive brain stimulation enhances memory formation and cortical plasticity for a variety of tasks including visuo-motor coordination (3), implicit motor learning (4) and probabilistic classification learning (5) in healthy volunteers. Here we tested whether tDCS affects also use-dependent plasticity, i.e. increase in neural and behavioral efficiency as a consequence of repeated practice. Therefore, either real or sham tDCS was applied during a single training session of practicing ballistic thumb movements. Retention was quantified 30 min after training, at the next day and one week later. Methods: 14 healthy young volunteers (age 18 to 30, 6 females, 4 left handed) were involved in the study. The subjects had to practice to flex their non-dominant thumb as fast as possible for 10 blocks, consisting of 20 movements each (1 movement every 3s). Between blocks there were 1min breaks to prevent fatigue, such that the overall duration of the training was 20min. In half of the subjects, real tDCS was applied during the first training session with the anode mounted over the contralateral primary motor cortex and the cathode placed on the contralateral shoulder. After the training, tDCS was removed and subjects performed a retention test of 1 block after a 20min break. Long-term retention was tested at the next day and one week later, each consisting of 3 blocks separated by 1 min breaks. The other half of the subjects followed the same experimental protocol but received sham tDCS which induces the same sensation without effectively stimulating the cortex. After a break of at least 3 months, subjects returned to the lab and re-learned the same task following the identical protocol but receiving the complementary stimulation intervention. This cross-over design allowed us to quantify the effect of real tDCS within subjects. In a subgroup of the subjects TMS was used to measure (1) cortical excitability, (2) SICI, and (3) ICF at a baseline and during 30min after the training (post). One TMS run consisted of 48 pulses (16 for each condition) and lasted approx. 7 min. After the training 4 post runs were recorded (separated by 1 min breaks). Results: First analyses indicate that real tDCS enhanced retention performance 1 day and particularly 1 week after training, as indicated by a significant training x stimulation interaction (F(16,160)=4.09, p<0.0001) which was revealed by an analysis of variance for repeated measurements. TMS measurements executed during the first 30 min after training did not reveal differences between the tDCS interventions. Conclusions: Our data show for the first time that tDCS has a positive effect on use-dependent plasticity. Importantly, tDCS did not affect training performance per se but rather memory consolidation. Its beneficial effect seems to be enhanced when the motor memory is repeatedly reactivated even if reactivation occurs one day after the tDCS intervention. Our results might have important implications for increasing the efficiency of motor practice in healthy subjects and rehabilitation settings. References: N° 1. Paulus W. Transcranial direct current stimulation (tDCS). Suppl Clin Neurophysiol 56: 249–54., 2003. 2. Nitsche M. A., Paulus W. Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. J Physiol 27: 633–9., 2000. 3. Reis J. et al. Noninvasive cortical stimulation enhances motor skill acquisition over multiple days through an effect on consolidation. PNAS 106: 1590-1595., 2009. 4. Nitsche M. A. et al. Facilitation of implicit motor learning by weak transcranial direct current stimulation of the primary motor cortex in the human. J Cogn Neurosci 15: 619–26., 2003. 5. Kincses T. Z. et al. Facilitation of probabilistic classification learning by transcranial direct current stimulation of the prefrontal cortex in the human. Neuropsychologia 42:113–7., 2004.