title: Bihemispheric transcranial direct current stimulation enhances effector-independent representations of motor synergy and sequence learning.
abstract: Complex manual tasks-everything from buttoning up a shirt to playing the piano-fundamentally involve two components: (1) generating specific patterns of muscle activity (here, termed "synergies"); and (2) stringing these into purposeful sequences. Although transcranial direct current stimulation (tDCS) of the primary motor cortex (M1) has been found to increase the learning of motor sequences, it is unknown whether it can similarly facilitate motor synergy learning. Here, we determined the effects of tDCS on the learning of motor synergies using a novel hand configuration task that required the production of difficult muscular activation patterns. Bihemispheric tDCS was applied to M1 of healthy, right-handed human participants during 4 d of repetitive left-hand configuration training in a double-blind design. tDCS augmented synergy learning, leading subsequently to faster and more synchronized execution. This effect persisted for at least 4 weeks after training. Qualitatively similar tDCS-associated improvements occurred during training of finger sequences in a separate subject cohort. We additionally determined whether tDCS only improved the acquisition of motor memories for specific synergies/sequences or whether it also facilitated more general parts of the motor representations, which could be transferred to novel movements. Critically, we observed that tDCS effects generalized to untrained hand configurations and untrained finger sequences (i.e., were nonspecific), as well as to the untrained hand (i.e., were effector-independent). Hence, bihemispheric tDCS may be a promising adjunct to neurorehabilitative training regimes, in which broad transfer to everyday tasks is highly desirable.
authors.affiliation: Institutes of Cognitive Neuroscience and Neurology, University College London, London WC1N 3AR/WC1N 3BG, United Kingdom, and Nuffield Department of Clinical Neurosciences and Department of Experimental Psychology, Oxford University, Oxford OX3 9DU/OX1 3UD, United Kingdom.
authors.forename: Sheena
authors.initials: S
authors.lastname: Waters-Metenier
authors.forename: Masud
authors.initials: M
authors.lastname: Husain
authors.forename: Tobias
authors.initials: T
authors.lastname: Wiestler
authors.forename: Jörn
authors.initials: J
authors.lastname: Diedrichsen
mesh_terms: D015331:Cohort Studies; D004311:Double-Blind Method; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D007839:Functional Laterality / Q000502:physiology*; D006801:Humans; D007858:Learning / Q000502:physiology*; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation / Q000379:methods*; D055815:Young Adult
keywords: generalization; learning; motor cortex; motor sequence; motor synergy; tDCS