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Anodal transcranial direct current stimulation (tDCS) is a reliable technique to improve motor learning. We here wanted to test its potential to enhance associative verbal learning, a skill crucial for both acquiring new languages in healthy individuals and for language reacquisition after stroke-induced aphasia. We applied tDCS (20 min, 1 mA) over the posterior part of the left peri-sylvian area of 19 young right-handed individuals while subjects acquired a miniature lexicon of 30 novel object names. Every subject participated in one session of anodal tDCS, one session of cathodal tDCS, and one sham session in a randomized and double-blinded design with three parallel versions of the miniature lexicon. Outcome measures were learning speed and learning success at the end of each session, and the transfer to the subjects' native language after the respective stimulation. With anodal stimulation, subjects showed faster and better associative learning as compared to sham stimulation. Mood ratings, reaction times, and response styles were comparable between stimulation conditions. Our results demonstrate that anodal tDCS is a promising technique to enhance language learning in healthy adults and may also have the potential to improve language reacquisition after stroke.

Dopamine is a potent modulator of learning and has been implicated in the encoding of stimulus salience. Repetition, however, as required for the acquisition and reacquisition of sensorimotor or cognitive skills (e.g., in aphasia therapy), decreases salience. We here tested whether increasing brain levels of dopamine during repetitive training improves learning success. Forty healthy humans took 100mg of the dopamine precursor levodopa or placebo daily for 5 days in a randomized double-blind and parallel-group design. Ninety minutes later on each day, subjects were trained on an artificial vocabulary using a high-frequency repetitive approach. Levodopa significantly enhanced the speed, overall success, and long-term retention of novel word learning in a dose-dependent manner. These findings indicate new ways to potentiate learning in a variety of domains if conventional training alone fails.

German Federal Ministry of Education and Research and the German Society for Aphasia Research and Treatment.

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The ability of the central nervous system to form motor memories, a process contributing to motor learning and skill acquisition, decreases with age. Dopaminergic activity, one of the mechanisms implicated in memory formation, experiences a similar decline with aging. It is possible that restoring dopaminergic function in elderly adults could lead to improved formation of motor memories with training. We studied the influence of a single oral dose of levodopa (100mg) administered preceding training on the ability to encode an elementary motor memory in the primary motor cortex of elderly and young healthy volunteers in a randomized, double-blind, placebo-controlled design. Attention to the task and motor training kinematics were comparable across age groups and sessions. In young subjects, encoding a motor memory under placebo was more prominent than in older subjects, and the encoding process was accelerated by intake of levodopa. In the elderly group, diminished motor memory encoding under placebo was enhanced by intake of levodopa to levels present in younger subjects. Therefore, upregulation of dopaminergic activity accelerated memory formation in young subjects and restored the ability to form a motor memory in elderly subjects; possible mechanisms underlying the beneficial effects of dopaminergic agents on motor learning in neurorehabilitation.