The role of the cerebellum in motor learning of dexterous control and interaction with aging remains incompletely understood. We compared the effect of age and cerebellar transcranial direct current stimulation (CRB-tDCS) on motor learning in two different manual dexterity tasks, visuomotor force control vs. effector selection (independent finger movements).

Twenty younger and 20 older adults were randomized (double-blinded) to anodal or sham CRB-tDCS during dexterity training over three consecutive days, and followed-up at day 10. Motor learning was measured as (i) overall learning (across 10 days), (ii) within-day (short-term) learning, (iii) between-day learning (consolidation), and (iv) retention (long-term learning; day 3 to day 10).

Younger and older subjects showed significant overall learning in both tasks. Subjects with poor initial performance showed stronger learning. No effects of CRB-tDCS were observed in younger adults. A significant Age*CRB-tDCS interaction showed that CRB-tDCS improved within-day learning in finger independence (improved reaction time in effector selection) in older adults. However, a significant Age*CRB-tDCS interaction showed that CRB-tDCS impacted consolidation negatively in older subjects. No stimulation effects were found on retention. Finally, we found that degree of within-day learning in finger independence (change in reaction times) correlated with baseline (pre-training) reaction times in both young and old subjects.

The results suggest that CRB-tDCS may improve short-term learning of manual dexterity in older adults in a task-dependent manner, specifically in difficult tasks requiring effector (action) selection. However, cerebellar tDCS stimulation may also interfere with consolidation in older subjects. These results need confirmation in a larger sample.