Cognitive Contributions to Motor Learning

Abstract

Transcranial direct current stimulation (tDCS) can facilitate motor learning. However, the tDCS literature scarcely addresses whether stimulation to prefrontal brain regions affects motor learning, whether chunking together of individual actions can be influenced by tDCS, and whether there are age differences in how stimulation affects sequence learning. Here we completed a series of studies that examined the application of tDCS to the prefrontal cortex (PFC), motor cortex (M1), or the presupplementary motor area (preSMA) and its impact on motor sequence learning to understand the neural bases of motor learning. First, we found both left and right PFC stimulation slowed reaction time decreases and chunking. Stimulation to the preSMA lowered reaction time but came at the expense of a higher number of chunks. and tDCS over M1 helped with reaction time decreases and chunking. Further, contrasts revealed the M1 group had overall faster reaction times and fewer chunks. In order to understand the sequence learning impairment of left PFC anodal tDCS group, we added a left PFC cathodal montage. The left PFC cathodal group demonstrated impaired learning, with longer reaction time and a greater number of chunks, results similar to the left PFC anodal montage. In experiment two, participants from the left PFC, M1, and sham tDCS groups returned for a fourth session to assess long-term effects of tDCS. Participants completed a single session of practice without tDCS on the same sequences assigned to them the year before. We found the M1 tDCS group reduced reaction time at a faster rate relative to sham and the left PFC group demonstrated less forgetting over the course of a year, but overall slower reaction times. Finally, we determined how tDCS applied to the same four brain regions as in the first study affected sequence learning and chunking in older adults. We found no age differences regarding stimulation effects on reaction time reductions; both age groups benefited from M1 stimulation, whereas stimulation to the prefrontal cortices impaired learning. However, we did find age-group differences in chunking. Stimulation to M1 helped chunking processes for both age groups and to a greater extent for older adults. Thus, our findings suggest that regardless of age, stimulation to prefrontal cortices impairs learning, likely interfering with the automatization of sequence, whereas stimulation to M1 facilitates learning, especially in chunk formation. In light of our findings, we suggest the Cognitive framework for Sequential Motor Behavior (C-SMB), a framework that accounts for motor sequence learning should be modified to account for our findings.