Multi-session Transcranial Direct Current Stimulation Over Primary Motor Cortex Facilitates Sequence Learning, Chunking, and One Year Retention

Abstract

Transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) can facilitate motor learning, but it has not been established how stimulation to other brain regions impacts online and offline motor sequence learning, as well as long-term retention. Here, we completed three experiments comparing the effects of tDCS and sham stimulation to the prefrontal cortex (PFC), M1, and the supplementary motor area complex to understand the contributions of these brain regions to motor sequence learning. In Experiment 1, we found that both left and right PFC tDCS groups displayed a slowing in learning in both reaction time and number of chunks, whereas stimulation over M1 improved both metrics over the course of three sessions. To better understand the sequence learning impairment of left PFC anodal stimulation, we tested a left PFC cathodal tDCS group in Experiment 2. The cathodal group demonstrated learning impairments similar to the left PFC anodal stimulation group. In Experiment 3, a subset of participants from the left PFC, M1, and sham tDCS groups of Experiment 1 returned to complete a single session without tDCS on the same sequences assigned to them 1 year previously. We found that the M1 tDCS group reduced reaction time at a faster rate relative to the sham and left PFC groups, demonstrating faster relearning after a one-year delay. Thus, our findings suggest that, regardless of the polarity of stimulation, tDCS to PFC impairs sequence learning, whereas stimulation to M1 facilitates learning and relearning, especially in terms of chunk formation.

Anodal Transcranial Direct Current Stimulation Over Prefrontal Cortex Slows Sequence Learning in Older Adults

“…Similar to what we reported with young adults (Greeley et al, 2020), we observed impaired sequence learning after the application of anodal tDCS over the left or right PFC for older adults. In combination, these two studies suggest a role for the bilateral prefrontal cortices in the early stages of sequence learning, regardless of age.…”

“…The resulting current distribution is similar between our left and right prefrontal stimulation montages, making it less surprising that the two groups would show similar behavioral effects. In our prior work with young adults, we found that both anodal and cathodal stimulation applied to the left PFC resulted in slower sequence learning (Greeley et al, 2020), further supporting the complex effects arising with prefrontal stimulation. Regardless, it is important to know for the future design of potential therapies combining brain stimulation with training that both young and older adults exhibit sequence learning impairments with prefrontal tDCS.…”

“…Unlike reaction time, however, we observed that this impairment was no different between the two age groups. Impaired chunking in the left and right PFC tDCS groups in older adults is surprising considering the involvement of the prefrontal cortices during chunking (Pammi et al, 2012;Wymbs et al, 2012), the positive relationship between working memory and chunking in older adults , and our previously reported findings demonstrating that prefrontal tDCS impaired chunking in young adults (Greeley et al, 2020). It remains an open question, however, whether using inhibitory stimulation (cathodal) over the prefrontal cortices would enhance chunking and whether it would affect older and young adults similarly.…”

“…We observed that right prefrontal anodal tDCS impaired learning as measured by both reaction time and the number of chunks limited to session one. Using the same design, we previously reported that anodal tDCS over the right PFC also impaired learning for young adults in session one (Greeley et al, 2020), indicating that the right PFC is involved for both age groups. However, it is of importance to note that when compared to young adults, right prefrontal stimulation was especially harmful to early learning (session one) to older adults as indexed by slowed reaction time.…”

Black-box testing in motor sequence learning

“…Neural responsiveness decreases with healthy aging, which is why enhanced plasticity induced by atDCS 60 likely benefits older adults 3 . Greeley and colleagues 61 reported improved motor chunk formation related to atDCS applied to M1 in young adults.…”