M1 recruitment during interleaved practice is important for encoding, not just consolidation, of skill memory

Abstract

The primary motor cortex (M1) is crucial for motor skill learning. We examined its role in interleaved practice, which enhances retention (vs. repetitive practice) through M1-dependent consolidation. We hypothesized that cathodal transcranial direct current stimulation (ctDCS) to M1 would disrupt retention. We found that ctDCS reduced retention due to weakened encoding during acquisition, not disrupted consolidation. These results highlight M1's broad role in encoding and retention of novel motor skills.

Fig. 1. The experimental procedure.

a Motor sequence learning task (specifically, a discrete sequence production task) was used^–, each of which required the execution of six key presses with the left index finger only that was directed by the presentation of a visual signal on the computer display. b During the practice phase, individuals in the IP-ctDCS group were administered cathodal stimulation at the right M1. Practice included nine training blocks of 21 trials organized in either a repetitive or an interleaved format. Tests were administered prior to training (Pre) and following training at Post_5min, Post_6h, and Post_24h. All tests consisted of 21 trials that included seven trials of each of the three sequences in a repetitive format. Stimulation (real or sham) was only present during the training blocks.

Fig. 2. Total response time (ms) for baseline (Pre), training, and post-practice time points (Post_5min, Post_6h, Post_24h).

a IP led to greater long-term performance due to offline improvement in the immediate (5 min), medium-term (6 h), and long-term (24 h) compared to the final block of practice, whereas RP shows significant forgetting at all post-practice recall time points compared to the end of practice. This replicates previous findings on the influence of practice structure on learning novel motor skills^–. b IP-ctDCS at M1 disrupted the encoding of a motor sequence memory for later offline consolidation. This was reflected in IP-ctDCS group showing significantly greater TT than IP-Sham for all training blocks except the final one. However, inhibitory ctDCS to M1 did not prevent subsequent offline consolidation, as shown by continuing post-training decreases in TT (i.e., performance improvements) for both IP groups in the medium-long term (6–24 h).

Fig. 3. tDCS montage used during practice in IP.

a Cathodal stimulation for the interleaved practice, real tDCS condition involved placement of the cathode 20% of the auricular measurement from Cz (determined based on the International 10–20 system) which placed this electrode above C4 with a reference electrode at the left supraorbital region. Participants in sham stimulation conditions (Repetitive-Sham, Interleaved-Sham) involved the same electrode configuration, but stimulation was only delivered for 30-s at the beginning and end of the training period. b, c The expected field intensity of 2 mA tDCS at C4 and the associated current flow for this electrode montage was modeled using HD-Explore^TM (Soterix Medical Inc., New York, NY). This figure illustrates the right M1, noted with a white circle.