The cerebellum is involved in implicit motor sequence learning

Abstract

Background: Implicit motor sequence learning (IMSL) is a cognitive function that allows us to execute multiple movements in a specific sequential order and plays a crucial role in our daily functional activities. Although the role of the basal ganglia network in IMSL is well-established, the exact involvement of the cerebellar network is less clear.

Aim: Here, we aimed to address this issue by investigating the effects of cerebellar transcranial direct-current stimulation (tDCS) on IMSL.

Methods: In this sham-controlled, crossover study in 45 healthy young adults, we used mixed-effects models to analyze sequence-specific (primary outcome) and general learning effects (secondary outcome) in the acquisition (during tDCS), short- (five minutes post-tDCS) and long-term consolidation (one week post-tDCS) phases of IMSL, as measured by the serial reaction time (SRT) task.

Results: Analyses based on response times (RTs) revealed that anodal tDCS over the cerebellum significantly increased sequence-specific learning during acquisition, compared to sham (anodal: M = 38.24 ms, sham: M = 26.78 ms, p = 0.032); did not affect general learning; and significantly slowed overall RTs (anodal: M = 362.03 ms, sham: M = 356.37 ms, p = 0.049). Accuracy-based analyses revealed that anodal tDCS reduced the probability of correct responses occurring in random trials versus sequential trials by 1.17%, p = 0.009, whereas sham tDCS had no effect, p = 0.999.

Conclusion: Our finding of enhanced sequence-specific learning, but not general learning, suggests that the cerebellar network not only plays a role in error correction processes, but also serves a sequence-specific function within the integrated motor learning network that connects the basal ganglia and cerebellum.

Keywords: basal ganglia; cerebellum; implicit motor sequence learning; motor learning; non-invasive brain stimulation; tDCS.

Figure 1

Schematic representation of the SRT task. [Left] On each trial, participants had to react as quickly as possible to the target location by pressing the spatially corresponding response key with the index finger of their dominant hand - in this example of three consecutive trials: C, B, V. Unknown to the participants, the target location followed a repeating sequence (e.g., 132342134142, with 1–4 referring to the target locations from left to right). [Right] Only the response keys C, V, B, N were visible to the participants. Taken from Firouzi et al. (2024).

Figure 2

Visualization of sequence-specific and general learning effects in the SRT task. Visualization of SSLE (primary outcome) and GLE (secondary outcome) in the eight- (used during acquisition and long-term consolidation) and three-block (used during short-term consolidation) SRT tasks. The decrease in RTs with repetition of the sequence across sequential blocks reflects general learning. The relative increase in RTs when the sequence is interrupted in the random block relative to preceding and subsequent sequential blocks reflects SSLE. The three-block task does not allow for analysis of GLE, as the second random block introduces noise in the data. Taken from Firouzi et al. (2024).

Figure 3

Schematic representation of the experimental design. Representation of the sham-controlled, counter-balanced experimental design.

Figure 4

Sequence-specific learning effects. Interaction effects between the factors stimulation condition, block type, and session. Estimated marginal means are displayed. Red lines denote random trials, blue lines denote sequential trials. Sequence-specific learning occurred at each point in time, and for each condition. SSLE was significantly larger in the anodal condition compared to sham during acquisition, as marked by an asterisk at the bottom of the leftmost panel. Abbreviations: SSLE sequence-specific learning effects.

Figure 5

General learning effects. Interaction effects between the factors stimulation condition, block, and session. Estimated marginal means are displayed. Red lines denote anodal tDCS, blue lines denote sham tDCS. Although anodal tDCS slowed overall motor performance, regardless of other factors, there was no significant influence of stimulation condition on general learning (secondary outcome).

Figure 6

Accuracy. Interaction effects between the factors stimulation condition, block, and session. Red lines denote random trials, blue lines denote sequential trials. Probabilities of correct responses are displayed. Error bars represent 95% confidence intervals. Asterisks indicate significant contrasts.