Interleaved Training and Cerebellar Transcranial Direct Current Stimulation Can Enhance Fine Motor Transfer Learning
A crucial goal of many real-world sequence learning tasks is the transfer of knowledge to novel sequences. The current work utilized the serial reaction time task, in which participants press keys in response to visual cues that appear in sequence. The first study examined the ability to transfer to new sequences after practicing sequences in a repetitive order compared to a non-repeating interleaved order. Interleaved practice resulted in better performance on repetitive or interleaved novel sequences than repetitive practice. Interleaved training may reduce interference from sequence-specific knowledge when the learner is faced with new variations of a task. In the second study, participants practiced interleaved sequences in a functional magnetic resonance imaging (fMRI) scanner and received a transfer test of novel sequences. Transfer ability was positively correlated with cerebellar blood oxygen level dependent (BOLD) activity during practice, indicating that greater engagement of the cerebellum during training resulted in better subsequent transfer performance. The cerebellum is thought to contribute to error-based learning through the instantiation of internal models (Wolpert, Miall, & Kawato, 1998). When novel variations of a task are encountered, it is hypothesized that relevant features from other previously-formed internal models are selected and form the basis for performance on the novel task (Imamizu, Higuchi, Toda, & Kawato, 2007). Those learners in which internal models are more readily formed may be better at later tests of transfer. Based on these results, the cerebellum was targeted with transcranial direct current stimulation (tDCS) during training in order to enhance transfer learning. Results indicate that anodal tDCS can lead to better transfer to novel sequences, but this change was apparent only after a delay. On the other hand, cathodal tDCS had a detrimental online effect on learning. These results are consistent with the theory that the cerebellum plays a large role particularly in the early learning stage of fine motor skills. Overall, this work demonstrates that easily implementable and relatively inexpensive manipulations can improve transfer learning of motor sequences.