Understanding how to make learning more efficient and effective is an important goal in behavioral neuroscience. The notion of "desirable difficulties" asserts that challenges for learners during study result in superior learning. One "desirable difficulty" that has a robust benefit on learning is contextual interference (CI), in which different tasks are practiced in an interleaved order rather than in a repetitive order. This study is the first to combine functional imaging and paired-pulse transcranial magnetic stimulation to analyze the neural basis of the CI effect in skill learning. Difficulty during practice of a serial reaction time task was manipulated by presenting sequences of response locations in a repetitive or an interleaved order. Participants practiced 3 sequences for 2 days and were tested on day 5 to examine sequence-specific learning. During practice, slower response times (RT), greater frontal-parietal blood-oxygen-level-dependent (BOLD) signal, and higher motor cortex (M1) excitability were found in the interleaved condition compared to the repetitive condition. Consistent with the CI effect, we found faster RT, decreased BOLD signal in frontal-parietal regions, and greater M1 excitability during the day 5 retention task when subjects had practiced interleaved sequences. Correlation analyses indicated that greater BOLD signal in contralateral sensorimotor region and M1 excitability during interleaved practice were interrelated. Furthermore, greater BOLD signal in prefrontal, premotor and parietal areas and greater M1 excitability during interleaved practice correlated with the benefit of interleaved practice on retention. This demonstrates that interleaved practice induces interrelated changes in both cortical hemodynamic responses and M1 excitability, which likely index the formation of enhanced memory traces and efficient long-term retrieval.
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