Age related differences in the neural substrates of motor sequence learning after interleaved and repetitive practice.

Practice of tasks in an interleaved order generally induces superior retention compared to practicing in a repetitive order. Younger and older adults practiced serial reaction time tasks that were arranged in a repeated or an interleaved order on 2 successive days. Retention was tested on Day 5. For both groups, reaction times in the interleaved condition were slower than the repetitive condition during practice, but the reverse was true during retention on Day 5. After interleaved practice, changes in M1 excitability measured by paired-pulse TMS were greater than after repetitive practice, and this effect was more pronounced in older adults. Moreover, the increased M1 excitability correlated with the benefit of interleaved practice. BOLD signal was also increased for interleaved compared to repetitive practice in both groups. However, the pattern of correlations between increased BOLD during practice and subsequent benefit of the interleaved condition differed by group. In younger adults, dorsolateral-prefrontal activity during practice was related to this benefit, while in older adults, activation in sensorimotor regions and rostral prefrontal cortex during practice correlated with the benefit of interleaving on retention. Older adults may engage compensatory mechanisms during interleaved practice such as increasing sensorimotor recruitment which in turn benefits learning.