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.

Enhanced motor learning in older adults is accompanied by increased bilateral frontal and fronto-parietal connectivity.

We recently demonstrated that older adults can benefit as much as younger adults from learning skills in an interleaved manner. Here we investigate whether optimized learning through interleaved practice (IP) is associated with changes in inter-regional brain connectivity and whether younger and older adults differ in such brain-behavior correlations. Younger and older adults practiced a set of three 4-element motor sequences in a repetitive or in an interleaved order for 2 consecutive days. Retention of the practiced sequences was evaluated 3 days after practice with functional images acquired simultaneously. A within-subject design was used so that subjects practiced sequences in the other condition (repetitive or interleaved) 2-4 weeks later. Using the psychophysiological interaction (PPI) analysis approach, we found that IP led to higher functional connectivity between the right and left dorsal lateral prefrontal cortex (DLPFC) and between the dorsal premotor cortex (PMd) and inferior parietal lobule (IPL) in older adults. Moreover, increased connectivity between these regions was significantly associated with the learning benefits of IP. In contrast, in younger adults, enhanced learning as a result of IP was associated with increased connectivity between DLPFC and the supplementary motor area (SMA) and the inferior frontal gyrus. These data suggest that though younger and older gain similar behavioral benefits from interleaved training, aging may alter the operation of brain networks underlying such optimized learning.