White matter integrity correlates of implicit sequence learning in healthy aging.

Previous research has identified subcortical (caudate, putamen, hippocampus) and cortical (dorsolateral prefrontal cortex, DLPFC; frontal motor areas) regions involved in implicit sequence learning, with mixed findings for whether these neural substrates differ with aging. The present study used diffusion tensor imaging (DTI) tractography to reconstruct white matter connections between the known gray matter substrates, and integrity of these tracts was related to learning in the alternating serial reaction time task (ASRT) in younger and healthy older adults. Both age groups showed significant sequence learning (better performance to predictable, frequently occurring vs. less frequent events), with an age-related difference in the late learning stage. Caudate-DLPFC and hippocampus-DLPFC tract integrity were related to ASRT sequence learning, and these brain-behavior relationships did not differ significantly between age groups. Additionally, age-related decreases in caudate-DLPFC tract integrity mediated age-related differences in late stage sequence learning. Together, these findings complement studies of gray matter substrates underlying implicit sequence learning, and provide evidence for similar white matter integrity-sequence learning relationships in younger and healthy older adults.