Our procedure of adapting list length minimized task perform

Our procedure of adapting list length minimized task performance difference across groups, which carries important advantages for interpreting group differences in activation (see Nagel et al., 2009; Luna et al., 2010; Poldrack, 2015). However, there are limitations to consider. First, we could not fully equalize the total number of to-be-remembered word pairs across age groups, as additional runs would have been too strenuous for the children and the older adults. This might have lowered the power of detecting SME in older adults due to having the least number of trials. However, given that older adults showed as robust an SME as younger adults, we think that the validity of the present results is not compromised. We also ran validation analyses by leaving out trials in children and younger adults so that all age groups were represented by the same number of trials for the first level R>O nmda (see Supplementary Material). Results remained largely unchanged, with similar SME regions found as well as same patterns of age differences in ROI analysis, supporting the validity of the analyses. Nevertheless, future lifespan comparisons will benefit from other routes to adapt task difficulty without confounding number of trials. The second limitation refers to the fact that our samples, particularly the older adults, were highly selective. In total, 97 out of 165 older adults were not invited to participate in the fMRI study due to failure in meeting the screening criteria. Similarly, children retained were possibly less representative of their cohort than the younger adults (although the latter were university students and hence also positively selected). It is likely that the selection procedures used in this study, while contributing to its internal validity, reduced the generalizability of the present findings to the general population. For older adults, Salami et al., (2012) showed that more activity in cognitive-control networks (including frontoparietal cortices) correlated negatively with memory performance. That is, among older adults, better performing persons showed less additional engagement of the control network relative to younger adults. In light of this finding, it is conceivable that the high-functioning nature of the older adults has contributed to the similarity of their SME patterns to the patterns observed in younger adults. This is in line with the between-person aspect of brain maintenance proposed by Nyberg et al. (2012). Given that maintenance likely does not exist in absolute terms, future studies are needed to delineate the boundary conditions within which healthy older adults may exhibit youth-like neural patterns. Likewise, the observed fMRI differences between children and younger adults may not be restricted to differences in successful memory formation. In particular, the high-performing children participating in the present study may not have under-recruited the middle frontal gyrus during SME relative to younger adults, but rather may have been quicker to disengage from encoding after the memory was formed. Finally, to reduce movement artifacts that are particularly prominent in children, their retrieval was conducted outside the scanner, leading to a change in retrieval context and slightly longer delay between encoding and retrieval that might have lowered children’s performance. Although we think that this cannot fully account the age difference found uniquely in middle frontal gyrus, but not other frontal or MTL regions, such difference in procedure is best avoided in future studies. Taken together, it is a challenging task to investigate age-related differences in memory mechanisms across the lifespan. Participants of different ages differ in a wealth of factors in addition to the phenomenon of interest (see Shing and Lindenberger, 2011). Our study is the first attempt at creating experimental setups that allow meaningful cross-sectional age comparisons in successful memory formation across the human lifespan. We believe that the utility and feasibility of such an approach are demonstrated, while the challenges and limitations that we encountered, particularly the selection bias across age groups and procedural differences as confounds, serve as important lessons for future studies, and indicate that there is room for further improvements in research design.