It has been proposed that cognitive reserve is supported by two neural mechanisms: neural compensation and neural reserve. The purpose of this study was to test how these neural mechanisms are solicited in aging in the context of visual selective attention processing and whether they are inter- or intra-hemispheric. Younger and older participants were scanned using fMRI during a visual letter-matching task with two attentional load levels. The results show that in the low-load condition, the older participants activated frontal superior gyri bilaterally; these regions were not activated in the younger participants, in accordance with the compensation mechanism and the Posterior-Anterior Shift in Aging (PASA) phenomenon. However, when task demand increased, the older participants recruited the same regions (parietal) as the younger ones, showing the involvement of a similar neural reserve mechanism. This result suggests that successful cognitive aging relies on the concurrent use of both neural compensation and neural reserve in high-demand tasks, calling on the frontoparietal network. In addition, the finding of intra-hemispheric-based neurofunctional reorganization with a PASA phenomenon for all attentional load levels suggests that the PASA phenomenon is a function more of compensation than of reserve.
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