The cognitive reserve hypothesis proposes that the brain actively attempts to cope with age-related changes by using pre-existing cognitive networks (neural reserve) or enlisting compensatory processes (neural compensation). In a context of visual selective attention, the current study compared task-related activation with BOLD fMRI signals in younger (N=16) and older (N=16) adults using a letter-name-matching task with two attentional load levels. In the low-load condition, the target letter might share the same identity (e.g., a/A) with one of two probes in the display, while in the high-load condition the display included four probes. The results suggest that there is an age-related change within the frontoparietal network that underlies visual selective attention processing. In the low-load condition, the older group needed to recruit more bilateral frontal regions to successfully perform the task, while the younger participants recruited more bilateral occipital regions, in agreement with the PASA (Posterior-Anterior Shift in Aging) phenomenon and the neural compensation hypothesis of cognitive reserve. In addition, in the high-load condition, we found a load-dependent posterior-anterior shift in the older participants, which was not present in the younger ones, involving the anterior part of the cingulate cortex. By showing a load-dependent PASA, our study indicates that the PASA phenomenon is supported more by the compensation mechanism (solicited exclusively in older participants) than by the reserve.
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