Age-differences in functional asymmetry during memory retrieval revisited: no evidence for contralateral over-activation or compensation
Brain asymmetry is inherent to cognitive processing and seems to reflect processing efficiency. Lower frontal asymmetry is often observed in older adults during memory retrieval, yet it is unclear whether lower asymmetry implies an age-related increase in contralateral recruitment, whether less asymmetry reflects compensation, is limited to frontal regions, or predicts neurocognitive stability or decline. We assessed age-differences in asymmetry across the entire cerebral cortex, using fMRI data from 89 young and 76 older adults during successful retrieval, and surface-based methods that allowed direct homotopic comparison of activity between hemispheres. An extensive left-asymmetric network facilitated retrieval in both young and older adults, whereas diverse frontal and parietal regions exhibited lower asymmetry in older adults. However, lower asymmetry was not associated with age-related increases in contralateral recruitment, but primarily reflected either less deactivation in contralateral regions reliably signalling retrieval failure in the young, or lower recruitment of the dominant hemisphere--suggesting that functional deficits may drive lower asymmetry in older brains, not compensatory activity. Lower asymmetry neither predicted current memory performance, nor the extent of memory change across the preceding [~]8 years in older adults. Together, these findings are inconsistent with a compensation account for lower asymmetry during retrieval and aging.