Causes and Consequences of Dedifferentiation in the Aging Brain.

Abstract

Cognitive performance declines across the adult lifespan. According to the dedifferentiation hypothesis of cognitive aging, age-related cognitive impairments reflect reductions in the fidelity of neural representations. However, behavioral tests of this hypothesis have yielded mixed results. Thus, the present research sought to explore age-related dedifferentiation using pattern classification of neural activity, which may yield a more direct measure of representational fidelity. Three studies examined age differences in the fidelity of the neural representations of visual stimuli, motor actions, and cognitive task sets, respectively. Study 1 showed that multi-voxel activation patterns evoked by presentation of face and house stimuli were less distinctive in older adults than in young adults. No regions showed greater distinctiveness in older adults than in young adults, and the spatial pattern of category information was similar across age groups, suggesting that older adults do not compensate for low- fidelity representations in visual cortex by forming higher-fidelity representations elsewhere in the brain. Study 2 extended these results to the domain of motor control, using multi-voxel pattern analysis to distinguish between left- and right-hand finger movements. Older adults showed reduced distinctiveness throughout a network of regions related to motor representation and control; again, no regions showed greater distinctiveness in older adults. Study 3 further investigated age differences in neural representations in the context of verbal and spatial working memory tasks. Results from memory encoding and retrieval were consistent with Studies 1 and 2, with reduced discrimination of verbal versus spatial information in older adults. In contrast, results from working memory maintenance showed that representational fidelity was decreased in older adults at high levels of task demand but increased in older adults at low levels of demand. Overall, results from perceptual and motor tasks were consistent with the dedifferentiation hypothesis, while results from memory maintenance were more consistent with compensation-related accounts of cognitive aging. These results suggest that both dedifferentiation- and compensation-based accounts can explain some phenomena, but that neither theory can offer a comprehensive account of age differences in neural representation. Future research should investigate the generalizability of the present results across analysis methods, cognitive tasks, and participant populations.

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