Hemispheric asymmetry and aging: right hemisphere decline or asymmetry reduction

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Abstract

We review evidence for two models of hemispheric asymmetry and aging: the right hemi-aging model, which proposes that the right hemisphere shows greater age-related decline than the left hemisphere, and the hemispheric asymmetry reduction in old adults (HAROLD) model, which proposes that frontal activity during cognitive performance tends to be less lateralized in older than in younger adults. The right hemi-aging model is supported by behavioral studies in the domains of cognitive, affective, and sensorimotor processing, but the evidence has been mixed. In contrast, available evidence is generally consistent with the HAROLD model, which is supported primarily by functional neuroimaging evidence in the domains of episodic memory encoding and retrieval, semantic memory retrieval, working memory, perception, and inhibitory control. Age-related asymmetry reductions may reflect functional compensation or dedifferentiation, and the evidence, although scarce, tends to support the compensation hypothesis. The right hemi-aging and the HAROLD models are not incompatible. For example, the latter may apply to prefrontal regions and the former to other brain regions.

Section snippets

Right hemi-aging model

The right hemi-aging hypothesis states that age-related cognitive declines affect functions attributed to the right hemisphere to a greater degree than those associated with the left hemisphere [5], [6]. The validity of this hypothesis has been investigated in various functional domains. In this article we briefly review evidence from the areas of verbal/spatial, affective, and sensorimotor functions, which are lateralized in young adults and relevant for aging research.

Hemispheric asymmetry reduction in older adults (HAROLD)

As mentioned earlier, the HAROLD model states that, under similar conditions, prefrontal cortex (PFC) activity tends to be less lateralized in older adults than in younger adults. This model is supported by functional neuroimaging evidence in the domains of episodic memory encoding and retrieval, semantic memory retrieval, working memory, perception, and inhibitory control (Table 1). Moreover, evidence has also been obtained using other methods, such as electrophysiological [38] and behavioral

Conclusion

The two models reviewed here provide different views of age-related changes in hemispheric asymmetry. The right hemi-aging model proposes that the right hemisphere shows greater age-related decline than the left hemisphere [5], [6], whereas the HAROLD model [7] proposes that frontal activity during cognitive performance tends to be less lateralized in older than in younger adults. Whereas empirical evidence relevant to the right hemi-aging model has been mixed, empirical evidence relevant to

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