Comment on Greenwood (2007): Which Side of Plasticity?

Brain aging is characterized by a complex pattern of decline and stability that includes differential regional shrinkage as well as redistribution of functional activation and deactivation. Many have noticed an apparent paradox of age-related increase in activation of structures that undergo the greatest structural deterioration. P. M. Greenwood (2007) proposes a theoretical framework for explaining that paradox by postulating plastic compensatory remodeling in the aging brain. This is a worthy first step, although several problems in formulating a testable set of hypotheses remain to be addressed. The working definition of plasticity that is driving this theory needs to be clarified. The biological basis, strength, and replicability of structure-function relationships derived from neuroimaging, on which Greenwood relies in her theorizing, remain to be established. The extrapolation of plasticity phenomena observed in acute lesions to gradual aging process needs to be justified. All in all, this is a positive development in the field that may be ripe for theoretical generalizations.