Degeneration vs. degeneracy: a neural systems approach to functional reorganization
The term functional reorganization has been used to describe a number of dynamic neural processes related to changes in brain-behaviour connectivity. These include process effi ciency, such that localized metabolic work decreases with expertise; process redundancy, whereby subtly different neural networks are able to resolve a given computational problem; and degeneracy, in which novel brain-behaviour relationships can be expressed in response to lesion or challenge. These processes are argued
... o reside within the framework of a type of 'neural Darwinism': dynamic brain-behaviour relations are in a constant state of competition for the most effective resolution of a given computational task, with selection pressure at the level of metabolic economy. Methods: Exemplars of process effi ciency, redundancy and degeneracy will be presented from a functional neuroimaging study of working memory and association learning processes in both healthy older subjects and those with mild cognitive impairment. Results: Individuals with either no neurological deficit or incipient Alzheimer's disease (as indicated by medial temporal lobe atrophy) show a large degree of variance in response to increasingly diffi cult working memory tasks, as assessed by a multivariate analysis of brain-behaviour correlations. Conclusions: The principles of effi ciency, redundancy and degeneracy are applicable to the functional response to any static or progressive brain lesion. Novel approaches to describing and quantifying these processes may therefore improve our understanding of the clinicopathological disconnect observed in a number of clinical areas, in particular neurodegeneration.