16 Multiple neural systems sustaining cognitive functions

The remarkable resilience of cognitive functions to focal brain damage suggests that multiple neural systems can sustain the same function either via similar mechanisms or by implementing different cognitive strategies. Two classes of ‘multiple neural systems’ can be distinguished based on the time courses of their underlying neural mechanisms. First, multiple degenerate neural systems might be present in the normal brain where they are either co-activated or remain latent during task performance. In this case plastic changes are immediate and rely on unmasking of a pre-existing but functionally latent system. Second, there might be only one system in the normal brain while an alternative system emerges in the pathological brain owing to functional reorganization, i.e. long-term plastic changes induced by the underlying pathological process. We discuss how multiple neural systems can be revealed using (1) intersubject variability or (2) an iterative approach integrating information from lesion and functional imaging studies.