Task switching capacities in persons with Alzheimer's disease and mild cognitive impairment.

Task switching is an executive capacity that relies on a set of separate components implicating a fronto-parietal network of brain areas. In the present study, different components implicated in task switching were assessed in persons with Alzheimer's disease (AD), persons with mild cognitive impairment (MCI), and their matched healthy controls. The procedure implicated presentation of a two-digit stimulus, and task switching involved either conceptual or spatial switching. Global switching was measured by comparing blocks that involved non-switch trials to blocks that included switch trials, whereas local switching was measured by comparing performance across single trials in the switch blocks. Furthermore, the paradigm measured practice effects. Persons with AD showed larger local and global switch cost than healthy controls suggesting that their deficits encompass both reconfiguration of new action sets and maintenance of potentially relevant tasks within working memory. Importantly, the deficit was large in spatial switching but negligible in the conceptual condition. Persons with MCI only showed global switching impairment, suggesting a deficit restricted to the concurrent maintenance of two relevant task sets, and as in AD, this impairment was limited to spatial switching. Interestingly, persons with MCI, but not AD patients, improved their switching capacities upon practice. These findings indicate that switching deficit is selective in both MCI and AD persons, and is thus supportive of the notion that different mechanisms are involved in task switching. The pattern across condition is coherent with a continuum between those two clinical groups.