Old age is associated with a pattern of relatively intact and relatively impaired task-set switching abilities.

In three experiments, we examined the effects of old age on the reaction time (RT) decrement associated with task alternation. Old age was associated with increased mixing-cost, which is the RT difference between two conditions: mixed-task, where trials involving two tasks were intermixed, and single-task, where all the trials involved the same task. Old age was also associated with an increased switching-cost, which is the RT difference between trials in which the task was just changed and trials in which it was repeated. There was also indication of a slowed passive dissipation of task set adopted in the preceding trial. In contrast to these impairments, old age was also associated with an almost intact ability to prepare for an upcoming task switch. This ability was indicated by a normal reduction in switching-cost due to an increase in the time allowed to prepare for the switch. We discuss the implications of the results in relation to theories of task-switching and to the underlying brain mechanisms, especially with respect to the effect of old age on the prefrontal cortex.