Task practice differentially modulates task-switching performance across the adult lifespan

Cued-trials task-switching paradigms have been used extensively to examine ageing-related changes in cognitive control. Many studies report an increase in mixing cost (i.e., cost of repeating the same task in a single-task vs. a mixed-task block) and a less reliable increase in switch cost (i.e., cost of switching vs. repeating tasks in a mixed-task block) in old as compared to young adults. However, there is substantial variability between studies in the emergence and size of age effects on mixing and/or switch cost. In this study, we examined variation in mixing cost and switch cost as a function of task practice and preparation interval across the adult lifespan (18-79 years) using a paradigm that promotes advance preparation and reduces cue encoding differences between switch and repeat trials. Both preparation interval and task practice modulated mixing cost and switch cost-but task practice mediated the effects of preparation interval and age differentially for mixing cost and switch cost. Mixing cost was consistently larger in older participants, reduced with preparation and varied little with task practice. In contrast, the effect of preparation interval on switch cost varied with task practice. Reduction in switch cost with preparation interval emerged in younger participants by the second practice session and even later in older participants. When fully practiced, older participants showed greater mixing cost but less switch cost than younger participants. Age effects on both mixing cost and switch cost were mediated by changes in processing of repeat trials, indicative of reduced differentiation between switch and repeat trials in mixed-task blocks. This is consistent with reduced cognitive flexibility with increasing age.