Within the last decade or so, there has been a renewed interest among cognitive psychologists and cognitive aging researchers in executive control functions, that is those functions which are concerned with the selection, scheduling and coordination of the computational processes that are responsible for perception, memory, and action. The interest in the executive control of cognitive processes has been reflected in the development of models of cognition that reserve an important role for executive control functions as well as in the detailed empirical examination of a subset of executive control processes of young and old adults.

In the studies which we report here we were particularly interested in examining the influence of aging as well as the potentially beneficial effects of practice on the efficacy of executive control processes of young and elderly adults. Given that executive control of cognition is most likely not unitary in nature but instead involves a number of different control processes we decided to focus our investigation on a specific paradigm, referred to as task switching, which has been relatively well characterized in terms of executive control. In this paradigm, introduced by Jersild (1927), subjects perform two relatively simple tasks such as judging whether a letter is a vowel or a consonant, judging the number of elements present on a display, or comparing two multidimensional stimuli. On the non-switch trials subjects either perform these two tasks in separate blocks of trials or perform one of the tasks several times before switching to the other task. The main point here is that subjects need only perform a single task on the non-switch trials. On the switch trials subjects either continuously alternate between two tasks (i.e. in a switch block) or instead switch to a new task after performing some number of trials on another task (i.e. in a block in which switch and non-switch trials can occur). The time required to complete the executive control processes necessary to switch from one task to the other task, such as the selection from long-term memory and configuration in working-memory of the appropriate processing algorithms and the inhibition of previously used but now inappropriate processing algorithms, is inferred from the increased response time observed when a task switch occurs compared with the response time for the same task performed separately or in a run of trials of the same task.

Several interesting results been observed in the task switching paradigm. First, several investigators have obtained data which suggests that executive control processes and task component processes (i.e. processes used to perform the separate tasks such as encoding, stimulus evaluation, response selection and response execution) are functionally independent. For example, Gopher (1996) reported that instructions concerning the likelihood and nature of a switch influenced switching time but not component task time (i.e. non-switch time). Rogers and Monsell (1995) found that the time allotted to prepare for a task switch had a substantially larger influence on switch time than it did on component task time while the presence of a warning cue influenced component task time but not switching time. Rubinstein, Meyer and Evans (in press);  reported a number of dissociations between non-switch and switch performance across 14 different task pairs in five separate studies. These dissociations included: (a) different component tasks with the same non-switch times but different switch times and (b) different component tasks with different non-switch times and the same switch times. In summary, the results obtained in a number of studies suggest that the processes which support switching performance are distinct, at least in part, from those processes that support performance in the separate tasks.

Researchers have also argued that multiple executive control processes can be distinguished in the task switching paradigm. Rogers and Monsell (1995) had subjects perform a sequence of trials in which they alternated between sets of two trials in which they decided whether a letter was a vowel or a constant and then two trials in which they decided whether a digit was odd or even. Alternation between the two tasks continued over a block of 24 trials. Within this paradigm Rogers and Monsell manipulated the response stimulus interval (RSI) between tasks from 150 and 1200 ms and found substantial decreases in switch costs as the RSIs increased from 150 to 600 ms but relatively stable switch costs between RSIs of 600 and 1200 ms. On the basis of these results the authors concluded that two different executive control processes could be distinguished in the task switching paradigm. They proposed that the decrease in switch costs observed when the RSI was increased from 150 to 600 ms were compatible with an endogenous, stage-like process of task re-configuration which can be carried out in anticipation of the stimulus. This stage like process likely requires the loading of processing algorithms required for the new task into working memory and the inhibition of the processing algorithms that are no longer appropriate (i.e. the processing algorithms that had been used for the previously performed task). However, such an endogenous preparatory process cannot be the whole story since a substantial and stable switch cost remained as RSIs were increased from 600 to 1200 ms. To account for this effect the authors argued that a component of the task reconfiguration process cannot be executed in advance of the stimulus but instead is triggered only exogenously by the appearance of a stimulus associated with the task to be performed.