Running span can be performed by either passively listening to to-be-remembered items or actively updating the target set during presentation. This choice of strategy is influenced by the rate of presentation in the task. Previous research suggests that the active updating process is demanding and time-consuming. It is favored at relatively slow rates of presentation, while the passive strategy is more successful when applied at fast rates. In two experiments the time course of resource demand during task performance and its sensitivity to presentation rate was examined. We hypothesized that running span imposes a high cognitive load only when active updating is employed. Participants performed running span simultaneously with a spatial reaction time (RT) task, and RTs on the concurrent task were used to index the resource demands of the memory task. A slow-paced running span exhibited a large overall resource demand in comparison with the serial recall tasks (Experiment 1) and fast-paced running span (Experiment 2). This demand was observed from the position in the list from which participants are presumed to start updating, suggesting a cognitive shift to a demanding mode of updating. In addition, a demand burst was found approximately 1000ms following item onset at these later positions. These data establish that the process of active updating in running span task is slow and cognitively demanding and indicate that this limits its application during fast presentation rates.

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Running span can be performed by either passively listening to memory items or actively updating the target set. Previous research suggests that the active updating process is demanding and time-consuming and is favored at slow rates of presentation while the passive strategy is employed at fast rates. Two experiments examined the time course of recruitment of resources during task performance and its sensitivity to presentation rate. In Experiment 1, participants performed one of three serial recall tasks: running span, simple span, and modified span. The tasks were completed at the same time as a choice reaction time (RT) task and the RTs were used to index the resource demands of the memory task. Running span generated higher RT costs than simple span. The costs were present only for positions at and beyond the point in the sequence when the target memory set was changed, indicating a shift to a more cognitively demanding mode of updating. At these positions there was a generalized increase in RT costs that peaked 1000 ms following item presentation. In Experiment 2 the resource demands of running span varied with presentation rate and a peak demand at 1000 ms was again evident, but only with a slow presentation rate. In conjunction with strategy reports, these data establish that the process of active updating in running span is slow and cognitively demanding, making it difficult to use when presentation rates are fast.