Involuntary strategy-dependent dual task performance.

Previous studies showed that some dual tasks can be performed simultaneously without costs. Yet, a variable SOA between the inputs to such tasks leads to strategic, often involuntary, prioritization of one of the two tasks. Here we explore the boundary conditions for this involuntary or exogenous strategy. In Experiment 1, subjects were initially trained on dual task performance where the input to the two tasks is presented simultaneously (0 SOA). We used two tasks that under such conditions can be performed without costs and indeed subjects displayed perfect sharing of the tasks. Subjects then performed the same two tasks but with a variable SOA (0, 50, 150, 800 ms). This manipulation led to a serial-like performance of the two tasks even in trials with 0 SOA. In Experiment 2, subjects participated in eight sessions. Within each session, they performed in alternation blocks with a fixed 0 SOA and blocks with a variable SOA. Subjects displayed perfect sharing in the pure 0 SOA blocks but performed the two tasks serially in the mixed SOA blocks despite receiving identical instructions. These findings demonstrate that task context is a powerful factor in dual task performance and may lead subjects to involuntarily exhibit dual task costs even in conditions where they can perform the tasks without any costs. Moreover, these findings strongly suggest that costs observed in PRP studies reflect the use of such exogenous strategies rather than a general structural dual task limitation.