Fast cars, slow words? Tracking processing of concurrent speaking and driving in real time.

The study of multitasking provides a unique window into the interaction of cognitive processes and systems and as such, into the cognitive architecture of the human mind. Previous studies reported that when participants speak and do something else at the same time, task performance often suffers. In the present study, we examine the mechanisms leading to such interference in multitasking in an ecological setting. We compare speaking performance across easy and difficult driving tasks and track measures of speech fluency and driving efficiency throughout the experiment. Interference is observed only under difficult driving conditions. Moreover, fluctuations in speaking performance predict fluctuations in driving performance and vice versa. When speaking efficiency decreases, so does driving efficiency. These findings have a straightforward explanation in accounts of dual-task interference, in which interference arises when task demands exceed available cognitive resources. They further suggest that when this limit is reached, the system downregulates resources on both tasks.