Serial or parallel processing in dual tasks: what is more effortful?

Recent studies indicate that dual tasks can be performed with a serial or parallel strategy and that the parallel strategy is preferred even if this implies performance costs. The present study investigates the hypothesis that parallel processing is favored because it requires less mental effort compared to serial processing. A serial or parallel processing strategy was induced in a sample of 28 healthy participants. As measures of mental effort, we used a rating as well as heart rate (HR) and electrodermal activity. Parallel processing again showed performance costs relative to serial, whereas serial processing was judged as more effortful. Also tonic HR and phasic HR deceleration were increased with a serial strategy. Thus the preference for a parallel strategy in dual tasks likely reflects a compromise between optimizing performance and minimizing the amount of mental effort. This aspect is neglected in current dual task accounts so far.