In overlapping dual-task (DT) situations, a bottleneck causes serial processing of task 1 and task 2 processes and leads to increased reaction times of task 2 (RT2) with decreasing interval between both tasks.
Here, we investigated how monetary reward affects DT processing and more specifically, whether rewarding either task 1 or task 2 as compared to rewarding both tasks at the same time yields to different improvements of DT processing. Previous studies showed that the application of reward to either task 1 or task 2 leads to equal improvements of RTs, with a localization of the main reward effect at task 1 before the bottleneck; the observed RT1 shortening propagated via the bottleneck to task 2, which shortened RT2 in both conditions. Based on that, we predicted that rewarding both, i.e. task 1 and task 2 at the same time should sum-up the reward effects for both tasks onto task 1 processing and this should result in double-sized monetary-reward effects compared to conditions with only one rewarded task.
Experiment 1, tested the effects of rewarding task 1 and task 2 with 1 Euro per task, summing up to 2 Euros distributed across the tasks. Experiment 2 tested reward effects with 2 Euros for task 1 but no-reward for task 2. The results showed twice as large reward effects in Experiment 1 compared to Experiment 2. This indicates a summing up of the magnitude of the reward effects localized at task 1, if task 1 and task 2 are separately rewarded.