In an attempt to examine whether stimulus detection in a simple reaction time task is expedited by an imagined stimulus, participants were asked to imagine a familiar stimulus in one sensory modality (e.g., sound) while waiting to signal the onset of a target stimulus in the other modality (e.g. vision). Contrary to what could be expected from research into multi-sensory integration and what was replicated in our baseline conditions where physical signals were presented, imagination of an additional stimulus did not lead to redundancy gains, but decreased response speed. The result is explicable by a mechanism that requires the dynamic allocation of a limited processing capacity - attentional weight - to sensory modalities, which results in reaction time costs when observers are required to switch from imagination in one modality to perception in the other modality. In two follow-up experiments the imagination task was exactly the same as in experiment 1, but the signal to be detected was either a bimodal signal or a signal of the same modality as the signal to be imagined. Here, no RT costs were found compared to the respective baseline conditions where the target to be detected was the same but no imagination task was to be executed. Consequently the effect found in experiment 1 does not appear to be due to an increase in unspecific cognitive load, task switching or task coordination demands but due to the switch in modalities between the imagery induced working memory content and the physically perceived detection target.

modality switching; imagery; attention; integration