It's all in your head: Expectations create illusory perception in a dual-task setup.

Predictions about the world can guide our perception and action, but they can also misguide us. We developed novel dual-task setups where the participants are occupied by a primary task and are from time to time queried about the phenomenal contents of the auxiliary task. We show that "hallucinating" the presence of an actually absent stimulus is not an exception, but a common phenomenon (more than 90% of participants experienced illusory objects at least once). Additionally, in experiment 1 we found a negative correlation between the amount of illusory perceptions and the Autism Spectrum Quotient score. People who scored higher on the questionnaire, were less likely to experience illusory objects. Finally, we observed no effect of spatial attention on expectation-based illusory presence of an object. More generally our results demonstrate that expectations misguide perception when attention is diverted to another task.