Why do we see what's not there?

Conscious perception is not the result of passively processing sensory input, but to large extent of active inference based on previous knowledge. This process of inference does go astray from time to time, and may lead to illusory perception: sometimes people see things that are not there. In a recent study we have shown that this inference may also be influenced by mood. Here we present some additional data, suggesting that illusory percepts are the result of increased top-down processing, which is normally helpful in detecting real stimuli. Finally, we speculate on a possible function of mood-dependent modulation of this top-down processing in social perception in particular.