Mental imagery, which involves the internal simulation of scenarios or stimuli that are not physically present, engages many of the brain systems involved in perception of the external world. Dijkstra et al. now identify a neural mechanism that allows the brain to avoid confusion resulting from this overlap by determining when perceptual signals reflect external reality.

The authors devised a task in which participants viewing a screen tried to determine the presence or absence of a visual grating amid a background of dynamic noise that made it difficult to discern. The stimulus was presented in half of the trials and omitted in the other half of the trials. In some trials, they were asked to simultaneously imagine a grating. After each trial participants stated whether they believed a grating was presented (reality judgement) and rated the vividness of their mental image.