Separating Component Signals of Episodic Simulation Using a Catch Trial Design

Tasks that require mentally simulating events, such as remembering events from one’s past and imagining events from one’s future, have been shown to involve a highly overlapping set of brain regions. Across a growing number of studies, relatively few regions have been found that show differences in activity between remembered and imagined events. However, studies have not disambiguated neural activity related to task orientation: i.e., preparing to remember events from the past or imagine events in the future) from activity related simulating events, per se. The current experiment uses functional MRI and employs a catch trial design to test the hypothesis that by separating orientation and simulation related activity, novel differences might be found between the acts of remembering and imagining events. We find that regions typically shown to activate above baseline in simulation tasks actually deactivate slightly in response to orientation cues, and that by accounting for this activity, regions in bilateral parahippocampal and right retrosplenial cortex show increased activity for the simulation of past events relative to the simulation of future events. This finding suggests that multiple, temporally overlapping processes exist in regions involved in episodic simulation, and that these differences concealed a network of regions sensitive to situations in which information from one’s past is explicitly retrieved