Fixation-related potentials : Foveal versus parafoveal target identification

The P300 event-related potential (ERP) can be used to infer whether an observer is looking at a target or not. Common practice in P300 BCIs and experiments is that observers are asked to fixate their eyes while stimuli are presented. First studies have shown that it is also possible to distinguish between target and non-target fixations on the basis of single ERPs in the case that eye movements are made, and ERPs are synchronized to fixations (fixation-related potentials or FRPs) rather than to stimulus onset. However, in these studies small object sizes ensured that participants could only identify whether the object was a target or non-target after fixating on it. We here compare (non-)target FRPs when objects are identified before versus after fixation. We also examine ERPs from static eyes conditions. FRP shapes are in accordance with the notion that the late component of the P300 is associated with identifying a target, and eye movements do not substantially affect the P300. Even when the time of object identification is unknown, it is possible to distinguish between target and non-target FRPs on a single FRP basis. These results are important for fundamental science and for developing applications to covertly monitor observers’ interests