The FN400 tracks the familiarity of task-irrelevant distractors in the environment

Abstract

The frontal N400 (FN400) is an event-related potential (ERP) typically observed approximately 300 to 500 ms post-stimulus and characterized by less negative (more positive) amplitudes for previously encountered stimuli relative to novel stimuli. The classic interpretation of this effect is that it is an index of familiarity. Here, we ask what exactly the FN400 is tracking the familiarity of? Is it just target objects, or might this ERP component provide useful episodic information about the familiarity of the context in which we find task-relevant information? Across two experiments, we manipulated how many times task-irrelevant stimuli were shown to test the possibility that the FN400 tracks the familiarity of the task-irrelevant context. In Experiment 1, familiarity was varied parametrically in a visual search task by embedding 0 to 3 previously encountered distractors in search arrays. The FN400 activity became increasingly positive as the number of repeated distractors increased. Experiment 2 pushed this further and tested the sensitivity of this component to task-irrelevant repetition by repeating only a single distractor from the immediately preceding trial; this minimal repetition reliably produced a more positive FN400 than all-novel displays. These results show that FN400 amplitude provides a graded familiarity signal of the task-irrelevant context, providing new information about the details of the episodes to which this memory component has access. The present results suggest that attention does not filter out contextual distractors from the memories accessed by the FN400. The results clarify a long-standing theoretical debate by showing that the FN400 indexes a rapidly updating familiarity signal that extends to task-irrelevant distractor objects.