Task context dissociates the FN400 and the N400.

In event-related potential studies, familiarity-based recognition has been associated with the FN400, that is, more positive-going waveforms for old items than new items 300-500 ms post-stimulus onset, maximal at frontal electrodes. We tested the proposition that the FN400 reflects the attribution of unexpected processing fluency to familiarity. This implies that the FN400 is greater when fluency is less expected, that is, for less familiar stimuli. Moreover, the FN400 should be modulated by the goal of remembering and only elicited when fluency is correctly attributed to the past, that is, by correct old responses in recognition memory tests. In the absence of a retrieval task, enhanced fluency for repeated items should be associated with an N400 attenuation as no episodic attribution takes place. In an incidental study-test design with words of low and high life-time familiarity, participants made pleasantness judgments for half of the studied words. The other half re-appeared in a recognition test. Only in the latter task, participants had the goal of remembering. As both tasks included also new words, we could compare old/new effects under conditions in which both effects are driven by increased fluency for repeated words. We did not find the expected differences in the FN400 for low vs. high life-time familiarity items. However, as expected, we found a frontally distributed FN400 in the recognition test whereas the old/new effect in the pleasantness task resembled an N400 effect. This supports the view that the FN400 occurs when fluency is attributed to familiarity during a recognition decision.