Neural correlates of familiarity and conceptual fluency in a recognition test with ancient pictographic characters.

Familiarity and conceptual priming refer to distinct memory expressions and are subtypes of explicit memory and implicit memory, respectively. Given that the neural events that produce conceptual priming may in some cases promote familiarity, distinguishing between neural signals of these two types of memory may further our understanding of recognition memory mechanisms. Although FN400 event-related potentials observed during recognition tests have often been ascribed to familiarity, much evidence suggests that they should instead be ascribed to conceptual fluency. To help resolve this controversy, we studied potentials elicited by unrecognizable ancient Chinese characters. These stimuli were categorized as high or low in meaningfulness based on subjective ratings. Conceptual priming was produced exclusively by repetition of characters high in meaningfulness. During a recognition test in which recollection was discouraged, FN400 old-new effects were observed, and amplitudes of the FN400 potentials varied inversely with familiarity confidence. However, these effects were absent for old items given low meaningfulness ratings. For both high and low meaningfulness, late positive (LPC) potentials were found in old-new comparisons, and LPC amplitudes were greater when higher familiarity confidence was registered during the recognition test. These findings linked familiarity and conceptual fluency with different brain potentials - LPC and FN400, respectively - and provide additional evidence that explicit memory and implicit memory have distinct neural substrates.