Neuroimaging evidence for dissociable forms of repetition priming

Repetition priming has been characterized neurophysiologically as a decreas ed response following stimulus repetition. The present study used event-rel ated functional magnetic resonance imaging to investigate whether this repe tition-related response is sensitive to stimulus familiarity. A right fusif orm region exhibited an attenuated response to the repetition of familiar s timuli, both faces and symbols, but exhibited an enhanced response to the r epetition of unfamiliar stimuli. Moreover, both repetition effects were mod ulated by lag between successive presentations. Further experiments replica ted the interactions between repetition, familiarity, and lag and demonstra ted the persistence of these effects over multiple repetitions. Priming-rel ated responses are therefore not unitary but depend on the presence or abse nce of preexisting stimulus representations.