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Like yin and yang, novelty and familiarity are typically described as separate-yet-complementary aspects of an experience, two ends of a single continuum. However, novelty and familiarity are also multifaceted. For instance, novelty can sometimes result in enhanced mnemonic performance, whereas at other times familiarity is better remembered. As previous investigations focused primarily on the experimental aspect of novelty, the mechanisms supporting conceptual novelty (the novel combination of two previously unrelated existing concepts) remain unclear. Importantly, conceptual novelty can be recognized as such only when compared with preexperimental familiar knowledge, regardless of experimental status. Here we applied a combined repetition suppression/subsequent memory fMRI paradigm, focusing on the conceptual aspect of novelty and familiarity as the subject matter. Conceptual novelty was characterized by sustained neural activity; familiarity, on the other hand, exhibited repetition effects in multiple cortical regions, a subset of which was modulated by successful encoding. Subsequent memory of novelty was associated only with activation differences in a distinct set of regions, including the hippocampus and medial cortical regions. These results suggest that conceptual novelty (a) does not (easily) trigger the repetition suppression phenomenon but requires sustained neural recruitment and (b) activates dedicated encoding mechanisms. Conceptual familiarity, in contrast, allows rapid neural processing that depends upon existing neural representations. Overall, these findings challenge the definition of novelty as a unitary concept. Furthermore, they bear important implications for research into the neural bases of knowledge representation and recognition memory.

Recognition tests in which participants indicate whether they recognize items using binary yes/no response options have typically yielded "yes" responses at equal rates for unattended old items and new items. Because most responses to unattended stimuli in such tests are "no" responses, we reasoned that a closer examination of "no" responses might reveal memory for unattended items. We modified a classic paradigm to allow participants to indicate high and low levels of confidence in their responses. As in earlier studies, the overall proportion of "yes" responses did not differ between unattended old items and new items. However, there was a crossover effect in the "no" responses: More high-confidence "no" responses were given for new items than for unattended old items, whereas more low-confidence "no" responses were given for unattended old items than for new items. These results indicate explicit memory for unattended material presented under high perceptual load. Our findings suggest that the null effects obtained in previous studies may not have stemmed from failures of perception or memory, but rather may have been due to insufficiently sensitive memory assessment.