Although reconsolidation opens up new avenues to erase excessive fear memory, subtle boundary conditions put constraints on retrieval-induced plasticity. Reconsolidation may only take place when memory reactivation involves an experience that engages new learning (prediction error). Thus far, it has not been possible to determine the optimal degree of novelty required for destabilizing the memory. The occurrence of prediction error could only be inferred from the observation of a reconsolidation process itself. Here, we provide a noninvasive index of memory destabilization that is independent from the occurrence of reconsolidation. Using this index, we show in humans that prediction error is (i) a necessary condition for reconsolidation of associative fear memory and (ii) determined by the interaction between original learning and retrieval. Insight into the process of memory updating is crucial for understanding the optimal and boundary conditions on reconsolidation and provides a clear guide for the development of reconsolidation-based treatments.

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Although disrupting reconsolidation is promising in targeting emotional memories, the conditions under which memory becomes labile are still unclear. The current study showed that post-retrieval changes in expectancy as an index for prediction error may serve as a read-out for the underlying processes engaged by memory reactivation. Minor environmental changes define whether retrieval induces memory reconsolidation or the initiation of a new memory trace even before fear extinction can be observed.[Supplemental material is available for this article.]The recent appreciation of memory as a dynamic rather than static process inspires new fundamental questions about memory malleability. After the formation of associative fear memory, a reencounter with the feared stimulus can lead to various outcomes; retrieval only, reconsolidation, or new learning. Insights into how the system determines whether a memory trace should be updated or an additional memory trace should be formed are essential not only for our understanding of the fundamentals of learning and memory but also for the development of reconsolidation based treatment.Prediction error (PE)-a mismatch between what is expected based on previous experiences and the actual state of events (Rescorla and Wagner 1972)-is a necessary condition for inducing reconsolidation of human associative fear memory upon retrieval (Sevenster et al. 2013). Previously, we have shown the utility of retrieval-induced changes in the expectancy ratings of the unconditioned stimulus (US) during presentation of the conditioned stimulus (CS) as a measure of PE (Sevenster et al. 2013). This behavioral and noninvasive index of PE may indicate whether the original memory trace was destabilized by the retrieval experience, independent of the outcome of the reconsolidation process itself. It may, however, be questioned whether PE is a sufficient condition for reconsolidation, given that PE can also give rise to new learning instead (e.g., extinction learning) (Rescorla and Wagner 1972). For example, extinction training, which involves the formation of a new inhibitory memory trace (Bouton 2002), puts a constraint on reconsolidation (Eisenberg et al. 2003;Lee et al. 2006;Bos et al. 2012). During extinction training, repeated or prolonged unreinforced exposure generates multiple PEs, which eventually reduces both threat expectancy and fear responding. However, the transition from updating of the original memory trace to the formation of a new memory trace may occur long before the expression of the inhibitory extinction memory can be observed. Until now, the restraint that new learning puts on reconsolidation could only be inferred from extinction of the fear behavior itself. In the current study, we tested whether changes in threat expectancy could serve not only as an index of memory destabilization, but could also reveal a boundary condition of reconsolidation independent from the fear reduction itself. Hereto, we established the optimal parameters for fear acquisition and memory reactivati...