Kuhl et al. find that the competition in the brain between old memories and new ones that are associated with the same thing (for example, an old versus a new password, or yesterday's versus today's space in the parking lot) can be observed in fMRI. They found competition between visual memories was captured in the relative degree to which target vs. competing memories were reactivated within the ventral occipitotemporal cortex. When lowered VOTC reactivation indicated that conflict between target and competing memories was high, frontoparietal mechanisms were markedly engaged, revealing specific neural mechanisms that tracked competing mnemonic evidence.
In another study on memory Diekelmann et al. show that memory reactivation has opposing effects on memory stability during wakefulness and sleep. Reactivation during slow-wave sleep following learning can stabilize memories. Reactivation during wakefulness has the opposite effect, rendering memories labile and susceptible to modest modification.
Finally Benedict Carey points to a study by Shema et al. showing that increasing levels of a brain enzyme (a protein kinase C isoform) involved in memory formation enhances long term memory. Also, Chen et al. show that injections of a different protein, a growth factor involved in memory formation (insulin-like growth factor II) can have the same effect.