Memories are formed through long-term changes in synaptic efficacy and are stored in the brain in specific neuronal ensembles called engram cells. However, neural mechanisms underlying memory association had been unclear. Using optogenetics and behavioral conditioning tests, we found as follows. First, an association of two different memories is caused by increasing overlapping neurons which encode both memories. Second, it is possible for us to artificially connect the link between two different memories through synchronous activation of distinct cell ensembles. Third, synapse-specific plasticity guarantees the identity and storage of individual memories. These results may give insight into therapeutic treatments for post-traumatic stress disorder.