Memory involves the complex systems by which a person perceives, remembers and recalls information and experiences. Memory is not a single process but consists of inter-related subsystems. The stages of memory include attention, encoding, storage, and retrieval. "Encoding" is the process of registering information, "storage" refers to maintaining it in memory and "retrieval" is the act of recalling it again. These stages are closely linked and interact with one another. Different brain areas have been identified that are involved in different stages of memory.