Scientists used to think of memories as being arranged together tidily in one place, like documents in a filing cabinet. This consensus was overturned in the early 1970s when cognitive neuroscientist Professor Endel Tulving proposed his theory that memories actually belong to one of two distinct groups.
What Tulving called “semantic memory” refers to general facts that have no real bearing on personality, being independent of personal experience. “Episodic memories”, meanwhile, consist of recollections of life events or experiences. The fact that the Natural History Museum is in London is a semantic memory. The time that I visited it on a school trip at the age of 11 is an episodic one.
Aided by advances in neuroimaging, Tulving discovered that episodic memories are generated as small pieces of information at different points across the brain and then reassembled into a coherent whole. He saw this process as akin to actually experiencing episodic memories again. “Remembering,” he said in 1983, “is mental time travel, a sort of reliving of something that happened in the past.”
Many of these memory signals arose from the hippocampus and the area surrounding it, suggesting that the hippocampus is the brain’s librarian, responsible for receiving information already processed by the temporal lobe, then sorting, indexing and filing it as episodic memory.
Just as a librarian might order books by subject matter or author, so the hippocampus identifies common features between memories. It might use analogy or familiarity, for example grouping all memories of various museum visits together in one place. These commonalities are then used to link the constituent parts of episodic memories together for future retrieval.