Where do old memories go?

One popular theory proposes that the hippocampus is critical only for recent memories, but not older ones. Over time, the hippocampus teaches the surrounding brain – the cortex – how to represent a memory. As the memory matures, the hippocampus kicks it out to reside independently in the cortex. If you lost your hippocampus today, you could still remember your childhood vacation to Florida, but the memory from last weekend’s dinner party would be lost. This is the exact pattern scientists have observed in many patients, including the celebrated amnesic H.M. After surgery to remove a large chunk of his hippocampus, H.M. could recall some very old memories, but could no longer make new memories or remember the years leading up to his surgery.

But researchers have seen other patients with hippocampal damage who have memory deficits extending back through most of their life. An alternative theory accounts for these discrepancies by proposing that the hippocampus selectively stores one type of memory – “episodic” – while the surrounding cortex stores another – “semantic.” Episodic memories are usually rich in details about our past experiences, whereas semantic memories are based on impersonal, factual knowledge. As a memory ages, the model proposes, it is copied many times in both the hippocampus and the cortex. All of those cortical copies generate a new semantic memory, representing only the gist or key facts about the experience, without all of its elaborate episodic details. Intriguingly, this theory is also backed by patients who exhibit memory problems depending on the type of memory, rather than its age. Without a hippocampus, such patients couldn’t remember the experience of their childhood Florida trip – an episodic memory – but could remember the fact that they visited Florida – a semantic memory.