Think back to your first childhood beach vacation. Can you recall the color of your bathing suit, the softness of the sand, or the excitement of your first swim in the ocean? Early memories such as this often arise as faded snapshots, remarkably distinct from newer memories that can feel as real as the present moment. With time, memories not only lose their rich vividness, but they can also become distorted, as our true experiences tango with a fictional past.
The brain’s ability to preserve or alter memories lies at the heart of our basic human experience. The you of today is molded not only by your personal history, but also by your mental visits to that past, prompting you to laugh over a joke heard yesterday, reminisce about an old friend or cringe at the thought of your awkward adolescence. When we lose those pieces of the past we lose pieces of our identity. But just where in the brain do those old memories go? Despite decades studying how the brain transforms memories over time, neuroscientists remain surprisingly divided over the answer.
Some of the best clues as to how the brain processes memories have come from patients who can’t remember. If damage to a particular brain area results in memory loss, researchers can be confident that the region is important for making or recalling memories. Such studies have reliably shown that damage to the hippocampus, a region nestled deep inside the brain, prevents people from creating new memories. But a key question, still open to debate, is what happens to a memory after it’s made. Does it stay in the hippocampus or move out to other areas of the brain? To answer this, scientists have studied old memories formed before brain damage, only to discover a mix of inconsistent findings that have given rise to competing theories.
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.