Why Your Memories Are Fading

One bummer of the human existence is how our memories—which are such important parts of our identities and experience—are always fleeting, fading, and totally faliable. The Monday after your vacation, you’ve got funny stories about arguments you had with the locals, the meals that you loved or that made you nauseated, and recommendations that go, “Oh, you haven’t had key lime pie until you’ve stepped over an alligator to get it down at Bubba's.” A year or two later it’s down to just one or two anecdotes, “Uh, it was good. I’d go back,” A couple years after that, it’s “Yeah, I’ve been to Florida. Or was it Georgia?”

How do memories move from vivid and rich with detail to just the bare, dull fact of something? As it turns out, this is actually a controversial topic in neurological circles. There are at least two major camps with competing models of how memory works and they are split mostly on the question of what role a specific part of the brain called the hippocampus plays. And now, a new study from researchers at Johns Hopkins attempts to leave them both behind and forge a new path.

Studies show that damage to the hippocampus can prevent people from making new memories, but researchers are torn as to what—if anything—the hippocampus does when we recall existent, and, especially, old memories. Is it still involved? Does its seemingly diminished role in older, or remote, memories explain why they decay like they do?

One model says that the hippocampus is part of the making of the memory, but over time the memory becomes independent of the hippocampus and depends on cortical connectivity. The other model says that episodic memories—ones that are full of the personal and visceral details—are always in the hippocampus, regardless of age, while retrieving the impersonal, semantic memories doesn’t require the hippocampus.

Researchers at Johns Hopkins who found both models lacking are proposing a new model of memory, which explains why the memories degrade like they do. Emilie Reas at Scientific American explained it well:

According to [the new] theory, a memory is first encoded by the coordinated activity of neurons in the hippocampus and cortex. The hippocampus acts as the brain’s director, telling the cortex which particular neurons to activate. Each time we recall that memory, a similar, but not identical set of neurons are activated. Neurons that are frequently activated become part of the permanent memory trace in the cortex, while the rarely activated ones are lost. Every reactivation re-encodes the memory, and depending on what cortical neurons are engaged, can strengthen, weaken or update particular memory features.

The hippocampus, then, isn’t a site where trace memories are stored, it’s the point guard facilitating the neocortex putting the memory together. The major parts of the memory become lodged well-enough through repetition that eventually they no longer need the hippocampus, but they also lose the context clues. The memory then becomes “I have been to Florida.”

Meanwhile, each time the memory is recalled, the contextual features—the episodic details—can be written over, or be given competing details. The main part of the pathway becomes a well-worn road, while the details, which are not worn down with each recollection, become grown over, and can be confused as parts of other memories, with things we simply imagine, or see in movies, or are just straight up forgotten. So things like the bedspread in the motel, the unpleasant feeling of having sand everywhere until you showered, fade.

The researchers’ goal was to better explain how patients with amnesia hold on to what they hold on to (the older, more semantic memories) and why they lose what they lose (the ability to make new memories, the context, and episodic memory of older ones too).

As T.S. Eliot, ever the phenomenologist, observed in Rhapsody on a Windy Night, “Midnight shakes the memory, As a madman shakes a dead geranium.” This new research, published this month in Frontiers in Behavioral Neuroscience, aims to explain why the shaking from the recollection can cost us some petals, leaving a dried stem and just the facts.