A little while back (853 days ago(!) WHAT HAVE I BEEN DOING WITH MY LIFE?!), I wrote a post on the neuroscience of dreaming. After writing that post, rereading it approximately 15x (it’s amazing how little you remember from your own writing) and thinking about my buddy Peytonz’s* comments, a related question crept into my mind: where do “lost” memories come from? And not just random lost memories, but random ones from long ago? What spurs their retrieval?
Let me give some examples to make sure we’re on the same page. I started thinking about such “lost” memories a few years ago and now I (with surprising frequency) catch myself having recalled them. When I notice that I’ve recalled one of these obscure memories, I’ll try sifting through recent thoughts to see if I can figure out how it was cued. Like a few weeks ago I was eating pasta, thinking about the bitterness of the sauce and remembered something I hadn’t thought of for over 20 years: when my aunt served us homemade tomato sauce and I made a fuss because it tasted weird to me and she heated up Ragu to put on my spaghetti. In this case the cue was clear: I was eating pasta with a bitter sauce and this reminded me of a time when I didn’t have a taste for good, homemade tomato sauces (sorry Aunt RoRo!**). But this isn’t quite the kind of memory I want to discuss.
The kind of “lost” memory I’d really like to explore is when you recall something out of the blue that as far as you know is totally unrelated to what you were thinking about. An uncued memory, essentially. A recent one I had: I was at work sorting through pictures of generic shapes that I was creating for a task and I suddenly recalled throwing a baseball with my buddy Brianz* in Raleigh’s Jaycee Park ~7 years ago. In this case, as hard as I tried, I couldn’t come up with how ‘baseball’, ‘Brianz’, ‘Jaycee Park’ or anything related had gotten into my head. In essence: the memory appeared to come from nowhere. Now that you’ve read this, try catching yourself uncovering an uncued memory in your own life. They happen surprisingly often (note I’m not talking about “repressed” memories, which might not even be a real thing).
And now that I’ve identified this exact kind of memory—those of the “lost” uncued variety—let’s explore where I think they might come from. Please note that these are speculations based only somewhat on established science.
Idea 1: There was a cue, but you didn’t have conscious access to it. Your brain has many different parts with many different ‘jobs’ being performed. Since you take in so much data through your senses at every moment of every day, in addition to a whole bunch of data in storage for you to access, your brain needs to make complicated decisions by sorting through what all these data streams tell you. And because you take in more data than you could possibly use, your conscious brain only has access to a limited stream of (highly processed) information. As an analogy, when you’re the boss of a large company, you don’t have time to micromanage every single person/report/price/whatever so you have various managers accumulate information and execute smaller decisions for you while only informing you when grand decisions need to be made (feel a bit more important now, don’t you?). With this idea in mind (consciousness meta-alert), we can imagine how your senses frequently pick up information that you are not privy to, which can then spur memories to be recalled and brought to your attention without you having any conscious access to what caused the original cue. What kind of “unknown” sensations am I talking about? Take subliminal images as an example. Studies have shown that when you flash an image very quickly on a screen people will report they have not seen it. However, fMRI scans show that brain regions notice these images, and people are even able to win money by using information from these images without actually being able to recognize the images. At some point in our evolutionary past our ancestors had no use for single images that showed up and then disappeared in 10 milliseconds, so the visual stream in your brain, which does have access to such quickly-disappearing images, can decide that they’re not important enough to pass on to higher regions. So, to tie things up, when I had that memory of throwing a baseball with my friend, maybe one of the shapes I was looking at reminded me of a baseball. Or the shape was weird like Brianz’s throwing motion. In which case even though I didn’t consciously make the connection, the memory had an explainable cue and only seemed to come out of nowhere.
Idea 2: You’re dumb and forgot the connection between what you sensed and what you remembered. Just kidding. You’re smart, as evidenced by you making it this far into my post. And good-looking!
Idea 3: Coincident connections in your neural network. In my post on the neuroscience of dreaming, I brought up replay events that we’ve found occur in rodents when they seem to be reimagining their behavior during rest. Here’s an image of one of these events:
In these events, each neuron fires spikes one after another, with the key being that they fire in this same order both when the rodent was running AND later on when it was resting, as if to say the rodent’s brain was recapitulating the rodent’s path in memory by replaying the sequential cell activity. There isn’t causal evidence yet for sequential cell firing actually being an essential signature of memories (Loren Frank is trying to make this happen, though!), but the specific code doesn’t matter for this thought experiment. Let’s just take it for granted that some network of N neurons is responsible for one of your episodic (autobiographical) memories. In this case, being fussy about bitter pasta sauce at your Aunt’s house when you were little. Now, I sensed the bitter pasta sauce in the present day by seeing/tasting/smelling it, which activated mnemonic representations in my visual/gustatory/olfactory long-term storage systems. Our brains likely store memories redundantly, in which similar memories are stored in similar engrams by a similar group of neurons. And some percentage of those N neurons were activated by ‘pasta with sauce’—in this case ‘bitter pasta sauce.’
There’s a putative process in the hippocampus called “pattern completion”, which is when a partial cue can recapitulate a whole memory. So when I am sensing this present-day bitter pasta sauce that I’m eating, maybe 5% of those N neurons were active (many of which are always active when you eat pasta sauce, just like neurons we’ve found in human brains were always active to specific concepts like Rachel from Friends or The Simpsons). And, by some chance, this small group of neurons “pattern completed” from the initial cue into the full reinstatement of the N neurons that collaboratively hold the episodic memory of me rejecting bitter pasta in my Aunt’s house. This gives some intuition for how related memories can be recalled.
Now, that gives a possible explanation for related memories. But what about seemingly unrelated memories? What I’m postulating is that with the redundancy necessary in our mnemonic system, in which each neuron is almost certainly part of many memories that are completely unrelated, the right pattern of activation of some subset of neurons could ‘pattern complete’ into a memory unrelated to current sensations. So, for my example above of tossing a baseball with my friend: let’s say 127 neurons are key for me to recall that memory. And, based purely by chance, some stimulus or thought I had happened to activate 42 of those same neurons. Through pattern completion my hippocampus could have taken the signal from those 42 and, due in part to the randomness of neural spiking within the structure of a neural network, happened to reactivate the other 85 neurons necessary to recollect that specific episodic memory. The fact this coincident neural firing occurred 7 years after the fact would therefore be a chance event.
These thoughts on chance happenings in the brain remind me of a fascinating topic in neuroscience: randomness in our neural code might well be an essential feature for survival. In fact, some smart cookies even consider the concept of randomness in the brain as one of the major paradigm shifts in neuroscience the past 15 years. While this seems a bit counterintuitive, if an organism is 100% predictable in its actions, then its predators will be 100% likely to learn how to eat it (as Kevin Mitchell put it, the collective noun for animals with no randomness: lunch). And beyond tricking predators, randomness can also help animals like rats explore new strategies or even aid fruit flies in achieving more successful love songs. And in the case of memories, maybe coming up with random ideas once in a while could provide some survival advantage to species as well. Obviously you want your most salient memories to be robustly recalled (from what I remember baby bears usually means there are mother bears, and mother bears EAT people like me), but for those of less obvious importance, it’s interesting to think the randomness of our memories—our creativity, to some degree—might be seen in our occasional ‘lost’ memories.
*names changed for anonymity.
**name not changed for anonymity, since this is a funny name.