Where is my memory going?

Probably the biggest symptom of aging is memory decline. Everyone knows it, someone with it, or about it. Nothing in nature lasts forever, and our brains are no exception.

Of course, the brain is super powerful and truly amazing with everything it controls, but it’s not immortal. After 80-100 years of use, it makes sense that it starts to fall apart a little. But maybe understanding why memory decline happens could help us prevent the effects, or at least, slow it down. For some context, I will begin by talking about the different types of memory in the brain.

Implicit vs. Explicit Memory

There are actually two types of memory in the brain (cool!) called short-term and long-term memory. The names are a little intuitive, but short-term memory is mainly sensory memory and things that you only need to remember for a short amount of time. Long-term memory is, well, long-term, and can be divided again into declarative (or explicit) and non-declarative (or implicit) memory.

Explicit memories are those you can declare, hence the name “declarative”, which basically means they are memories that you can remember remembering. That’s a little confusing, so here’s an example. Remember when you got lost in a shopping mall when you were little? Or the name of all the continents that you learned in grade school? That’s an explicit memory.

Implicit memories are the ones you can’t speak out loud, hence non-declarative, but you still know how to do. What do I mean by that? Well, you probably know how to tie your shoes properly. And maybe you had a bad experience with a dog when you were a kid, and now 20 years later, you’re deathly afraid of dogs and will avoid them at all costs. These are things that you can’t explain immediately, and you just know or do. Sure, if you’re asked by a kid how to tie their shoelaces, you can probably teach them. But what makes this an implicit memory is that when you’re tying your shoelaces, you don’t do it by relying on a memory (of how to tie your shoelaces) in a step-by-step process. It just comes to you.

Now, explicit memories are the big flashy ones that everyone talks about when you think about memory decline. These memories are known to get worse when we age, for example, forgetting your keys at home after driving a couple miles away, or forgetting the name of a close friend. Losing these memories probably seems harmless at first, but when the memories that are being forgotten have to deal with loved ones or safety, things get dark pretty fast. For example, older people or people with dementia disorders might forget the decades they spent with their spouse, or that they should turn off the gas stove after using it.

Like I said earlier, no one really talks about how implicit memories are affected by age. That’s why we are! To talk about this, I’m going to describe a psychological research technique called priming. This is where responses by subjects are created by the experimenter, for example, a participant primed to a lake (by being shown a picture of a lake) is theoretically more likely to remember details about a story involving a lake (this would be a story provided before the priming). So, this is a pretty good measure of “implicit memory.” Studies in labs generally find no relationship between age and primed responses. This means no matter the age, a person will still remember more things about a lake story if they were first primed to a lake.

There’s another interesting thing about implicit memory and age. Studies show that primed memories can become clearer and more detailed (meaning the people can remember more) after doing more tests. This is a “practice effect,” and it means that older people can still improve their implicit memory through practice, which, in my opinion, is a pretty hopeful finding.

Now that we have talked about the different memory systems in the brain and how each one might respond differently to age-related memory loss, we will discuss a newer theory that could help us prevent age-related memory loss (wow, really? Tell me more).

Neural Compensation

Obviously, we can’t always just perform surgery on seemingly healthy people just because we want to look at their brains. So, techniques called neuroimaging have become super popular in the research community, and researchers are now able to look at the brain on a scan without a scalpel (the knife they use in surgeries).

One method is called PET and it measures brain area activation. This method is a little outdated, but a lot of studies in the 1990s or so used it to conclude older adults use brain areas differently than younger adults, even though the people are doing the same things. Weird, right? Scientists thought so too, so they studied it so we can know more.

With enough studies, a hypothesis around this formed, and researchers voted to call it the compensation- related utilization of neural circuits hypothesis. If you think this is too long to remember, maybe the abbreviation will help: CRUNCH. Right now, people think CRUNCH is a result of hemispheric asymmetry (we know the brain has two sides. So, if older adults use structures on the left side of the brain to do a drawing task, younger adults might use the right side. So says the theory, anyway).

I said earlier that PET is pretty much no longer used in research. This is because it uses a radioactive isotope to get all the nitty-gritty data images and that’s both expensive and complicated. Nowadays, we have something called TMS, which is also non-invasive (meaning there are no knives involved) and is basically a procedure where electrodes are used to send teeny tiny magnetic pulses to areas in the brain that can either activate (turn on) or deactivate (turn off) these regions. Researchers using TMS in their studies have written things that are really important to the CRUNCH theory. One group, Rossi et. al. used TMS to show older adults use the front part of their brains (on both hemispheres, or sides) when doing memory tasks, while younger adults tend to use the left side of their brains more often.

In this article, we talked about how the two parts of memory differ when it comes to age-related losses. Something called explicit memory tends to decrease when people age, but the system called implicit memory is pretty much untouched and can even improve, with practice! That sounds like good news for memory, and maybe makes aging a little less scary for people really worried about losing their memory.

We also talked about a new(er) theory in memory research, called CRUNCH. Basically, the theory says when the areas of the brain that younger adults use for memory don’t really work as well anymore, older adults will “call in” other parts of their brain to help with the same memory tasks. Same results, just older people might take a different pathway or a longer time to do it. Again, also good news for memory!

And remember to be kind and patient to older people, since they are literally thinking differently than you, for all things, not just memory!

— Allie Yuxin Lin