When cognitive neuroscientist Charan Ranganath meets someone for the first time, he's often asked, "Why am I so forgetful?" But Ranganath says he's more interested in what we remember, rather than the things we forget.

"We're not designed to carry tons and tons of junk with us. I don't know that anyone would want to remember every temporary password that they've ever had," he says. "I think what [the human brain is] designed for is to carry what we need and to deploy it rapidly when we need it."

Ranganath directs the Dynamic Memory Lab at the University of California, Davis, where he's a professor of psychology and neuroscience. In the new book, Why We Remember, he writes about the fundamental mechanisms of memory — and why memories often change over time.

Ranganath recently wrote an op-ed for The New York Times in which he reflected on President Biden's memory gaffes — and the role that memory plays in the current election cycle.

"I'm just not in the position to say anything about the specifics of [either Biden or Trump's] memory problems," he says. "This is really more of an issue of people understanding what happens with aging. And, one of the nice things about writing this editorial is I got a lot of feedback from people who felt personally relieved by this because they're worried about their own memories."

Interview highlights

On instituting a cognitive test for candidates running for president

I think it would be a good idea to have a comprehensive physical and mental health evaluation that's fairly transparent. We certainly have transparency or seek transparency about other things like a candidate's finances, for instance. And obviously health is a very important factor. And I think at the end of the day, we'll still be in a position of saying, "OK, what's enough? What's the line between healthy and unhealthy?" But I think it's important to do because yes, as we get older we do have memory problems. ...

On why you can sometimes only remember the first letter of something, like a name

You get what's called partial retrieval, where you get a piece of the information but not the whole thing. ... Memories compete with each other. And this is true for a name. This could be true for memory, for an event. And so if you have learned multiple names that start with the letter K, now what happens is you have this competition where essentially they're fighting with each other.

On the tip-of-the-tongue phenomenon

They call it the tip-of-the-tongue phenomenon ... where you know the information is there, you're aware of something, but it just doesn't. You don't have proof of its existence. You're just working on this complete faith that it exists. There's many reasons why this happened. One of the big ones is you pull out the wrong information. When you pull out the wrong information, what happens is it makes it much harder to find the right information. So in other words, if you're looking for someone named "Fred" and you accidentally pull out "Frank" and you know that's not the name. Now, Frank is very big in your consciousness, and it's fighting against the other memory that you have. And so as a result, you're going to have some trouble. Now, later on, what happens is your mindset changes and you're no longer stuck in that previous mistake. And that's why it can pop up. So what can sometimes happen is that we're looking for something, but then we get the wrong thing. And that leads us so far in the wrong direction that the competition in memory works against us.

On how interruption hurts our ability to remember

This is the reality of modern life, is that we're constantly being interrupted. Now, sometimes those interruptions are in our world and not of our own making. So any person with a newborn child, for instance, can relate to this idea of you're trying to do something and all of a sudden your child starts crying and your brain is telling you, "Forget everything else. Let's focus on this." Then there's things that we do to ourselves, like, we just have other thoughts that come into our head or we start daydreaming about things. But then I think the most insidious of all are the alerts and the distractions that we put upon ourselves with smartphones and smartwatches where there's things constantly buzzing and grabbing our attention, and then people start to get bad habits like checking texts and emails. For instance, I'll sit in academic talks and I see people checking email during a talk, and I can guarantee you they're not remembering either the email or the talk after they've left the place.

On how stress interferes with memory

Stress has a bunch of complex effects on memory. So if you have a severely stressful experience, sometimes you can remember that experience better than if it was not stressful. And so this happens a lot in cases of traumatic memories. But the other part of it is that stress makes it harder to pull out the information you need when you need it. ... It shuts down the prefrontal cortex. And under those states of stress, you're prioritizing things that are more immediate, your knee-jerk responses to things. And so that makes it harder to remember stuff that happened before you were under stress.

Then there's the issue of chronic stress, where we know that chronic stress can be actually neurotoxic for areas of the brain that are important for memory, like the prefrontal cortex and another area called the hippocampus. And that is really, I think, part of the problem that you see in people with PTSD, for instance. If you're under chronic stress for a long period of time, there's a whole series of stress-related hormones that are bathing your brain in these stress-related hormones. And what can happen is, this can be causing damage to areas like the hippocampus and the prefrontal cortex so that they're no longer functioning as efficiently as you would hope they would. And you can see this in many different animal models of stress.

On why sleep is so important to memory

One of the fascinating things about sleep is we tend to think, oh, nothing's happening. I'm not getting anything done. But your brain is hugely at work. There are all these different stages of sleep where you can see these symphony of waves, where different parts of the brain are talking to each other, essentially. And so, we know for a fact that some of these stages of sleep, what happens is the brain will flush out toxins, like the amyloid protein that can build up over the course of a day. So just by virtue of that function, sleep is very important. But then on top of it, what we can see is that the neurons that were active during a particular experience, have come back alive during sleep. And so there seems to be some processing of memories that happen during sleep, and that the processing of memories can sometimes lead to some parts of the memory being strengthened, or sometimes you're better able to integrate what happened recently with things that happened in the past. And so, sleep scientist Matt Walker likes to say that sleep converts memory into wisdom, for instance.

[Sleep is] an investment. Because you're depriving your brain of all this, information processing that can happen in your sleep. And I do believe it's controversial, but I do believe in the idea that sometimes you can wake up and through that memory processing, actually have the ability to solve a problem that you couldn't do when you were, before you went to sleep. I mean, the other part of sleep, I think that's very important is when we're sleep deprived it's just terrible for memory. All the circuitry that's important for memory does not function as well, and memory performance really declines.

Sam Briger and Thea Chaloner produced and edited this interview for broadcast. Bridget Bentz, Molly Seavy-Nesper and Carmel Wroth adapted it for the web.