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How do our brains decide to remember something positively or negatively?

(SOUNDBITE OF ALARM RINGING)

AYESHA RASCOE, HOST:

What happens when you hear that sound in the middle of the day? Do you feel dread or anger? Or maybe if you're a morning person, it makes you feel excited because you associate it with the start of your day. Be it positive or negative, a new study shows how the brain associates the sound of an alarm or any memory with a feeling. Hao Li is a researcher at the Salk Institute for Biological Studies and one of the authors of this study. He joins us now. Welcome.

HAO LI: Hi.

RASCOE: So your study refers to something called emotional valence. Can you first tell me what that is?

LI: Yeah. So essentially, all the behaviors in our lab have been motivated by either there's a good memory or bad memory. So emotional valence just refers to the value that we put on our memories.

RASCOE: OK. So this makes me think of that song "The Way We Were" by Barbra Streisand. Stay with me, now. I've been listening to the version by Gladys Knight, and she says memories may be beautiful, and yet what's too painful to remember, we simply choose to forget. So it's talking about the different types of memories and why we might remember good things over bad things. Is that kind of why you wanted to do this research?

LI: Yes, exactly. It's really close to our daily life. So that's one of the big motivations for me to study neuroscience, particularly study how our brain thinks and how the memory is stored.

RASCOE: And so let's say I smell or taste something that makes me feel comforted. What's happening in my brain that associates that feeling with that memory?

LI: So this goes back to our 2015 study where we identified these two populations of neurons in the amygdala that selectively responds to good or bad memories. So it's almost like two separate railway tracks that one leads to reward and one leads to punishment.

RASCOE: What was your approach to finding out how this emotional valence is determined?

LI: Essentially, we use rodents models, specifically mice. We train the mice to perform this task where they hear a sound, and a few seconds later, the sound is followed by either a sugar water reward or a mild shock punishment. So over multiple trials, we'll learn that sound A is predictive of reward, which is the sugar. The sound B is predictive with a full shock, which is punishment. So we can then observe the animals' behavior during the presentation of this tone to see whether or not animal have learned that this is a good sound or otherwise the bad sound.

RASCOE: And what you found was that they were able to associate that sound with either a good thing or a bad thing.

LI: Yeah. Essentially, we use this task to model whether the animal have learned a good and bad memory. And what we found is this signaling molecule that's called neurotensin that can work as almost a switch operator that can turn on and off good and bad memory.

RASCOE: And, I mean, do we know if the same rules apply to human brains?

LI: Yes. Yes. A lot of studies have shown that human brains are using a very similar circuit to rodent brains in terms of emotional or memory processing or learning those basic survival skills.

RASCOE: So would there be, like, an evolutionary reason for these findings? I mean, it seems like it would make sense that if you hear something that should be associated with danger, it would be good to remember that.

LI: Yeah, I think it's most likely it's a revolutionary reason, right? Because, you know, if there's a danger, you want to avoid, run away, but if you're hungry, you can wait. But you don't want to wait if the tiger is coming at you. So evolutionarily, the brain circuits have been prompt to punishment over reward.

RASCOE: So then I got to ask because it almost seems like in modern day with humans, we can be anxious and remember those bad experiences that we don't want to remember as much. Are there practical ways that scientists or medical professionals could use your findings to help with people who may have a lot of negative feelings?

LI: Yeah. So I think there's a big potential because if you think all those mental health disorders, there's either too much reward or too much negative feeling, right? But if there's a molecule that can reverse this and make them feel happy again or make them learn things to be happy, they can potentially help them to overcome or recover from these diseases.

RASCOE: Hao Li is a researcher with the Salk Institute for Biological Studies. Thank you so much for talking to me about this. It was very interesting.

LI: Thank you for having me. Transcript provided by NPR, Copyright NPR.

Ayesha Rascoe is a White House correspondent for NPR. She is currently covering her third presidential administration. Rascoe's White House coverage has included a number of high profile foreign trips, including President Trump's 2019 summit with North Korean leader Kim Jong Un in Hanoi, Vietnam, and President Obama's final NATO summit in Warsaw, Poland in 2016. As a part of the White House team, she's also a regular on the NPR Politics Podcast.
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