why and how we remember

08 May 2024

why and how we remember

0 COMMENTS

Today's selection-- from Why We Remember by Charan Ranganath. Noradrenaline, the hippocampus, and the amygdala:

“Why do our most emotionally intense experiences—times when we were livid with anger, petrified with fear, or in shock from witnessing something terrible—seem to be indelible in memory? The answer to that question is fundamental to the very reason we evolved the capability to remember: our memories hold the key to our survival.

“As we've learned, the brain is constantly prioritizing what it thinks is important and letting us forget what isn't. So, it makes sense that we tend to remember the events associated with intense emotions, but that's only part of the story. Emotions, the conscious feelings we experience based on myriad combinations of internal and situational factors, are central to the human experience, but in and of themselves they aren't necessarily important for our survival—feeling a little guilty or embarrassed isn't going to put food on the table or a roof over your head. Rather, the critical influence of emotional experiences on memory has to do with what neuroscientist Joe LeDoux calls survival circuits.

“Our emotions, as well as the actions and choices they influence, are shaped by basic survival circuits in the brain that motivate us to avoid threats, find sustenance, and reproduce. When these circuits go into overdrive, we tend to experience intense emotions, such as elation, lust, panic, anxiety, or disgust. It makes perfect sense that these are experiences we remember most vividly. Events that intensely activate our survival circuits are worth remembering because they usually provide valuable information that we can use in the future to stay safe, thrive, and reproduce. We might not have survived as a species if our cave-dwelling ancestors hadn't found encounters with saber-toothed tigers to be particularly memorable.

“When a survival circuit in the brain gets revved up, say by the terror of a face-to-face encounter with a predator, or the joy of holding. your child in your arms for the first time, your brain gets flooded with neuromodulators. Neuromodulators are chemicals that influence the functioning of neurons, but they don't simply increase or decrease neural activity. Neuromodulators have more complex effects that fundamentally change how neurons process information. Some neuromodulators are like the hot sauce on your tacos-they change the flavor, add heat, and cause us to sit up and pay attention. Neuromodulators also promote plasticity, meaning they enable significant, long-lasting changes in the connections between neurons in the cell assemblies that are activated when we learn something new.

“Noradrenaline (also known as norepinephrine) is one well-studied neuromodulator that influences how we learn and remember. You've probably heard of the fight-or-flight response. When we experience threats, the adrenal glands mobilize us into action by pumping out adrenaline, which raises our heart rate, blood pressure, and breathing rate. Noradrenaline, in turn, is released all over the brain. Adrenaline and noradrenaline are the chemical costars of the fight-or-flight response, contributing to the feelings of arousal and immediacy you might experience if you go bungee jumping or get into a shouting match with a driver who cut you off.

A dog and a cat expressing the fight (top) and flight (bottom) response simultaneously

“Psychologist Mara Mather has shown that emotional arousal ratchets up the stakes for attention, making us better at perceiving the salient things that are important or stand out in some way. So, emotional arousal influences the outcomes of those ‘neural elections’ that determine what we will perceive, funneling resources to the strongest candidates.

“Because emotional arousal constrains what we will pay attention to, we can expect that it should change what we will remember, rather than simply how much. For instance, if you're getting mugged at gunpoint, your attention will be captured by the weapon pointed at you, possibly at the expense of the shoes the mugger is wearing. Just as increasing the contrast in a photograph makes some information stand out and pushes other information into the background, noradrenaline jacks up the contrast of our memories, highlighting the significant details.

“The effects of noradrenaline keep going even after an emotionally intense event has passed. Over a few hours, noradrenaline triggers a cascade of events in the cell assemblies that were active during the event, turning on genes that manufacture proteins that ultimately tighten up the connections between those neurons so the memory remains robust over time. If you witness a horrible car crash outside your local grocery store, the release of noradrenaline promotes changes in the connections between cells in your brain so your memory for that particular trip would be more likely to stick around than if you had walked into the store without incident. This is a key reason it's so easy to forget more mundane experiences yet so hard to let go of a traumatic memory: our brains are designed to hold on to the events that revved us up, ostensibly because remembering those events has survival value.

“Emotional arousal does more than simply affect the memorability of an event. When we recall a traumatic experience, we don't just remember what happened, we reexperience the sensations vividly and with immediacy. It feels as if the memory is stored in our bodies as well as our minds. To survivors of combat situations or sexual assault, the physical experience of reliving those traumatic events can become debilitating, effectively retraumatizing them each time the event is recalled.

“This visceral experience of remembering emotional events hinges on a key part of the brain: the amygdala. This almond-shaped area (hence the name, which is Latin for ‘almond’) sits just inside the temporal lobes, right in front of the hippocampus, and it is a core part of the brain's circuitry that responds to threats. The amygdala has a close line of communication with the different neuromodulatory systems in the brain as well as the peripheral glands that bring about the fight-or-flight response. When we recall a time our survival circuits were called into action, whether it was a harrowing car crash or the adrenaline rush of zip-lining through a forest, the amygdala and hippocampus work as a team. As the hippocampus forms memories that capture the context at that moment, the amygdala is connecting those memories with the survival circuits that generate the raw sensations. Later on, as the hippocampus helps us mentally time travel back to those events, the amygdala brings us back to the heat of the moment, making us feel as if we are vividly reexperiencing the event.

“The division of labor between the hippocampus and the amygdala suggests that there is an important distinction between what we remember and how we feel when we remember. For instance, people with amygdala damage due to a rare disease called Urbach-Wiethe syndrome can remember past events, but unlike individuals with a functioning amygdala, they don't seem to respond any differently to a gruesome photo of surgery on a car crash victim than they would to a mundane picture of a mother and child in a car. Conversely, people with damage to the hippocampus can have no memory of getting an electric shock but still experience an unconscious threat response when reminded of the event. In the healthy brain, the hippocampus and the amygdala work together, so we can recall what happened and also reexperience how we felt when it happened.

“Our understanding of how the amygdala and hippocampus work gives us insights into how we can understand and process memories for emotionally painful events. That vivid jolt we get upon remembering an emotional experience can trick us into thinking that it is fundamentally a part of our memory of what happened, but that is not the case. In my work with L.C. and others, recovery was not about forgetting the past but rather about managing the intense emotions brought about by the past."