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 prioritising 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 flavour, 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 mobilise 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.
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, funnelling 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 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.
Excerpted with permission from Why We Remember: The Science of Memory and How it Shapes Us, Charan Ranganath, Faber.
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