The “Fight or Flight” Idea Misses the Beauty of what the Brain Really Does

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The brain's primary job is to reduce uncertainty in an ever-changing world

 

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When a person views a photograph of a hairy, looming spider or a slithering snake in a laboratory experiment, scientists usually see markers of increased electrical activity deep in that person’s brain, in a region called the periaqueductal gray (PAG). When a caged mouse smells a cat and freezes, scientists observe similar changes in the mouse’s PAG. What’s the obvious conclusion? The PAG controls fight-or-flight responses of mammals in threatening situations.

 

But what if brains don’t have dedicated circuits for fighting and fleeing? People clearly experience threats, but is threat detection really a primary mode of the brain with its own neural circuitry? A body of recent evidence from my lab and elsewhere suggests that we don’t go through life constantly detecting threats and reacting with flight-or-fight circuits. Rather brains operate mainly by prediction, not reaction.

 

All brains constantly anticipate the needs of the body and attempt to meet those needs before they arise. They seek to reduce uncertainty to survive and thrive in circumstances that are only partially predictable.

 

Reducing uncertainty requires energy. It’s a costly metabolic outlay for a brain, and if intense or persistent enough—as in times of political chaos, economic or personal hardship, pandemic or climate change—the metabolic burden can feel distressing or utterly exhausting. The suffering signifies that you’re doing something really hard: foreseeing the future in an ever changing, uncertain world.

 

Why do people believe that fight-or-flight circuits exist? This notion belongs to a popular but outdated idea about brain evolution and function called the “triune brain.” According to this idea, the human brain evolved in three layers—one for instincts and urges, one for emotion and one for rationality—arranged like a layer cake. The deepest layer, your so-called lizard brain, which includes the PAG, supposedly houses neural circuits that evolved in reptiles to avoid threats, seek food and compete for mates. Over time, the story goes, ancient mammals evolved a layer of brain circuits for emotion called the “limbic system.” This includes a region called the amygdala that’s been dubbed the home of fear or emotion. Eventually, certain mammals, notably humans, supposedly evolved a topmost layer of circuits for higher-order reasoning located in a brain area called the neocortex (where “neo” means “new”). According to this idea, the neocortex strives to keep our so-called inner beast in check.

 

The roots of this three-layered confection of instinctive, emotional and rational parts stretch back to a morality tale told by Plato in Ancient Greece. He described the human psyche as two wild horses controlled by a human charioteer. In modern terms, the tale goes like this: Your brain is a battleground between rationality on one hand and urges and emotions on the other. When your rational brain keeps your inner beast brain under control, you’re a good person: healthy, mature and just. On occasions when your beast gains the upper hand—poetically called “amygdala hijack”—you’re childish or wicked. And if rationality cannot contain your inner beast, it could even be an indication of mental illness.

 

If you know anything about evolution, you might already suspect that something in this story is amiss. For starters, mammals didn’t evolve from reptiles. The only animal on this planet with a lizard brain is a lizard. But here’s something you might not know: Thanks to advances in molecular genetics, scientists discovered that nonhuman mammals have the same kinds of neurons that humans do—even those in the misnamed “neocortex.” The varied brains of humans, mice and other mammals look quite different to the naked eye. Yet their biological building blocks are the same. (What differs is the timing as those brains develop.) Even reptiles have many of the same building blocks that we do.

 

Also, scientists have known for some time that there’s no such thing as a unified limbic “system” dedicated exclusively to emotions. And the misnamed neocortex, which is properly called the cerebral cortex, is not the home of rationality. So where does this leave the fight-or-flight story? We can find answers in three fairly recent sources of scientific evidence.

 

The first source comes from powerful brain-scanning devices. One very popular scanning technology called functional magnetic resonance imaging (fMRI) uses a large magnet to measure changes in blood flow in the brain. These blood flow changes are thought to reflect changes in certain types of neural activity. Magnet strength is measured in units called teslas. Typical brain imaging experiments use a three-tesla magnet, which cannot precisely localize small and deep brain regions such as the PAG. With the advent of more powerful seven-tesla magnets, however, scientists can peer into deep clumps of neurons with far greater precision. My lab has recently observed that changes in PAG activity occur in nonthreatening, mundane tasks, such as comparing a letter of the alphabet to another letter on a screen.

 

So if the PAG is not dedicated to fighting or fleeing, what is it doing? Anatomical connections suggest that the PAG regulates and coordinates your heart, lungs and other systems of your body with your so-called limbic system and neocortex. And it does so not just when you experience a threat but all the time—whether you’re highly emotional, deep in thought, sound asleep or reading a fascinating article about the brain. In moments of threat, such as in the mouse and human experiments I described earlier, the PAG is simply working harder. (Incidentally, this explanation suggests one reason why so-called antianxiety drugs manage anxiety disorders without curing them. They target brain circuits that aren’t dedicated to anxiety or fight-or-flight but that simply regulate the body.)

 

The second reason we should reevaluate the fight-or-flight story comes from studies of mice. In these investigations, the animals are free to roam around, and when these mice encounter a potential threat, such as the odor of a predator or something novel, they typically don’t freeze, fight or hightail it out of there in one simple movement. More often, they flit away and then dart forward, repeatedly yo-yoing back and forth between avoiding and approaching. When my daughter was a little girl playing at the beach, she’d do something similar at the edge of the water: running toward and then away from the ocean waves and dipping her toes in the surf over and over. Similarly, the mice appear to be foraging for more information, dipping their toes into an ambiguous situation, trying to reduce the uncertainty of their immediate environment.

 

The third piece of evidence comes from an avalanche of research over the past 20 years on the brain’s powers of prediction. In daily life, you might feel like you react to all sorts of things that are threatening or safe. You see a car swerve toward you, and you flinch. You read a text from a loved one, and you smile. You hear a sneeze and turn away to avoid germs. You smell the aroma of a chocolate cake, and you salivate. In scientific terms, you encounter a stimulus and then respond. The best available evidence suggests, however, that your brain doesn’t react to the world—it predicts in advance how to act and what to experience in the next moment.

 

This guessing happens so quickly and efficiently that you don’t experience yourself doing it. You don’t react to someone’s sneeze by turning away. As they slightly move their head, preparing to sneeze, your brain uses its abundant past experiences to predict the probability that a sneeze is coming and creates an action plan for you to turn away. These preparations for movement also prepare you to hear and see the sneeze, all before the sneeze happens. The signals coming from the retina in each eye and the cochlea in each ear either confirm those predictions or adjust them so your brain can predict better next time. The fancy name for this is “learning.”

 

Like every other animal, you have basic needs. To eat, drink and deal with other imperatives of life, you must move about in the world—a world that is ever changing and only partly predictable, populated with other living creatures who might ignore you, eat you or be tasty to eat. This world is inherently uncertain, and uncertainty is expensive, metabolically speaking. In an uncertain situation, your brain must assemble multiple predictions with multiple action plans and maintain them for an extended time through neuronal activity and other brain functions. All this takes energy, more than you’d need to hold fewer action plans for a shorter time. Also, uncertainty translates into a slower response. If you truly are facing a germ-filled sneeze, a swerving car or some other threat, a delay could be life-threatening.

 

A core task of every brain is to reduce uncertainty so it, and the rest of the body, can operate in a metabolically efficient manner. This increases the odds of good health, survival and reproduction. This more justified scientific explanation might not reflect the dramatic feeling of being pulled in two directions at once by emotion and reason. But the experiences crafted by your brain don’t reveal how it works.

 

Moreover, your brain, which does much more than avoid threats, eat and mate, also lives in a complex social world with abundant ambiguities to deal with. That’s a recipe for stress. And what is stress? It’s just your brain anticipating the need to expend energy and preparing accordingly. Spending energy isn’t limited to avoiding threats, either. It also covers exercising, getting out of bed in the morning and reading this article. Additionally, it’s the cost of learning in uncertainty because your brain spends energy to hone its predictions to be more effective in similar situations in the future. There’s also a sustained metabolic cost if you don’t learn and instead remain in never-ending soup of uncertainty.

 

Metabolic expense, if it drags on for long enough, can feel unpleasant. When your brain attempts to learn in uncertainty, it releases chemicals that increase your arousal level, and you may feel worked up and agitated. Your brain often makes sense of this arousal as anxiety, but that’s not mandatory. You can also make sense of arousal as regular uncertainty and forage for information like a free-roaming mouse. Uncertainty can be a good thing. People even seek it out—and its positive cousin, novelty—when they try new foods, watch new movies, meet new people and learn new skills. Uncertainty, like all things that are metabolically costly, needs to be managed in a metabolically efficient way.

 

Uncertainty is a normal condition of life, but these days, with social media and round-the-clock news coverage, it sometimes bombards us. At every moment, there’s a crisis somewhere in the world: war, political chaos, climate-induced fires and floods and school shootings, not to mention the occasional pandemic. Too much uncertainty is metabolically draining and can leave you feeling distraught and worn out. But these feelings don’t emerge from mythical, overtaxed fight-or-flight circuits. They may just mean, in an ever changing and only partly predictable world, that you’re doing something really hard.

 

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