The Fear Trigger The Trusty Little Amygdala Helps Keep Us Out Of Harm’s Way.
Mother Nature has provided each of us with a miniature “watchdog” in our brain, an almond-sized organ whose job is to warn us of imminent peril.
This subconscious system has been part of our makeup since our remote ancestors, the first mammals, were dodging dinosaurs more than a hundred million years ago.
The little lump of tissue is where our brains deal with fear, the oldest and most powerful of all human emotions.
Fear might be unpleasant, but it is essential to survival, helping keep us safe from guns, knives, speeding cars and deadly vipers. It provides extra energy to muscles so we can run faster or hit harder. It suppresses pain, allowing, for instance, a wounded soldier to keep fighting.
“We don’t have to learn how to be afraid,” said Joseph LeDoux, a leading fear researcher at New York University. “We are born with this in our brain.”
Until recently, emotions like fear were the province of psychologists and psychiatrists. Little work was done to understand the biological nature of fear.
But now, powerful new electronic tools let scientists observe the amygdala at work deep inside the living brain. Their research could help doctors diagnose and treat victims of head injuries or emotional disorders.
“Ultimately our work may lead to knowledge that will help us prevent some of the psychiatric and psychological afflictions that currently cause an enormous amount of suffering among people,” said Hanna Damasio, a neurologist at the University of Iowa’s College of Medicine in Iowa City.
Scientists call the center of fear the amygdala (ah-MIG-dah-la), for the Greek word for almond, which it vaguely resembles. There are actually two of them, one on each side, buried deep below the ears.
“A very careful watchdog seems to be exactly what nature has installed in our brains to protect ourselves from threat and danger,” psychologist Sabine Windmann said in a report to an international convention of neuroscientists in New Orleans last month.
“It is able to detect potential threats in the environment immediately and to respond to them automatically, even before higher developed parts of the brain can notice and identify them,” said Windmann, a researcher at the University of Saarlandes in Hamburg, Germany.
Like a real watchdog that barks at the postman as well as a burglar, the amygdala is quick to respond but is sometimes mistaken. Occasionally it overreacts, causing phobias, panic attacks or the post-traumatic stress disorders afflicting war veterans and rape victims.
“While fear is a part of everyone’s life, too much or inappropriate fear accounts for many common psychiatric problems,” LeDoux said.
In a lecture to the neuroscientists, LeDoux described the amygdala as “the hub of a wheel of fear.” It takes in danger signals from your senses - like the sight of a snake or the sound of a gunshot - and flashes messages through the subconscious nervous system, alerting your body to respond.
Immediately hormone glands start pumping out adrenaline, producing “the taut stomach, racing heart, clammy hands and feet and dry mouth that typify fear in humans,” LeDoux wrote in a book, “The Emotional Brain,” published last year by Simon & Schuster.
Researchers have discovered two paths by which danger signals reach the amygdala. Both run from the eyes, ears and other sense organs to a central clearinghouse deep in the brain called the thalamus. Then they divide.
One path, which LeDoux calls the “high road,” leads upward from the thalamus through the frontal cortex, the part of the brain involved in thought, memory and consciousness.
The “low road” is shorter, running directly from the thalamus to the nearby amygdala. Precious microseconds are saved by this short cut, but the message can be inaccurate because it does not go through the frontal cortex to be interpreted.
“There are many things we are better off not thinking about, like blinking when objects come near the eye,” LeDoux said. “The thalamic pathway is a quick-and-dirty processing system. It cannot tell the amygdala exactly what is there, but can provide a fast signal that warns that something dangerous may be there.”
Say you are walking in the woods and see an object that might be a snake or a stick. Before the higher levels of your brain decide which it is, your automatic fear system makes you react as if it were a snake.
“You’re better off treating a stick as a snake than a snake as a stick,” LeDoux said.
Because of our evolutionary history, researchers say humans are preconditioned to fear heights, spiders and snakes, just the way mice freeze at the sight of a cat. When our distant grandparents still lived in trees, falling from a height was an ever-present danger. Spiders probably were perilous to cave dwellers.
For similar reasons, our distant relatives, monkeys, still fear snakes. If a monkey’s amygdala is removed, it loses its fear.
Until recently, most fear studies were done with rats, usually by administering electric shocks and observing the way the animals’ behavior changed. Occasionally a person with a brain injury could be tested for unusual emotional reactions.
But the latest electronic techniques, particularly magnetic resonance imaging, allow safe, ethical experiments on normal people. For example, MRI scans show which areas of the brain respond to a mild electric shock or a sudden puff of air near the eyes.
“It is now possible to view the brain images elicited during anxiety-provoking situations in humans,” said Kevin LaBar, a researcher at Northwestern University in Evanston, Ill.
A rapid response to danger has been essential to survival throughout evolution. A single electric shock is usually enough to train a rat to avoid a certain passage in a maze, for example.
“An animal in the wild doesn’t have an opportunity to practice trial after trial,” said LeDoux. “If he escapes a predator once, he’d better remember it the next time.”