You are on a plane, thirty thousand feet above ground. Four hundred and fifty snakes crawl into the passenger cabin. You think this is terrifying? Hollywood producers certainly gambled on that when they released the 2006 summer blockbuster “Snakes on a Plane.”  Israeli scientists, however, have come up with an even creepier scenario.

You are in an MRI machine. Your head is fixed in a round cage. Your body is rolled into a narrow tube. Magnetic pulses are beamed into your brain. A meter-and-a-half-long snake is strapped with Velcro atop a small box on a conveyor belt just inches behind your head. Your eyes meet the snake’s beady gaze through a tiny mirror above your head. You can’t move.

Why would Uri Nili and Yadin Dudai, two scientists from the Weizmann Institute of Science in Rehovot, Israel, want to put a snake in the MRI scanner with you? Obviously, not to scan the snake’s brain (although this might be an interesting possibility). They wanted to scan your brain while you perform an act of courage. They wanted to push research on fear one step further – from understanding how we passively react to fear, through actively avoiding it, to actually confronting it.

FBI agent Neville Flynn (Samuel L. Jackson) could have been an ideal candidate for the experiment. Grabbing and fighting the snakes on the plane with his bare hands, Flynn came to the rescue of the passengers on red-eye flight 121. But there was no FBI or Mossad agent at the Weizmann Institute. The participants in the experiment had to face the snake on their own. All they had were two buttons. Pressing one would roll the snake closer. Pressing the other would slide it away. ‘Advance’ or ‘Retreat’, were their two options. They could choose either one, instructed only to do their best in pulling the snake toward their heads. (See the video here.)

“Courage,” wrote Mark Twain, “is resistance to fear, mastery of fear, not absence of fear.” And so, the participants who chose to ‘Advance’ the snake closer and closer to the backs of their heads did it despite being afraid. They were now scientifically courageous and their courage was quantitative and measurable (in ‘Snake-Advance’ units).

With those courage units in hand, Nili and colleagues thoroughly scrutinized the participants who were sliding the snake back and forth. They measured their brain activation with the MRI scanner, hooked them up with electrodes to measure how much they were sweating, and gave them questionnaires to fill out about how fearful and anxious they felt.

The results, reported in Neuron, revealed an interesting dissociation between fear reactions, a sort of internal disagreement paving the way to courageous acts.

Fear of the snake manifests itself in two ways – either you simply say, “I’m afraid,” or your body says it for you, with sweat. When Nili and colleagues analyzed the questionnaires  (in which participants rated their level of fear) and the electrical resistance of the skin (indicating how much they sweated), they realized that the two facets of fear do not always go together.

You could say that you are not afraid but sweat a lot, or say that you are freaked out and sweat not at all. But here is the interesting thing: as long as these two disagree, you would act courageously. It is only when you scored high on both, sweat and fear, that you would succumb to cowardice. It is as if you have two brakes. Release either one, and you could drive on.

Where is this driver in the brain? Imagine you stick a pencil straight into the bridge of your nose between your eyes. You push it in and stop right before the line between your ears. There it is, a brain region called the subgenual anterior cingulate cortex (sgACC, to make it short).

The sgACC was the only part of the brain whose activation went hand in hand with courageous acts. Bringing the snake closer to the head strongly activated the sgACC. The more the participants were afraid but did not succumb to fear, the more active was their sgACC, as if more “mental effort” was required to act in the face of fear.

When they did succumb to fear, another region came into play: the amygdala, an area known as a seat of primitive fear, among other things.  (To find it, now imagine you take two pencils, stick one in your eye and the other in your ear; the amygdala is where the pencils meet). Only a strongly active sgACC silenced the amygdala.

The mechanics of courage in the brain, it seems, involves a competition. When fear reaches a certain threshold, pushing both your subjective feeling of it and your bodily sweat, you would succumb. Your amygdala drives that fear, but internal disagreement overcomes it. The agent behind this disagreement is the sgACC. It acts to control and suppress bodily fear responses, and sends nerve projections into the amygdala that shut it down.

Why should we care about this? Of course there is a “brain correlate” to whatever we say, think, or do. So what? Well, every car can break down and so does our brain. Sometimes the brakes in the brain are stuck and you are perpetually petrified. Knowing how the brain works up the courage to confront fear could help. Perhaps someday, to help people with anxiety disorders, we  could inject drugs, insert electrodes, or just train the brain to act differently. With this knowledge, perhaps, eventually, fear will not prevail.

Does finding a basis for courage in the brain cast a shadow on the great heroes of history? Were they merely men of great and powerful sgACCs? Such a conclusion would be way overboard. There is no evidence for actual genetic, anatomical or physiological differences between the sgACCs of the brave and the fearful. Perhaps any sgACC could work up enough courage. We don’t know. But more importantly, do we really care?

What matters is that we have seen courage at work in the brain, and we all have the same basic neural equipment. From this point on, it’s up to us. Perhaps one day there will be a “courage pill” or maybe we could electrically stimulate the sgACC to boost up our courage. Whether we choose to do this or not is a matter of broader social and ethical debate over our growing ability to fiddle with the plasticity of the brain. At least, if we choose to try to enhance courage, we now know where to begin.

Oh, and one final note: no snakes were harmed in the course of the experiment.