What's scarier, a deadly snake slithering across your path during a hike or watching a 1,000-point drop in the stock market? Although both may instill fear, researchers disagree over the nature and cause of this very powerful emotion.

"When you see the stock market fall 1,000 points, that's the same as seeing a snake," says Joseph LeDoux, professor of neuroscience and psychology the Center for the Neuroscience of Fear and Anxiety based at New York University. "Fear is the response to the immediate stimuli. The empty feeling in your gut, the racing of your heart, palms sweating, the nervousness—that's your brain responding in a preprogrammed way to a very specific threat."

LeDoux adds: "Since our brains are programmed to be similar in structure, we can assume that what I experience when I'm threatened is something similar to what you experience."

Fear even affects different species in similar ways. "We come into the world knowing how to be afraid, because our brains have evolved to deal with nature," LeDoux says, noting that the brains of rats and humans respond in similar ways to threats, even though the threat itself might be completely different.

Other researchers find fear to be a vastly personal experience. Whereas some people become terrified watching a scary film, others may be more afraid to walk back to their cars in a dark parking lot after the movie ends.

If you ask a group of people to catalogue the things that make them afraid, you are likely to get a very different list from each person, says Michael Lewis, director of the Institute for the Study of Child Development at Robert Wood Johnson Medical School in New Brunswick, N.J. "Introspectively, we can agree that fears of an IRS [Internal Revenue Service] audit or mugging may feel the same," he says. "The problem is we don't have a good physiological measure of fear or any emotion."

He notes that the behavior of people around us may influence our responses to threatening situations. "We learn to become fearful through experience with the fear event, or learning from those people around us like our parents, our siblings, our colleagues," Lewis says. "Fear has a certain contagious feature to it, so the fear in others can elicit fear in ourselves. It's conditioning, like Pavlov and the salivating dog."

Other researchers turn to technology to help them better understand trepidation. "It's very hard to define that emotion in terms of the feeling it evokes," says Joy Hirsch, professor of functional neuroradiology, neuroscience and psychology, and director of the program for Imaging and Cognitive Sciences at Columbia University in New York City. "Can you define what pain is? Can you define what the color red is? These are the essential sensations that represent the hardest problems in neuroscience."

To find out more about what keeps us up at night, Hirsch and her team use fMRI (functional magnetic resonance imaging) to probe how our brains are wired. "The circuitry that underlies the sensation of fear is quite readily activated with a specific stimuli," Hirsch says. In her research, subjects are shown a photograph of another person's face with a frightened look. A standard library of stimuli to elicit fear activity uses actors to make facial gestures that communicate fearfulness.

In the fMRI research, the reaction to fear-inducing stimuli shows up in the amygdala, an almond-size mass beneath the temporal lobe also known as the brain's fear center. Hirsch says the amygdala is the first responder to threatening stimuli.

The fMRI scanner tracks the change in blood flow to the amygdala. "We are looking at signal changes in particular parts of the brain," Hirsch says, "The signal means increased neural activity." The magnetic resonance signal is responding to the amount of blood that is being recruited to the local area. The photograph of a fearful face elicits a greater amount of blood flow and a higher signal during the scanning period than a neutral face photograph."

Critics of fMRI-based research point out that it is not always clear what the flow of blood in a brain region means. But Hirsch dismisses naysayers. "We use very carefully chosen stimuli that don't necessarily scare people [who are] in the scanner, but arouse systems that are involved in fear activity if you were scared," she says. "The interpretation of another fearful face arouses the system of neural regions that respond to fear in the observer."

Hirsch notes that the amygdala responds to more than just facial expressions. "If you were in a dark alley and something scary jumped out at you," she says, "it would be the amygdala that would contribute to your decision to run."

Fear is as basic a human process as breathing or digestion, yet science's ability to completely understand and describe it remains elusive. "That is the $64,000 question," Lewis says, noting that despite more than 100 studies into how the body reacts to fear, there still is no way to quantify fear itself.