Image: Brain study
Brain imaging studies in volunteers indicated that the amygdala, shown as a red and yellow spot in this graphic, responded specifically to the wide-eyed image that was interspersed with images of neutral faces.
updated 12/16/2004 7:16:36 PM ET 2004-12-17T00:16:36

If you look into the eyes of someone who is frightened, your brain will pick up on the fear in a split second, well before you can consciously put a name to the emotion, scientists say.

Why such a hair-trigger response to what someone else is feeling? Recognizing a fearful expression on another person’s face might save your skin some day, because whatever has spooked your friend might also be a danger to you.

According to a new study in the journal Science, published by AAAS, the nonprofit science society, seeing the enlarged whites of fear-widened eyes is enough to activate a fear-related structure in the brain called the amygdala.

Fear Central
The amygdala, an almond-shaped nugget buried deep in the brain, is an ancient structure found in all vertebrates.

Scientists have learned much about the amygdala by studying human patients with damage to this part of the brain. These patients are remarkably normal in most respects. When they look at fearful facial expressions, however, they often have difficulty recognizing that fear is the emotion being expressed. Or they can’t distinguish between expressions of mild fright and sheer terror.

Some evidence suggests that faulty signaling by the amygdala may be involved in autism, a disorder that affects, in part, verbal and nonverbal communication abilities. Likewise, too much activity by this structure may play a role in anxiety disorders, according to Paul Whalen of the University of Wisconsin at Madison.

Because fear can help you avoid danger, it’s no surprise that the amygdala is “Fear Central” for the brain. It’s involved in other emotions too, but it’s crucial for feeling fear and priming the body to respond physically to danger.

If an object comes flying at your head, “you don’t need to know that it’s a brick or a tennis ball, you need to start buckling your knees,” said Whalen.

Whalen also thinks the amygdala is important for learning to avoid dangerous situations in the future, which would help explain why it’s so highly attuned to fearful and surprised facial expressions.

In response to these expressions, the amygdala basically says, “That person has detected an important event close by and I should really find out what it is.”

The amygdala on autopilot
Whalen described the amygdala as somewhat like an idiot savant: It’s neither versatile nor flexible, but it does its job exceedingly well. It’s so good at its task, in fact, that it senses fear in others and responds automatically, perhaps even before parts of the brain involved in conscious thought have figured out what’s going on.

“People automate thousands of things, like driving. Sensing fear is important enough that we’ve automated it,” Whalen said.

Scientists have been investigating the automatic nature of the amygdala using a technology called functional magnetic resonance imaging, or fMRI, to measure the blood flow in specific regions of the brain.

Whalen and his colleagues, for example, have found that volunteers’ amygdalae responded to images of fearful faces — even though the faces appeared too quickly for the volunteers to be aware of them.

The reason for showing the images so quickly was to eliminate conscious thoughts that would confuse the results. With this subliminal-image approach, known as “backward masking,” the researchers could be sure that the fMRI device wasn’t recording the brain activity associated with the volunteers’ thinking “Hey, that face looks like my Uncle Ed.”

As with other fMRI studies, more research will be needed to uncover the actual nature of the brain activity that the device picks up. The notion that the amygdala might be part of a circuit that could bypass detailed processing through the cortex in humans remains controversial.

The whites tell the story
Facial expressions can be extremely complex and subtle. Often it’s difficult to tell just what part of a face is sending a particular message, though the eye region of the face seems to be most important.

In order to determine what part of a fearful face was stimulating the amygdala, Whalen and his colleagues considered other facial expressions. Angry faces don’t activate the amygdala as strongly, for example, but surprised faces do.

The eyes — specifically, the whites of the eyes — were indeed the key.

For example, a pair of fearful or surprised eyes has larger whites than eyes from other expressions. Happy expressions tend to have eyes showing the smallest amount of white, according to Whalen.

In their new Science study, Whalen’s team examined whether eye whites alone were sufficient to trigger the amygdala. They used the backward-masking approach on their volunteers, interspersing pictures of fearful or happy eye whites for a split-second in between longer images of neutral faces.

The eye white stimuli were derived from actual photographs of actors making afraid and happy faces, developed by Paul Ekman at the University of California at San Francisco.

The volunteers’ amygdalae responded only to the fearful eyes, the results suggested. Thus, instead of having to sort through many subtle variations in facial expression to locate a fear signal, the brain can just use information about eye-white size.

“We suspected that the amygdala was not all that bright.  The eye-white finding offers a simple rule that the amygdala could handle,” Whalen said.

While some experts might be tempted to say this automatic fear-sensing ability is something we’re born with, Whalen said it’s too early to tell for sure.

Just because a brain response is automatic doesn’t mean it can’t be something we learn how to do with experience. Understanding facial expressions is essential for human communication, so Whalen thinks we may learn to do this very early on, even as infants staring into our parents’ faces.

Interestingly, most monkeys and apes don’t have lighter-colored scleras (or “eye whites”). So if this type of fear sensing is hard-wired in humans, it must have evolved relatively recently.

© 2013 American Association for the Advancement of Science


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