Darwin’s spirit lives on in everything from the Human Genome Project to medicine to conservation biology–the three topics I covered in my post on Friday. It also lives on in brain scans.
While Darwin is best known for The Origin of Species, he also wrote a lot of books in later years, most of which explored some aspect of nature that he showed revealed the workings of evolution. His examples ranged from orchids to peacock tails. In his 1872 book, The Expression of the Emotions in Man and Animals, he proposed that the expressions we humans use–our smiles, our frowns, and so on–are part of a heritage that dates back millions of years, a heritage shared by other mammals. He pointed out that our face and bodies are partly controlled by reflex responses, controlled by nerves that are similar in man and beast. (Darwin here was building on the work of Thomas Willis, a personal hero of mine and the chief subject of my book Soul Made Flesh.) Darwin explored the baring of teeth, the widening of the eyes, and other the expressions made by people as well as cats, dogs, and other animals. Evolution, in other words, was as plain as the face in the mirror.
Expressions convey information, and the human brain is exquisitely sensitive to them. In recent years, neuroscientists have begun to map out the circuits dedicated to this information processing. The sight of a face, for example, activates a tiny region on the brain’s underside called the facial fusiform area. Another region high on the side of the brain, the superior temporal sulcus, is sensitive to moving lips and eyes. And most interestingly of all, an almond-shaped clump of neurons called the amygdala buried deep within the brain, becomes active at the sight of emotions on faces–particularly the scowls of anger and bared teeth of fear.
The amygdala is an extraordinary bit of brain, shared by humans, monkeys, rats, fish, and all other vertebrates. In the 1970s, neuroscientists first recognized it as Fear Central. A rat sees a light go on and gets a shock. It sees the light go on again and gets another shock. After a while, the rat becomes terrified at the sight of the light. But if you take out the rat’s amygdala, it never makes this fearful connection. Not all fears are learned, though. Monkeys, for example, are scared at the sight of snakes (including fake ones) even when they haven’t seen one before. Monkeys without amygdalas will run up to a fake snake and play with it. Later research showed that it becomes active in human brains at fearful sights, such as a pointed gun. (A good place to learn about the amygdala is Joseph LeDoux’s web site at NYU.)
When incoming signals activate the amygdala, it sends out signals to other regions of the brain. Hormones start to race through the body, and the brain becomes more alert to sights and sounds. It can even control the brain and body without our awareness. A quick flash of a scary sight is enough to activate it, even if it is too quick to register in the conscious brain. Scientists now believe that the amygdala gets a brief early edition of the news coming into the brain before the same information gets processed through higher regions in the cortex.
The amygdala lets us take quick actions that make the difference between life and death. It also lets us recognize other important signs of danger, such as angry or frightened faces on other people. These faces may be reacting to a threat (a lion bursting out of the brush), or they may belong to a person with malice on his mind. In this work the amygdala joins forces with the facial fusiform area, which responds more strongly when the amygdala responds strongly.
But Darwin had more than faces in mind when he first explored the evolutionary heritage. He pointed out that the body has its own expressions of fear, joy, and anger. They often come together in reflexive packages–think of the hair rising up on a cats back as it bares its teeth. Given what we know now about facial expressions in the brain, another possibility naturally emerges: if the brain has dedicated circuits sensitive to information in the face, perhaps there are also circuits dedicated to picking up emotional information in the body.
In today’s issue of the journal Current Biology, two Harvard neuroscientists put this idea to the test. They placed people in an MRI scanner and then showed them a series of pictures. Some of the pictures were of people in fearful positions, with their faces blurred out (like the one at the top of this post). In other pictures, people raised their arms in meaningless ways. The neuroscientists then compared the responses of their subjects’ brains to both groups of images and found indeed that certain regions were sensitive only to the sight of fearful bodies. Remarkably, they were the amygdala and the facial fusiform area, the same regions that are sensitive to fearful faces. The scientists conclude that these regions work together to get as much information they can about emotions such as fear, whether in the geometry of widened eyes or cowering shoulders. It’s an example–one of many these days–of how Darwin’s musing are being mapped onto the anatomy of the thinking brain.