Human infants barely out of the delivery room, at the age of one-and-a-half days, will automatically cry in response to the distress cries of other babies; and they immediately distinguish between a baby's cry and a computerized simulation of one. That's contagious
empathy as it appears among Homo sapiens--and it seems remarkably similar to what we see in other mammals. Mice, for example.
When you inject a mouse with a 0.8 percent solution of acetic acid, you cause a painful sensation that is marked by rhythmic abdominal constrictions, or writhing. The writhing can be measured, and so it gives researchers a convenient way of identifying the intensity of a mouse's pain. Using a writhing count as their measuring device, scientists at McGill University recently placed mice in pairs inside transparent Plexiglass cylinders, and then they injected one of each pair with a particular amount of acetic acid solution. They discovered that the injected mouse who had been paired with another mouse writhed with a significantly greater intensity than a mouse injected with the same amount of acetic acid but kept in isolation. Moreoever, when the McGill scientists injected both mice in a pair, they found that each mouse writhed more intensely than a mouse injected with the same amount but paired with a non-injected mouse. "These findings imply," the researchers conclude, "the communication of pain from one mouse to the other."
Was this merely a case of simple imitation? Probably not, since the researchers also found that an acetic-acid induced pain in one mouse would affect the responsiveness to heat-induced pain in a second mouse paired with the first. The empathetic communication of pain, moreover, increased if the paired mice knew each other (that is, had been cagemates for a particular number of days). Inject two cagemates with the acetic acid solution, and they will each show significantly greater writhing than two mice in the same circumstances who are strangers to each other. Additional experiments showed that the only way to reduce this communication of pain was to place a visual barrier between the paired mice. In other words, the contagion spread through vision--watching another mouse in pain--rather than through hearing or smell.
The ability to feel pain is an obvious example of evolutionary continuity. It's a neurological, physical, and psychological feature with broad distribution across a great zoological swath. Pain engages a part of the brain called the anterior cingulate cortex, which processes the sensory input and introduces a coherent mental-and-physical experience often expressed automatically with a cry, a scream, a yelp--whether you're a person or a dog or a nocturnal prosimian living on a tiny island off the coast of Madagascar. Such is evolutionary continuity; and a number of scientists believe that contagious empathy, the automatic ability to feel someone else's pain emotionally, is also evolutionarily continuous.
Evolutionary continuity in a psychological experience implies neurological continuity, and the neurological mechanism involved in empathy could be associated with specialized brain cells known as mirror neurons. Mirror neurons were discovered and named by Giacomo Rizzolatti and his colleagues little more than a decade ago. The initial discovery began as an accident that happened in a neuroscience laboratory at the University of Parma, in Italy, while Rizzolatti and his colleagues were mapping out the function of cells in a brain region (identified as F5) in the premotor cortex of rhesus monkeys. This part of the monkey's brain is associated with grasping and moving a seen object. When a laboratory monkey saw and moved his hand to a piece of food and grasped it, the neurons in area F5 would fire. The researchers could observe this neurological activity because they had implanted electrodes into that region of the brain, and thus neural excitations would be registered on an external monitor. The monkey grabs a peanut, brings peanut to mouth, and the monitor indicates that neurons in region F5 are firing.
Then one hot day, a researcher in the laboratory walked in grasping and licking an ice cream cone, and the same neurons in the monkey's brain started firing.
It was astonishing. The monkey was merely watching someone else grasp and move an object to his mouth. And when the researchers tried the same thing with nuts and bananas and raisins, they got the same results. Those F5 neurons were activated both when the monkey grasped the nuts or bananas or raisins and drew them toward his mouth, and when the monkey watched someone else do it. This general principle was true whether the monkey was watching the grasping hand of a person or another monkey. It was true whether the seen object was near or far away. And it was true whether the object was food or not food. Nor was there any relationship between this neurological finding and any reward. The laboratory monkeys would respond the same whether or not they got to eat the banana or peanut or raisin after it was over.
When the researchers began studying another part of region F5, they found neurons that were specifically related to actions of the mouth. About one quarter of them had mirror properties, with some of the mirror neurons responding to eating activities of the mouth and others responding to communication actions (smacking lips, for example). Later examinations of another part of the brain, the superior temporal sulcus (STS), found neurons that will respond to other body actions, such as walking, arm movements, head turning, and torso bending--done by the monkey and seen by the monkey when others do it. Monkey see and monkey do were neurologically the same thing.
"It took us several years to believe what we were seeing," Rizzolatti recently told a reporter for The New York Times. But what they were observing was less remarkable than what the presence of mirror neurons seemed to imply. Mirror neurons appear to enable some animals (including humans) to learn through imitation. I watch someone do something, and I can acquire enough knowledge strictly from that observation to do it myself. Among people, this uncanny ability is present from the start. Babies in the delivery room stick out their tongues in response to tongue-waggling grown-ups.
Mirror neurons also provide a quick way to predict the actions of others. Whenever we see a person grasp a coffee cup in one particular fashion and drink from it, we review a vivid neural representation of the act that includes both the visual and the motor aspects. Next time we see someone grasp a similar cup in a similar fashion, we have the knowledge to predict quickly and accurately that his or her intention is to drink from it, rather than, say, throw it at us.
But mirror neurons may also improve our understanding of social behavior more generally, since they give us a way to understand other people's emotions, in part, perhaps, from witnessing the visual display of emotions on the face. In essence, we transform the experience of seeing someone else have an emotion--experiencing a humiliating rejection, for instance--into experiencing it ourselves. We can be entertained by televised sports because we are able not only to see but to experience mentally, through activated neurons, the motor skills involved in athletic performance; and we are entertained by film and theatre because we can experience emotionally the emotional lives of others. We feel this woman's magnificent triumph, just as we experience that man's humiliating rejection. Mirror neuron systems, in short, may provide an internal simulation device by which one person can appreciate and imitate the actions, perceptions, and emotions of another; and, thus, contagious empathy may be grounded in our own bodily experience at the neurological level.
Adapted from my latest book, The Moral Lives of Animals.
