Understanding empathy

The Economist, 12 May 2005
ALISON MOTLUK

CHRISTIAN KEYSERS has a good way of making his point. He shows his audience a clip from a James Bond movie in which a large, hairy spider is climbing over our hero's naked body. He then asks the audience what they think the actor playing Bond is feeling.

It is impossible to tell, of course, whether Sean Connery was really revolted and fearful when the scene was being shot, or whether he was actually indifferent, but just acting well. The point is that the observer can feel—literally feel—Bond's fear. This ability not merely to know in an intellectual sense what someone else is feeling, but actually to feel it with them, is an important social attribute. Dramatists, novelists and psychologists have known about it for centuries, of course. And those who lack it, such as people who are autistic, are at a social disadvantage. But it is only in the past few years that its neurological basis has begun to be understood. It seems to rely on a type of nerve cell known as a mirror neuron. Dr Keysers, who works at the University of Groningen, in the Netherlands, is one of a band of neurologists that is studying them.

A mirror neuron is one that is active when the individual whose brain it is in is engaged in some action or experiencing some sensation or emotion, and also when that particular action, sensation or emotion is being observed in someone else. Action-sensitive mirror neurons were the first to be found, and they were discovered in rhesus monkeys, one of the mainstays of animal laboratory research.

When a monkey reaches out for something—a piece of food, for example—a particular group of nerve cells in its brain fires off lots of electrical signals. The activity of individual neurons within such a group of action-sensitive cells can be traced with electrodes that have tips so fine that they can be placed against a single cell. Most such cells fire only in response to the action. But about 20% of them also fire in exactly the same way if the monkey sees another monkey (or, indeed, a human) reaching out for food. This empathic firing "mirrors" the way the cells behave when they are involved in an action.

Sticking electrodes into human brains in this way is not on, of course. But modern brain-scanning techniques can be used to look for mirror activity in particular parts of the brain, even if they cannot pick out individual nerve cells. So Dr Keysers uses brain scanners to study the role of mirror neurons in human emotional and sensational empathy, such as the audience feels with Connery/Bond.

Measuring fear by letting a venomous spider crawl over the body of an experimental subject is no more likely to get past an ethics committee than is sticking electrodes in his brain, so Dr Keysers chose to study another emotion, disgust, instead. He put his volunteers in a brain scanner and wafted disgusting odours such as rancid butter and rotten eggs into their nostrils (he wafted some non-disgusting ones in, too, as a control). The disgusting odours, he found, activated part of the brain called the anterior insula. He then played film clips of people's faces registering disgust to his volunteers, and found activity in exactly the same part of the brain.

The sense of touch, too, is mirrored in this way. Though no spiders were involved, Dr Keysers found that part of the brain that was activated by touching the leg of a person in a brain scanner also reacted if the subject was shown film of another person being touched on the leg. All this suggests that understanding the experiences and emotions of others involves the same neural circuitry that we require to have those experiences and emotions ourselves—in other words, that it is mediated by mirror neurons.

Mirror, mirror on the wall

Such observations lead to bigger questions, and one of the most pertinent concerns "theory of mind", a grandiloquent term used to describe the extent to which one individual can understand and anticipate the intentions of another.

Two recent papers address this question. Marco Iacoboni, of the University of California, Los Angeles, and his colleagues employed a similar methodology to Dr Keysers's to study the human brain. Meanwhile Leonardo Fogassi and his colleagues at the University of Parma, in Italy, used monkeys and electrodes to watch the process in individual nerve cells (indeed, it was this group, led by Giacomo Rizzolatti and Vittorio Gallese, which was responsible for discovering mirror neurons this way in the first place).

Both papers showed that the mirror-neuron activity is context-dependent in a way that suggests the experimental subjects not only recognise particular movements, but also understand the intention behind them. Watching someone grasping food or drink is a well-known stimulus of mirror-neuron activity. Dr Iacoboni's study, published in Public Library of Science Biology, showed, though, that there is far more such activity in someone's brain when they see a teacup being grasped in the context of a scene that includes biscuits, milk and a teapot (which suggests the grasping hand belongs to someone who is about to drink and eat), than when the scene contains empty plates and vessels (which suggests the hand belongs to someone who is clearing up).

Dr Fogassi's paper in Science has similar results for monkeys (though the context is grasping a pellet that sometimes is and sometimes is not made of food, rather than a tea party). This suggests that monkeys' mirror neurons, too, are capable of distinguishing intentions.

The idea that a lack of mirror-neuron activity is at least part of the cause of autism, has also received support recently. Eschewing brain scanners and implanted electrodes, Vilayanur Ramachandran and his colleagues at the University of California, San Diego, studied brainwaves believed to be associated with mirror neurons by pasting surface electrodes on their volunteers' scalps and faces, and monitoring them while those volunteers performed different tasks.

Ten of the volunteers were men and boys of normal intelligence, but who suffer from autism (not all those with the condition have other, more damaging, symptoms such as low intelligence as well). The other ten were individuals of similarly normal intelligence who had no autistic symptoms. The researchers were interested in the so-called mu-wave (an electrical oscillation in the brain that has a frequency of between eight and 13 cycles a second). In healthy people mu-waves are suppressed not only when actions are executed, but also when they are observed or even simply imagined. It is this suppression that has led researchers in the field to believe mu-waves might be connected with mirror-cell activity. Dr Ramachandran and his colleagues therefore wanted to see what happened to mu-waves in people with autism.

Once they had wired their subjects up, they asked them to perform four tasks. One was for the subject to watch one of his own hands as he opened and closed it in a sort of slow-motion shadow-puppet routine, about once a second. The other three tasks involved watching video clips. These clips were of someone else making the same hand motion, of balls bouncing into each other and apart, and of visual "static" (the sort of thing seen on a badly tuned television).

As the team report in their paper in Cognitive Brain Research, the non-autistic individuals all responded in the expected way: both moving their own hand and watching someone else's hand move caused mu-suppression in their brains, while the other two video clips had no effect. But in people with autism, only their own hand movements caused the mu-waves to be suppressed. Watching other people's hands move had no more effect than watching the balls and the static. That suggests there is something awry with their mirroring system.

This finding followed on the heels of another study investigating mirror-neuron activity in autists, published in Current Biology by Hugo Théoret and his colleagues at Harvard University. Dr Théoret wanted to see whether watching video clips of people moving their fingers changed the excitability of neurons in the part of the brain where action-sensitive mirror neurons are found. This experiment also studied ten autists of normal intelligence and ten controls.

Once again, the mirror neurons in the autistic volunteers failed to respond to the hand actions of others in the way that those of the controls did.

All of these experiments are focused on relatively simple stimuli that researchers can reproduce and measure easily. Whether mirror neurons are involved in more complex calculations of motive—and, most significantly, in those calculations made when someone is trying to manipulate the behaviour of someone else—remains to be seen. But it seems a plausible hypothesis, and the tools to test it more thoroughly are now in place. Understanding what someone else thinks is the necessary first step to deceiving or even controlling them. The actions of mirror cells may have wide ramifications.