The Economist, 12 May 2005
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
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.
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 wallSuch
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).
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).
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.
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.
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.
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.
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.
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.