Read my mind
27 Jan 01
Imagine you had cells in your brain that could read other people's minds. Well, you
do. And they could be the key to human language, empathy, even society, says
Alison Motluk
A CHILD watches her mother pick up a toy. The child smiles: Mum wants to play. A
husband watches his wife pluck car keys from a table. He shivers: she really is leaving
this time. A nurse watches a needle being jabbed into an elderly patient. She flinches: it
must have hurt.
How do these people know what the other person is thinking? How do they judge
intentions and feelings, or assign goals or beliefs to the other? It sounds simple, but the
child could just as easily have decided that Mum was leaving or the husband that his wife
wanted to play. Yet they didn't. They knew.
"Reading" the minds of others is something we take for granted. Yet philosophers,
psychologists and neuroscientists alike have been baffled by our ability to anticipate other
people's behaviour and empathise with their feelings. Now a team of Italian
neurophysiologists may have stumbled on the key to this mystery.
Vittorio Gallese, Giacomo Rizzolatti and their colleagues at the University of Parma have
identified an entirely new class of neurons. These neurons are active when their owners
perform a certain task, and in this respect are wholly unremarkable. But, more
interestingly, the same neurons fire when their owner watches someone else perform that
same task. The team has dubbed the novel nerve cells "mirror" neurons, because they
seem to be firing in sympathy, reflecting or perhaps simulating the actions of others.
Many neuroscientists are starting to think that in higher primates, including humans,
these neurons play a pivotal role in understanding the intentions of others. "Mirror
neurons may be one important part of the mosaic that explains our social abilities," says
Gallese. Vilayanur Ramachandran of the University of California at San Diego goes
further. He believes that mirror neurons will answer important questions about human
evolution, language and culture-and may take us to the heart of what it means to be
human. "I predict that mirror neurons will do for psychology what DNA did for biology,"
he says. "They will provide a unifying framework and help explain a host of mental
abilities that have hitherto remained mysterious."
Gallese and his colleagues didn't set out to find anything so radical when, in the early
1990s, they started recording the activity of neurons in a macaque's brain. They were
tapping into the signals emitted from nerve cells in a part of the monkey's brain known as
F5. This is part of a larger region called the premotor cortex, whose activity is linked to
planning and making movements. Some years earlier, the same researchers had
discovered that neurons in F5 fired when an animal performed certain goal-oriented
motor tasks using its hands or mouth, such as picking things up, holding or biting them.
They wanted to learn more about F5 neurons-how they responded to different objects
with different shapes and sizes, for example. So they presented monkeys with things like
raisins, slices of apple, paper clips, cubes and spheres. It wasn't long before they noticed
something odd. As the monkey watched the experimenter's hand pick up the object and
bring it close, a group of the F5 neurons leaped into action. But when the monkey looked
at the same object lying on the tray, nothing happened. When it picked up the object, the
same neurons fired again. Clearly their job wasn't just to recognise a particular object.
All fired up
The neurons turned out to be quite fussy about what they reacted to. Those that responded
to an experimenter plucking a raisin from a tray, for instance, failed to react when the
experimenter dug the same raisin out of a small well with his finger. Some neurons fired
when the experimenter held a few slices of apple, but not when he placed the apples on
the tray-other neurons fired for that.
Most importantly, the very same action that made a neuron fire when a monkey
performed it would almost always make that neuron fire if the monkey saw the
experimenter doing the same thing. It soon became clear that the motor system in the
brain is not limited to controlling movements. In some way it is also reading the actions
of others.
In 1998, Gallese gave a talk about mirror neurons at a meeting on the "Science of
Consciousness" in Tucson, Arizona. Alvin Goldman, a philosopher from the University
of Arizona, listened with interest. Afterwards, he approached Gallese and they spoke
about the potential of these cells for reading the minds of others. "He wasn't familiar with
the mind-reading literature in philosophy," says Goldman.
Mind-reading, or theory of mind, is an ability that all healthy humans possess. We are
particularly good at representing the specific mental states of others. These can be basic,
such as seeing someone crying and understanding that they are sad, or realising that when
someone is yelling and gesticulating wildly at you they may be angry and might even
mean to harm you. But we intuitively understand more complex mental states too. When
a mother loses a baby, other parents get lumps in their throats. When you hear that a
colleague has been cheating on their spouse, you share the hurt and shame.
A debate rages over whether other primates, such as chimps, can understand other minds,
even in the simplest ways. And even in humans, while almost everybody agrees that
some measure of mind-reading goes on, there is little agreement on how it happens. One
theory, sometimes called "theory theory", holds that people build up common-sense
hypotheses to explain why other people do what they do. Like physicists using rules and
laws to explain observable phenomena, we all use our experiences to develop a set of
explanatory laws for others' behaviour.
Another dominant theory, championed mainly by philosophers like Goldman, is known
as simulation theory. It's based on the idea that people understand what is going through
the minds of others by mentally mimicking what the other is thinking, feeling or doing-in
essence, putting themselves in the other's shoes. The discovery of mirror neurons backs
up this theory nicely.
As the suspicion grew that these neurons might have something to do with the
complexities of mind-reading, the burning question became whether human brains had
mirror neurons too. But finding out wasn't easy-humans aren't keen on having electrodes
implanted into their brains, even for the lofty purposes of science.
Luciano Fadiga, now at the University of Ferrara in Italy, was the first to find some
evidence that humans may have a system analogous to that found in monkey brains,
when he measured the excitability of particular muscles in the hand. He found that when
the volunteers were watching grasping actions, the very muscles that would be needed to
copy that movement seemed primed to act-as if they were preparing to make the same
movement themselves. "The interesting thing was that the pattern of activated muscles
changed according to the observed actions," says Fadiga. But while this suggested that a
mirror system might exist in human brains too, it didn't yield any information about
where it might reside.
Several brain-imaging studies followed, the first led by Rizzolatti, and another by Scott
Grafton, then at the University of Southern California in Los Angeles. Both found that
watching an experimenter pick up and handle objects activates two regions of the brain
behind the temples on the left side: the superior temporal sulcus and, just above it, a part
called Broca's area.
An even more recent study by Marco Iacoboni at the Los Angeles School of Medicine
confirmed that Broca's area was active while volunteers either watched images of
someone drumming their fingers, and when they also tried to imitate the movement they
saw (Science, vol 286, p 2526).
Finding the words
The finding that Broca's area was activated was doubly intriguing. For one thing, F5 in
monkeys is considered an analogue for Broca's area in humans. But even more suggestive
was the fact that, while F5 is associated mainly with hand movement, Broca's area is
traditionally thought of as a speech-production area. This raised questions about what a
mirroring system might have to do with language-and language with mind-reading.
Rizzolatti and Arbib think that mirror neurons may have provided the bridge from
"doing" to "communicating". The relationship between actor and observer may have
developed into one involving the sending and receiving of a message. In all
communication the sender and receiver have to have a common understanding about
what's passing between them. Could mirror neurons explain how this is achieved?
Rizzolatti and Arbib think the answer is yes.
They suggest that it is probably no coincidence that the area which links action
recognition and action production in the monkey brain is exactly the same area that in
humans has been linked to speech production. They think that the development of human
speech was made possible by the fact that F5, the precursor of Broca's area, was endowed
with this mirroring mechanism for recognising actions made by others. This, they say,
was a prerequisite for the development of communication and ultimately of speech. It
made us "language-ready", says Arbib.
Most of the time, a strong spinal cord inhibition prevents you from involving your own
motor neurons in activity you are merely observing, according to work by Fadiga soon to
be published in the European Journal of Neuroscience. But sometimes the premotor
cortex allows a brief snippet of the movement-like the twitchy feeling you get when
you're watching someone struggling to open a packet of crisps or untie a knot.
This slight movement, says Arbib, tips off the person carrying out the action that the
watcher knows what's going on, in a sort of primitive dialogue. "This dialogue forms the
core of language," he says. "Perhaps we evolved some crude form of communication
based in sign, then built speech," says Arbib. Imagine an early human chipping away at a
stone, he says, and that this person wants to communicate something else while
demonstrating this skill. Or perhaps he wants to communicate in the dark or at a distance.
In both cases, using sign or gestures doesn't work so well. If the brain could allow the
person to develop speech through the same neural apparatus earlier primates were already
using to communicate manually or through lipsmacks, so much the better.
The exciting news is that mirror neurons may not be limited to these motor regions.
Gallese, for one, suspects that they are found in other areas. "My belief is that this may
apply also to other modalities, for instance sensory modalities," he says. Gallese points to
recent work by William Hutchison, a physiologist at the University of Toronto. He and
his colleagues studied humans who were conscious while undergoing brain surgery. They
discovered neurons in the anterior cingulate cortex, a region thought to be involved in
perceiving pain, which fired both in response to a finger being pricked and also when
patients saw the experimenter prick himself.
Gallese sees this as tantalising, preliminary evidence of a far-reaching neural mechanism.
Could this explain how we are able to "feel" what others feel? Could it underpin the
sensations behind empathy?
Ramachandran also believes that mirror neurons play a bigger role than is generally
appreciated. He thinks these exciting nerve cells don't just provide a missing link between
gesture and language, but they go a great way towards explaining human learning,
ingenuity, and culture in general. "Their emergence and further development in hominids
was a decisive step," he says.
He says mirror neurons and the way they facilitate imitative learning help to explain why
we only developed things like tool use, art and mathematics about 40,000 years ago,
despite the fact that our brains had reached their full size some 150,000 years earlier.
These cultural inventions, he contends, probably popped up accidentally, but they were
disseminated quickly because of our amazing, imitative, learning brains-made possible by
a more sophisticated version of the monkey mirror neuron system.
He admits that mirror neurons probably aren't the whole story-necessary, but perhaps not
sufficient-but insists they could be a big part of it. Language, imitative learning and
mind-reading, seemingly unrelated human developments, may all be shown to be linked
through these intriguing nerve cells. "These are all human qualities. All mysterious
qualities," he says. "Mirror neurons may provide the key."
Mind blind
SOME researchers have suggested that the inability to "read" the minds of others, dubbed
"mind-blindness", is one of the main deficits of autism. Interestingly, a common
symptom of the disease is "echolalia", the automatic repeating back of what someone
says without understanding it. One intriguing suggestion is that the brain's mirroring
system might be defective. "It's not that they can't mirror," says Chris Frith at the Institute
of Neurology in London, "it's that they're mirroring in the wrong way."
Both old and emerging data on autism also fit rather well with the suggestion that mirror
neurons might be the key to mind-reading and language development. As Frith points
out, language development is often delayed in autistic children, and even after it
develops, they have problems speaking normally. Also, he hints that his own brainimaging studies of people with autism have uncovered something unusual about their
superior temporal sulcus-one of the regions where mirror neurons are thought to be. His
findings are due to be published in a few months.
Vilayanur Ramachandran and his postdoctoral student Eric Altschuler, from the
University of California at San Diego, have indirect evidence that autistic people may
indeed have anomalous mirror neuron systems. They had previously found that a
characteristic human brainwave pattern called the "mu" wave, picked up in an
electroencephalograph (EEG) recording, is suppressed not only when people move, but
also when they watch others move. The two researchers naturally began to wonder if muwave suppression is associated with activity in the mirror neuron system.
Intriguingly, they have now found that in one autistic child, watching movement failed to
block the mu wave. They presented these findings at the annual meeting of the Society
for Neuroscience in New Orleans last November. "[The child] didn't show the expected
suppression," says Ramachandran. "Maybe the system is malfunctioning in autistics."
They stress that these mu waves haven't been studied in children before, but they are now
recruiting more subjects to study the blocking of the waves more thoroughly.
Further reading:
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: Mirror neurons and imitation learning as the driving force behind "the great
leap forward" in human evolution by V. S. Ramachandran at
www.edge.org/documents/archive/edge69.html
Mirror neurons and the simulation theory of mind-reading by Vittorio Gallese
and Alvin Goldman, in Trends in Cognitive Sciences, vol 2, p 493 (1998)
Language within our grasp by Giacomo Rizzolatti and Michael Arbib, in Trends
in Neurosciences, vol 21, p 188 (1998)
Alison Motluk
From New Scientist magazine, vol 169 issue 2275, 27/01/2001, page 22
© Copyright New Scientist, RBI Limited 2001