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.
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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
(New Scientist, 8 April 2000, p 30).
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
(New Scientist, 8 May 1999, p 17).
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.”
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 brain-imaging 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 mu-wave 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.
Mind blind
-
Further reading:
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)