|Bron bij Neurologie: spiegelneuronen||
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Scientific American, nov. 2006, door Giacomo Rizzolatti, Leonardo
Fogassi en Vittorio Gallese
Overview | Meeting the minds
■ Subsets of neurons in human and monkey brains respond when an individual
performs certain actions and also when the subject observes others performing
the same movements.
■ These "mirror neurons" provide a direct internal experience, and therefore
understanding, of another person's act, intention or emotion.
■ Mirror neurons may also underlie the ability to imitate another's action, and
thereby learn, making the mirror mechanism a bridge between individual brains
for communication and connection on multiple levels.
Mirrors in the mind
A special class of brain cells reflects the outside world, revealing a new
avenue for human understanding, connecting and learning
Tussentitels: The pattern of activity was a true representation in the brain of the act
regardless of who was performing it.
When people use the expression "I feel your pain," they may not realize
literally it could be true.
John watches Mary, who is grasping a flower. John knows what Mary is doing - she
is picking up the flower - and he also knows why she is doing it. Mary is smiling
at John, and he guesses that she will give him the flower as a present. The
simple scene lasts just moments, and John's grasp of what is happening is nearly
instantaneous. But how exactly does he understand Mary's action, as well as her
intention, so effortlessly?
A decade ago most neuroscientists and psychologists would have attributed an
individual's understanding of someone else's actions and, especially, intentions
to a rapid reasoning process not unlike that used to solve a logical problem:
some sophisticated cognitive apparatus in John's brain elaborated on the
information his senses took in and compared it with similar previously stored
experiences, allowing John to arrive at a conclusion about what Mary was up to
Although such complex deductive operations probably do occur in some situations,
particularly when someone's behavior is difficult to decipher, the ease and
speed with which we typically understand simple actions suggest a much more
straightforward explanation. In the early 1990s our research group at the
University of Parma in Italy, which at the time included Luciano Fadiga, found
that answer somewhat accidentally in a surprising class of neurons in the monkey brain that fire when an individual performs simple goal-directed motor
actions, such as grasping a piece of fruit. The surprising part was that these
same neurons also. fire when the individual sees someone else perform the same
act. Because this newly discovered subset af cells seemed to directly reflect
acts performed by another in the observer's brain, we named them mirror neurons.
Much as circuits of neurons are believed to store specific
memories within the brain, sets of mirror neurons appear to encode templates
for specific actions.
This property may allow an individual not only to perform basic motor procedures
without thinking about them but
also to comprehend those acts when they are observed, without any need for
explicit reasoning about them. John grasps Mary's action because even as it is
happening before his eyes, it is also
happening, in effect, inside his head. It is interesting to. note that
philosophers in the phenomenological tradition long ago posited that one had to experience
something within oneself to truly comprehend it. But for neuroscientists, this finding
of a physical basis for that idea in the mirror
neuron system represents a dramatic change in the way we understand the way we
Our research group was not seeking to support or refute one philosophical
position or another when we first noticed mirror neurons. We were studying the
brain's motor cortex, particularly an area called F5 associated with hand and
mouth movements, to learn haw commands to perform certain actions are encoded
by the firing patterns of neurons. Far this purpose, we were recording the
activity of individual neurons in the brains of macaques. Our laboratory
contained a rich repertoire of stimuli for the monkeys, and as they performed
various actions, such as grasping for a toy or a piece of food, we could see
that distinct sets of neurons discharged during the execution of specific motor
Then we began to notice same thing strange: when one of us grasped a piece
of food, the monkeys' neurons would fire in the same way as when the monkeys
themselves grasped the food. At first we wandered whether this phenomenon could
be the result of same trivial factor, such as the monkey performing an
unnoticed movement while observing our actions. Once we managed to
rule out this possibility and others, including food expectation by the monkeys,
we realized that the pattern of neuron activity associated with the observed
action was a true representation
in the brain of the act itself, regardless of who was performing it.
Often in biological research, the most direct way to establish the function of a
gene, protein or group of cells is simply to eliminate it and then look for
deficits in the organism's health or behavior afterward. We could not use this
technique to determine the role of mirror neurons, however, because we found
them spread across important regions on both sides of the brain, including the
premotor and parietal cortices. Destroying the entire mirror neuron system would
have produced such broad general cognitive deficits in the monkeys that teasing
out specific effects of the missing cells would have been impossible.
So we adopted a different strategy. To test whether mirror neurons play a role
in understanding an action rather than just visually registering it, we assessed
the neurons' responses when the monkeys could comprehend the meaning of an
action without actually seeing it. If mirror neurons truly mediate understanding,
we reasoned, their activity should reflect the meaning of the action rather than
its visual features. We therefore carried out two series of experiments.
First we tested whether the F5 mirror neurons could "recognize" actions merely
from their sounds. We recorded the mirror neurons while a monkey was observing a
hand motor act, such as ripping a sheet of paper or breaking a peanut shell,
that is accompanied by a distinctive sound. Then we presented the monkey with
the sound alone. We found that many F5 mirror neurons that had responded to the
visual observation of acts accompanied by sounds also responded to the sounds
alone, and we dubbed these cell subsets audiovisual mirror neurons.
Next we theorized that if mirror neurons are truly involved in understanding an
action, they should also discharge when the monkey does not actually see the
action but has sufficient clues to create a mental representation of it. Thus,
we first showed a monkey an experimenter reaching for and grasping a piece of
food. Next, a screen was positioned in front of the monkey so that it could not
see the experimenter's hand grasping the food but could only guess the action's
conclusion. Nevertheless, more than half the F5 mirror neurons also discharged
when the monkey could just imagine what was happening behind the screen.
These experiments confirmed, therefore, that the activity of mirror neurons
underpins understanding of motor acts:
when comprehension of an action is possible on a nonvisual basis, such as sound
or mental representation, mirror neurons do still discharge to signal the act's
Following these discoveries in the monkey brain, we naturally
wondered whether a mirror neuron system also exists in humans. We first obtained
strong evidence that it does through a series of experiments that employed
various techniques for detecting changes in motor cortex activity. As volunteers
observed an experimenter grasping objects or performing meaningless arm gestures,
for example, increased neural activation in their hand and arm muscles that
would be involved in the same movements suggested a mirror neuron response in
the motor areas of their brains. Further investigations using different external
measures of cortical activity, such as electroencephalography, also supported
the existence of a mirror neuron system in humans. But none of the technologies
we had used up to this point allowed us to identify the exact brain areas
activated when the volunteers observed motor acts, so we set out to explore this
question with direct brain-imaging techniques.
In those experiments, carried out at San Raffaele Hospital in Milan, we used
positron-emission tomography (PET) to observe neuronal activity in the brains of
human volunteers as they watched grasping actions performed with different hand
grips and then, as a control, looked at stationary objects. In these situations,
seeing actions performed by others activated three main areas of the brain's
cortex. One of these, the superior temporal sulcus (STS), is known to contain
neurons that respond to observations of moving body parts. The other two-the
inferior parietal lobule (IPL) and the inferior frontal gyrus (IFG)-correspond,
respectively, to the monkey IPL and the monkey ventral premotor cortex,
including F5, the areas where we had previously recorded mirror neurons.
These encouraging results suggested a mirror mechanism at work in the human
brain as well but still did not fully reveal its scope. If mirror neurons permit
an observed act to be directly understood by experiencing it, for example,
we wondered to what extent the ultimate goal of the action is also a component
of that "understanding."
Returning to our example of John and Mary, we said John knows both
that Mary is picking up the flower and that she plans to hand it to him. Her
smile gave him a contextual clue to her intention, and in this situation, John's
knowledge of Mary's goal is fundamental to his understanding of her action,
because giving him the flower is the completion of the movements that make up
When we perform such a gesture ourselves, in reality we are performing a series
of linked motor acts whose sequence is determined by our intent: one series of
movements picks the flower and brings it to one's own nose to smell, but a
partly different set of movements grasps the flower and hands it to someone else.
Therefore, our research group set out to explore whether mirror neurons provide
an understanding of intention by distinguishing between similar actions with
For this purpose, we returned to our monkeys to record their parietal neurons
under varying conditions. In one set of experiments, a monkey's task was to
grasp a piece of food and bring it to its mouth. Next we had the monkey grasp
the same item and place it into a container. Interestingly, we found that most
of the neurons we recorded discharged differently during the grasping part of
the monkey's action, depending on its final goal. This evidence illustrated that
the motor system is organized in neuronal chains, each of which encodes the
specific intention of the act. We then asked whether this mechanism explains how
we understand the intentions of others.
We tested the same grasping neurons for their mirror properties by having a
monkey observe an experimenter performing the tasks the monkey itself had done
earlier. In each instance, most of the mirror neurons were
activated differently, depending on whether the experimenter brought the food to
his mouth or put it
in the container. The patterns of firing in the monkey's brain exactly matched
those we observed when the monkey itself performed the acts - mirror neurons that
discharged most strongly during grasping-to-eat rather than grasping-to-place did
the same when the monkey watched the experimenter perform the corresponding
A strict link thus appears to exist between the motor organization of
intentional actions and the capacity to understand the intentions of others.
When the monkeys observed an action in a particular context, seeing just the
grasping component of the complete movement activated mirror neurons forming a
motor chain that also encoded a specific intention. Which chain was activated
during their observation of the beginning of an action depended on a variety of
factors, such as the nature of the object acted on, the context and the memory
of what the observed agent did before.
To see whether a similar mechanism for reading intentions exists in humans, we
teamed with Marco Iacoboni and his colleagues at the University of California,
Los Angeles, for a functional magnetic resonance imaging (fMRI) experiment on
volunteers. Participants in these tests were presented with three kinds of
stimuli, all contained within video clips. The first set of images showed a hand
grasping a cup against an empty background using two different grips. The second
consisted of two scenes containing objects such as plates and cutlery, arranged in one instance as though they
were ready for someone to have afternoon tea and in the other as though they
were left over from a previously eaten snack and were ready to be cleaned up.
The third stimulus set showed a hand grasping a cup in either of those two
We wanted to establish whether human mirror neurons would distinguish between
grasping a cup to drink, as suggested by the ready-for-tea context, and grabbing
the cup to take it away, as suggested by the cleanup setting. Our results demonstrated not only that they do but also that the mirror neuron system
responded strongly to the intention component of an act. Test subjects observing
the hand motor acts in the "drinking" or "cleaning" contexts showed differing
activation of their mirror neuron systems, and mirror neuron activity was
stronger in both those situations than when subjects observed the hand grasping
a cup without any context or when looking only at the place settings [see box on
Given that humans and monkeys are social species, it is not
difficult to see the potential survival advantage of a mechanism, based on
mirror neurons, that locks basic motor acts onto a larger motor semantic
network, permitting the direct and immediate comprehension of others' behavior
without complex cognitive machinery. In social life, however, understanding
others' emotions is equally important. Indeed, emotion is often a key contextual element that signals the
intent of an action. That is why we and other research groups have also been
exploring whether the mirror system allows us to understand what others feel in
addition to what they do.
Connect and Learn
As with actions, humans undoubtedly understand emotions in more than one way.
Observing another person experiencing emotion can trigger a cognitive
elaboration of that sensory information, which ultimately results in a logical
conclusion about what the other is feeling. It may also, however, result in
direct mapping of that sensory information onto the motor structures that would
produce the experience of that emotion in the observer. These two means of
recognizing emotions are profoundly different: with the first, the observer
deduces the emotion but does not feel it; via the second, recognition is
firsthand because the mirror mechanism elicits the same emotional state in the
observer. Thus, when people use the expression "I feel your pain" to indicate
both comprehension and empathy, they may not realize just how literally true
their statement could be.
A paradigmatic example is the emotion of disgust, a basic reaction whose
expression has important survival value for fellow members of a species. In its
most primitive form, disgust indicates that something the individual tastes or
smells is bad and, most likely, dangerous. Once again using fMRI studies, we
collaborated with French neuroscientists to show that experiencing disgust as a
result of inhaling foul odorants and witnessing disgust on the face of someone
else activate the same neural structure - the anterior insula - at some of the very
same locations within that structure [see box]. These results indicate
that populations of mirror neurons in the insula become active both when the
test participants experience the emotion and when they see it expressed by
others. In other words, the observer and the observed share a neural mechanism
that enables a form of direct experiential understanding.
Tania Singer and her colleagues at University College London found similar
matches between experienced and observed emotions in the context of pain. In
that experiment, the participants felt pain produced by electrodes placed on
their hands and then watched electrodes placed on a test partner's hand followed
by a cue for painful stimulation. Both situations activated the same regions of
the anterior insula and the anterior cingulate cortex in the subjects.
Taken together, such data strongly suggest that humans may
comprehend emotions, or at least powerful negative emotions, through a direct
mapping mechanism involving parts of the brain that generate visceral motor
responses. Such a mirror mechanism for understanding emotions cannot, of course,
fully explain all social cognition, but it does provide for the first time a
functional neural basis for some of the interpersonal relations on which more
complex social behaviors are built. It may be a substrate that allows us to
empathize with others, for example. Dysfunction in this mirroring system may
also be implicated in empathy deficits, such as those seen in children with
autism [see "Broken Mirrors: A Theory of Autism," by Vilayanur S. Ramachandran
and Lindsay M. Oberman].
Many laboratories, including our own, are
continuing to explore these questions, both for their inherent interest and
their potential therapeutic applications. If the mirror neuron template of a
motor action is partly inscribed in the brain by experience, for instance, then
it should theoretically be possible to alleviate motor impairments, such as
those suffered following a stroke, by potentiating undamaged action templates.
Recent evidence indicates, in fact, that the mirror mechanism also plays a role
in the way we initially learn new skills.
Although the word "ape" is often used to denote mimicry, imitation is not an
especially well developed ability among nonhuman primates. It is rare in monkeys
and limited in the great apes, including chimpanzees and gorillas. For
humans, in contrast, imitation is a very important means by which we learn and
transmit skills, language and culture. Did this advance over our primate
relatives evolve on the neural substrate of the mirror neuron system? Iacoboni
and his group provided the first evidence that this might be the case when they
used fMRI to observe human subjects who were watching and imitating finger
movements. Both activities triggered the IFG, part of the mirror neuron system,
in particular when the movement had a specific goal.
In all these experiments, however, the movements to be imitated were simple and
highly practiced. What role might mirror neurons play when we have to learn
completely new and complex motor acts by imitation? To answer this question,
Giovanni Buccino at our university and collaborators in Germany recently used
fMRI to study participants imitating guitar chords after seeing them played by
an expert guitarist. While test subjects observed the expert, their
parietofrontal mirror neuron systems became active.
And the same area was even more strongly activated during the subjects'
imitation of the chord movements. Interestingly, in the interval following
observation, while the participants were programming their own imitation of the
guitar chords, an additional brain region became active.
Known as prefrontal area 46, this part of the brain is traditionally associated
with motor planning and working memory and may therefore play a central role in
properly assembling the elementary motor acts that constitute the action the
subject is about to imitate.
Many aspects of imitation have long perplexed neuroscientists, including the
basic question of how an individual's brain takes in visual information and
translates it to be reproduced in motor terms. If the mirror neuron system
serves as a bridge in this process, then in addition to providing an
understanding of other people's actions, intentions and emotions, it may have
evolved to become an important component in the human capacity for
observation-based learning of sophisticated cognitive skills.
Scientists do not yet know if the mirror neuron system is unique to primates or
if other animals possess it as well.
Our own research group is currently testing rats to see if that species also
demonstrates mirror neuron responses.
Such internal mirroring may be an ability that developed late in evolution,
which would explain why it is more extensive in humans than in monkeys. Because
even newborn human and monkey babies can imitate simple gestures such as
sticking out the tongue, however, the ability to create mirror templates for
observed actions could be innate.
And because lack of emotional mirroring ability appears to be a hallmark of
autism, we are also working with young autistic children to learn whether they
have detectable motor deficits that could signal a general dysfunction of the
mirror neuron system.
Only a decade has passed since we published our first discoveries about mirror
neurons, and many questions remain to be answered, including the mirror system's
possible role in language one of humanity's most sophisticated cognitive skills.
The human mirror neuron system does include Broca's area, a fundamental
language-related cortical center. And if, as some linguists believe, human
communication first began with facial and hand gestures, then mirror neurons
would have played an important role in language evolution. In fact, the mirror
mechanism solves two fundamental communication problems: parity and direct
comprehension. Parity requires that meaning within the message is the same for
the sender as for the recipient. Direct comprehension means that no previous
agreement between individuals - on arbitrary symbols, for instance - is needed
for them to understand each other. The accord is inherent in the neural
organization of both people. Internal mirrors may thus be what allow John and
Mary to connect wordlessly and permit human beings in general to communicate on
GIACOMO RIZZOLATTI, LEONARDO FOGASSI and VITTORIO GALLESE work together at the
University of Parma in Italy, where Rizzolatti is director of the neurosciences
department and Fogassi and Gallese are associate professors. In the early 1990s
their studies of motor systems in the brains of monkeys and humans first
revealed the existence of neurons with mirror properties. They have since
continued to investigate those mirror neurons in both species as well as the
role of the motor system in general cognition. They frequently collaborate with
the many other research groups in Europe and the U.S. now also studying the
breadth and functions of the mirror neuron system in humans and animals.
More to explore
Action Recognition in the Premotor Cortex. Vittorio Gallese, Luciano Fadiga,
Leonardo Fogassi and Giacomo Rizzolatti in Brain, Vol. 119, No. 2, pages
593-609; April 1996.
A Unifying View of the Basis of Social Cognition. V. Gallese, C. Keysers and G.
Rizzolatti in Trends in Cognitive Sciences, Vol. 8, pages 396-403; 2004.
Grasping the Intentions of Others with One's Own Mirror Neuron System. Marco lacoboni et al.
in PLoS Biology, Vol. 3,Issue 3, pages 529-535; March 2005.
Parietal Lobe: From Action Organization to Intention Understanding. Leonardo Fogassi et al.
in Science, Vol. 302, pages 662-667; April 29, 2005.
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