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IntroductionAuthor(s): Christopher D. Frith and Daniel M. WolpertSource: Philosophical Transactions: Biological Sciences, Vol. 358, No. 1431, Decoding, Imitatingand Influencing the Actions of Others: The Mechanisms of Social Interaction (Mar. 29, 2003),pp. 431-434Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/3558123
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Published nline14 February2003qu THE ROYAL*ff SOCIETY
Introduction
In the past few decades there have been enormous
advances in our understanding of the links between the
mind, the brain and behaviour. Sensory systems,
especially the visual system, have been explored in detail,
leading to a much greater understanding of the mech-
anisms underlying visual perception (Zeki 1993). In
addition, we know much more about the mechanisms bywhich our motor system allows us to reach and grasp
objects (Jeannerod et al. 1995). Progress has been made
in our understanding of the higher cognitive functions
involved in solving novel problems (Shallice 1988). Most
remarkable of all has been the enthusiasm with which neu-
roscientists have embarked on the search for the neuralcorrelates of consciousness (Crick Koch 1998). How-
ever, a striking feature of these approaches is that peopleare considered as strictly isolated units. For example, in a
typical experiment, a volunteer might sit at a bench or lie
in a brain scanner, watching abstract shapes appear on a
screen and pressing a button when a target shape appears.
By contrast, experiments that involve thinking about and
interacting with other people are rare, even though this is
what we spend much of our time doing in everyday life.
It is this social interaction that forms the main theme of
this special issue.
Humans, like other primates, are intensely social crea-
tures. One of the major functions of our brains must beto enable us to be as skilful in social interactions as we are
in recognizing objects and grasping them. Furthermore,
any differences between human brains and those of our
nearest relatives, the great apes, are likely to be linked to
our unique achievements in social interaction and com-
munication rather than our motor or perceptual skills. In
particular, humans have the ability to mentalize, that is to
perceive and communicate mental states, such as beliefs
and desires. The acid test of this ability is the understand-
ing that behaviour can be motivated by a false belief
(Dennett 1978). Deception, for example, depends uponsuch understanding. This ability is absent in monkeys and
exists in only rudimentary form in apes (Povinelli Bering2002). A key problem facing science, therefore, and one
that is at least as important as the problem of conscious-
ness, is to uncover the mechanisms underlying our abilityto read other minds and to show how these mechanisms
evolved. To solve this problem we need to do experimentsin which people (or animals) interact with one another
rather than behave in isolation. Such experiments are, of
course, taking place in increasing numbers and many of
the leading exponents of such experiments have contrib-
uted to this special issue.
Among humans, social interactions predominantlyinvolve language. However, in this issue we have chosen
One contribution of 15 to a Theme Issue 'Decoding, imitating and
influencingthe actions of others:the mechanisms of social interaction'.
Phil. Trans. R. Soc. Lond. B (2003) 358, 431-434 431
DOI 10.1098/rstb.2002.1260
to restrict ourselves largely to studies of social interactions
that do not involve language. The advantage of such stud-
ies is that they can more easily be applied to infants, ani-
mals and robots. Such application is essential for exploringin detail developmental trajectories, brain mechanisms
and computational processes. Furthermore, by concen-
trating on studies in which language is not involved we can
largely avoid discussion of the hard problem of semantics.
We have structured this special issue in terms of three
major developments in the twentieth century that have
had a considerable impact on research on the biologicalbasis of social interactions. These three developments
involve biological motion, mirror neurons and 'theory ofmind'. They also reflect different levels of complexity in
social interactions.
1. BIOLOGICALMOTION:
DECODINGSOCIALSIGNALS
The term 'biological motion' was coined by Johansson
(1973). He attached small points of light at the joints of
human actors, and filmed them moving about in the dark.
Typically all that is presented in such point-light displaysare a few moving dots, but the observer can instantly per-ceive the motion as a human figure, can see what the fig-ure is doing and can tell whether it is male or female
(Kozlowski Cutting 1977). This demonstrates thatthere is something special about the motion of living
things. This motion, in the absence of any other cues, can
convey detailed and specific information about what other
organisms are doing. At the lowest level we can detect
whether or not an object is animate. The movement of
inanimate objects like billiard balls is determined by out-
side forces while animate objects are self-propelled. At the
next level we can detect agency. The movements of agentsare determined by their goals. At the highest level we can
detect intentionality. The movements of intentional agentsare determined by their beliefs and desires.
Puce Perrett (2003) present evidence that there is a
dedicated neural system in the brains of primates, bothhuman and non-human, for detecting and interpreting
biological motion. Movements of hands, faces and eyesare of particular interest to this system, which lies in the
superior temporal sulcus (STS) adjacent to V5, an area
concerned with visual motion in general. This region of
STS does more than simply detect biological motion. It
also distinguishes between different types of biologicalmotion such as whether eyes are looking towards or awayfrom the observer. Furthermore, the late components of
EEG potentials evoked by biological motion are altered
by the context in which the motion occurs.
Csibra (2003) reports that, before they are 1 year old,
human infants can follow another person's gaze directionor pointing gesture. This behaviour implies that they are
already interpreting actions in terms of goals. In this case
the goal is communicative ('there is something interesting
( 2003 The Royal Society
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432 C. D. Frith and D. M. Wolpert Introduction
over here'). These infants can also interpret non-com-
municative actions as goal directed, as when a ball jumpsover a barrier 'in order to reach a target'. These attri-
butions are not based solely on the nature of the move-
ments observed, but also on the end state of the
movement and the context in which it occurs. However,
although infants under 1 year can attribute goals to mov-
ing objects, they do not seem to attribute mental states
such as beliefs and desires.
Frith Frith (2003) outline the developmental tra-
jectory of the ability to mentalize. This trajectory parallelsthe analysis of different levels of decoding social signals,
starting with biological motion, followed by agency detec-
tion and finally attribution of intentionality or mentalizing.
Mentalizing becomes explicit at the age of 4-6 years when
children are able to explain the misleading events that giverise to a false belief. Mentalizing depends upon a more
complex brain system than the detection of biologicalmotion. However, the mentalizing system includes STS as
one of its components. Another component is located in
the temporal poles and may be concerned with the context
in which the observed behaviour is occurring. Medial pre-frontal cortex seems to have a special role in the mentaliz-
ing system. This area is activated when mental states of
the self as well as others are represented and may have a
role in signalling that mental representations do not neces-
sarily correspond to the actual state of the world.
Rittscher et al. (2003) describe computational approachesthat have been used for the machine recognition and
interpretation of human actions. They use, for example,motion contour tracking (examining how a smooth curve
that encompasses the outline of an actor changes over time)to identify the nature of biological motion. They indicate
that semantic context needs to be taken into account to pro-vide a higher level interpretation of the observed motion.
Their approach uses a small collection of low-level models
encoding a set of motion primitives that are then interpretedin terms of the semantic context in which they occur.
Detection of biological motion seems to be sufficient for
the attribution of animacy, but, for the attribution of goalsand intentions, the context in which the movement occurs
must also be taken into account.
2. MIRRORNEURONS:IMITATINGHE BEHAVIOUROF OTHERS
In 1996 the Parma group reported the serendipitous
discovery of neurons that respond when monkeys see
someone else performing a specific action (Gallese et al.
1996). As the same neurons also respond when the mon-
key itself performs that particular action they were termed
'mirror neurons'. The discovery of mirror neurons
aroused great interest owing to their obvious relevance for
social interactions. In particular, such neurons provide a
neural mechanism that may be a critical component of
imitation. Studies in humans have shown that observingsomeone else's action primes the observer to perform the
same action (Fadiga et al. 1995) and it has long been
known thatpatients
with frontal lobe lesions can some-
times automatically and inappropriately imitate the
actions of others (Lhermitte et al. 1986).When we imitate someone we take the next step beyond
the simple observation of biological motion. We observe
the action and then we try to reproduce it. This leads to
a fundamental problem. What we see is a series of con-
figurations of the person in space, but what we have to do
is to issue a series of motor commands. How can we trans-
late what we see into what we need to do? The discoveryof mirror neurons demonstrated that a mechanism for
translation is present in the primate brain and is automati-
cally elicited when viewing the actions of others. A fre-
quent theme in the contributions to this special issue is
that this mirroring system could underlie the developmentof empathy and other forms of inter-subjectivity.
Meltzoff Decety (2003) present the evidence that this
mirroring ability is innately wired and ready to interact
with others from birth. Studies of adults show that imi-
tation depends upon a neural system that is shared
between the recognition and production of action. How-
ever, regions of the parietal lobe also have a critical role
in distinguishing between actions of the self and others.
This combination of systems that represents others as 'like
me' and as 'different from me' is fundamental for inter-
subjectivity.In spite of their mirror neurons there is no evidence that
monkeys can learn new skills by imitation and it has been
suggested that true imitation learning cannot occur unless
the learner can attribute intentions and understand cause-
effect relationships. Byrne (2003) analyses in detail the
processes by which mountain gorillas can use imitation to
learn how to prepare nettles for eating and proposes that
this learning occurs without any attribution of intentions
or causal understanding. He suggests that imitation in this
case depends on the perceptual ability to parse a complexaction into a sequence of more primitive actions, and
detect hierarchical organization underlying the action's
original production. This ability might be a necessary pre-
requisite to attributing intention and cause.
Wohlschliger et al. (2003) show that children, in con-
trast to gorillas, clearly make attributions about intentions
when imitating the actions of others. As a result they make
predictable 'mistakes' during imitation. On seeing an adult
press a button with her left hand, children interpret the
task to be imitated as 'pressing the button' and use which-
ever hand is most convenient. In this respect they are
behaving like the infants of Csibra (2003) who expect
goals to be achieved by the most efficient means. How-
ever, if the form of the movement is seen as the goal of
the action then the movement will be imitatedcorrectly.Here, the context in which the movement is made also
has a role in determining the goal that will be attributed
and hence the level at which the imitation occurs.
Gallese (2003) proposes that the mirroring system in
the brain applies to emotions and intentions as well as
actions. These mirror effects are automatic and uncon-
scious simulations. When we see an action, this automati-
cally triggers action simulation at a covert level. This
involves not only the motor system, but also systems con-
cerned with the sensory consequences of the action beingsimulated. Similar effects occur when we see an expressionof emotion. These automatic effects ensure that we share,to some
degree,the inner states of the
peoplewith whom
we are interacting, a necessary starting point for attribu-
ting mental states to others.
Schaal et al. (2003) discuss the computational methods
that have been used to control robots that can learn by
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Introduction C. D. Frith and D. M. Wolpert 433
imitation. Such learning seems to be best achieved if
movements are decomposed into a set of movement primi-tives that can be observed in the robot teacher as well as
generated in the robot student. A common framework for
observation and production can be achieved by expressing
these movement primitives in task space, i.e. the series of
movements made by the pole in a pole-balancing task.
Such task-level imitation requires prior knowledge of how
movements of the pole can be converted into movements
of the arm that is doing the balancing. Using this method,
robots can successfully learn by imitation. This learning,sometimes called 'mimicking' can occur without any
knowledge of goals, but cannot generalize to new contexts.
Much more robust learning by imitation can be achieved
if the task goal is known so that imitated movement tra-
jectories can be improved by trial and error learning. Even
better imitation can be achieved if the movement primi-tives are used to make predictions about the behaviour of
the robot teacher (see also Wolpert et al. (2003)).
3. MENTALIZING:
CLOSINGTHE COMMUNICATION OOP
A provocative paper entitled 'Does the chimpanzee have
a theory of mind?' (Premack Woodruff 1978) ushered in
a new era of research on social cognition. The somewhat
misleading phrase 'theory of mind' has been widely used
ever since to refer to the propensity to understand behav-
iour in terms of desires and beliefs. Having a theory of
mind is the same as 'taking an intentional stance' (Dennett
1996). We prefer the word 'mentalizing', which refers
more specifically to the ability to attribute mental states
such as beliefs and desires to another person. For instance,
in order to understand deception it is necessary to recog-
nize that someone's behaviour can be determined by their
belief about the state of the world even when we know
this belief to be false. In the 25 years since Premack
Woodruff (1978) posed their question no definitive evi-
dence has emerged to indicate that chimpanzees are able
to mentalize (Povinelli Bering 2002). Alternatively,
human children show signs of implicit mentalizing from
about 18 months and pass the acid test of understanding
the effects of false beliefs from the age of 4 years (Frith
Frith 2003).The interest in mentalizing received a further boost with
the discovery that children with autism acquire this ability
very late if at all while their other abilities can remain
intact (Baron-Cohen et al. 1985). This observation led to
the hypothesis that mentalizing depends upon a dedicated
brain system rather than on some more general problem-
solving mechanism (see Frith Frith 2003). Whether or
not this is the case we are still left with the question of how
mentalizing is possible since the mental states of others are
hidden. We believe that to discover the mechanisms that
underlie mentalizing, it will be necessary to study the
closed loop of social interactions. In most studies of imi-
tation this loop remains open. The transmitter (or teacher)
displays some action and then the receiver (or learner)imitates that action. To close the
loopthe transmitter
must observe the imitation and then respond in some wayto the receiver. A prototype of such a communicative loopis seen when a mother teaches her infant to pronounce a
word correctly by exaggerating certain acoustic features of
the word (Burnham et al. 2002). Through a series of iter-
ations the transmitter and receiver can reach a consensus
as to the nature of the action being imitated. Through this
mutually contingent behaviour the hidden purpose of the
action is passed from transmitter to receiver. The con-
tributors to the final part of this special issue are con-
cerned with interactions in which the communicative loop
is closed in this way.
Johnson (2003) shows that infants will treat a novel,
amorphously shaped object as an agent with goals if it
interacts contingently with them or with another person,i.e. if it moves in response to another agent's actions.
Infants can use the object's environmentally directed
behaviour to determine its attentional orientation and
object-oriented goals. Adults will also treat objects that
behave contingently as agents in spite of knowing that the
objects are artefacts. This indicates that this agent-detec-tion mechanism is a module that is hard-wired in the
brain. However, while this mechanism may be necessary,it does not seem to be sufficient to support advanced men-
talizing ability.Blair (2003) shows that emotional expressions are com-
municative gestures with specific roles in social interact-
ions. Confronted with an expression of anger the receiver
will stop performing his current action in order to changethe expression of the transmitter. Expressions of embar-
rassment after the commission of a social solecism are
designed by the transmitter to prevent further criticism
from the receiver. Thus, emotional expressions and empa-
thy permit the rapid modification of behaviour duringsocial interactions. Disorders in the perception of
emotional expressions involve a failure to recognize the
intention behind these expressions and can have devastat-
ing effects including persistent antisocial behaviour as in
psychopathy.While social interactions may be a novel topic of study
for neuroscientists, well-established techniques for such
studies have been developed in the social sciences. Grif-
fin Gonzalez (2003) describe a series of formal
approaches for the design and analysis of studies of dyadicinteractions. They show how these methods permit the
measurement of interdependence and social influence.
Sally (2003) considers the various interactive games that
have been developed by economists such as the Prisoner's
Dilemma. These games require that each player predictwhat the other will do in order to work out an appropriate
strategy. A consistent observation is that most players do
not adopt the optimum strategy as defined by the Nash
equilibrium. In part this seems to be owing to the players
attributing beliefs and intentions to each other that extend
beyond the narrow confines of the game. Sally (2003) con-
siders the various factors that cause players not to adoptthe optimum economic strategy.
Wolpert et al. (2003) present a computational account
of interactions that can be applied to robots as well as
people. Fundamental to this account is the idea of the
'forward model' that predicts the consequences of issuinga particular command to the motor system. The current
context in which theagent
is acting can be discoveredbyrunning multiple forward models to see which one gives
the best prediction. Each forward model (or predictor) is
paired with a controller that is used to issue motor com-
mands. Through prediction the most appropriate control-
Phil. Trans. R. Soc. Lond. B (2003)
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434 C. D. Frith and D. M. Wolpert Introduction
ler can be identified for any point in an action sequence.These multiple predictor-controller pairs can also be used
for imitation. Through prediction the receiver (or learner)can estimate which controller he must use to generatewhat the transmitter (or teacher) is doing at each point in
the movement sequence. As long as the motor control sys-tem in the learner is sufficiently similar to that in the
teacher, then the learner can reproduce the movement by
using this sequence of controllers. However, the teacher
can also observe the learner and, in the same way, estimate
the sequence of controllers the teacher would use to gen-erate the learner's movement. If communication has been
successful then the sequence that the teacher estimates
should correspond to the sequence he originally used. In
this way the communicative loop is closed and the success
of the communication can be checked.
We suggest that we have here the rudiments of a mech-
anism by which intentions can be transmitted from one
mind to another. Such a mechanism could be the basis
for some of the most intricate and complex human
social interactions.
Christopher D. Frith1
Daniel M. Wolpert2 December2002
1WellcomeDepartment of Imaging Neuroscience, and 2Sobell
Department of Motor Neuroscience and Movement Disorders,Institute of Neurology, University College London, London
WC1N 3BG, UK
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