Below is a literature review of the research on Mirror
Neurons I have yellow highlighted the
interesting parts.
Mirror neurons and their clinical relevance
Giacomo
Rizzolatti*, Maddalena Fabbri-Destro and Luigi Cattaneo About the authors
Correspondence *Department of Neuroscience, University of Parma,
39 via Volturno, 43100 Parma, Italy
Email giacomo.rizzolatti@unipr.it
Email giacomo.rizzolatti@unipr.it
Summary
One of
the most exciting events in neurosciences over the past few years has been the
discovery of a mechanism that unifies action perception and action execution. The essence of this 'mirror'
mechanism is as follows: whenever individuals observe an action being done by
someone else, a set of neurons that code for that action is activated in the
observers' motor system. Since the observers are aware of the outcome of their
motor acts, they also understand what the other individual is doing without the
need for intermediate cognitive mediation. In this Review, after
discussing the most pertinent data concerning the mirror mechanism, we examine
the clinical relevance of this mechanism. We first discuss the relationship
between mirror mechanism impairment and some core symptoms of autism. We then
outline the theoretical principles of neurorehabilitation strategies based on
the mirror mechanism. We conclude by examining the relationship between the
mirror mechanism and some features of the environmental dependency syndromes.
- The mirror
mechanism is a neural system that unifies action perception and action
execution
- The mirror
mechanism is organized into two main cortical networks, the first being
formed by the parietal lobe and premotor cortices, and the second by the
insula and anterior cingulate cortex
- The role of
the mirror mechanism is to provide a direct understanding of the actions
and emotions of others without higher order cognitive mediation
- Limited
development of the mirror mechanism seems to determine some of the core
aspects of autism spectrum disorders
- The recently
demonstrated link between limited development of the mirror mechanism and
that of some aspects of the motor system suggests that rehabilitation in
children with austism spectrum disorder should take into account both
motor and cognitive strategies
- The use of
action-observation-based protocols could represent a new rehabilitation
strategy to treat motor deficits after stroke
Review criteria
PubMed
was searched using Entrez for articles published up to September 2008. The
search term was "mirror neuron" OR "mirror neurons" OR
"mirror neuron system" OR "mirror system". Owing to
limitations on the number of references, we cited only articles that we judged
to be most important from a theoretical or clinical point of view.
Keywords:
autism, environmental dependency syndromes, mirror neurons, neurorehabilitation, utilization behavior
Introduction
Traditionally, it has been assumed that the understanding
of actions performed by others depends on inferential reasoning.1, 2, 3 Theoretically, when we witness the actions
of others, the information could initially be subjected to sensory processing
and then be sent to higher order 'association' areas where it is elaborated on
by sophisticated cognitive mechanisms and compared with previously stored data.
At the end of this process, we would know what others are doing.4
It is possible that this cognitive operation might indeed
occur in some situations when the behavior of the observed person is difficult
to interpret.5, 6, 7 However, the ease with which we usually
understand what others are doing suggests that an alternative mechanism might
be involved in action perception. The essence of this alternative system is
that actions performed by others, after being processed in the visual system,
are directly mapped onto observers' motor representations of the same actions.
The observers are aware of the outcomes of
their own actions, so the occurrence of a neural pattern similar to that
present during their own voluntary motor acts will enable them to understand
the actions of others.
Evidence in favor of the existence of this direct
sensory–motor mapping mechanism came from the discovery of a set of motor
neurons, known as mirror neurons, that fire both when a monkey performs a given
motor act and when it observes another individual performing an identical or
similar motor act.8, 9 In this article, we will first review the
basic properties of this mechanism, which is known as the mirror mechanism. We
then examine the relevance of the mirror mechanism for the interpretation of
clinical syndromes such as autism, and for the development of motor
rehabilitations strategies.
Mirror neurons in
the monkey
Mirror neurons were originally discovered in the ventral
premotor cortex (area F5) of the macaque monkey.8, 9 The defining characteristic of these neurons
is that they discharge both when the monkey performs a motor act and when the
monkey, at rest, observes another individual (a human being or another monkey)
performing a similar motor act (Figure 1). The degree of similarity that is required
between executed and observed motor acts in order to trigger a given mirror
neuron varies from one neuron to another. For most mirror neurons, however, the
relationship between the effective observed and executed motor acts is based on
their common goal (e.g. grasping), regardless of how this goal is achieved
(e.g. using a two-finger or a whole-hand prehension). Importantly, mirror
neurons do not discharge in response to the presentation of food or other
interesting objects.
Figure 1 A cytoarchitectonic
map of the monkey cortex and an example of a mirror neuron.
The
upper part of the figure shows the activity of a mirror neuron recorded from
area F5. The neuron discharges both when the monkey grasps an object (A)
and when it observes the experimenter grasping the object (B). (C)
The cytoarchitectonic parcellation of the agranular frontal cortex and the
parietal lobe. PE, PEc, PEip, PF, PFG and PG are parietal areas. An enlargement
of the frontal region (inset on the left) shows the parcellation of area F5
into three parts: F5c, F5p and F5a. The mirror neurons are typically found in
F5c. The inset on the right shows the areas buried within the intraparietal
sulcus. Abbreviations: AI, inferior arcuate sulcus; AIP, anterior intraparietal
area; AS, superior arcuate sulcus; C, central sulcus; FEF, frontal eye field;
IO, inferior occipital sulcus; IP, inferior precentral sulcus; L, lateral
sulcus; LIP, lateral intraparietal area; Lu, lunate sulcus; MIP, medial
intraparietal area; P, principal sulcus; STS, superior temporal sulcus; VIP,
ventral intraparietal area. Permission obtained from Elsevier Ltd © Rizzolatti
G and Fabbri-Destro M (2008) Curr Opin Neurobiol 18: 179–184.
Mirror neurons have also been described in the PFG and
anterior intraparietal areas of the inferior parietal lobule (IPL; Figure 1). The general properties of parietal mirror
neurons seem to be similar to those of mirror neurons in the premotor cortex.
Like the latter neurons, the parietal mirror neurons code for the goals of
motor acts rather than the movements from which they are constructed.8, 9
The PFG and anterior intraparietal areas are both
connected with the F5 area and the cortex of the superior temporal sulcus.
Neurons in the superior temporial sulcus have complex visual properties, and
some respond to the observation of motor acts done by others.10, 11 However, they lack the motor properties that
are defining features of mirror neurons, and cannot, therefore, be considered
to be part of the mirror system.
The organization of the cortical motor system
To understand the functional role of mirror neurons in
the premotor cortex and IPL, it is necessary to frame them within the modern
conceptualization of the organization of the cortical motor system. Clear
evidence exists that most of the parietal and frontal motor areas code for
motor acts (i.e. movements with a specific goal) rather than mere active
displacement of body parts.12, 13, 14, 15, 16, 17, 18 Even in the primary motor cortex,
approximately 40% of neurons code for motor acts.15, 18
Studies in which the properties of single neurons were
studied in a naturalistic context have been particularly important for
establishing this new view on cortical motor organization.12 These studies showed that many neurons
discharge when a motor act (e.g. grasping) is performed with effectors as
different as the right hand, the left hand, or the mouth. Furthermore, for the
vast majority of neurons, the same type of movement (e.g. an index finger
flexion) that is effective at triggering a neuron during one particular motor
act (e.g. grasping) is not effective during another motor act (e.g. scratching).
By using motor acts as classification criteria, premotor neurons have been
subdivided into various categories such as 'grasping', 'reaching', 'holding',
and 'tearing' neurons.
Recently, evidence was provided that both inferior
parietal and premotor (area F5) neurons are organized in motor chains.19, 20 Grasping neurons recorded from these areas
were tested in two main conditions (Figure 2). In one condition, a monkey reached and
grasped a piece of food located in front of it and brought it to its mouth. In
the other condition, the monkey reached and grasped an object and placed it
into a container. The results showed that the majority of the recorded neurons
discharged with a different intensity according to the final goal of the action
(e.g. eating or placing) in which the grasping motor act was embedded
('action-constrained' neurons). This 'chained' organization seems to be
particularly well adapted for providing fluidity to action execution.
Individual neurons not only code for specific motor acts, but, by virtue of
being wired to neurons that code for the subsequent motor acts, they facilitate
the activity of these downstream neurons, thereby ensuring smooth execution of
the intended action.
Figure 2 Action-constrained
neurons in the monkey IPL.
(A)
Apparatus and paradigm used for a task designed to demonstrate
action-constrained neurons. The monkey starts from the same position in all trials,
reaches for an object (1) and brings it to the mouth (2a) or places it into a
container (2b). (B) Activity of three IPL neurons during the motor task
in conditions 2a (grasp to place) and 2b (grasp to eat). Raster histograms are
synchronized with the moment when the monkey touched the object to be grasped.
Unit 67 fires during grasping to eat and not during grasping to place. Unit 161
is selective for grasping to place. Unit 158 does not show any task preference.
(C) Visual responses of IPL mirror neurons during the observation of
grasping to eat and grasping to place performed by an experimenter. Unit 87 is
selective for grasping to eat, unit 39 is selective for grasping to place and
unit 80 does not display any task preference. Abbreviation: IPL, inferior
parietal lobule. Permission obtained from American Association for the
Advancement of Science © Fogassi L et al. (2005) Science 308:
662–667.
The functional role of the mirror neurons
The existence of a class of motor neurons that discharge
during the observation of actions done by others is not as bizarre as it might
initially seem. While it is true that an action done by others could be
recognized by inference on the basis of previous visual experience without
involving the motor system, visual perception per se does not provide
the observer with the experiential aspects of the action. Furthermore, the
mirror system provides a particularly efficient way to establish links between
the observed action and other actions with which it is functionally related.21
Evidence in favor of the notion that mirror neurons
mediate action understanding came from experiments in which monkeys were not
allowed to see the actions performed by others, but were given clues for
understanding them. In one series of experiments, monkeys were presented with
noisy motor acts (e.g. peanuts breaking, tearing a piece of paper), which they
could either both see and hear or only hear.22 The researchers found that many mirror
neurons in area F5 responded to the sound of the motor act, even when it was
not visible.
In another series, F5 'grasping' and 'holding' mirror
neurons were tested both when the monkey observed the experimenter grasping a
piece of food and when the monkey was prevented from seeing the experimenter's
hand movements by use of a black screen.23 Despite the fact that the monkey could not
see the hand–object interaction (the visual triggering feature of the recorded
neurons) in the latter condition, many mirror neurons in F5 were active in this
situation. The neurons typically began to discharge at the beginning of the
hand-reaching movement, indicating that the monkey had a representation of the
action performed behind the screen, even when it could not see the performed
motor act.
The activity of mirror neurons per se describes
only what is happening in the precise moment of occurrence of the observed
actions. There is, however, a broader function of mirror neurons. This function
is related to the recent discovery that most action-constrained neurons (see
above) have mirror properties and selectively discharge when the monkey
observes motor acts embedded in a specific action (e.g. grasping for eating but
not grasping for placing; see Figure 2).19 The activation of action-constrained mirror
neurons, therefore, codes not only 'grasping', but 'grasping for eating' or
'grasping for placing'. This
coding implies that when the monkey observes grasping done by another, it is
able to predict, on the basis of contextual cues (e.g. repetition, presence of
specific objects), what will be the individual's next motor act. In other
words, the monkey is able to understand the intentions behind the observed
motor act.
The mirror system in
humans
Understanding of goals and intentions
A large number of studies based on noninvasive electrophysiological
(e.g. EEG, magnetoencephalography [MEG]) or brain imaging (e.g. PET, functional
MRI [fMRI]) techniques have demonstrated the existence of the mirror mechanism
in humans.8, 9 Brain imaging studies have enabled the
mirror areas to be located. These studies showed that the observation of
transitive actions done by others results in an increase in blood oxygen
level-dependent (BOLD) signal not only in visual areas, but also in the IPL and
the ventral premotor cortex, as well as the caudal part of the inferior frontal
gyrus (IFG). These latter three areas have motor properties and closely
correspond to the areas that contain mirror neurons in the monkey (Figure 3).
Figure 3 The parietofrontal
mirror system in humans.
Lateral
view of the human cerebral cortex showing Brodmann cytoarchitectonic
subdivision. The areas in yellow correspond to areas that respond to the
observation and execution of hand motor acts. The left-hand panel shows an
enlarged view of the frontal lobe. The possible homology between monkey and
human premotor cortex is indicated by arrows. Note that in monkeys area F5
consists of three subareas: F5c, F5p and F5a. Area 44 is considered to be the
most likely human homolog of area F5. Abbreviations: C, central sulcus; FEF,
frontal eye field; IF, inferior frontal sulcus; IP, inferior precentral sulcus;
PMd, dorsal premotor cortex; PMv, ventral premotor cortex; PrePMd, pre-dorsal
premotor cortex; SF, superior frontal sulcus; SP, upper part of the superior precentral
sulcus. Permission obtained from Elsevier Ltd © Rizzolatti G and Fabbri-Destro
M (2008) Curr Opin Neurobiol 18: 179–184.
Both the premotor and the parietal areas of the human
mirror system show a somatotopic organization.24 Observation of motor acts done with the leg,
hand or mouth activates the precentral gyrus and the pars opercularis of
the IFG in a medial-to-lateral direction, as in the classical homunculus model
of Penfield25 and Woolsey.26 In the IPL, mouth motor acts are represented
rostrally, hand and arm motor acts are represented caudally, and leg motor acts
are represented even more caudally and dorsally, extending into the superior
parietal lobule.
Most studies on the mirror mechanism in humans have
investigated transitive movements such as grasping. In a recent fMRI study in
which volunteers were asked to observe video clips showing a hand transport
movement without an effector–object interaction, activations were found in the
dorsal premotor cortex and also in the superior parietal lobule, with the
activation extending into the intraparietal sulcus.27 This finding indicates that the human brain
is endowed with a reaching mirror mechanism that is anatomically separated from
the mirror mechanism that codes for the distal motor act.
As in the monkey, the parietal and frontal mirror areas
in humans code mostly for the goals of motor acts. Gazzola et al.28
instructed volunteers to observe either a human or a robot arm grasping
objects. In spite of differences in shape and kinematics between the human and
robot arms, the parietofrontal mirror network was activated in both conditions.
Further evidence in favor of goal coding was obtained in an fMRI study based on
repetition suppression29—a technique that exploits the trial-by-trial
reduction of a physiological response to repeated stimuli. The results showed
that repeated presentation of the same goal caused suppression of the
hemodynamic response in the left intraparietal sulcus, but this region was not
sensitive to the trajectory of the agent's hand.
The study of aplasic individuals born without arms and
hands provided further evidence in favor of a goal-coding mirror mechanism.30 During MRI scanning, two aplasic individuals
and a group of nonaplasic volunteers were instructed to watch videos showing
hand actions. All participants also made actions with their feet, mouths, and,
in the case of the nonaplasic volunteers, hands. The results showed that in
aplasic individuals, the observation of hand motor acts, which they had never
themselves performed, activated the mirror areas. The communality of goals
between the never-executed hand motor acts and those performed with the mouth
and feet was the most probable explanation for this activation.
Growing evidence exists that, in addition to goal coding,
the human mirror mechanism has a role in the ability to understand the
intentions behind the actions of others. In an fMRI study, volunteers observed
motor acts (e.g. grasping a cup) embedded in specific contexts (a condition in
which the agent's intention could be easily understood) or devoid of context (a
condition in which the agent's intention was ambiguous).31 The results showed that the mirror network
was active in both conditions. However, the understanding of intention produced
a stronger signal increase in the caudal IFG of the right hemisphere.
The importance of the mirror system in understanding the
intentions of others was confirmed by a repetition-suppression fMRI experiment.32 Participants were asked to observe repeated
movies showing either the same movement or the same action outcome regardless
of the executed movement. The result showed activity suppression in the right
IPL and the right IFG when the outcome was the same.
Movement, emotions and language
As we have discussed, the mirror mechanism located in the
parietal and frontal areas codes mostly for the goals of observed motor acts.
However, studies that involved transcranial magnetic stimulation (TMS) have
shown that the human motor system also responds to the observation of movements
devoid of a goal.33, 34 This 'movement mirror mechanism' seems to be
extremely sensitive to movement kinematics. Dayan et al.35 studied brain responses to the observation
of curved hand movements that either obeyed or disobeyed the law—known as the
2/3-power law—that describes the coupling between movement curvature and
velocity. Mirror hand areas were more active during the observation of
movements that obeyed this law than during other types of motion.
The mirror mechanism is located not only in centers that
mediate voluntary movement, but also in cortical areas that mediate
visceromotor emotion-related behaviors.36, 37 Brain imaging studies showed that when an individual feels or observes
emotions in others caused by disgusting stimuli or stimuli representing pain,
there is activation in two structures: the cingulate cortex and the insula.
Interestingly, the same voxels are activated in these two structures in both
'feeling' and 'observing' conditions. This finding strongly suggests that
feeling emotions and recognizing them in others are mediated by the same neural
substrate.
It should be made clear that the anterior insula, where
the aforementioned activations were found, has a dysgranular–agranular
structure,38 and is, therefore, cytoarchitectonically
similar to motor areas. Electrical stimulation of the insula in the monkey
produces movements of various body parts, accompanied by a variety of
visceromotor responses.39, 40 Similar effects have also been described in
humans.41, 42 It is, therefore, appropriate to define
these structures as 'mirror areas' in which the motor response includes a
visceral component.
In humans, the mirror mechanism is also located in
Broca's area, which is involved in language processing and speech production.
Evidence for a mechanism that translates heard phonemes into the motor programs
necessary to produce them has been provided by TMS experiments.43 The mouth motor field was stimulated in
volunteers while they heard words containing phonemes requiring tongue
movements (e.g. "birra") or not requiring tongue movements (e.g.
"baffo"). Motor evoked potentials recorded from the tongue muscles
increased with the presentation of verbal material containing a double 'r'
relative to those containing a double 'f'.
The mirror system in
neurology
The mirror system and autism
Autism spectrum disorder (ASD) is a heterogeneous
developmental syndrome characterized by a marked impairment in social
interaction and communication.44 Communication deficits include disturbances
in most domains of language and are not limited to its pragmatic aspects.45 Impairment in the domains of affective links
and emotion recognition is another important component of ASD.46 A restricted repertoire of activity and
interests, repetitive motion, and hypersensitivity to certain sounds are other
symptoms that are often present in ASD.
Autism affects a variety of nervous structures, from the
cerebral cortex to the cerebellum and brainstem.47 However, in a context of a broader
neurodevelopmental deficit, a set of ASD symptoms (impairment in communication,
language and emotion, as well as in the capacity to understand others) seems to
match the functions mediated by the mirror mechanism. A hypothesis has,
therefore, been advanced that this set of deficits might depend on an
impairment of the mirror mechanism,48,
49 and there is growing evidence to support
this view.50, 52, 53
One classical EEG observation is that mu rhythm (an EEG
rhythm recorded from the motor cortical areas) is blocked when a person makes a
voluntary movement. This rhythm is also suppressed when a person observes
another person performing a movement. Oberman et al.50
used this phenomenon to test the mirror mechanism in children with ASD. The
results showed that although individuals with ASD exhibited a suppression of mu
rhythm during voluntary movements, this suppression was absent when they watched
some one else performing the movement (Figure 4). Martineau et al.54 have reported similar observations.
Figure 4 Absence of mirror EEG
responses in autism.
The
charts show suppression of the mu rhythm in controls (A) and patients
with autism spectrum disorder (B) during observation of movement of an
inanimate object (ball, pale green) or movements made with a hand (hand,
green), and during active hand movements made by the individual from whom
recordings were being taken (move, red). The bars represent the amount of mu
activity in central scalp locations; C3, Cz and C4 refer to scalp coordinates
of the 10/20 EEG system. Significant suppression of this activity, indicated by
asterisks, is present for the hand observation condition only in controls,
showing that patients with autism spectrum disorder fail to respond in a
standard way to the observation of other people's actions. Permission obtained
from Elsevier Ltd © Oberman LM et al. (2005) Brain Res Cogn Brain Res
24: 190–198.
Oberman et al.55
recently reported an interesting observation concerning the mirror system of
children with ASD. The authors investigated how familiarity between an
observing individual and a person performing a movement modulates the entity of
mu rhythm suppression. Typically developing children and children with ASD
viewed video clips showing the hand of a stranger performing a grasping action,
the hand of a child's guardian or sibling performing the same action, and the
participant's own hand performing the action. The study revealed that mu
suppression depended on the familiarity of the observer with the agent, and
that children with ASD showed mu suppression when a familiar person performed
the action but not when it was performed by an unfamiliar person.
An fMRI study has provided strong evidence in favor of a
deficit of the mirror mechanism in ASD. High-functioning children with ASD and
matched controls were scanned while they imitated and observed emotional
expressions. The results showed a markedly weaker activation in the IFG in
children with ASD than in typically developing children. Most interestingly,
the degree of activation was inversely related to symptom severity.53
Impaired motor facilitation during action observation has
been reported in individuals with ASD by use of TMS.52 Furthermore, unlike typically developing
individuals, children with ASD tend not to imitate other individuals in a
mirror fashion when viewing them face-to-face.56 This imitation peculiarity is probably
attributable to a deficit in the ability of the mirror mechanism to superimpose
another person's movements on one's own.
Deficits in the mirror mechanism in ASD have also been
addressed from another perspective.57 Typically developing children and children
with ASD were tested while they observed an experimenter either grasping a
piece of food for eating or grasping a piece of paper to place it into a
container (Figure 5). The EMG activity of the mylohyoid muscle,
which is involved in opening of the mouth, was recorded. The results showed
that observation of food grasping produced activation of the mylohyoid muscle
in typically developing children, but not in children with ASD. In other words,
whereas the observation of an action done by another individual intruded into
the motor system of a typically developing observer, this intrusion was lacking
in children with ASD. This finding indicates that, in this disorder, the mirror
system is silent during action observation, and that the immediate,
experiential understanding of the intentions of others is absent.
Figure 5 Motor behavior in
typically developing children and children with ASD.
This
experiment was designed to assess whether an action-constrained motor
organization is present in typically developing children and children with ASD.57 (A) Schematic representation of the
tasks. The individual reaches for an item on a plate and either brings it to
their mouth or puts it into a container placed on their shoulder. Time course
for typically developing children (B) and children with ASD (C)
of the rectified electromyographic activity of mouth-opening muscles during the
execution (left side) and observation (right side) of the
'bringing-to-the-mouth' action (red line) and of the 'placing' action (blue
line). All curves are aligned with the moment of object lifting from the
touch-sensitive plate (time = 0). The results demonstrate a lack of
anticipatory motor activity during execution and a lack of mirror motor
activation during observation of a given action in children with ASD.
Abbreviations: ASD, autism spectrum disorder; EMG, electromyography.
Both children with ASD and typically developing children
were also asked to perform the two actions described above (grasp to eat and
grasp to place) while the EMG activity of the mylohyoid muscle was recorded.57 In typically developing children, the muscle
became active as soon they moved the arm to reach the food. By contrast, no
mylohyoid muscle activation was observed during food reaching and grasping in
children with ASD; activation of the muscle was evident only when these
children brought the food to their mouths. These data indicate that children
with ASD are not only unable to organize their own motor acts into a unitary
action characterized by a specific intention, but that they also show a deficit
in the mirror mechanism, as reflected in the absence of motor activation of the
muscles involved in an observed action.
These findings show an apparent contradiction between the
cognitive capacities of children with ASD to report the purpose of an
experimenter's action and their lack of motor resonance with the action. To
clarify this incongruity, a further experiment was performed in which typically
developing children and children with ASD observed an actor performing
goal-directed motor acts and were asked to report what the actor was doing and
why he was doing it (Rizzolatti G et al., unpublished data). These tasks
test two different abilities: the ability to recognize a motor act (e.g.
grasping an object) and the ability to understand the intention behind it (e.g.
grasping to eat). The results showed that both typically developing children
and children with ASD were able to recognize what the actor was doing, but
children with ASD failed to recognize why the act was being performed. Children
with ASD systematically attributed to the actor the intention that could be
derived by the semantics of the object—for example, an intention to cut when
scissors were shown—regardless of how the object was grasped. This finding
indicates that children with ASD interpret the behavior of others on the basis
of the standard use of objects rather than the actual behavior of a person performing
a task. Children with ASD, therefore, seem to lack the ability to read the
intentions of others on the basis of behavior.
The mirror mechanism and motor rehabilitation
As well as having a role in action understanding, the
mirror mechanism also modulates the motor behavior of the observer. This
function forms the basis for the imitation of simple motor acts58 and for learning through imitation.59 Particularly interesting from a clinical
point of view was the demonstration that the mirror mechanism is involved in
the building of motor memories. The most convincing evidence for such a role
came from studies by Stefan et al.60, 61 that involved TMS. The authors showed that
when participants simultaneously performed and observed congruent movements,
the learning of these movements was potentiated with respect to learning
through motor training alone. These findings indicate that the coupling of
observation and execution strongly facilitates the formation of motor memories.
Could this mechanism be exploited for motor
rehabilitation? Many current behavioral neurorehabilitation techniques use
strategies that induce long-term plasticity in the motor cortex either by
depressing activity on the unaffected side or by potentiating activity on the
affected side.62 The possibility that plasticity might be
induced in the motor cortex by coupling action observation and execution
represents the theoretical basis of a recent study that examined the effect of
an 18-day cycle of active motor training with the paretic limb in two groups of
patients with chronic stabilized stroke in the middle cerebral artery
territory.63 The test group was required to perform hand
motor acts prompted by movies showing similar motor acts, whereas the control
group performed the same motor training without any visual cues. Functional
assessment of the upper limb showed a significant improvement in the test group
relative to the control group.
The mirror mechanism probably also forms the neurophysiological
basis for 'mirror therapy' (the word 'mirror' being used here in its literal
sense), which has been shown to improve upper-limb function in patients with
stroke.64, 65 In mirror-therapy protocols, the patients
are required to perform movements with their nonparetic hand while watching the
hand and its reflection in a parasagittal mirror. This procedure gives a visual
illusion of movement of the paretic hand. The generation of cortical plasticity
and the consequent rehabilitative results strongly suggest a role in patient
improvement for a mechanism that matches seen and executed actions, thereby
implicating the mirror mechanism in this process.66
Deficits in the control of mirror mechanisms
Clinical observations have shown that frontal lesions can
cause a series of disturbances characterized by the appearance of forced motor
behavior triggered by external stimuli.67 Among these manifestations, imitation
behavior is particularly interesting in relation to the mirror mechanism. The
main feature of this syndrome is the spontaneous imitation of motor acts done
by others, and it is considered to be part of the so-called 'environmental
dependency syndrome'.68 The condition arises from unilateral, or,
more frequently, bilateral prefrontal lesions.68, 69 Imitation behavior is generally attributed
to an imbalance between exogenously and endogenously determined behaviors. The
observation of actions done by others leads to the coding of potential motor
acts in the parietal and premotor mirror areas by means of the mirror
mechanism. These potential motor acts typically do not determine overt
movements in the healthy adult brain because the manifestation of these acts is
suppressed by the frontal lobe. Damage to this lobe would destroy this control
mechanism, thereby transforming the potential motor acts into actual motor
behavior. In view of the temporal latency between observation and imitation
that patients often show, an additional mechanism could be also involved, but
the essence of the phenomenon seems to depend on a release of potential motor
acts.
Echopraxia is a term that describes forced and uncritical
imitation of behaviors. The exogenously triggered behavior is sustained through
endogenous mechanisms, resulting in its perseveration. In view of the
simplicity of the imitated behaviors, combined with the total lack of criticism
of the patient to the imitated behavior, echopraxia is perceived as a distinct
disorder from imitation behavior. Echopraxia can arise in the context of basal
ganglia dysfunction, as well as after frontal lobe damage. It is probable,
however, that in both cases the mechanism that underlies echopraxia is a
disinhibition of the mirror areas through loss of suppression by the frontal
lobe.70
Conclusions and
future prospects
The discovery of the mirror mechanism radically changed
our views on how individuals understand actions, intentions and emotions. The
identification of this mechanism has had a profound impact on a variety of
disciplines, ranging from cognitive neurosciences to sociology and philosophy.
Until recently, this discovery had influenced clinical research to a much
lesser degree. However, it has now provided deeper insights into the interpretation
of certain neurological syndromes, such as the environmental dependency
syndrome, and has provided a new theoretical basis for establishing
rehabilitation techniques in patients with motor deficits following stroke.
Autism is one condition in which the discovery of the
mirror neuron mechanism could have important practical implications in the
future. Recent experimental data suggest that individuals with ASD have a
deficit in representing goal-directed actions, both when the actions are performed
and when they are observed. Children with ASD, therefore, show impairments in
organizing their own motor acts according to an action goal, as well as in
using this motor mechanism to understand the intentions of others. This new
view on ASD could be used to establish new rehabilitation strategies based on a
motor approach. The rationale of such an approach is that if the motor
knowledge of individuals with ASD is improved, their social knowledge and
behavior would also be enhanced.
Key points
- The mirror mechanism
is a neural system that unifies action perception and action execution
- The mirror
mechanism is organized into two main cortical networks, the first being
formed by the parietal lobe and premotor cortices, and the second by the
insula and anterior cingulate cortex
- The role of
the mirror mechanism is to provide a direct understanding of the actions
and emotions of others without higher order cognitive mediation
- Limited
development of the mirror mechanism seems to determine some of the core
aspects of autism spectrum disorders
- The recently
demonstrated link between limited development of the mirror mechanism and
that of some aspects of the motor system suggests that rehabilitation in
children with austism spectrum disorder should take into account both
motor and cognitive strategies
- The use of
action-observation-based protocols could represent a new rehabilitation
strategy to treat motor deficits after stroke
Acknowledgments
This study was supported by European Union Contract
012738, Neurocom, by PRIN 2006 to GR and by Fondazione Monte Parma. MF-D was
supported by Fondazione Cassa di Risparmio di Ferrara.
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Competing
interests
The authors declared no competing interests.
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Patti Wood, MA, Certified Speaking Professional - The Body Language Expert. For more body language insights go to her website at www.PattiWood.net. Check out Patti's website for her new book "SNAP, Making the Most of First Impressions, Body Language and Charisma" at www.snapfirstimpressions.com. Also check out Patti's YouTube channel at http://youtube.com/user/bodylanguageexpert.
