Download Mirror neuron functioning: an explanation for

Mirror neuron functioning: an explanation for gender differences in empathy?
Author: Marja Nab, 314483
Supervisor: Drs. A. H. M. van Boxtel
Bachelor thesis in cognitive Neuroscience
Department Psychology and Health, Cognitive Neuroscience, Tilburg University
June, 2010
Mirror neuron functioning: an explanation for gender differences in empathy?
Abstract
Even though we are not completely sure that mirror neurons exist in human beings, a lot of
theories are proposed in which they are involved in different aspects of our behavior. This
literature study aims at the assumed role for mirror neurons in the process of empathy.
Considering the evidence it seems they are a part of our empathy mechanism. Therefore, the
social problems that arise in Autism Spectrum Disorder (ASD) patients are frequently attributed
to a deficit in their mirror neuron system (MNS). It is proposed that the sex difference, that is
well known in empathy, could also be caused by a difference in mirror neuron functioning. This
study argues that the found gender difference in mirror neurons might resemble the difference
between mirror neuron functioning in healthy people and ASD-patients. It is indicated that a
lower level of mirror neuron activity is present in two overlapping areas in the male- and ASDpatients‟ brain, being the sulcus temporalis superior and the inferior frontal gyrus. For ASDpatients obviously the mirror neuron deficit is present to a greater extent than it is in males. The
fact that the male MNS functions on a lower level than the female one, and the fact that ASDpatients have even less activity in this area contributes to the extreme-male-brain theory of
autism that was proposed by Baron-Cohen.
Empathy, mirror neurons, gender difference, autism
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Mirror neuron functioning: an explanation for gender differences in empathy?
Table of contents
Abstract ........................................................................................................................................... 2
Table of contents ............................................................................................................................. 3
1. The mirror neuron system in humans ......................................................................................... 4
1.1 Evidence for a mirror neuron system in the human brain ................................................. 4
1.2 Conclusiveness of the found evidence ............................................................................... 5
1.3 Measuring the level of mirror neuron activity................................................................... 5
2. The importance of mirror neurons in everyday functioning ....................................................... 7
2.1 Mirror neurons and imitation ............................................................................................ 7
2.2 Mirror neurons and empathy ............................................................................................. 9
3. Mirror neurons and autism spectrum disorders ........................................................................ 10
4. Gender differences .................................................................................................................... 13
5. The autistic male brain .............................................................................................................. 16
Discussion ..................................................................................................................................... 19
Summary ....................................................................................................................................... 21
References ..................................................................................................................................... 22
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Mirror neuron functioning: an explanation for gender differences in empathy?
The mirror neuron system in humans
Evidence for a mirror neuron system in the human brain
The first article on mirror neurons was published eighteen years ago; tungsten
microelectrodes were used to record 184 single neurons from inferior area 6 (F5) of the
macaque‟s brain, while acting and observing actions. The results showed that specific neurons in
the monkeys‟ brain, that discharge during making certain goal directed movements, also
discharge when observing another individual making these movements (Pellegriono, Fadiga,
Fogassi, Gallese and Rizzolati, 1992). These neurons were called mirror neurons, and it was
stated that they become activated during movement observation as well as during execution of
that exact same movement.
After this concept of mirror neurons was born, a lot of research followed, trying to prove
the presence of these neurons in human beings. The first experiment on human mirror neurons
contained a magnetic stimulation study (Fadiga, Fogassi, Pevesi and Rizzolati, 1995). While
being stimulated on the left motor cortex, subjects observed another person making goal directed
(the grasping of an object), and pointless (the tracing of geometrical figures in the air)
movements. Stationary objects and light dimming were observed as a control condition. The
motor evoked potentials of four different muscles (involved in the movements that the
participants were observing) that were recorded during these tasks, showed a significantly
increase in the first conditions in comparison to the control conditions. This outcome indicates
that the activity of the motor system had increased during observing movements made by another
person, and that therefore it could be possible for humans, to be in the possession of a system
resembling to the one found in monkeys.
This involvement of the motor system in movement observation was also demonstrated
with the use of a quantified electro encephalography (qEEG). Cochin, Barthelemy, Lejeune,
Roux and Martineau (1998) recorded qEEG signals from participants while watching an
individual perform actions and while observing moving and stationary objects as control
conditions. Neural activity in centro-parietal regions of the brain was found in the experimental
condition, were this was not found in the control conditions, indicative of involvement of the
sensimotor cortex in movement observation.
Neuro-imaging studies also contributed to the increased probability of the existence of a
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Mirror neuron functioning: an explanation for gender differences in empathy?
MNS (mirror neuron system) in human beings, for example with the use of functional magnetic
resonance imaging. Iacoboni and colleagues (1999) performed an fMRI on subjects while they
were watching someone else move their fingers and while performing finger movements of their
own. During this action and action-observation two areas were found in which overlap in activity
was present; the inferior frontal cortex and rostal-most region of the right superior parietal lobe.
This overlap in activity could indicate that neurons in those areas were firing in both conditions
and that therefore mirror neurons are present in those specific regions of the human brain.
Conclusiveness of the found evidence
Research as described above does makes it likely to believe that humans are in the
possession of a system resembling to the one found in monkeys. However, a recent article by
Turella and colleagues (2007) argues that from the great range of research on the topic of mirror
neurons in humans, due to inaccurate designs and methodological problems no study is sufficient
enough to prove this existence of mirror neurons. Additionally, even though many different
techniques were used in attempt to prove the presence of a MNS in human beings, al these
methods (EEG‟s, MRI‟s, TMS- and PET-studies) are only capable of providing indirect evidence
for this concept, since they are unable to record single neurons. Therefore, the evidence of
previously described research cannot be seen as conclusive, and it cannot be concluded that
human beings are in the possession of a so called mirror neuron system. Unfortunately, single
neuron recording, that was used to prove the mirror neuron existence in monkeys, is unethical in
human beings, so the question remains if there ever will be conclusive evidence for the existence
of human MNS. Currently the mirror neuron debate is still going on; a consensus has not been
reached. However, the many research concerning this issue does make a reasonable case for its
existence in human beings. And even though the presence of mirror neurons in humans cannot be
directly proved, this study starts from the idea that our brain is in the possession of them.
Measuring the level of mirror neuron activity
The research discussed above, is indirect prove of a MNS in human beings. It is expected that the
amount of mirror neuron activity is different for different people, brain areas and situations.
However, to measure the level of mirror neuron functioning, the methods used above are not
sufficient enough. For measuring the functioning of the MNS, the electroencephalographic μrhythm has been proposed. The μ-rhythm is a specific α-rhythm that can be measured with the
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Mirror neuron functioning: an explanation for gender differences in empathy?
use of an EEG-scan. This EEG μ-rhythm is produced in the sensorimotor cortex and ranges
between 8 and 13 Hz. During rest, this rhythm is most prominent, when moving or observing
biological movements it is attenuated (Muthukumaraswamy, Johnson and McNair, 2004). It is
proposed that attenuation of the EEG μ-rhythm reflects desynchronization of the cell assemblies
lying underneath. This desynchronization therefore suggests that the load on these cells has
increased (Pfurtscheller, Neuper, Andrew & Edlinger, 1997).
The exact relationship between the μ-rhythm and mirror neurons is not clear. However, it
has been proposed that the μ-rhythm might reflect ‘downstream modulation of motor neurons by
cells in the premotor cortex, some of which are mirror neurons’ (Muthukumaraswamy, Johnson
and McNair, 2004). Even though the relationship between the MNS and the μ-rhythm is unclear,
for this study, μ-rhythm is considered to be a good method for indicating the level in which
mirror neuron activity is present.
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Mirror neuron functioning: an explanation for gender differences in empathy?
The importance of mirror neurons in everyday functioning
Mirror neurons and imitation
Going from the concept that mirror neurons become activated during observing as well as
during execution of an action, a hypothesis followed arguing that these neurons are at the base of
an observation-execution matching mechanism. The idea of this mechanism is that during action
observation the motor circuit produces an “activation pattern” that is also produced when the
observer executes this particular action (Gallese, 2001). In other words; our motor system
becomes activated as if we are performing an action, when in fact we are only observing one.
This observation-executing matching system is thought to be important when it comes to actionunderstanding and -recognition (Muthukumaraswamy, Johnson & McNair, 2004).
A similar idea was also proposed when trying to explain the process of imitation. During
our daily activities imitation is an important quality to have at ones disposal. The process of
imitation contains observation of a specific action, processing and executing. It is a process in
which the brain can „plan‟ a movement after only observing one. The „direct matching
hypothesis‟ was proposed; a hypothesis similar to the one described above. It also states that in
the human brain, due to mirror neuron activity, observed actions are directly matched with a
neural representation of that action. It is believed that this lies at the base of the ability to imitate
observed behavior, and that it takes place whenever we are observing an action (Iacoboni and
colleagues, 1999), This idea was tested in numerous studies and is in fact found to be a correct
one.
Iacoboni and colleagues, for example, assigned subjects to either observation or
observation-execution conditions. The subjects in observation-execution conditions were then
subdivided into two groups. There was an imitative group, in which subjects had to perform
finger movements after observing them, and there was a non-imitative group, here the subjects
performed finger movement after a different kind of cue. For all groups, the brain activity was
measured with the use of an fMRI and was found in different areas. The left frontal operculum
and the right anterior parietal region were activated in both observation and observationexecution tasks. The right parietal operculum was activated in observation-execution tasks only.
According to the „direct-matching hypothesis‟ brain areas would become activated during finger
movement, with no regard to the way it was evoked. This activity however, would increase when
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Mirror neuron functioning: an explanation for gender differences in empathy?
the cue for this movement was the observation of another individual making this same movement.
Results indeed show this increase; after comparing the different executive groups it was
concluded that the imitation task accounted for larger signal intensity than the non-imitative
groups in the left inferior frontal cortex and the right superior parietal lobule. So, watching a
finger movement and then imitating accounted for a greater increase than making a finger
movement without watching one. Another interesting outcome is the fact that during both action
execution and observation conditions, the sum of the signal increase in inferior frontal and rostral
posterior parietal areas (both parts of the imitation process) was approximately equal to the
signal increase during imitation. According to the outcomes of this study, a representation
mechanism, as proposed in the direct mapping hypothesis, is thought to be present in these
specific areas of the brain.
A causal relationship between imitation and the inferior frontal mirror neuron area was
indicated during a TMS study by Heiser et al (2003). Repetitive transcranial magnetic
stimulation, a way of causing a „virtual lesion‟, was used on eight participants. rTMF was aimed
at the left and right pars opercularis (regions in the inferior frontal area) and the occipital lobe
(an area not involved in motor behavior) while subjects were asked to perform two tasks. In one
task, they were asked to imitate key-pressing as shown on a video. The other task (the control
condition) was based on a video containing a red dot that would appear above the key that the
subjects were supposed to press. Apart from the different videos both tasks were exactly the
same. Compared to the rTMF over the occipital lobe, a significant impairment in pressing the
right key was found in the imitation-condition during rTMF over the right and left pars
opercularis. This impairment was not present in the control condition, meaning that the right pars
opercularis and left pars opercularis are involved in imitation. These regions belong to the
inferior frontal cortex, an area in which mirror neuron activity was proposed. Therefore it was
concluded that a causal relationship is present between imitation and the inferior frontal area.
Judging by the research above, it can be concluded that different areas of the brain, in
which mirror neurons are present, are of great interest when it comes to imitative behavior. This
type of behavior is not only important for imitative learning, but is also thought to be essential in
other types of behavior; like inner-imitation. Inner-imitation implies the process in which one
subconsciously imitates someone else‟s behavior and is thought to be most important when it
comes to empathy related qualities.
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Mirror neuron functioning: an explanation for gender differences in empathy?
Mirror neurons and empathy
Empathy plays a big role in our everyday life; being able to identify with someone else‟s
feelings, motivations and thoughts is necessary to survive in the social society that we live in.
According to Wied, Gispen-de Wied and van Boxtel (2010) the construct of empathy can be
divided into two types. First there is cognitive empathy, also known as „theory of mind‟, which
concerns the ability to be aware of someone else‟s internal state, for example; understanding the
fact that someone else may have a different point of view. Second is affective empathy; being
able to identify with emotions experienced by someone else (e.g. crying during a sad movie).
Theodore Lipps was the one to introduce the concept of empathy; translated from his
concept "Einfühlung", in 1903 (as cited by Gallese, 2001). He also pointed out the idea of inner
imitation, where people subconsciously imitate the behavior observed from their interaction
partner. This idea was later called the „perception-behavior link‟ and is assumed to be the base of
the „chameleon effect‟. An effect that refers to the way people imitate other people‟s postures,
expressions and other behaviors, without being aware of it. In emphatic individuals this effect is
present in a greater amount than in less empathic persons (Chartrand & Bach, 1999).
After the discovery of mirror neurons, and the importance of mirror neurons in imitation,
Carr, Iacoboni, Dubeau, Mazziotta and Lenzi (2003) tried to link the MNS to empathy, and
therefore to inner-imitation. They proposed that action-representation (as takes place in an
observation-matching system) is of great importance to this process. Meaning that when we are
observing a facial expression, sadness for example, our mirror neurons system becomes activated
as if we are expressing a sad face ourselves. The idea was that, if their hypothesis was true,
during observation of facial expressions the same brain areas would be activated as during the
actual expression of these faces. An fMRI-scan performed on subjects who were either observing
or imitating facial expressions, indeed showed this overlap, and therefore the importance of
action representation in understanding other people‟s emotions. This action representation took
place trough a specific circuit in the brain containing the inferior frontal and posterior parietal
neurons (containing mirror neuron activity) and the superior temporal neurons which are only
active during observation. According to Carr and colleagues, the critical areas, being the superior
temporal and inferior frontal cortices, are connected to the limbic system by the insula and form
a mechanism essential for empathy.
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Mirror neuron functioning: an explanation for gender differences in empathy?
Mirror neurons and autism spectrum disorders
Given the evidence above it is plausible that the mirror neuron system is part of the
process of empathy. Therefore it could be the case, that people who are having problems with
experiencing empathy are experiencing a dysfunction in their mirror neuron system. In order to
test this hypothesis a basis was needed of a group of people incapable of experiencing empathy;
the autism spectrum disorder patient group.
According to the RIVM (Rijksinstituut voor Volksgezondheid en Milieu), 1.3 out of 1000
children gets diagnosed with autism. Another 1 to 2 kids in a 1000 children suffers from other
autism spectrum disorders, for example Pervasive Developmental Disorder -Not Otherwise
Specified (PDD-NOS) or the Asperger syndrome. These disorders are found to be four times
more common in boys than in girls. Autism spectrum disorders are severe conditions that are of
great influence when it comes to social behavior. An autism spectrum disorder (ASD) manifests
its main features in three different domains: abnormal or impaired development in social
behavior, communication deficits, and abnormal patterns of behavior, interests and activities.
The first domain, social dysfunction, can be subdivided into different specific traits (BaronCohen, 2008). For example:
-
Having trouble with assessing someone else’s feelings or thoughts.
-
Not knowing how to react to other people’s behavior.
-
Having trouble with reading other people’s emotions from their facial expressions, voice
or posture.
-
Having trouble accepting the fact that there are different views on a topic instead of one
right view.
According to the previously described definition of empathy it can be stated that these traits fall
in (both types of) the empathy concept. Therefore, a following conclusion is that the social
problems that people with an autism spectrum disorder are suffering from are partly due to
deficits in their empathy mechanism. Assuming that mirror neurons are of great importance
when it comes to this mechanism, it could be the case that a mirror neuron dysfunction is present
in people suffering from these disorders.
The possible deficit in mirror neurons in ASD was tested in numerous studies. For
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Mirror neuron functioning: an explanation for gender differences in empathy?
example the research carried out by Bernier, Dawson, Webb and Murias (2007). They performed
an investigation concerning two tasks; a behavior-imitation task in which individual imitation
skills were determined, and an EEG task is which brain waves were measured during observation
and imitation. Participants were 14 high-functioning ADS-patients, and a control group
containing 15 healthy adults with matched age, IQ and handedness. With the behavior-imitation
task the imitation skills of the subjects were observed, after coding of the imitations it was found
that the ASD-group performed significantly worse than the typically control group. During the
second task (the EEG task) the subjects‟ μ-rhythm was measured with the use of an EEG-scan
during four different conditions. These conditions were resting, observing, executing and
imitating. When comparing the different conditions, for both groups a significant μ-rhythm
attenuation was found in the execute condition compared to the rest-condition. However, when it
came to observing, the two groups did differ; typical adults showed greater attenuation of the μrhythm than the ASD-patients. Also, positive correlations were found between μ-rhythm
attenuation and the scores on the behavior-imitation task. This study indicates that in autistic
patients, significantly lower μ-rhythm attenuation is present. And since μ-rhythm is found to be a
good indicator for mirror neuron activity, it is proposed that there is a deficit in mirror neurons
when comparing these patients to healthy people. However, even though this mirror neuron
dysfunction is found in ASD-patients, it is not possible to attribute the empathy- related
problems they are suffering from to this deficit. The study was based on mirror neurons that are
active during observing and executing the gripping of a manipulandum, so therefore we can tell
that there is a matter of dysfunction in their MNS, but we cannot say anything about the
problems they are having when it comes to empathic behavior.
A second investigation concerning autism spectrum disorders and its relation to the MNS,
came from Dapretto and colleagues (2005). An event-related fMRI was used in other to compare
the MNS of autistic children and that of a matched control group. Subjects were shown pictures
of facial expressions and were then asked to observe and imitate them. Analysis of the fMRIscans learned that the autistic children, in contrast to the control group, showed reduced activity
in their inferior frontal gyrus (pars opercularis), an area that is proposed to be including mirror
neurons. The relationship between the amount of mirror neuron activity and the level of ASDsymptom gravity was also examined. A negative correlation was found, meaning that smaller
mirror neuron activity accounts for more severe symptoms. Unlike the study described above,
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Mirror neuron functioning: an explanation for gender differences in empathy?
the method of this study did include aspects of empathy: observing and imitating facial
expressions. Therefore it is possible to say something about the found deficit in relation to the
empathy deficits in ASD. Because of the reduced mirror neuron activity that is found in ASDpatients, and the fact that this negatively correlates with the symptom gravity, this research
provides evidence for the MNS deficit as a possible cause of the empathy problems found in
autism spectrum disorders.
It was pointed out that the deficit in mirror neurons in ASD-patients can also show itself
in an anatomic way. The brain of patients suffering from ASD significantly has less gray matter
in areas belonging to the MNS, than that of healthy people does (Hadjikhani, Joseph, Snyder &
Tager-Flusberg, 2005). In 28 persons (of which 14 were suffering from ASD and of which 14
were healthy matched control persons) the cortical thickness was measured. Regions in which
this cortical thinning was found included the inferior frontal gyrus (pars opercularis), the superior
temporal sulcus and the inferior parietal lobule. As stated in chapter 2, these first two areas are
likely to be involved in emphatic behavior. Similar to the previous research, the dysfunction in
the MNS (which in this case expresses itself in the found cortical thinning) correlates with the
symptom severity of the patients. Based on parental reports of the behavior of the participants
when they were between 4 and 5 years old, social and communication scores were correlating
with the amount of cortical thinning in inferior frontal gyrus (IFG), inferior partial lobule (IPL)
and superior temporal sulcus (STS). It was also stated that various other areas in the autisticbrain are smaller than in the regular brain. Cortical thinning of some other areas, involved in face
perception and phonological processing also showed correlations with symptom severity.
These different studies make a case for the idea that a dysfunction in their MNS could be
at the base of the empathy problems that ASD patients are suffering from. Of course the empathy
mechanism is more extensive than these mirror neuron areas. Many other different brain regions
(e.g. the insula and the limbic system) are also involved in experiencing empathy. Also, empathy
consists of many different kinds of behavior that may all involve different brain areas. This
means that even though it may seem plausible that a deficit of mirror neurons is causing the
difficulties in social interaction, the real origin of these problems in patients with ASD might be
much more extensive and/or complicated.
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Mirror neuron functioning: an explanation for gender differences in empathy?
Gender differences
Stereotypically females are more sensitive, emotional and empathic than males. Over the
years, research on whether there actually is a sex difference in empathy is frequently carried out.
By comparing over 20 different studies, Hoffman was able to draw some conclusions about this
much-discussed topic (1977). First he stated that women tend to be more empathic than males; a
vicarious affective response to another one‟s emotion and feeling is more common in females.
However, when it comes to assessing someone else‟s perspective, both spatial and cognitive,
there does not seem to be a difference between males and females. For example, they will
perform equally well in situations of evaluating someone‟s thoughts or spatial position. Starting
from these results it seems that there is a gender difference when it comes to affective empathy
but that there is no significant sex difference in theory of mind.
If a dysfunction in the MNS causes ASD-patients to have problems with empathy,
possibly individual differences in empathy in healthy people can also be attributed to differences
in mirror neuron functioning. Since males tend to be less empathic than females it could be the
case that their MNS is not functioning as well as that of woman. Research indeed shows that
there are a number of differences between male and female mirror neuron functioning.
According to Cheng and colleagues (2008) there is a significant difference in de μrhythm between males and females. Their participants (20 males and 20 females) were asked to
watch videos of hand actions and (as a control condition) a moving dot. The subject‟s EEG data,
which was recorded during this assignment, showed that in females there was greater μ-rhythm
suppression than in males while observing the hand actions. The μ-rhythm suppression was equal
in males and females for the moving dot. An additional outcome is an observed correlation
between suppression and some personal aspects of empathy, that was determined a week before
the EEG measurements based on self-report questionnaires. After statistical analysis a correlation
was found between suppression and the personal distress subscale of interpersonal reactivity
(positive) and between suppression and the systemizing quotient (negative). This means that
greater μ-rhythm attenuation indicates a higher score on that subscale of interpersonal reactivity.
On the opposite, a greater μ-rhythm attenuation accounts for a smaller „systemizing quotient‟,
which stands for the tendency to analyze or construct systems. It was indicated that in general
males score higher on this concept. This is also found for ASD-patients when compared to
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Mirror neuron functioning: an explanation for gender differences in empathy?
healthy persons (Baraon-Cohen, Richler, Bisarya, Gurunathan & Wheelwright, 2003). This
study indicates that there is a gender difference in the mirror neuron system, and because of the
found correlation it is proposed that this difference is involved in social behavior (empathy). The
same conclusion could be drawn from a magneto-encephalography survey on this same topic
(Cheng, Tzeng, Decety, Imada, & Hsieh, 2006).
A method similar to the one previously
described, was carried out; however instead of an EEG, their cortical magnetic signals were
recorded (MEG). Their outcomes also showed greater μ-rhythm suppression in females than in
males.
These two studies make it likely to believe that a difference in mirror neuron functioning
is present between males and females. However, both studies concern mirror neurons that
become activated during the observation and the execution of hand actions. It can be asked here,
whether these results are meaningful for this study, since their experimental condition is not
associated to empathy. Since the investigations presented above do not contain any empathyrelated constructs, it is peculiar that in the first study a correlation was found between μ-rhythm
attenuation and qualities belonging to social behavior. Therefore the quality of this research is
questionable, and conclusions drawn from them cannot be taken for granted.
A study that did found a gender difference in mirror neurons that might be related to
differences in empathy was carried out by Schulte-Rüther and colleagues in 2008. Their fMRI
study measured brain activity in males and females during a so called self- and other-task,
respectively tasks in which they had to focus on their emotional state, elicited by facial
expression of others, or in which they had to assess the emotional state of the shown faces. Same
brain regions in males and females became activated, involving medial and lateral prefrontal,
parietal and temporal areas. In females, however, during the self-task, the right inferior frontal
cortex and the STS showed greater neural activity in comparison to males. As stated earlier,
these are two areas for which it is indicated they (partly) consist of mirror neurons. On the
contrary, during this self-task, females showed smaller activation in the left temporoparietal
junction than males did. The increased activity in the right inferior frontal cortex in females
(relative to males) also showed itself in the other-task. From this data the authors suggested that
during both types of tasks, females activate the brain areas in which mirror neurons are present to
a higher degree than males do. Prior to the fMRI scans, with the use of the Balanced Emotional
Empathy Scale (BEES) empathy scores were determined, which in general were higher for
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Mirror neuron functioning: an explanation for gender differences in empathy?
females than they were for males. Those scores positively correlated with the found activation of
the bilateral IFG and the left STS.
These outcomes all lead to the conclusion that there are differences in the way the mirror
neuron system functions in males and females. The last study (Schulte-Rüther et el. 2008) also
suggests a gender difference in mirror neurons involved in empathy. Of course, since the
existence of mirror neurons is not even convincingly proved, this cannot be stated with complete
certainty. However, for this research let us assume that in females, there is increased mirror
neuron activity in regions critical for experiencing empathy when compared with males. The
question here is whether this gender difference could be causing the difference found in empathy.
There is a knowledgeable gender difference in empathy and a knowledgeable gender difference
in mirror neuron functioning in areas related to empathy. Also there is well established theory
that mirror neuron functioning is crucial for empathy. All this together makes it likely to assume
that the sex difference in empathy in fact can be attributed to mirror neuron functioning. The
correlation that Schulte-Rüther showed between aspects of empathy and mirror neuron
functioning makes it even more likely to believe that differences in the MNS are causing the
gender difference in empathy. However it is also a prerequisite to keep in mind that this
assumption is based on one single study and that therefore one must be careful with drawing
conclusions on this topic.
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Mirror neuron functioning: an explanation for gender differences in empathy?
The autistic male brain
When it comes to autism spectrum disorders, Simon Baron-Cohen is a well known
specialist on this field of interest. In 1999 his paper on the „extreme-male-brain theory of autism‟
was published, in which he hypothesized that the autistic profile could be seen as an extreme
version of the general male profile. He pointed out that during assessing the „autistic profile‟, a
lot of components were found that corresponded to an extreme version of components that are
present in the general male profile. For example there is fact that in general, males are superior to
females when it comes to spatial tasks. Comparing ASD-patients with healthy people it is found
that they are even more superior on these tasks (Jolliffe and Baron-Cohen, as cited by BaronCohen, 1999). Another fact that Baron-Cohen points out, is that, overall, males are slower than
women in language development and that children suffering from ASD are even slower with this
development (Rutter, as cited by Baron-Cohen, 1999). Comparable differences were also found
in other fields, concerning social development, mindreading (Baron-Cohen et al, as cited BaronCohen 1999), neurological differences like the size of the corpus callosum (Egaas, Courchesne,
and Saitou, as cited by Baron-Cohen, 1999) and the weight of the brain (Bailey et al, as cited by
Baron-Cohen, 1999), left handedness (Fein, Humes, Kaplan, Lucci and Waterhouse, as cited by
Baron-Cohen, 1999) and mathematical/spatial and mechanical professions (Baron-Cohen,
Wheelwright, Bolton, Stott & Goodyes, as cited by Baron-Cohen, 1999) . Besides these divisions,
the fact that autism spectrum disorders are found to be four times more common in boys than
they are in girls, also contributes to the likelihood of the extreme-mail-brain theory of autism.
In this final chapter Baron-Cohen‟s proposed theory is looked at in relationship to this
study. Therefore let us assume that the gender difference in empathy is in fact caused by a
difference in mirror neuron functioning. The empathy difference between healthy people and
ASD-patients can also be attributed to a difference in their mirror neuron functioning. The
question that can be asked here, is whether the gender difference in mirror neurons is comparable
to the difference found in autistic and healthy people? In other words, does the male brain have
MNS-deficits that resemble the deficits found in the mirror neuron system of ASD-patients?
Facing this question it is necessary to compare research concerning the male MNS with studies
on the MNS of ASD-patients. Three of the discussed studies are considered to provide direct
evidence concerning mirror neurons and empathy and therefore were used in the comparison.
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Mirror neuron functioning: an explanation for gender differences in empathy?
Table 1 provides an overview of the differences as well as the similarities of these studies. As
shown, the way that the male brain differs from a women‟s in situations of face expression,
recognition and imitation, expresses itself in lower levels of mirror neuron activity in two
different areas: the STS and the IFG. When comparing an „ASD‟ brain with a healthy brain,
lower levels of mirror neuron activity are also found in the IFG. Additionally, ADS-patients
showed a cortical thinning in the STS. So simply said, these are the areas in which the ASDpatients and the males differ from their comparison group (relatively a healthy matched control
group and an all female control group).
Going from the idea that these studies all met quality standards, and that a proper
comparison was made, it can be stated that males and ASD-patients are dealing with matching
deficits during empathy related behavior. Overlapping areas in which these deficits are present
are the IFG and the STS. Correlations between mirror neuron deficits and symptom severity (in
ASD-patients) and an empathy score (in males and females) were pointed out in each study,
meaning that lower scores of empathy stand for greater deficits in mirror neuron functioning.
Therefore one can assume that the deficits are found in the same brain areas, but the extent to
which they are present is greater for ASD-patients than for males. This conclusion supports the
hypothesis proposed by Baron-Cohen. Therefore, next to brain-weight and corpus callosum size,
the amount of mirror neuron activity in the IFG and STS can be added to his list of theories
supporting his idea.
17
Mirror neuron functioning: an explanation for gender differences in empathy?
Table 1.
Comparison of three different studies concerning mirror neuron functioning
Design
Authors
Dapretto
Hadjikhani
SchulteRüther
Results
Experimental group
Control group
Imaging method
Conditions
Results relative to control group
Autistic children
Healthy,
fMRI scan
Observing and imitating of facial
Reduced functioning in IFG during
(10)
matched control
expressions (neutral, happiness,
observation and expression.
group (10)
sadness, anger, or fear)
Autistic patients
Healthy,
Magnetization
(14)
matched control
Prepared Rapid
group (14)
Gradient Echo
Females (14)
fMRI scan
Males (12)
-
Cortical thinning; among others, in
IFG (pars opercularis), IPL and STS.
Other condition: assessing shown
Other: weaker activation in IFC.
emotional expressions (anger or
Self: weaker activation of right IFC,
fear)
right STS and right cerebellum.
Self condition: assessing own
Stronger activation of the left TPJ.
emotions elicited by the shown
expressions
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Mirror neuron functioning: an explanation for gender differences in empathy?
Discussion
Goal of this study was to examine whether mirror neurons could be seen as a cause of the gender
difference in empathy. It is found that there is a knowledgeable sex difference in mirror neuron
functioning that correlates with the level of empathy related qualities. The deficits that were
found in males (relative to females) can be seen as a minor version of the deficits that were
showed to be present in ASD-patients. This statement fits the extreme-male-brain theory that was
proposed by Baron-Cohen and therefore supports his hypothesis. However, an important
comment that has to be made is that large parts of this literature study are based on assumptions.
Therefore, the ideas that are proposed here cannot be seen as well-founded facts. This study
however does indicate the importance of subsequent research.
In chapter one, for example, it is stated that mirror neuron activity in human beings is not
conclusively proved. More research on this subject should follow, trying to bring watertight
prove that these neurons are present in human beings. Ideally a new participant group would be
found in patients in whom we can measure on single-cell level. This could imply neurological
patients who already are going to have brain surgery. Nevertheless, this proposal does raise
ethical questions, and therefore it is not likely to expect this type of research in the near future.
Even though single cell recording would be the most ideal way to prove mirror neuron existence,
it is also discussed that there are ways in which the recent research can be improved. As
discussed in chapter 1, large parts of research on this topic do not meet quality standards on
fields of design and methodology, and therefore these studies should be critically looked at and
be repeated with different methods.
Another assumption that was made concerns the involvement of mirror neurons in
empathy. Even though the abundant research on this topic makes that involvement sound very
likely, again this is not completely proven. For future research, studies could use „virtual lesions‟
for example, to contribute to the increase of evidence on this topic.
In the final chapter of this study, an evaluation was made of similarities and differences
between the male and ASD mirror neuron deficits. However, there are some comments that can
be placed on the comparison that was made. First of all, when looking at the different
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Mirror neuron functioning: an explanation for gender differences in empathy?
experimental conditions in the fMRI studies, both concerned facial expressions. The research
carried out by Dapretto however, contained five different emotions that were positive, negative
and neutral. Schulte-Rüther‟s method on the contrary, contained only two emotional states that
were both of negative nature. Perhaps it is the case that different natures of emotion activate
different areas in the brain and therefore different mirror neurons; if that is the case we cannot
base conclusions on outcomes of this comparison. Another comment on the comparison is that
even though in both experiments the tasks the subjects were asked to perform were linked to
social behavior and empathy, they were not totally the same. We can also ask ourselves the
question if it is valid to compare children (Dapretto) with adults (Hadjikani & Schulte-Rüther).
After comparing the different studies a conclusion was drawn. This conclusion was based on
only three different studies, that all consisted of 20 to 30 participants. However, managing to
draw valid conclusions requires more extensive research. This research should involve a
comparison of more different studies concerning males relative to females and ASD-patients
relative to a healthy control group. These studies should all use exactly the same method and
parallel subjects. In support of increasing statistical power, future research should also make use
of more subjects.
Another way to see if the gender difference in MNS is comparable to the MNS difference
in ASD-patients (compared to healthy persons) is to assess males, females and ASD-patients in
one design. After measuring the degree to which their mirror neuron system functions (during
tasks related to empathy) a comparison of the three groups can be made. Based on this literature
study it is expected to find the highest level of mirror neuron functioning in females, the second
in males and the lowest level in ASD-patients.
To summarize, this study shows that there is an indication of similarities between the
male mirror neuron system and the mirror neuron system of ASD patients. This contributes to the
plausibility of Baron-Cohen‟s extreme-male-brain theory. However being able to provide
conclusive evidence on this matter requires more extensive research.
20
Mirror neuron functioning: an explanation for gender differences in empathy?
Summary
Even though the presence of mirror neurons in humans is not completely certain, it is suggested
that they play an important role in our empathy mechanism. It is proposed that, among other
areas, the superior temporal and inferior frontal cortices, containing mirror neurons, are essential
to the process of experiencing empathy. Great deficits are proposed in the MSN of patients
suffering from autism spectrum disorders, which could be the base of their social behavior
problems. This deficit shows itself in the IFG (pars opercularis), STS and IPL. When looking at
the gender differences that are found in mirror neuron functioning during empathy-related tasks,
it also concerns the IFG and the STS, in which males show less activity than females. Positive
correlations that are found between empathy and mirror neuron activity, indicate that this
difference in mirror neuron functioning might be causing the gender difference in empathy.
Because of the found area-overlap in which males differ from women and ASD-patients differ
from healthy persons during empathy related behavior (the superior temporal sulcus and the
inferior frontal gyrus) it could be the case that this concerns lower levels of mirror neuron
activity in the exact same areas, but only to a different degree. If this is the case, the fact that
males tend to be less empathic than females could be due to the fact that males show minor
autistic symptoms. This proposal fits to the extreme-male-brain hypothesis suggested by BaronCohen (1999), arguing that the autistic profile is an extreme version of the regular male profile.
This literature review provides evidence in favor of his extreme-male-brain-theory of autism.
However, the provided evidence is slim, and it should also be noted that during the process of
this study, many assumptions were made that were not based on conclusive evidence. Due to of a
lack of quality research on this topic, the amount of sources used was too small to base actual
conclusion on. It also remains questionable if the comparison of the three different studies in the
final chapter was justified, since their methodological approaches were different. For this reason
their results might not reflect the same neural mechanism.
It can be stated that ever since Pellegrino‟s discovery in 1992, studies on mirror neurons
were frequently carried out. However, being able to draw actual conclusions on the human
mirror neuron system requires more sufficient research on every different aspect of this topic.
21
Mirror neuron functioning: an explanation for gender differences in empathy?
References
Baron-Cohen, S. (1999). The extreme-male-brain theory of autism. University of Cambridge,
Cambridge.
Baron-Cohen, S., Richler, J., Bisarya, D., Gurunathan, N., & Wheelwright, S. (2003). The
systemizing quotient: an investigation of adults with Asperger syndrome or highfunctioning autism, and normal sex differences. Philosophical Transactions of the Royal
Society, 358, 361-374.
Baron-Cohen, S. (2008). Autism and Asperger syndrome: The facts. Oxford: Oxford Universty
Press.
Bernier, R., Dawson, G., Webb, S., & Murias, M. (2007). EEG mu rhythm and imitation
impairments in individuals with autism spectrum disorder. Brain and Cognition, 64, 228237.
Buccino, G., Vogt, S., Ritzl, A., Fink, G.R., Zilles, K., Freund, H., & Rizzolatti, G. (2004).
Neural circuits underlying imitation learning of hand actions: an event-related fMRI
study. Neuron, 42, 323-334.
Carr, L., Iacoboni, M., Dubeau, M., Mazziotta, J.C., & Lenzi, G.L. (2003). Neural mechanisms
of empathy in humans: A relay from neural systems for imitation to limbic areas. PNAS,
100(9), 5497-5502.
Chartrand, T. L., & Bargh, J.A. (1999). The chameleon effect: the perception-behavior link and
social interaction. Journal of Personality and Social Psychology, 76(6), 893-910.
Cheng, Y., Tzeng, O.J.L., Decety, J., Imada, T., & Hsieh, J. (2006). Gender differences in the
human mirror system: a magnetoencephalography study. Cognitive Neurosciene and
Neuropsycholgy, 17(2), 1115-1119.
Cheng, Y., Lee, P, Yang, C., Lin, C., Hung, D., & Decety, J. (2008). Gender differences in
the mu rhythm of the human mirror-neuron system. PLoS ONE, 3(5).
22
Mirror neuron functioning: an explanation for gender differences in empathy?
Cochin, S., Barthelemy, C., Lejeune, B., Roux, S., & Martineau, J. (1998). Perception of motion
and
qEEG
activity
in
human
adults.
Electroencephalography
and
Clinical
Neurophysiology, 107, 287-295.
Dapretto, M., Davies, M.S., Pfeifer, J.H., Scott, A.A., Sigman, M., Bookheimer, S.Y., &
Iacoboni, M. (2005). Understanding emotions in others: mirror neuron dysfunction
in children with autism spectrum disorders. Nature Neuroscience, 1-3
di Pellegrino, G., Fadiga, L., Fogassi, L., Gallese, V. & Rizzolatti, G. (1992). Understanding
motor events: a neurophysiological study. Experimental Brain Research, 91, 176-180.
Fadiga, L., Fogassi, L., Pavesi, G., & Rizollati, G. (1995). Motor facilitation during action
observation: A magnetic stimulation study. Journal of neurophysiology, 73(6), 26082611.
Gallese, V. (2001). The 'shared manifold' hypothesis: from mirror neurons to empathy.
Journal of Consciousness Studies, 8, (33-50).
Hadjikhani, N., Joseph, R.M., Snyder, J., & Tager-Flusberg, H. (2006). Anatomical differences
in the mirror neuron system and social cognition network is autism. Cerebral Cortex, 16,
1276-1282.
Heiser, M., Iacoboni, M., Maeda, F., Marcus, J., & Mazziotta, J.C. (2003). The essential role of
Broca's area in imitation. European Journal of Neuroscience, 17, 1123-1128.
Hoffman, L. M. (1977). Sex differences in empathy and related behaviors. Psychological
Bulletin, 84(4), 712-722.
Iacoboni, M., Woods, R.P., Brass, M., Bekkering, H., Mazziotta, J.C., & Rizzolatti, G. (1999).
Cortical mechanisms of human imitation. Science, 286, 2526-2528.
Muthukumaraswamy, S. D., Johnson, B.W., & McNair, N.A. (2004). Mu rhythm modulation
during observation of an object-directed grasp. Cognitive Brain Research, 19, 195-201.
Pfurtscheller, G., Neuper, C., Andrew, C., & Edlinger, G. (1997). Foot and hand area mu
rhythms. International Journal of Psychophysiology, 26, 121-135.
23
Mirror neuron functioning: an explanation for gender differences in empathy?
Rijksinstituut voor volksgezondheid en milieu, http://www.rivm.nl/, 2010.
Rizzolatti, G., Fadiga, L., Matelli, M., Bettinardi, V., Paulesu, E., Perani, D., & Fazio, F. (1996).
Localization of grasp representations in humans by PET: 1. Observation
versus
execution. Experimental Brain Research, 111, 246-252.
Rizzolatti, G., & Craighero, L. (2004). The mirror-neuron system. Annual Review of
Neuroscience, 27, 169-192.
Schulte-Rüther, M., Markowitsch, H.J., Shah, N.J., Fink, G.R., & Piefke, M. (2008). Gender
differences in brain networks supporting empathy. NeuroImage, 42, 393-403.
Turella, L., Pierno, A.C., Tubaldi, F., & Castiello, U. (2007). Mirror neurons in humans:
consisting or confounding evidence? Brain and Language, 108, 10-21.
Wied, M., Gispen-de Wied, C., van Boxtel, A. (2010). Empathy dysfunction in children and
adolescents with disruptive behaviour disorders. European Journal of Pharmacology,
626, 97-103.
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