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Transcript
MIRROR NEURON FUNCTION: AN EXAMINATION OF DIFFERENCES
RELEVANT TO EMPATHY AND AUTISM
HONORS THESIS
Presented to the Honors Committee of
Texas State University
in Partial Fulfillment
of the Requirements
for Graduation in the Honors College
by
Erin Celeste Timperlake
San Marcos, Texas
May 2014
MIRROR NEURON FUNCTION: AN EXAMINATION OF DIFFERENCES
RELEVANT TO EMPATHY AND AUTISM
Thesis Supervisor:
________________________________
Crystal D. Oberle, Ph.D.
Department of Psychology
Approved:
____________________________________
Heather C. Galloway, Ph.D.
Dean, Honors College
COPYRIGHT
by
Erin Celeste Timperlake
2014
FAIR USE AND AUTHOR’S PERMISSION STATEMENT
Fair Use
This work is protected by the Copyright Laws of the United States (Public Law 94-553,
section 107). Consistent with fair use as defined in the Copyright Laws, brief quotations
from this material are allowed with proper acknowledgment. Use of this material for
financial gain without the author’s express written permission is not allowed.
Duplication Permission
As the copyright holder of this work I, Erin Celeste Timperlake, authorize duplication of
this work, in whole or in part, for educational or scholarly purposes only.
ACKNOWLEDGEMENTS
I would like to express my very great appreciation to my thesis supervisor, Dr.
Crystal Oberle, for her guidance and encouragement in completing this project. I would
also like to thank Professors John Hood and Diann McCabe for their help and support in
the development of this topic. My grateful thanks are also extended to Dr. Heather
Galloway and the Texas State University Honors College. Special thanks should be given
to Genelle Sanders for her valuable and constructive suggestions and keen proofreading
skills. I would also like to take this moment to thank my college sponsors, Dr. and Mrs.
Roger Timperlake, who have provided me with endless support and wisdom throughout
my lifetime. Lastly, I would like to thank my roommates and friends, Amanda Dorta and
Sarah Miller, for tolerating my obscure sleep schedule and interest in this subject. I
promise we can find something else to talk about now.
v
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS .................................................................................................v
ABSTRACT...................................................................................................................... vii
CHAPTER
I. INTRODUCTION ................................................................................................1
Background on Mirror Neurons ...................................................................1
Previously Proposed Relationships to Empathy and Autism.......................3
Purpose and Significance of the Study ........................................................4
II. LITERATURE REVIEW ....................................................................................6
Mirror Neurons and Behavior ......................................................................6
Mirror Neurons and Empathy ......................................................................7
Sex Differences in Empathy ......................................................................12
Autism and Mirror Neuron Dysfunction....................................................14
Sex Differences in Autism .........................................................................16
III. DISCUSSION ..................................................................................................19
Discussion of Past Research ......................................................................19
Implications and Applications ...................................................................20
Future Research .........................................................................................21
Conclusion .................................................................................................22
REFERENCES ..................................................................................................................23
vi
ABSTRACT
This thesis presents a comprehensive review of past research to investigate
whether evidence supports that the mirror neuron system of the brain is the physiological
mechanism behind our ability to empathize with others. Regarding sex differences in
empathy, females are regarded to be more empathetic than male counterparts. Research
indicates females frequently score higher on empathy quotients, social sensitivity, and
emotion recognition than males, and these behavioral differences correspond to changes
in the mirror neuron system. Regarding autism and empathy, research indicates
individuals diagnosed with autism have markedly diminished levels of empathy, and
possible dysfunction of the mirror neuron system. Sex differences in mirror neuron
function may further be related to the greater prevalence of these disorders in males than
females.
Key words: Mirror neurons, empathy, autism, sex differences
vii
I. INTRODUCTION
Background on Mirror Neurons
Mirror neurons were discovered in the early 1990s at the University of Parma
while researchers were investigating macaque monkeys’ action performance (di
Pellegrino, Fadiga, Fogassi, Gallese, & Rizzolatti, 1992; Rizzolatti, Fadiga, Gallese, &
Fogassi, 1996). Electrodes implanted in the F5 pre-motor cortex of the monkeys’ brains
revealed an interesting reaction from the neurons. It was observed that the same neuron
fired both when performing an action as well as when that same action was observed. The
neurons were believed to be a part of the brain's motor system and correlated with
specific goals rather than specific actions. The discovery of these mirror neurons has
altered the way psychologists believe humans interact with others.
Mirror neuron activation in response to observed action is more than just motor
preparation, but also understanding. The activation of the mirror neurons in observation is
representative of an individual’s mental processing as a formation of an image of the
action (Rizzolatti et al., 1996). This mental image formed in observation will later be
used when planning or preparing for the action (Rizzolatti et al., 1996). In the observer,
the meaning of the observed action does not result from the emotion it evokes, but by
matching of the observed action with the motor activity that occurs when the observer
performs the same action (Rizzolatti et al., 1996).
1
The evidence that distinguishes mirror neurons from pre-motor neurons that fire
in advance of movement execution comes from further study by Rizzolatti and collegues
(1996). Using electroencephalography (EEG) measures of macaques’ motor processes,
activation of mirror neurons was observed in response to observation of an action.
However, this activation of the neurons was not followed by the action for which the
monkey had mentally prepared. The monkey observing another monkey grabbing for
food does not move his fingers. Another important aspect is that activation ceases when
the food is made available to him. Whereas, if these neurons were related to pre-motor
neurons then activation would have increased in response to preparation of movement
execution, not decreased. This supports the idea that mirror neurons are involved in more
than just motor preparation, but also awareness of another’s perception in performing an
action.
Although humans were presumed to have a mirror neuron system (MNS), for
many years, it was difficult to find conclusive evidence. The invasive measure of singlecell recordings, used to directly study neurons, has been avoided. Since mirror neurons
could not be directly recorded, mu rhythm suppression has been utilized in studies. Mu
rhythm is the EEG measurement of the pattern of oscillating electrical frequencies
(Braadbaart, Williams, & Waiter, 2013; Cuevas, Cannon, Yoo, & Fox, 2014). Motor
neurons are normally suppressed by input through action observation or movement
execution, which is why it is used indirectly to measure mirror neuron function
(Braadbaart et al., 2013).
Studies using mu rhythm suppression in relation to MNS function were met with
skepticism since evidence of mirror neurons in humans was lacking. Multiple studies
2
determined that suppression of mu rhythm did indicate an activation of the MNS
(Braadbaart et al., 2013; Oberman et al., 2013). However, concrete proof of the existence
of the MNS in humans could not be provided until single cell recordings could be
conducted.
With the development of new techniques to placate symptoms of epilepsy,
opportunities to conduct single-cell recordings of mirror neurons in humans have become
possible. Research conducted at the UCLA Medical Center on patients with intractable
epilepsy attempted to identify seizure foci in the brain using intracranial depth electrodes
implanted in the brain (Wheeler, 2013). Electrodes were implanted with the intent to
develop a surgical treatment; however, their use provided neuroscientists with the
opportunity to directly observe mirror neurons in humans (Wheeler, 2013). This study
provided a unique chance to demonstrate the existence of the MNS.
UCLA’s single-cell recording study provided direct electrophysiological evidence
for the existence of mirror neurons in the human brain (Keysers & Gazzola, 2010).
Results confirmed these neurons are located in areas of the brain associated with the
MNS, the ventral pre-motor cortex and inferior parietal lobe, as well as extending beyond
into other areas of the brain (Keysers & Gazzola, 2010). Establishing evidence of mirror
neurons in the human brain further spurred research towards understanding the function
of these neurons. However, additional research still needs to be conducted to fully
understand the presence of the MNS in the human brain.
Previously Proposed Relationships to Empathy and Autism
When Giacomo Rizzolatti and his colleagues first discovered mirror neurons at
the University of Parma, they believed these neurons had the potential to explain how and
3
why we empathize with others (di Pellegrino et al., 1992; Rizzolatti et al., 1996;
Winerman, 2005). According to these researchers, observing a particular action and
performing the same action invokes the same activation in these neurons. This may
explain why we flinch when we see another individual trip or cry when watching a sad
movie. Mirror neurons are implicated in the study of empathy for this reason.
Other researchers proposed that a defect in the MNS of individuals with autism or
autism spectrum disorder (ASD) could explain their difficulty in translating action
observation to action understanding (i.e., interpreting the actions and intentions of others)
and their diminished empathizing ability (Gallese, Rochat, & Berchio, 2013; Hamilton,
2013). Individuals diagnosed with ASD, more commonly found in males than females
with a ratio of 5:1, share three similar impairments: social interaction, verbal
communication, and non-verbal communication (Baio, 2013; Gallese et al., 2013). These
communicative and social deficits encompass a lack of a "theory of mind" whereby
individuals with ASD frequently misunderstand others’ perspective.
Relevant to the MNS, research reveals that mirror neuron therapy (introducing
people to various stimuli to activate the MNS) has resulted in improvement of
comprehension of the intentions and actions of others (McGarry & Russo, 2011).
Examination of the successes and failures of these therapies could identify the function of
these neurons and further our understanding of the role they play.
Purpose and Significance of the Study
This thesis presents a comprehensive review of previous research to investigate
whether the MNS is responsible for the process of empathizing with others. Much of
research concerns sex differences in MNS function and the corresponding sex differences
4
in empathetic ability. Prevalence of ASD, which is associated with diminished
empathetic ability, is also studied.
Although mirror neurons have recently been discovered, there is an abundance of
research examining the function and process of these neurons. What we know about them
is rapidly increasing and the sooner we identify their role, the more we understand motor
learning and empathy, as well as organic causes and symptoms of ASD. Understanding
the relationship between mirror neuron function and sex will further current knowledge
of ASD, as well as sex differences in empathizing with others.
5
II. LITERATURE REVIEW
Mirror Neurons and Behavior
The relevance of mirror neurons is far reaching, with numerous studies examining
their role in motor learning, communication, and empathy. The most palpable role that
the neurons play is their role in motor learning. If mirror neurons comprise the
neurophysiological center transforming visual information into motor commands, then
they would indeed play a significant role in observational learning (Lago-Rodriguez,
Lopez-Alonso, & Fernández-del-Olmo; 2013). Observational learning has proven to
improve an observer’s efficiency in execution strategy and yields better performance than
physical practice alone (Al-Abood, Davids, & Bennett, 2001; Buchanan & Dean, 2010).
Observational learning incorporates imitation, which is an important development
technique in children, and mirror neurons are proposed as the mechanism behind this
method of motor learning (Braadbaart et al., 2013).
Related to motor learning, there is existing research supporting the idea that
mirror neurons are responsible for action identification and social learning (Burgess,
Arnold, Fitzgibbon, Fitzgerald, & Enticott, 2013). Studies have shown a positive
correlation between empathy self-report and MNS activation, as well as observations of
impaired imitation and empathy in ASD (Baird, Scheffer, & Wilson, 2011; Hunter,
Hurley, & Taber, 2013).
6
Mirror Neurons and Empathy
Empathy allows us to relate to others and our environment. If mirror neurons are
the mechanics behind this function, then they are essentially the key to what makes us
who we are by defining our interactions with others. While some psychologists support
this idea, it is still highly debated. Since the discovery of mirror neurons in humans,
research in the field of cognitive empathy has exploded. However, it is difficult to draw
supported conclusions with the little amount of understanding we have. Several theories
have been presented to explain the function and role of the MNS in humans.
The first individual to bring the idea of empathy to light was Theodore Lipps in
1903 (Montag, Gallinat, & Heinz, 2008). Empathy was perceived as how we recognize
the mental states of other people (Montag et al., 2008). Although the idea has evolved
over time, empathy is still defined as “the experience of understanding another person’s
condition from their perspective” (“Psychology Today,” 2013). In today’s field, there are
three different forms of empathy: cognitive empathy, motor (sensory) empathy, and
affective (emotional) empathy.
Cognitive empathy is known as the ability to recognize and attribute mental states
to others (Edele, Dziobek, & Keller, 2013). It can be defined as the ability to recognize
others intentions during their actions. Experiencing emotional states of others can lead to
an understanding of reasons for action, leading to identification of the behavior intentions
(Corradini & Antonietti, 2013). This form of empathy is based on our ability to (a)
process the actions of others through observation of their behavior and facial expressions
and (b) assign an emotional state in order to determine their intention. Cognitive empathy
7
is used in conversing with others. By observing facial expressions and posture, it is
possible to interpret another’s interest in the topic.
Motor, or sensory, empathy is the phenomenon of experiencing similar sensory
input as the individual experiencing the stimulus firsthand (Loggia, Mogil, & Bushnell,
2008). This is the form of empathy behind experiencing sensory pain when observing an
individual who is experiencing a type of pain. A matching sensory experience is
suggested to be the response of activation of the MNS during observation, as well as
during the immediate experience of the pain (Avenanti, Bueti, Galati, & Aglioti, 2005;
Hutchison, Davis, Lozano, Tasker, & Dostrovsky, 1999). This explains why we may
cringe when observing an individual trip or stub their toe.
Affective, or emotional, empathy is known as “the experience of feelings
congruent with another’s emotional situation” (Edele et al., 2013). This experience is also
referred to as emotional contagion, when you feel physically along with the other person
as if their emotions are contagious. An example of this would be crying in response to a
scene of a movie in which the character is sad or crying. Emotional empathy is a
connection between individuals resulting in a genuine concern for the other’s well being.
Understanding how these different forms of empathy function could potentially
illuminate the role of the MNS in the process of empathizing with others. The MNS is the
proposed biological center for humans’ use of empathy during interactions with others.
Evidence for this idea comes from (a) behavioral studies that examined the link between
imitation and empathy, (b) function neuroimaging studies that reveal a positive
correlation between mirror neuron activation and self-reported empathy scores, and (c)
8
observations of impaired imitation and empathy in individuals with ASD. These studies
provide insight into how activation of the MNS is expressed in human behavior.
The MNS seems to respond to observation of others' actions and processes the
visual information into identifiable goals by establishing a relationship between the
action and corresponding intentions (Corradini & Antonietti, 2013). This function can be
applied to both cognitive and affective empathy. Facial expression is a commonly used
communication technique allowing us to nonverbally express our thoughts and emotions.
An individual with a functioning MNS would be capable of identifying facial cues and
ascribing a specific intention or emotion to each expression.
Emotionally expressive facial expressions demand subtle shifts in expression,
which is why they are often used as a stimulus of imaging studies of mirror neurons to
identify the role of these neurons in social cognition (Corradini & Antonietti, 2013). A
study conducted using preadolescent participants supported the idea that the MNS is
linked to both the subjective experience of emotion as well as the identification of the
same emotion in other individuals (Pfeiffer, Iacoboni, Mazziotta, & Dapretto, 2008).
Functional MRI results revealed that activation of the MNS in subjects during
observation and imitation of facial expressions was positively correlated with the level of
empathetic skill (Pfeiffer et al., 2008). This supports the idea that the MNS is involved in
everyday social functions.
Empathy is part of a group of related processes, including imitation. Imitation is
an observational learning technique that has been linked to function of the MNS. A study
examining imitation of facial expressions revealed a correlation between imitation
accuracy and empathetic ability (Braadbaart, de Grauw, Perrett, Waiter, & Williams,
9
2014). Effective imitation of emotion in another's facial expression requires the observer
to have a previous understanding of the relationship between the emotional state behind
the expression as well as the motor plan for producing the expression (Braadbaart et al.,
2014; Carpenter, 2006).
Instances in which individuals lose the ability to imitate others provide further
support for a link between observation and perception in emotional recognition. Emotion
recognition was examined in individuals prevented from imitating muscle contraction in
facial expressions (Niedenthal, Barsalou, Winkielman, Krauth-Gruber, & Ric, 2005).
Participants’ accuracy in identifying the expressed emotion was significantly diminished.
Further support comes from individuals with Moebius syndrome in which the ability to
move facial muscles is hindered (Cole, 2001). Individuals with this syndrome were
unable to recognize the emotions expressed by others.
Considering the different types of empathy, mirror neurons may serve as the
unifying framework. Neuroimaging studies support this concept, revealing that in
different neural regions of the brain, pathways activate independently depending on the
type of empathy being observed (Bien, Roebroeck, Goebel, & Sack, 2009; ShamayTsoory, Aharon- Peretz, & Perry, 2008). Correlations have also been found between grey
matter in these mirror neuron areas and empathetic behavior (Cheng, Tzeng, Decety,
lmada, & Hsieh, 2006). A positive correlation was discovered between grey matter
volume and empathetic traits, which suggests that the MNS is involved in development of
empathetic traits (Cheng et al., 2009).
Chartrand and Bargh (1999) conducted a study observing unconscious motor
mimicry action, which they called the “chameleon effect.” Chartrand and Bargh observed
10
individuals’ tendency to automatically become engaged in behavior observed previously
in another. They considered this reaction to be a “perception-behavior” link. These
mimicked behaviors included actions as shaking one's foot and facial expressions such as
smiling. They considered this effect to be a passive and cognitive process requiring very
little association with an affective or an emotional state. They discovered a higher
frequency of motor imitation in individuals with higher cognitive empathy scores. Their
findings provide evidence for a link between observation and perception in regard to
empathy.
Further evidence for the MNS’s role in the different types of empathy comes from
imaging studies of motor empathy’s involvement with pain and disgust. Functional MRI
results revealed that the processing of one’s own emotional experience and the
observation of another individual undergoing a similar experience may engage
overlapping neural matter (Singer et al., 2004; Wicker et al., 2003) The results also
showed the relative activation of these neural networks correlated with participants'
individual differences in empathic abilities (Singer et al., 2004). These studies provide
evidence that the relationship between observation and perception involves empathy.
Discovering the neural networks for empathy is important to the field of mirror
neuron research and has supported the idea that the MNS is involved in the empathy
process. There is still limited knowledge of mirror neurons ascribing an emotional or
mental state to another individual. A fMRI study attempted to identify the differential
neural networks involved in “self” and “other” related attribution of emotional states
(Schulte-Rüther, Markowitsch, Fink, & Piefke, 2007). The researchers of the study
discovered attribution of emotion either to a facial expression or to the self activates brain
11
areas involved in emotional processing, as well as the MNS. Their study indicated that
mirror neuron mechanisms attribution of emotions in the self in the process of empathetic
intrapersonal cognition. These results provide support of the MNS’s involvement in
social cognition, including detection and identification of intent, emotional states, or
empathy. The MNS’s function in emotional social interactions substantiates the neural
origin of the observer’s resonance of the emotional state of another individual.
The current understanding of the MNS suggests that it is broadly involved in the
empathy process. The future for MNS research is to identify its role in the forms of
empathy, including motor, emotional, and cognitive empathy. Further research needs to
be conducted in order to identify the function of the processes involved with MNS and
empathy.
Sex Differences in Empathy
Culturally, it is a common belief that women are the more empathetic and
emotional of the sexes, and behavioral research supports this belief. Women’s
interactions tend to be more intimate than men’s, despite equal levels of motivation for
empathy in both sexes (Clark & Reis, 1988). Secondly, considerable research on
cognition has revealed that women tend to be superior in verbal and social abilities
(Halpern, 2000; Wakabayashi, Sasaki, & Ogawa; 2012). Women consistently
demonstrate greater accuracy in identifying and interpreting non-verbal cues than the
opposite sex (Hall, 1978). Finally, support for the belief that women are the more
empathetic sex was largely based on self-report data on empathy quotients, which may
have resulted in a bias due to the culturally supported idea that women were more
12
sensitive to emotions (Groen, Wijers, Tucha, & Althaus, 2013). Imaging techniques a
nonbiased support to research sex differences in empathetic abilities.
Recent neuroimaging studies, which remove the aforementioned bias, have only
begun to illuminate sex differences in empathetic ability and their underlying neural
mechanisms. Currently, the research focuses on sex differences in cognitive empathy,
including identification, inference, and imagination of the observer's feeling while
observing expressions of another individual (Schulte-Rüther, Markowitsch, Shah, Fink,
& Piefke, 2008). An imaging study required male and female participants to indicate the
extent of their emotional response to images exhibiting pain and suffering, in different
contexts, compared to emotionally neutral images of scenes and landscapes (Mercadillo,
Diaz, Pasaye, & Barrios, 2011). The results of this study revealed that females had more
activation in the brain regions related to cognitive empathy than their male counterparts.
As information of mirror neurons has increased, researchers have become better at
designing studies that identify and isolate mirror neuron activation providing better
insight into sex differences. Research examining sex differences in mirror neurons
reveals significant differences between the sexes; females showed significantly more
suppression of mu rhythm than male participants (Cheng, Tzeng, Decety, Imada, &
Hsieh, 2006). A similar study revealed both males and females exhibited activation
related to pain empathy, but females showed greater mu suppression for pain empathy, as
well as in non-pain simulations (Yang, Decety, Lee, Chen, & Cheng, 2008). This finding
supports the existence of sex difference in MNS activation.
A recent event-related potential (ERP) study examined sex differences in brain
activation to congruent and incongruent actions and attempted to isolate attention bias by
13
adding a second task. The secondary task separated selective attention from the actionprocessing task, which was automatic rather than guided by subjective evaluation
(Proverbio, Riva, & Zani, 2010). Females showed a significantly greater response in the
neural areas of the cortex associated with mirror neurons when viewing congruent
actions. The results of this study suggest that action-processing differs between males and
females. This data provides evidence that there exists a sex difference within the actionprocessing center of the brain, the MNS.
In order to determine the function of the mirror neurons in the human brain, it is
important to recognize how they may differ between the sexes. Clearly defining how
these neurons differ could unlock the potential to understand how and why they work to
connect us with others. Further evidence comes from individuals with a diminished MNS
function. Individuals with autism have a dysfunction in the MNS as well as an inhibited
ability to empathize with others.
Autism and Mirror Neuron Dysfunction
Autism or ASD is characterized by deficits in three areas: social interaction,
verbal communication, and non-verbal communication (Gallese et al., 2013). Among the
symptoms concerning these deficits is the diminished ability to empathize with others,
and impaired ability to interpret the actions and intentions of others. However, certain
characteristics remain prevalent. Exploring the connection between autism and MNS
function furthers the understanding of how symptoms of ASD arise.
Currently, the neurobiology behind ASD is little understood, but recent studies
suggest dysfunction of the MNS may be responsible for social-relating characteristics of
the disorder (Enticott et al., 2012). The broken mirror theory of autism has been proposed
14
as an explanatory model for these symptoms. The broken mirror theory suggests that a
dysfunction of the MNS may result in the chief diagnostic signs of ASD, which are social
isolation, lack of eye contact, poor language capacity, and absence of empathy
(Ramachandran, & Oberman, 2006). Mirror neurons have been implicated in perception
of other’s intention, interpretation of emotion, and empathy (Ramachandran & Oberman,
2006).
Evidence for the broken mirror theory comes from using imaging techniques to
compare activation in individuals with and without ASD. An electromyography (EMG)
recording study used healthy children and children with ASD as participants to examine
MNS activation during observation (Cattaneo et al., 2007). Observation of an action led
to mirrored activity in healthy participants, but this response was absent in those with
ASD. This study suggests a deficit in motor chains to understand intention.
EEG recordings have revealed disruption of mu rhythm suppression in individuals
with ASD. An early study conducted comparing individuals with ASD to a healthy
control group revealed intact mu suppression during execution (Oberman et al., 2005).
This response was absent in ASD participants during observation of action execution
(Oberman et al., 2005). Replication of this study revealed similar results and additionally
revealed a correlation between mu attenuation and imitative abilities (Bernier, Dawson,
Webb, & Murias, 2007).
Investigation into the MNS’s role in the symptoms of autism may offer an
explanation for the development of the disorder. Research suggests there are sexdifferences in the activation of mirror neurons and ASD has been linked to a dysfunction
of the MNS. Examining sex differences in ASD could further current understanding of
15
the MNS’s role in how symptoms of the disorder may arise. This could provide
information necessary for improving the course of diagnosis and potential treatments.
Sex Differences in Autism
The majority of individuals diagnosed with ASD are male. The current statistics
regarding male to female ratio of ASD is estimated to range from 5.5:1.4 to 16.8:4.0
(Center for Disease Control and Prevention, 2007). Examining the current evidence for
sex differences in mirror neuron function could lead us to evidence of why this disorder
tends to affect males more than females. Moreover, the broken mirror theory of ASD
proposes that MNS dysfunction is responsible for the core social and cognitive deficits in
individuals with ASD.
Sex differences in ASD symptoms are variable, but general patterns surface.
Females with ASD tend to have greater communication deficits than males, who tend to
experience more restricted, repetitive, and stereotyped behaviors than girls (Hartley &
Sikora, 2009). Young males with ASD were found to show an increased amount of
unusual motor movements and abnormal use of their body (Hartley & Sikora, 2009; Lord
et al., 1982). Sex differences were most prominent in young children (Hartley & Sikora,
2009). Using IQ as a control, a study of sex differences in ASD, conducted by Lord et al.
(1982), observed higher rates of restricted or repetitive behaviors, and inappropriate play
in males. A similar study conducted by McLennen, Lord, and Schopler (1993) yielded the
same results, but it was also observed that early adolescent males tended to display
greater deficits in reciprocal social interaction and communication.
Despite the research supporting the idea that there are sex differences in ASD,
there are few studies that focus on females with the disorder. The existing data suggests
16
that there are subtle differences between the sexes that could play a significant role in the
differential way that ASD manifests itself in males and females (Hartley & Sikora, 2009).
Further research with males, however, has revealed results that support the broken
mirror theory of ASD. A study of the effect of MNS activation during a perceived task
revealed a dysfunction in individuals with ASD (Cattaneo et al., 2007). The study
examined the normal activation of the MNS during task observation and task
performance and compared it to activation in individuals with ASD. In healthy
individuals, the observation of the action led to mirrored activation within the MNS, but
this mirrored activation was absent in individuals with ASD. The absence of activation in
individuals with ASD suggests a deficit in linking or representing motor chains for
understanding intention.
Mindblindness is defined as a lack of a “theory of mind” when one understand the
perspective (thoughts and feelings) of another. Having mindblindness is essentially the
inability to empathize with others (Baron-Cohen, 2011). Mindblindness is frequently
labeled as a symptom of ASD. Some individuals with the disorder severely lack the
ability to recognize and interpret the actions of others (Baron-Cohen, 2011). Current
studies involving theory of mind and ASD are attempting to provide concrete proof that
mindblindness is linked to the function of the MNS. A severe dysfunction in the MNS
has been observed in individuals with complete mindblindness, as well as a slight
dysfunction of the MNS in individuals who have difficulty interpreting explicit actions
(Baron-Cohen, 2011). This evidence supports the idea that mirror neurons play a role in
how we interpret and attribute others' emotions and actions.
17
The role of mirror neurons in ASD is still little understood. Researchers have only
managed to scrape the surface of both the MNS and ASD. However, there is a
considerable amount of evidence that supports the broken mirror theory of ASD.
Continued research will hopefully lead to a better comprehension of the disorder, as well
as potential for better methods of treatment and diagnosis.
18
III. DISCUSSION
Discussion of Past Research
Research has confirmed the presence of the MNS in the brain and has explored
its role in various forms of empathy. Whether or not the MNS is the neurological basis
behind the process of empathy has yet to be confirmed. Location of the MNS has found
to be extensive throughout the brain and activation tends to vary according to the form of
empathy being observed.
Although the link between the MNS and empathy has yet to be supported,
differences between the sexes suggest its role. Females are considered to be the more
empathetic sex. Correlation studies reveal a link between high self-reported empathy and
stronger MNS activation. While research continues to clarify the role of the MNS and
empathy, it does suggest that females tend to be the more emotionally intelligent sex.
Research supports the idea that females are more efficient at interpreting and
comprehending actions of others than their male counterparts.
Activation of the MNS in a wide range of action and observation of performance,
such as facial expression, interpretation, and motor mimicry, strongly suggests that the
MNS serves a neurological basis for action understanding. The activation of the MNS to
variable degrees for a variety of actions indicates the location of the neurons may serve a
19
specific purpose in identification. Location of mirror neurons in the brain may account
for the different types of empathy.
Studies conducted using individuals with ASD correlate the dysfunction of the
MNS in individuals with the disorder. A key symptom of ASD is the diminished ability
to comprehend or interpret another’s actions or intent. The correlation between the
dysfunction of the MNS and diminished empathetic ability suggests that the MNS plays a
significant role in empathy. Considering the significant difference in the prevalence of
ASD between the sexes, as well as increased activation of MNS in females compared
with males, sex differences in the MNS could potentially play a role in the development
of the disorder.
Implications and Applications
Identifying the sex differences of the MNS could possible provide insight into the
prevalence of ASD in males and its expression of symptoms between the sexes. ASD
tends to express itself differently among individuals, including between the sexes. The
current method of diagnosis relies on chief diagnostic signs, such as social isolation, lack
of eye contact, poor language capacity, and absence of empathy. Defining how these
symptoms express themselves between the sexes would lead to earlier, more efficient
forms of diagnosis.
Determining the role of the MNS in empathy could possibly provide new
information on how autism and its symptoms may arise. This knowledge could also
provide the information necessary to improve current treatment methods of the symptoms
of ASD. A current form of treatment, known as mirror neuron therapy, has had success in
improving the social functioning of an individual with ASD. This form of therapy
20
attempts to stimulate the regions of the brain overlapping with the MNS to strengthen
neural connections (Wan, Demaine, Zipse, Norton, & Schlaug, 2010). A study conducted
using music as the method for activating MNS found success in this form of therapy.
Individuals undergoing mirror neuron therapy were shown to have increased
communication, even in non-verbal individuals with ASD, and improved motor function.
The broken mirror theory provides a possible explanation for symptoms of ASD.
Evidence of the MNS’s role in action-understanding and empathy as well as evidence of
a dysfunction in individuals with ASD provide the main support for this theory. This
theory provides much insight into the symptoms of ASD, as well as a potential
explanation for how they may arise in individuals. However, it is unlikely that MNS
dysfunction alone can provide the answer for why this disorder may occur. It is necessary
to continue to explore the validity of the broken mirror theory, as well as the role of the
MNS in empathy and behavior.
Future Research
Science is far from determining the cause of ASD, but there is a growing need for
more information. The estimates for children affected with autism have been increasing
each survey year. It is currently estimated that one child in 68 has been identified with
autism (Baio, 2014). ASD and its relationship to the function of the MNS should continue
to be studied. The limitations of existing studies of MNS function often are small sample
sizes. A comprehensive study should be conducted in order to better identify the sex
differences in MNS function, furthering the knowledge of sex differences in empathy.
There is a lack of recent information on sex differences relating with symptoms of
individuals with ASD. There are very few studies that explore the relationship between
21
sex differences and ASD symptoms in depth. There is a significant difference in the ratio
of males and females diagnosed with the disorder, so few studies of autism represent both
sexes equally. Future studies should attempt to represent both sexes. Considering females
are diagnosed later than males, it is important to determine how symptoms of ASD
express themselves between the genders in order to improve current standards of
diagnosis.
Studies regarding the current methods of diagnosis are necessary in order to
improve their efficacy. The cost of behavioral treatment can also vary between $40,000
to $60,000 per child annually (Lavelle et al., 2014). These costs could potentially be
reduced if diagnosis and implementation of treatment occur earlier in children with the
disorder. The total social cost for care of children with ASD was an estimated 9 billion
dollars for 2011 (Lavelle et al., 2014). Considering this cost, it is necessary to increase
our understanding of autism in order to improve treatment and decrease costs of care.
Conclusion
The knowledge we have of the relationship between mirror neuron function,
empathy, and autism is far from complete. As research continues to explore the
relationship between mirror neuron function and empathy, the role of the MNS in
symptoms of autism will become more defined. Examination of previous research
suggests sex differences in MNS function, which could be potentially be related to
prevalence of autism in males. Future research regarding symptoms of autism in females
needs to be conducted before the existence of this relationship can be assumed.
22
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