Mirror neuron functioning: an explanation for gender differences in empathy?

Transcription

1 Author: Marja Nab, Supervisor: Drs. A. H. M. van Boxtel Bachelor thesis in cognitive Neuroscience Department Psychology and Health, Cognitive Neuroscience, Tilburg University June, 2010

2 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 2

3 Table of contents Abstract... 2 Table of contents The mirror neuron system in humans Evidence for a mirror neuron system in the human brain Conclusiveness of the found evidence Measuring the level of mirror neuron activity The importance of mirror neurons in everyday functioning Mirror neurons and imitation Mirror neurons and empathy Mirror neurons and autism spectrum disorders Gender differences The autistic male brain Discussion Summary References

4 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 4

5 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 μ- rhythm has been proposed. The μ-rhythm is a specific α-rhythm that can be measured with the 5

6 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. 6

7 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 7

8 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. 8

9 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. 9

10 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 (Baron- Cohen, 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 10

11 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 μ- rhythm 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, 11

12 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. 12

13 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 μ- rhythm 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 13

14 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 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 14

15 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. 15

16 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 Baron- Cohen, 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 Baron- Cohen 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 extrem -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. 16

17 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

18 Table 1. Mirror neuron functioning: an explanation for gender differences in empathy? Comparison of three different studies concerning mirror neuron functioning Design Results Authors Experimental group Control group Imaging method Conditions Results relative to control group Dapretto 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) Hadjikhani Autistic patients Healthy, Magnetization - Cortical thinning; among others, in (14) matched control Prepared Rapid IFG (pars opercularis), IPL and STS. group (14) Gradient Echo Schulte- Rüther Males (12) Females (14) fmri scan Other condition: assessing shown emotional expressions (anger or fear) Self condition: assessing own emotions elicited by the shown expressions Other: weaker activation in IFC. Self: weaker activation of right IFC, right STS and right cerebellum. Stronger activation of the left TPJ. 18

19 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 19

20 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

21 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 Baron- Cohen (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

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