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The role of the mirror neuron system in action understanding and empathy Bachelor thesis Cognitive Neuroscience Bachelor program Psychology and Health Department of Psychology Dirk van Duijnhoven ANR: 985752 March 2010 Supervisor: Bernard Stienen Summary Mirror neurons have been linked to action understanding in monkeys and humans. It has even been linked with empathy in humans. This review focuses on the literature on mirror neurons and the function of these neurons. First of all, the discovery of mirror neurons in Macaque monkeys and the possibility of the existence of mirror neurons in humans are described. Subsequently, the role of the mirror neurons in action understanding and in empathy is discussed. Based on the evidence that actions can be understood without the mirror neuron system, this paper concludes that it seems unlikely that the mirror neuron system is the underlying system for action understanding in monkeys and humans. Hence, it is also not likely that the mirror neuron system is the underlying system for empathy in humans. Mirror neurons could be formed through experience as a byproduct of associative learning. However, it cannot be excluded that the mirror neurons do enrich the experience of observing an action or emotion. Keywords: Mirror neurons, action understanding, empathy, mirror neuron system. Contents §1 Introduction 1 §2 Mirror Neurons in Macaque Monkeys 1 §2.1 Properties of the Mirror Neurons 2 §2.2 The Mirror Neuron System 3 §2.3 The role of the Mirror Neuron System in the Macaque Monkey 4 §3 Mirror Neurons in Humans 5 §3.1 Mirror Neurons in Humans 6 §3.2 Similarities and Differences between the Human and Macaque 8 Monkey’s Mirror Neuron System §4 Action Understanding and the Mirror Neuron System §4.1 The Possible Role of the Mirror Neuron System in 9 9 Action Understanding §4.2 Criticism on the Role of the Mirror Neuron System in 10 Action Understanding §5 Empathy and the Mirror Neuron System 13 §6 General Discussion 15 Summary 17 Literature 18 §1 Introduction Since the discovery of mirror neurons in Macaque monkeys there has been a lot of speculation about the possibility of a similar mirror neuron system in humans and the role of these neurons. One possible function of the mirror neurons could be the understanding of actions made by others. The linkage of motor and sensor areas of the brain could be the essential link in order to understand what somebody else is doing and for what purpose he or she is doing it. Some researchers have also suggested that empathy in humans is based on this mirror neuron system. For example, individuals with Autism seem to have problems with social recognition and empathy. Deficits in the mirror neuron system could be at the root of this problem. This thesis is aimed to investigate mirror neurons and its possible role in action understanding and empathy. In this paper it will be attempted to give an overview of the theories on this particular subject and to give some insights on the mirror neuron system functioning. First of all, the finding of mirror neurons in Macaque monkeys will be discussed together with its possible function. Subsequently, the possibility of human mirror neurons will be reviewed. Finally the role of the mirror neurons in action understanding and empathy will be discussed. §2 Mirror Neurons in Macaque Monkeys The frontal cortex of monkeys consists of several areas, among which the F5 area is especially of interest for its possible homology to Broca’s area in humans (Gallese, Fadiga, Fogassi, & Rizzolatti, 1996; Rizzolatti, Fadiga, Gallese & Fogassi, 1996). F5 is located in the ventro-rostral part of area 6. Stimulation and single cell recordings of neurons within this area have been conducted and showed that it is associated with hand and mouth movements (Hepp-Reymond, Hüsler, Maier & Qi, 1994; Rizzolatti, Scandolara, Gentilucci & Camarda, 1981). In F5 hand movements are represented more in the dorsal part and mouth movements are represented in the ventral part. Rizzolatti et al. (1996) found in Macaque monkeys that some neurons in the F5 area both fired during conduction of active movements and during observation of the same movements made by the experimenter. A neuron with these particular properties is called mirror neuron. 1 § 2.1 Properties of the Mirror Neurons Gallese et al. (1996) conducted a single cell recording study in the F5 area, in which they found that activation of mirror neurons was only caused by actions in which a hand or mouth interacted with an object. Hand movements that included grasping, manipulating and placing objects caused the strongest mirror neuron activation. The showing of objects alone was not effective in activating the mirror neurons, even if they were interesting stimuli like food items. Emotional gestures were also not effective. These gestures involved threatening the monkey and the displaying of unpleasant items. The use of tools, even if they were similar to the meaningful hand movements, was not effective. If the monkey saw the experimenter use pliers to grasp an object it did not activate the mirror neurons. Rizzolatti and Arbib (1998) showed that the mirror neurons did fire after repetitive observation of a meaningful action with a tool. Gallese et al. (1996) also showed that it did not matter if the monkey executed an action in a well-lit environment or in a dark environment. Therefore the mirror neurons in the monkey did not merely fire while observing its own actions, because it could not see the actions in the dark environment. This provided evidence that mirror neurons did not exclusively respond to observed actions, but also to the execution of actions. Gallese et al. (1996) and Rizzolatti et al. (1996) showed that there is a difference in the degree of congruence between mirror neurons. Strict congruent mirror neurons have a very specific connection between the motor properties of the neuron and the visual properties of the neuron. The relation in terms of general action between the observed and executed movement, for example grasping with the hand, is strict. Furthermore, they also have a strict relation in the manner in which the action was executed. An example is a neuron that fires only when a precision grip movement is performed with the hand. The other end of this spectrum consists of mirror neurons that have no connection between their visual and motor properties. They are noncongruent. These mirror neurons respond for example both to the observation of mouth and hand actions. The mirror neurons that are congruent, but not strict, are considered as broadly congruent. An example of a broad congruent mirror neuron is a neuron that fires both when a precision finger grasping movement is executed and when it is observed, but also fires when a whole hand grasping movement is 2 observed. Research has shown that 92% of mirror neurons are congruent, approximately 30% are strict congruent, 60,9% is broadly congruent and 7,6% are non-congruent. The discharge of mirror neurons is not dependent of the distance of the observed action. It has no effect on the discharge of mirror neurons if an observed action is presented far away from or close by to the observer (Rizzolatti & Graighero, 2004). § 2.2 The Mirror Neuron System in Macaque Monkeys Rizzolatti and Graighero (2004) state that other areas in the cortex of the Macaque monkey besides F5 were found to have neurons that respond to observed actions made by others. They suggest that there is a mirror neuron system (MNS) that has different cortical areas that contain mirror neurons. First of all, the superior temporal sulcus (STS) has neurons that respond to observed actions. This area sends output to the ventral premotor cortex, including F5. In comparison with mirror neurons in F5 however, the STS neurons do not have motor properties. Another cortical area where neurons were found that respond to actions made by others is area 7b. It is located in the rostral part of the inferior parietal lobule. It receives input from STS and sends output to the entire ventral premotor cortex. Some neurons in the 7b area do have mirror neuron qualities. The MNS thus includes: The rostral part of the inferior parietal lobule (7b) and the ventral premotor cortex (including F5). STS, although related because of the input it sends to the MNS, is strictly not a part of the MNS because it has no mirror neurons. Figure 1 shows the monkey’s cortex and the areas of the MNS. 3 Figure 1. The cortex of the Macaque monkey. Area F5 and 7b make up the core of the MNS. The STS is linked with the MNS, but not a part of it. Adapted from Rizzolatti et al. (1996). §2.3 The role of Mirror Neuron System in the Macaque Monkey Looking at the possible functional role of the MNS, Gallese et al. (1996) and Rizzolatti et al. (1996) proposed three possibilities. The first possibility is the preparation of movement. The preparation of the execution of an action could benefit from the fact that this action is observed before executing it. This makes it possible to execute the action as fast as possible. However, if the mirror neurons were related with the preparation of movement, they would continue firing between the observation and movement. According to Gallese et al. (1996) the mirror neurons do not continue firing in this period, thus it does not seem likely that the MNS is involved in the preparation of movement. The second possibility is learning through imitation. Human adults and children both learn trough imitation. Imitation could be based on a mechanism that links the observation and execution of an action. Mirror neurons could represent this link (Gallese et al., 1996). A study focused on imitation of Macaque monkeys and found that newborn Macaque monkeys do imitate facial gestures, but only for a short period of time. The MNS in macaque monkeys therefore could be a system that provides a role in early imitation. However, it is not evident that this role is still of importance in learning at a later age (Ferrari, Visalberghi, Paukner, Fogassi, Ruggiero & Suomi, 2006). Finally, a possible function is the understanding of observed action. If an observed action evokes a neural activity, which is the same activity that would be produced when one executes a similar action, the meaning of the observed action should be recognized. The similarity between the 4 neural activities could be the mechanism behind this recognition. A study provided evidence in favor of the action understanding theory. Even while a meaningful object was hidden and the last part of a movement could not be seen, some mirror neurons still fired. The monkeys saw beforehand if there was an object present or not. Mirror neurons did not fire when the object was not present. This shows that the motor representation of a meaningful action performed by another can be generated internally in the premotor cortex of the observer (Umiltà, Kohler, Gallese, Fogassi, Keysers & Rizzolati, 2001). In addition, researchers found that mirror neuron activity was also present when a sound was played that previously was paired with an action corresponding with the sound, e.g. ripping a piece of paper (Kohler, Keysers, Umiltà, Fogassi, Gallese & Rizzolatti, 2002). Possibly, the motor properties of that particular action were simulated in the monkey’s brain when only the sound was presented. The visual stimuli were not needed. Kohler et al. (2002) concluded that therefore the monkey ‘understood’ the action. However, it is difficult to decide whether or not the monkey really understood the action, or that it had associated the sound with the action through classical conditioning. The evidence is indirect and it is difficult to say when a monkey really understands an action. §3 Mirror Neurons in Humans If indeed the mirror neurons in monkeys could be at the base of action understanding, it could also be the case in humans. It could even be at the core of more complex cognitive functions that makes humans understand each other and feel empathy. The first thing to investigate is whether or not there are in fact mirror neurons in the human cortex. The possibility of mirror neurons in humans was already proposed in the first studies concerning mirror neurons of the Macaque monkey (Gallese et al., 1996; Rizzolatti et al. 1996). So far, there are no single cell recordings of mirror neurons in humans documented. Therefore there is no direct evidence of the existence of mirror neurons in humans. There is, however, indirect evidence of the existence of mirror neurons in humans (Rizzolatti & Graighero, 2004). Neurophysiological experiments show that the motor cortex becomes active when observing others performing an action, without the presence of overt motor activities (Rizzolatti & Graighero, 2004). 5 §3.1 Mirror Neurons in Humans Fadiga, Fogassi, Pavesi and Rizzolatti (1995) found through Transcranial Magnetic Stimulation (TMS) evidence in favor of the existence of mirror neurons in humans. TMS is a non-invasive method of stimulating the nervous system. Through this electric stimulation of the motor cortex they found enhanced motor evoked potentials (MEPs) distinct from spontaneous potentials in hand and arm muscles of normal human subjects while observing movement. The enhanced MEPs were only found during conditions in which movement was shown. An interesting fact was that it did not matter if the movements were transitive (movements with objects) or intransitive (meaningless movements without objects). The increased potentials were only found in the muscles that are active if a subject would execute the observed action. When subjects merely observed the objects used in the transitive movement condition or a dimming spotlight no MEPs were found. The dimming spotlight condition was added to show non-human movement. Maeda, Kleiner-Fishman and Pascual-Leone (2002) confirmed these findings in a TMS study that focused on simple non-goal directed movements. They found that there was enhancement of MEPs during observation of the action that was to be executed. Seeing the movement of the index finger produced a larger MEP in the muscles that would facilitate this movement. This provided evidence that the observation of movements activates the motor cortex. In an EEG study, Cochin, Barthelemy, Lejeune, Roux and Martineau (1998) found that while observing human motion there was a desynchronization of the socalled Mu-rhythm of the spectrum of brain waves in the human cortex. This desynchronization was not observed while observing a still scene (lake) or non-human movement (waterfall). The desynchronization of the Mu-rythm is associated with motor activity (Cochin et al., 1998). This study indicated that the motor cortex is activated while observing human movement, but not while observing non-human movement. To investigate why there is no overt muscle activity mimicking the observed action, Baldissera, Cavallari, Craighero and Fadiga (2001) focused on spinal cord excitability. During the activation through TMS of a stretching reflex in a finger muscle of the subject, the same or opposite action as the reflex was observed by the subject. The same action was opening of the hand and the opposite action was closing of the hand. The observation of the same action as the reflex caused the reflex to 6 decrease in size. The cortical excitability increased when the same action was observed, the spinal chord decreased. The spinal chord excitability therefore contradicted the cortical excitability. This suggests that there is a spinal mechanism that prevents the automatic overt mimicking of an observed action. Thus, the MNS can be active without interfering with the normal muscular activity. Jeannerod (2001) states that an inhibitory system could be present at the spinal chord level that coexists with the cortical MNS to suppress overt movements. The previous mentioned studies suggest that there is a MNS in humans. The evidence is indirect, but Rizzolatti and Graighero (2004) state that it is likely that there is an MNS in humans such as in Macaque monkeys. They proposed that the core of the MNS in humans consists of two main regions. The first one is the rostral part of the inferior parietal lobule (IPL) and the second region is located in the premotor cortex, containing the lower part of the precentral gyrus and the posterior part of the inferior frontal gyrus (IFG). The latter contains area 44, or Broca’s area. These areas are predominantly motor areas, but are also active during observation of movement (Rizzolatti & Graighero, 2004). Figure 2 shows the human MNS. Figure 2. The human MNS, with the STS. Adapted from Iacoboni and Dapretto (2006). The two main regions correspond with the regions of the MNS found in monkeys. It is plausible that the IPL corresponds with the PF area in monkeys (Rizzolatti & Craighero, 2004). F5 in monkeys is considered homologue to Broca’s Area (Gallese et al., 1996). Therefore the areas of the MNS are consistent with the areas of the Macaque monkey. The next step is to look at the properties of the human MNS and the possible differences between the human and Macaque MNS. 7 §3.2 Similarities and Differences between the Human and Macaque Monkey’s Mirror Neuron System Buccino et al. (2001) focused through fMRI on different parts of the premotor cortex and the parietal lobule during the observation of a variety of movements. They found that the MNS in humans is spatially organized, for a large range of movements. For example, the same areas that were activated in the premotor cortex during an execution of an arm movement are also activated during the observation of the same arm movement. This is consistent with the amount of congruence in the monkey’s mirror neurons (Gallese et al., 1996; Rizzolatti et al., 1996). It suggests that the MNS is spatially organized and codes for specific actions made by specific effectors. Buccino et al. (2001) also found that the IPL was only activated when a transitive action was observed. An observation of a mimed or intransitive action did not activate this area. The mimed action was the same action as the object-related action, but without the object. This result supports the view that the parietal lobule has a fundamental role in the description of objects for action (Jeannerod, 1994). It is also consistent with the observation that the Macaque monkey’s MNS is only active during the observation of transitive movements (Gallese et al., 1996; Rizzolatti et al., 1996). However, Buccino et al. (2001) did find IFG activation during the observation of mimed actions. Also, Maeda et al. (2002) found that the observation of intransitive movements and transitive movements both showed enhanced MEPs. These results are consistent with the earlier findings of Fadiga et al. (1995). This is a major difference between the human and Macaque monkey’s MNS; the monkey’s MNS did not respond to intransitive movements at all (Gallese et al., 1996; Rizzolatti et al., 1996). The human brain area IFG, corresponding with F5, does respond to intransitive actions. This difference between the Macaque monkey’s MNS and the human MNS could be based on the fact that Macaque monkeys do not use pantomime actions (Iacoboni, 2009). Humans do use pantomime action in a lot of situations. For example, a person mimes that he or she is going to sleep in some occasions by pressing the hands together and placing the head against the hands. This could explain the activation of mirror neurons during the observation of intransitive actions. The MNS in humans exists of the areas that correspond with the monkey’s MNS (Rizzolatti & Graighero, 2004). However, it could have evolved in humans in a 8 different manner than in Macaque monkeys (Hickok, 2009). §4 Action understanding and the Mirror Neuron System Action understanding is defined as: “The capacity to achieve the internal description of an action and to use it to organize appropriate future behavior” (Rizzolatti, Fogassi & Gallese, 2001). The mirror neuron theory on action understanding is: When an observed action evokes a neural activity which is normally evoked when the observed action is executed, the meaning of the action is understood because of the similarity between the two neural activities (Rizzolatti et al., 1996). Action understanding could be mediated by the MNS. §4.1 The Possible Role of the Mirror Neuron System in Action Understanding To investigate the role of the MNS in action understanding, Buccino, Binkofski and Riggio (2004) focused on the effect of the observation of different species that performed similar actions. A subject observed humans, monkeys and dogs. The observed action was either biting, or a specie-specific communicative action. Through fMRI, they found that during the observation of biting the two main regions of the human MNS were active, the IPL and the IFG. It did not matter what specie performed the biting. It was different for the observation of communicative actions. The frontal lobe was only activated during the observation of human (silent speech) and monkey’s (silent lip smacking) communicative actions. The dog’s barking, also silent, did not activate any frontal lobe activity. This indicates that different actions can be processed differently. If an action is shown that is in the motor repertoire of the person observing it, it is mapped on his or her motor system. The person recognizes an action because he or she can perform the same action. It is therefore “personal knowledge”. If an action is observed that is not in the motor repertoire, it is not mapped in the motor system. The properties of this action are indentified on a visual basis. Buccino et al. (2004) concluded that there seem to be two different means of understanding action, one that involves some sort of ‘resonance’ in the motor areas of the brain and one based more on visual information. This study provides evidence for the role of the MNS in action understanding in the sense that an internal description of the observed action is made, namely in the areas of the MNS. If an action is observed that lies in the motor repertoire of the observer the MNS is active and provides ‘motor resonance’. According to the mirror 9 neuron theory of action understanding, this resonance should induce the understanding of the action because of its neural similarity. The enhanced MEPs in the muscles that would have been active during the execution of the observed action, found through TMS studies, provide evidence for this statement (Baldissera et al., 2001; Fadiga et al., 1995; Maedea et al., 2002). A spinal chord inhibition of the muscles prevents overt mimicking of the action, allowing a functional motor resonance in the cortex, provided through the MNS (Baldissera et al., 2001). The question is: Is this internal description of the observed action necessary to understand the action? §4.2 Criticism on the role of the Mirror Neuron System in action understanding Hickok (2009) interestingly pointed out, as a criticism of the study of Buccino et al. (2004), that the lip smacking condition of the monkey holds less semantic information than the silent speech condition and the barking dog condition. These last two conditions are generally more present in everyday observations. The subjects understood that the person they observed was speaking, they also understood that the dog was barking. Lip smacking is in a lesser manner recognizable according to Hickok (2009), yet it produced more MNS activity than the barking dog. Actions can be understood without the MNS, the dog’s barking may be better understood than the monkey’s lip smacking. Catmur, Walsh and Heyes (2007) showed that the enhanced MEPs in the activated muscles could be changed through exercise. Subjects were taught to perform a movement of the little finger, while they observed the same movement of the index finger on the screen. The MEPs recorded of the little finger muscles were enhanced when an observation was made of the moving index finger and were not enhanced while observing little finger movements. This contrasts the findings of Fadiga et al. (1995). If the MNS was necessary in order to understand actions, the MEPs should not change because it represents mirror neuron activity. This activity should cause the person to understand the action before it produces his own action. The subjects understood that the index finger was moving, but there was no mirror neuron activity causing this understanding. If the MNS is a necessary system to understand actions, deficits in the production of actions should be correlated with deficits in action understanding. Researchers focused on 37 patients with unilateral brain lesions causing apraxic 10 impairments (Negri, Rumiati, Zadini, Ukmar, Mahon & Caramazza, 2007). The patients were tested on several tasks concerning action understanding. Correlations were found between, for example, pantomime imitation and pantomime recognition. This suggests the relationship between the MNS and action recognition, because the MNS was active during pantomime action (Buccino et al., 2001). However, there were individual patients that did not support these correlations. Dissociation between the individual patients caused Negri et al. (2007) to conclude that the ability to imitate mimed actions is not necessary in able to recognize mimed actions. The absence of working imitation skills did not disturb the action understanding. These findings suggest that the MNS is not necessary in understanding actions. A point of criticism on this study is that the patients had unilateral deficits. It cannot be excluded that the partial damage could have influenced the results. Lingnau, Gesierich and Caramazza (2009) concentrated, through fMRI, on blood-oxygen level dependent (BOLD) response adaptation in the areas of the MNS while first executing an action and then observing the same action. The adaptation occurs when the same neurons are repeatedly used (Ogawa, Lee, Nayak & Glynn, 1990). So, if the MNS hypothesis of action understanding is true, this adaptation should occur in the MNS whenever an action is first observed and then executed. It should also occur when the action is first executed and then observed. Adaptation did occur when the action was first observed and then executed or when two observations were made after each other and also when two actions were executed after each other. However, it did not occur when the action was first executed and then observed. This lack of adaptation challenges the theory that action understanding is facilitated through direct matching using the MNS. A critical note on this research is that there is adaptation when the action is first observed and then executed. Lignau et al. (2009) state that this could be helpful with the preparation of the execution of a movement. Remember that Gallese et al. (1996) excluded this possibility of the mirror neurons in Macaque monkeys, because there was no mirror neuron activity in the time between the observation and execution of movement. This raises questions on the interpretation of this adaptation. Also, note that Lingnau et al. (2009) used intransitive movement in their study, however as suggested earlier, to activate the entire MNS, transitive movements must be used (Buccino et al., 2001). The authors however noticed that some mirror neurons are active during intransitive actions. Earlier studies showed that there were enhanced 11 MEPs during intransitive movements (Fadiga et al., 1995). Lignau et al. (2009) further suggest that if the MNS is the key to understand actions, intransitive actions should also produce MNS activity. The mirror neurons are very interesting neurons in the sense of their double role. They play a role in the observation of action and the execution of the same action. Rizzolati et al. (1996) proposed that because of the neural similarities between the observation and execution of an action, the action is understood. In the study of Catmur et al. (2007) MEPs contradicted the expectations based on the mirror neuron theory on action understanding. The neural activity between the observation and the execution was not similar, but the subjects did understand the observed action. Negri et al. (2007) showed that patients with deficits in the production of movement did not necessarily have problems with the understanding of these movements. Thus the neural similarity between the observation and execution of an action, as stated in the theory of Rizzolatti et al. (1996), cannot be the only way to understand actions. Does this mean that the MNS does not contribute to action understanding? Haslinger et al. (2005) showed that there was more MNS activity in professional pianists in contrast with naive controls while observing finger movements of a pianist. It is conceivable that the expert pianist understands the observed action in a different manner than the control. Perhaps the motor resonance enriches the experience of an observation of an action, but the actual understanding does not depend on this motor resonance. Rizzolatti and Arbib (1998) showed that the MNS in Macaque monkeys could adapt. It can be activated when observing an action with a tool, whereas it first was not activated. Catmur et al. (2007) showed that the human MNS also could adapt. Heyes (2010) suggests that the MNS could be formed through experience. It could be a byproduct of associative learning, rather than a system designed for action understanding. This theory is consistent with the evidence that the MNS can adapt to new experiences. It is also consistent with the idea that we can understand actions even if they are not in our motor repertoire. Seeing a dog barking does not activate the MNS. We do however understand what the dog is doing. Also, if the dog is barking at a certain time of the day, we have learned through experience that the dog wants to be taken for a walk. Therefore we understand for what purpose the dog is barking. Lignau et al. (2009) also suggested that the mirror neuron activity could be a consequence of action understanding instead of the other way around. This is 12 consistent with the MNS as a byproduct of associative learning. It is also consistent with the findings of Kohler et al. (2002) in Macaque monkeys. They found that a sound could activate the mirror neurons that are normally activated when a particular action is observed. This sound could be linked with the action through experience and therefore produce mirror neuron activity. §5 Empathy and the Mirror Neuron System Not only is action understanding regarded as a possible function of the MNS, another function of the MNS in humans is proposed by Gallese (2003). He states that in a similar manner as the theory on action understanding proposed by Rizzolatti et al. (1996), the mirror neurons could be at the base of empathy. This theory was defined earlier: When an observed action evokes a neural activity which is normally evoked when the observed action is executed, the meaning of the action is understood because of the similarity between the two neural activities. Gallese (2003) proposes that the shared experience causes humans to understand each other. He calls this shared experience the shared manifold, the ability of different organisms to experience the same thing. He proposes that the MNS is the mechanism to share this experience and ultimately cause us to understand others. He states that the mirror neurons are not only used in order to understand each other’s actions, but also to understand each other’s emotions and to empathize with another human being. The first thing to notice is, that the mirror neurons in F5 of the Macaque monkey’s brain were not active if emotional gestures were used (Gallese et al., 1996; Rizzolatti et al., 1996). This means that if in humans the MNS is the underlying system in order to feel empathy, the human mirror neurons must be evolved in a different manner than in Macaque monkeys. This means that one must accept two major differences between the Macaque monkey’s MNS and the human MNS: The activation of the human MNS during intransitive actions and during emotional gestures. Gazolla, Aziz-Zadeh and Keysers (2006) found through fMRI that during the exposure to sounds of actions, the same brain areas were active that were active during the execution of the same actions. This provided evidence for human auditory mirror neurons, in correspondence with Macaque monkey’s MNs (Kohler et al., 2002). Furthermore, Gazzola et al. (2006) also found that individuals who scored higher on an empathy scale showed stronger MNS activity. This provided some 13 evidence that the MNS is linked with empathy. A critical note is that the number of participants of this study was rather low (n=14). Also, the reason that the higher scorers had more MNS activity is not fully understood. It could be that selective attention to the actions made by others plays a role. High scorers may be more aware of the actions of others and therefore have more MN activity. Therefore, this study does not provide compelling evidence in favor of the theory stated by Gallese (2003). If the MNS is the system that is responsible for empathy, the motor resonance that causes the shared experience should be responsible for the understanding of emotions (Gallese, 2003). Tamietto et al. (2009) focused on emotional contagion, which is the tendency to pair our facial expressions with those of other individuals. They used patients with unilateral damage to the visual cortex, causing affective blindsight, and found that observing bodily expressions triggered the automatic reflex of the emotion congruent facial muscles. Buccino et al. (2001) showed that the human MNS is organized is such a manner that it responds very specific to the effectors, for example hands, used in actions. The same reactions to different effectors therefore must be caused outside of the MNS. Tamietto et al. (2009) also found emotionspecific pupil reactions that coincided with the facial expressions. There was no evidence of motor activity prior to these reactions that could reflect motor resonance. The responses were evoked through the affective meaning of the bodily expressions and not through the motor properties of these expressions. This means that motor resonance is not at the base of emotional contagion. Another important conclusion from Tamietto et al. (2009) was that patients with affective blindsight could effectively recognize the emotions that were presented in the unseen visual field. The patients had automatic emotion-specific physical reactions to the presented stimuli in the unseen visual field, which indicated emotional changes. The recognition of emotions is an important point of the theory of Gallese (2003) and is therefore challenged by these results. Tamietto et al. (2009) do notice that it is not clear how this process works and that it needs more investigation in the future. Gazolla et al. (2006) state that individuals with autism spectrum disorder (ASD) show weak MNS activity while observing others. A key impairment in individuals with ASD is the disability to relate with another individual (Oberman, Ramachandran & Pineda, 2008). The shared manifest proposed by Gallese (2003) states that in order to feel empathy it is necessary to experience the same thing as another individual, thus 14 to relate with another individual. This shared experience, formed by the MNS, allows us to understand another person and to feel the same thing as somebody else. As individuals with ASD cannot relate to each other in a normal manner, the MNS must be impaired if the theory by Gallese (2003) is true. Oberman et al. (2008) investigated the MNS in individuals with ASD. They used EEG scans to investigate Mu-rhythm suppression. As discussed earlier, the Murhythm suppression is used to study the MNS in other studies (Cochin et al., 1998). Oberman et al. (2008) let individuals with ASD watch videos of grasping movements made by strangers or familiar individuals. They found that Mu-suppression does occur in individuals with ASD if the actors of the movements were familiar to them. This shows that the MNS in individuals with ASD is not impaired. It shows that the MNS in this group of people is only active if a socially important individual performs the action. The MNS is working but the activation of the MNS is impaired. The question that is raised through this study is whether or not the decreased MNS activity is the reason individuals suffer from ASD, or whether this decreased activity is caused by an underlying deficit in relating to unfamiliar people. §6 General discussion Since the discovery of mirror neurons there has been a lot of discussion on the role of these neurons. Gallese et al. (1996) focused on the role of the mirror neurons in action understanding in Macaque monkeys and also in humans. The statement that the MNS is the system underlying action understanding was quickly adopted in other studies. Moreover, the role of the MNS was not restricted to action understanding, Gallese (2003) states that the MNS is also the system that makes it possible to feel empathy for another human being. This theory is built upon the basic idea that the mirror neurons are necessary to understand actions. This thesis focused on the possible role of the MNS in action understanding and also in empathy. First off all, the MNS in humans must be different from the MNS in Macaque monkeys. The human MNS is active during the observation of intransitive movements (Buccino et al., 2001). It also must be active during emotional gestures to be the key system in empathy (Gallese, 2003). Because of the fact there is no direct evidence that a human MNS exists, the assumption must be made that if there is a MNS it is evolved differently in humans than in Macaque monkeys. Catmur et al. (2007) showed that the MNS activity could be changed through 15 exercise. Heyes (2010) pointed out that the MNS could be formed entirely through associative learning. It means that the MNS exist because of action understanding and not for action understanding. Evidence suggests that action understanding is possible without the MNS. Therefore the MNS cannot be the necessary system contributing to action understanding. It is however possible that the activation of the MNS enrich the experience of the observation of an action (Hickok, 2009). Haslinger et al. (2005) showed that expert pianists did have more MNS activity than novices, observing a pianist playing. The expert pianists could have a different understanding of the observation. Perhaps, the playing of the observed pianist is technically very good. Only the expert pianist may recognize this fact. If one does not accept the MNS as the necessary system responsible for action understanding, than one must question the MNS as the necessary system for empathy. This is because Gallese (2003) formed his hypothesis on the fact that the MNS is the key in understanding actions. The low MNS activity in individuals with ASD has been presented as evidence for the role of the MNS in empathy (Gazzola, 2006). Oberman et al. (2008) showed that the MNS activity in individuals with ASD is normal if someone familiar performs the action that is observed. Tamietto et al. (2009) showed that emotional contagion is not achieved through the activity of the MNS. They conclude that emotional contagion is achieved through the affective meaning of the observed bodily or facial expressions, and that emotions are recognized without the MNS. The MNS is therefore, in a similar manner as with action understanding, not the necessary system that underlies empathy. Heyes (2010) mentions that when a system is designed for a particular function, such as the understanding of actions, the system tends to be specific in its function. A byproduct tends have multiple uses and effects, but not necessary of sufficient to that function. This is consistent with the idea of the MNS as a byproduct of associative learning and not as a system designed for action understanding. A lot of properties have been attributed to the MNS, including action understanding and empathy. The basic idea of the MNS as a system responsible for action understanding is not backed by strong evidence; therefore it might not be justified. Building upon this statement, it can also not be justified to attribute the ability to feel empathy through the MNS. 16 Summary This thesis focused on mirror neuron system in Macaque monkeys and in humans. The focus is the function of this set of neurons. Firstly, the role of the mirror neuron system on action understanding is discussed. The evidence suggests that the mirror neuron system is not the necessary system for understanding actions. 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