The Mirror System in Humans--论文代写范文精选

那些受试者的肌肉运动发现变化。随后的实验中，证实了肌肉的选择性，激发和描述各种大脑皮层和脊髓兴奋性变化，引起的观察行为。下面的paper代写范文进行详述。

Abstract
There is rich evidence that a mirror system exists also in humans. Evidence for this comes from electroencephalography (EEG), magnetoencephalography (MEG), transcranial magnetic stimulation (TMS), and brain imaging studies (e.g., Fadiga et al., 1995; Rizzolatti et al., 1996b; Grafton et al., 1996; Decety et al., 1997; Hari et al., 1998; Cochin et al., 1999). Many of these studies have been reviewed recently (Rizzolatti et al., 2001). Here only those particularly relevant for imitation are examined.

Transcranial Magnetic Stimulation Studies The rationale of TMS studies of the mirror neuron system is the following. If there is a system endowed with mirror properties, the observation of an action performed by another individual should increase the motor-evoked potentials (MEPs) recorded from the observer’s muscles involved in producing that action. Fadiga et al. (1995) demonstrated that this is the case. Normal volunteers were required to observe an experimenter grasping different objects (transitive hand movements) or performing meaningless arm gestures in the air (intransitive arm gestures). As control conditions, detection of the dimming of a small spot of light or the presentation of 3-D objects was used. The results showed that observation of both transitive and intransitive actions produced an increase in the motor-evoked potentials recorded from the observers’ hand and arm muscles. 

The increase was found in those muscles that the subjects would use to produce the movements observed. Subsequent experiments confirmed the selectivity of the muscle excitation and described various cortical and spinal cord excitability changes caused by the observation of actions made by others (Baldissera et al., 2001; Gangitano et al., 2001; Maeda et al., 2002). Of these studies, the last two are of particular interest for imitation. Gangitano et al. (2001) recorded MEPs from the hand muscles of normal subjects while they were observing grasping movements made by another individual. The MEPs were recorded at different intervals following onset of the movement. The results showed that cortical motor excitability faithfully followed the phases of the observed grasping movement (figure 1.3). This finding indicates that in humans the mirror neuron system codes for the temporal aspects of the observed movements and not only the meaning of the observed action. Maeda et al. (2002) also recorded MEPs from two hand muscles of normal volunteers. 

The recordings were made while they observed video clips of different finger movements, such as thumb abduction or adduction. The finger movements were presented in two hand orientations: as if the actor were sitting next to the observer (hand ‘‘away’’ position) and as if the actor were in front of the observer (hand ‘‘toward’’ position). The results showed that the degree of cortical motor modulation depended on the orientation of the hand. Modulation was greater when the observed movement was performed in the hand away position (i.e., when the actor and the observer were in the same position) than in the hand toward position. Summing up, TMS studies have shown two important properties of human mirror systems that have not been observed in the monkey. First, intransitive meaningless movements produce mirror neuron activation (Fadiga et al., 1995; Strafella & Paus, 2000; Maeda et al., 2002). Second, the correlation between the time course of the observed movements and the MEPs facilitation suggests a mirror mechanism that also codes for the movements forming an action. I previously referred (see Rizzolatti et al., 2002) to the movement-related mirror mechanism as the ‘‘low-level resonance mechanism,’’ contrasting it with the ‘‘high-level resonance mechanism’’ of F5 where the coded element is the action. These properties of the human mirror neuron system, which may explain the great human capacity for imitation, are discussed in ch. 1.10.

Brain Imaging Studies 
Early brain imaging studies showed that the observation of hand actions activates (besides various occipital visual areas) the STS region, the inferior parietal lobe, and the ventral premotor cortex, including Broca’s area (see Rizzolatti et al., 2001). The finding of activation of Broca’s area during observation of hand action was rather unexpected. Although comparative cytoarchitectonic studies indicate that the pars orbicularis of Broca’s area (area 44) is the human homologue of area F5 (see Petrides & Pandya, 1994), the traditional view is that area 44 is the speech motor area. 

In recent years, however, rich evidence has been accumulating that, in addition to speech representation, area 44 contains, similarly to monkey area F5, a hand motor representation (Krams et al., 1998; Binkofski et al., 1999a; Iacoboni et al., 1999; Gerardin et al., 2000; Ehrsson et al., 2000; Schubotz & Von Cramon, 2001). The hand motor representation, albeit greatly overlapping with that of mouth, is situated dorsally to the latter, sometimes invading the adjacent ventral area 6, where proximal arm movements are located. It is interesting to note that precision grip is richly represented in area 44 (Ehrsson et al., 2000). The same overrepresentation of precision grip is found in the monkey area F5 (Rizzolatti et al., 1988). 

This activation of area 44 gave rise to some speculation about a possible exclusive role for this area in functions mediated by the mirror neuron system, with the explicitly stated doubt that in humans, verbal mediation rather than the mirror neuron system plays a fundamental role in these functions (see Heyes, 2001a). New experiments on the functional organization of the mirror system have shown that this view is wrong. Buccino et al. (2001) examined the general organization of the mirror neuron system using as stimuli mouth, hand, and foot actions. Transitive actions (directed toward an object) and intransitive actions were used. 

The following stimuli were presented: biting an apple or chewing; grasping a cup, grasping an apple or miming these actions; kicking a ball, and pushing a brake or miming these actions. Observation of an action was contrasted with the observation of a static face, hand, and foot, respectively. The observation of object-related mouth movements resulted in activation of areas 6 and 44 bilaterally. In addition, two activation foci were present in the parietal lobe. The rostral focus was located in area PF (BA 40), while the caudal one was (most likely) in area PG (BA 39). The observation of intransitive actions produced activation of the same premotor areas as the observation of transitive actions, but there was no parietal lobe activation. 

Observation of object-related hand and arm movements resulted in two areas of activation in the premotor cortex, one corresponding to area 44 and the other more dorsal in ventral area 6. Considering the motor organization of this region, it is likely that the former activation was caused by observation of grasping hand movements, while that of area 6 was caused by observation of reaching. As for mouth movements, there were two activation foci in the parietal lobe. The rostral focus was still in PF, but was more posteriorly located than the focus observed during mouth actions, while the caudal focus was essentially in the same location as that for mouth actions. During the observation of intransitive movements, the premotor activations were present, but not the parietal ones. (论文代写)

Finally, the observation of object-related foot actions resulted in activation of a dorsal sector of area 6 and activation of the posterior parietal lobe, in part overlapping with that seen during mouth and hand actions (BA 39), in part extending more dorsally. Nonobject-related foot actions produced the area 6 activations, but not the parietal ones. The results of this study are important for several reasons. First, they demonstrate that the mirror system includes a large part of premotor cortex and the inferior parietal lobule. It is not limited to Broca’s area. Second, they show that the activation map obtained during observation of actions made with different effectors is similar to the motor map (the so-called ‘‘homunculus’’) obtained with electrical stimulation of the same region. Finally, they allow one to rule out the idea advanced by some authors (see Gre`zes & Decety, 2001; see also Heyes, 2001a) that the activation of area 44 is due to internal verbalization. Verbalization cannot be present during the observation of hand movements and magically disappear during the observation of foot movements. In conclusion, the human mirror system is widespread and centered on the inferior parietal lobule and the premotor cortex, including area 44. The next section examines how this system is involved in imitation.(论文代写)

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