Imitation of Actions: Brain Imaging Experiments--论文代写范文精选

研究这两种运动行为，从镜像神经元的角度。在这篇心理report代写范文中，将讨论行为的观察，主要的可能机制可以解释，而不是实证研究的基础上。正如前面所讨论的,个体与对象交互的方式建立在规范的神经元上。

Abstract
Unlike psychologists, ethologists typically stress the learning aspects of imitation. Many consider ‘‘true’’ imitation to require the precise repetition of an observed action previously not present in the observer’s motor repertoire; the learning of actions with effects on the environment that are similar to the observed ones is not sufficient (A. Spence, 1937; Galef, 1988; Tomasello, 1990; R. Byrne & Tomasello, 1995). This view in large part relates to the need to exclude from imitation motor behaviors apparently learned by observation of action but in fact triggered by the meaning of the stimulus (A. Spence, 1937) or by its affordances (Tomasello, 1990).

Two different ways of learning a new motor behavior should be distinguished. One is substitution for the motor pattern spontaneously used by the observer in response to a given stimulus of another motor pattern, more adequate to reach the intended goal, on the basis of observation of the behavior of another individual. Examples could include the correct way to hold a tennis racket or to place a finger on a guitar’s neck (action adjustment). The second way is learning, by observation, a new motor sequence that is useful to reach a certain goal (sequence learning). The ability to open a box only if a certain action sequence is followed could be an example of this second type of imitation learning (see also the artificial fruit of Whiten and Custance, 1996). There are no experiments that I am aware of that have studied these two types of motor behavior from the perspective of mirror neurons. 

So in this section I discuss the issue of acquisition of new motor behaviors following observation of actions made by others mostly in terms of possible mechanisms that may explain them rather than on the basis of empirical studies. The neurophysiological network that should intervene in action is that formed by STS, PF, and F5. As discussed earlier, this circuit stores many visually described actions in its visual node, STS. STS neurons send information to PF, where there are neurons that receive, in addition to STS input, backward connections from F5. The way an individual interacts with an object before learning is established by F5 canonical neurons that specify which type of movement (e.g., a specific type of grip) has to be used on the basis of the object’s affordances (see Jeannerod et al., 1995). 

When the observer sees that another grip is more efficient than the one previously used to reach the goal of the action, this new grip is coded in STS. The learning process consists of the production of a motor pattern that activates, via backward connections, those PF neurons that receive the sensory copy of the desired action from STS. The comparison between the visual aspect of the performed action and the sensory copy of it will allow a modification of the internal motor pattern until this pattern produces an action similar to the observed one. This model is basically an internal forward model (see Wolpert et al., 1995; Wolpert, 1997; Kawato, 1999; Arbib & Rizzolatti, 1999). 

Its main biological constraints are, on the motor side, the motor repertoire present in PF and F5 and, on the sensory side, the variety of action prototypes coded in STS and their plasticity. The presence in humans of a rich representation of intransitive motor acts, shown by TMS studies, renders the human mirror neuron system much more apt for imitation than the analogous monkey system, where the poor representation of intransitive actions (or even its absence) and the apparent poverty (on the basis of available evidence) of mirror neurons coding for precise details of actions present serious limits to the capacity for imitation. Without the storage of intransitive actions to complement basic object-related actions and precise copies of actions, the capacity of the monkey system to imitate the behavior of others should be rather limited. Logically, the mechanism that is the basis of learning a sequence by imitation ought to be different. 

Here, unlike the case of action adjustment, the essential achievement is not the substitution of an action determined by an object’s affordances with a more effective action, but rather the capacity to replicate a series of actions previously never executed. An interesting hypothesis to explain how this type of imitation may occur has been recently advanced by Byrne (see R. Byrne, 2002c and chapter 9). According to Byrne, sequence learning by imitation is based on two operations. The first is the capacity to segment the perceived action into smaller units and to match them to ‘‘motor acts’’ already present in the motor repertoire of the observer. Mirror neurons are the elements that perform this matching. The other essential operation (‘‘string-parsing’’) consists of extracting the statistical regularities that characterize an action’s sequence. This operation imposes high-order organization on the observed action sequence and, if successful, mirrors the original planning structure that produced the behavior. On the basis of neurophysiological data indicating a role for the mesial cortical area in sequence learning and execution (see Hikosaka et al., 1995, 2000; Tanji, 1996; Tanji et al., 1996; Shima & Tanji, 2000), Byrne proposed that these areas also play a role in string parsing. An additional possible neural substrate for this operation is the basal ganglia, which also appears to play a role in sequence learning. Obviously, at present the proposed mechanisms for action adjustment and for sequential learning by imitation are both merely hypothetical. However, they suggest a series of brain imaging experiments that may be easily performed using the available technology.

Concluding Remarks 
A point central to this chapter’s attempt to give imitation a neurophysiological basis is that an understanding of actions preceded imitation in evolution. The mirror system evolved as a system whose main aim was to match sensory information to personal motor knowledge of action meaning. This system became progressively richer and more complex and, in humans, came to include intransitive actions and detailed specifications of how an observed action is executed. This evolved mirror system became the basis for reproducing actions performed by others; that is, for imitation. A possible criticism of this view is that some actions produce imitation without any evidence that they have been understood. 

There are several examples of this type of behavior. In many species of animals, for instance, the observation of a movement made by one individual is a signal for the rest of the group to start a similar movement (e.g., the behavior of shorebirds studied by Thorpe, 1963). Imitation of this type, that is, imitation without understanding the meaning of an action, is present in humans. A well-known example is the capacity of newborns, first described by Meltzoff and Moore (1977), to imitate buccal gestures. Other examples are laughing, yawning, crying, and, as shown by Dimberg et al. (2000), involuntary mimicking of facial expressions. It is likely that the main purpose of these behaviors is to create a link between individuals by facilitating affiliative behaviors and inhibiting aggressive behaviors. 

Is such imitation without understanding also dependent on mirror neurons? In the absence of empirical data, a response to this question can be, obviously, only hypothetical. It is tempting, however, to think that the same mechanism underlies these behaviors and action understanding. At this point an obvious conceptual difficulty arises. It is difficult to accept that relatively simple behaviors such the escape behavior of shorebirds mentioned earlier developed after action understanding and requires this understanding as its prerequisite. The interpretation given by Thorpe (1963, see also Tinbergen, 1953) in terms of releasing signals appears to be more parsimonious and convincing. 

A possible solution of this paradox might lie in the distinction between high-level and low-level resonance mechanisms (Rizzolatti et al., 2002).2 According to this proposal, there are neurons endowed with motor properties (motor neurons in a broad sense) that resonate when an appropriate stimulus is presented. The effect of this resonance is radically different according to the role that these neurons play in motor control. If they are close to the effectors, their low-level resonance elicits an actual motor action, with little if any cognitive effects. In contrast, if the neurons represent the action internally without necessarily causing motor effects (e.g., F5 mirror neurons), their high-level resonance would produce mostly cognitive rather than motor phenomena, such as action understanding.

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