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Researchers have measured human brain responses using a variety of methods such as functional magnetic resonance imaging (fMRI), which demonstrate reliable effects of changes in neural activity by changes in blood flow. Although slower to respond than measures of electrical activity, fMRI provides evidence about where neural activity in the human brain occurs. This kind of research does show that observing action produces activity in areas of the human brain more typically associated with executing action. While there may be some disagreement about which motor areas of the human brain are active while observing or imitating action and how these would correspond to areas in the monkey brain, there is good agreement that the human motor system responds for observation and imitation of action.⁴

There is quite a difference between recognizing an action and understanding that action. We can see a hand move through space with an open palm oriented with the flat of the palm moving toward the surface of an object and predict where the hand will strike the object and that it will apply force to the surface of that object. But understanding the same general action as pushing a door open with the intent to enter and slapping a person in the face is quite different. A ball can be thrown in a game of catch or as a missile intended to do harm. The actions may be similar but the intentions are different. Therefore it is important that some researchers have argued that mirror neurons respond to the intention as well as the action.⁵ However, to date there has been no clear evidence that such neurons respond to intention -- just that the sight of the action of reaching without an object to grasp, the sight of the object alone, and the sight of the action with the object present show different patterns of brain response. The fact that such different visual experiences lead to different patterns of brain activity does not provide clear evidence that intentions or goals are somehow part of the mirror neuron response to observed action.

Understanding spoken language as action understanding

The potential ambiguity of action is perhaps clearest if we consider language. Talking is a form of action and understanding speech might be a form of action understanding. In talking, mouth movements are made in such a way as to create sounds that will have some affect on the listener, as discussed in Howard Nusbaum’s chapter. Listeners must understand what was meant by making those sounds. However, if someone says, “It’s hot in here,” or “You are a great friend,” there can be ambiguity about the meaning. Such sentences could be straightforward observations as they seem to be or they could be something very different. We can ask to have a window opened or we can make a negative social comment using exactly the same sentences.

Nonetheless, there is good reason to believe that the human action system is involved in understanding speech as well as producing it. While some ambiguities in speech or behavior simply cannot be resolved without broader contextual knowledge, the motor system may be important in understanding. If you try to have a conversation in a noisy bar, looking at your friend talking makes it easier to understand what is being said. We have shown that the motor system contributes to this process of recognizing speech. When listeners can see someone’s face while talking, mouth

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