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Unit 158

Unit 158

but embedded in different actions, their visual properties were tested in the same two conditions as those used for studying their motor properties. The actions were performed by an experimenter in front of the monkey. In one condition, the monkey observed the experimenter grasping a piece of food and bringing it to his mouth; in the other, the monkey observed the experimenter placing an object into the container.

The results showed that more than two-thirds of IPL neurons were differentially activated during the observation of grasping in placing and eating conditions. Examples are shown in Figure 2. Neurons responding to the observation of grasping for eating were more represented than neurons responding to the observation of grasping for placing, again with a two to one ratio.

A comparison between neuron selectivity during the execution of a motor act and motor act observation showed that the great majority of neurons (84%) have the same specificity during grasping execution and grasping observation. Thus, a mirror neuron whose discharge was higher during the observation of grasping for eating than during the observation of grasping for placing also had a higher discharge when the monkey grasped for eating than when it grasped for placing. The same was true for neurons selective for grasping for placing.

The motor and visual organization of IPL, just described, is of great interest for two reasons. First, it indicates that motor actions are organized in the parietal cortex in specific chains of motor acts; second, it strongly suggests that this chained organization might constitute the neural basis for understanding the intentions of others.

In favor of the existence of action chains in IPL, it is not only the activation of neurons coding the same motor act in one condition and not in another, but also the organization of IPL neuron-receptive fields. This organization shows that there is a predictive facilitatory link between subsequent motor acts. To give an example, there are IPL neurons that respond to the passive flexion of the forearm, have tactile receptive fields on the mouth, and in addition discharge during mouth grasping (Ferrari et al., manuscript in preparation). These neurons facilitat the mouth opening when an object touches the mouth, but also when the monkey grasps it, producing a contact between the object and the hand tactile-receptive field. Recently, several examples of predictive chained organization in IPL have been described by Yokochi et al. (2003). If one considers that a fundamental aspect of action execution is its fluidity, the link between the motor acts forming an action and the specificity of neurons coding them appears to be an optimal solution for executing an action without having pauses between the individual motor acts forming it.

The presence of chained motor organization of IPL neurons has deep implications for intention understanding. The interpretation of the functional role of mirror neurons was, as described above, that of action understanding. A motor act done by another individual is recognized when this act triggers the same set of neurons that are active during that act execution. The action-related IPL mirror neurons allow one to extend this concept. These neurons discriminate one motor act from another, thus activating a motor act chain that codes the final goal of the action. In this way the observing individual may re-enact internally the observed action and thus predict the goal of the observed action. In this way, the observer can "read" the intention of the acting individual.

This intention-reading interpretation predicts that, in addition to mirror neurons that fire during the execution and observation of the same motor act ("classical mirror neurons"), there should be neurons that are visually triggered by a given motor act but discharge during the execution not of the same motor act, but of another one that is functionally related to the former and is part of the same action chain. Neurons of this type have been previously described both in F5 (Di Pellegrino et al. 1992) and in IPL (Gallese et al. 2002) and referred to as "logically related" mirror neurons. These "logic" mirror neurons were never theoretically discussed because their functional role was not clear. The findings just discussed allow us to not only account for their occurrence but also to indicate their necessity, if the chain organization is at the basis of intention understanding.

While the mechanism of intention understanding just described appears to be rather simple, it would be more complex to specify how the selection of a particular chain occurs. After all, what the observer sees is just a hand grasping a piece of food or an object.

There are various factors that may determine this selection. The first is the context in which the action is executed. In the study described above, the clue for possible understanding of the intention of the acting experimenter was either the presence of the container (placing condition) or its absence (eating condition). The second factor that may intervene in chain selection is the type of object that the experimenter grasped. Typically, food is grasped in order to be eaten. Thus, the observation of a motor act directed towards food is more likely to trigger grasping-for-eating neurons than neurons that code grasping for other purposes. This food-eating association is, of course, not mandatory but could be modified by other factors.

One of these factors is the standard repetition of an action. Another is, as mentioned before, the context in which the action is performed. Context and object type were found to interact in some neurons. For example, some neurons that selectively discharged during the observation of grasping for eating also discharged, although weakly, during the observation of grasping for placing when the object to be placed was food, but not when it was a solid. It was as if the eating chain was activated, although slightly, by food in spite of the presence of a contextual clue indicating that placing was the most likely action. A few neurons, instead of showing an intermediate discharge when the nature of the stimulus (food) and context conflicted, decreased their discharge with time when the same action was repeated. It was as if the activity of the placing chain progressively inhibited the activity of neurons of the eating chain.

Understanding "other minds" constitutes a special domain of cognition. Developmental studies clearly show that this cognitive faculty has various components and that there are various steps through which infants acquire it (see Saxe et al. 2004). Brain imaging studies also tend to indicate the possibility of an involvement of many areas in this function (Blakemore and Decety 2001; Frith and Frith 2003; Gallagher and Frith 2003).

Given the complexity of the problem, it would be naive to claim that the mechanism described in the present study is the mechanism at the basis of mind read-

Visual responses of mirror neurons

Grasp to eat Unit 87 Unit 39 Unit 80

Visual responses of mirror neurons

Grasp to eat Unit 87 Unit 39 Unit 80

Grasp to place

Grasp to place

Fig. 2. Visual responses of IPL mirror neurons during the observation of grasping-to-eat and grasping-to-place done by an experimenter. Condtions as in Figure 1. (From Fogasso et al. 2005)

Fig. 2. Visual responses of IPL mirror neurons during the observation of grasping-to-eat and grasping-to-place done by an experimenter. Condtions as in Figure 1. (From Fogasso et al. 2005)

ing. Yet, the present data show for the first time a neural mechanism through which an important aspect of mind reading, understanding the intention of others, may be solved.

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