The data discussed above indicate that the premotor and parietal cortices of primates contain a mechanism that allows individuals to understand the actions of others. Typically, an individual observing an action done by another person not only understands what that person is doing, but also why he/she is doing it. Let us imagine a boy grasping a mug. There are many reasons why the boy could grasp it, but the observer usually is able to infer why he did it. For example, if the boy grasped the cup by the handle, it is likely that he wants to drink the coffee, while if he grasped it by the top it is more likely that he wants to place it in a new location.
The issue of whether the intention comprehension (the "why" of an observed action) could be mediated by the mirror neurons has been recently addressed in a study in which the motor and visual properties of mirror neurons of the inferior parietal lobule (IPL) were investigated (Fogassi et al.2005).
IPL neurons that discharge during active grasping were selected. Subsequently, their motor activity was studied in two main conditions. In the first, the monkey grasped a piece of food located in front of it and brought it to its mouth (eating condition). In the second, the monkey grasped an object and placed it into a container (placing condition).
The results showed that the large majority of IPL grasping neurons (about 65% of them) were significantly influenced by the action in which the grasping was embedded. Examples are shown in Figure 1. Neurons coding grasping for eating were much more common than neurons coding grasping for placing, with a ratio of two to one.
Studies in humans showed that the kinematics of the first motor act of an action is influenced by the subsequent motor acts of that action (see Jeannerod 1988). The recordings of reaching-to-grasp kinematics of the monkeys in the two experimental conditions described above confirmed these findings. Reaching-to-grasp movement followed by arm flexion (bringing the food to the mouth) was faster than the same movement followed by arm abduction (placing the food into the container). To control for whether the differential discharge of grasping neurons in eating and placing conditions were due to this difference in kinematics rather than to the action goal, a newplacing condition was introduced (see Fig. 1). In this condition the monkey had to grasp a piece of food and place it into a container located near its mouth. Thus, the new place condition was identical in terms of goal to the original one but required, after grasping, arm flexion rather than arm abduction. The kinematics analysis of the reaching-to-grasp movement showed that the wrist peak velocity was fastest in the new placing condition, intermediate in the eating condition, and slowest in the original placing condition.
Neuron activity showed that, regardless of arm kinematics, the neuron selectivity remained unmodified. Neurons selective for placing in the far container showed the same selectivity for placing in the container near the monkey's mouth. Thus, it is the goal of the action that determines the motor selectivity of IPL neurons in coding a given motor act, rather than factors related to movement kinematics.
As in the premotor cortex, there are neurons in IPL that are endowed with mirror properties, discharging both during the observation and execution of the same motor act (Fogassi et al. 1998, Gallese et al. 2002). To see whether these neurons also discharge differentially during the observation of the same motor act
Fig. 1. A. Lateral view of the monkey brain showing the sector of IPL (gray shading) from which the neurons were recorded; cs, central sulcus; ips, inferior parietal sulcus. B Schematic drawing illustrating the apparatus and the paradigm used for the motor task. Left: starting position of the task. A screen prevents the monkey from seeing the target. Right: after the screen is removed, the monkey could release the hand from the starting position, reach and grasp the object to bring it to its mouth (Condition I) or to place it into a container located near the target (II) or near its mouth (III). C Activity of three IPL neurons during grasping in Conditions I and II. Rasters and the histograms are synchronized with the moment when the monkey touched the object to be grasped. Abscissa: time, bin = 20 ms; ordinate: discharge frequency. (From Fogassi et al. 2005)
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