No abstract

Our understanding of the functions of motor system evolved remarkably in the last 20 years. This is the consequence not only of an increase in the amount of data on this system but especially of a paradigm shift in our conceptualization of it. Motor system is not considered anymore just a "producer" of movements, as it was in the past, but a system crucially involved in cognitive functions. In the present study we review the data on the cortical organization underlying goal-directed actions and action understanding. Our review is subdivided into two major parts. In the first part, we review the anatomical and functional organization of the premotor and parietal areas of monkeys and humans. We show that the parietal and frontal areas form circuits devoted to specific motor functions. We discuss, in particular, the visuo-motor transformation necessary for reaching and for grasping. In the second part we show how a specific neural mechanism, the mirror mechanism, is involved in understanding the action and intention of others. This mechanism is located in the same parieto-frontal circuits that mediate goal-directed actions. We conclude by indicating future directions for studies on the mirror mechanism and suggest some major topics for forthcoming research.

Experiments in monkeys demonstrated that many parietal and premotor neurons coding a specific motor act (e.g., grasping) show a markedly different activation when this act is part of actions that have different goals (e.g., grasping for eating vs. grasping for placing). Many of these ''action-constrained'' neurons have mirror properties firing selectively to the observation of the initial motor act of the actions to which they belong motorically. By activating a specific action chain from its very outset, this mechanism allows the observers to have an internal copy of the whole action before its execution, thus enabling them to understand directly the agent's intention. Using electromyographic recordings, we show that a similar chained organization exists in typically developing children, whereas it is impaired in children with autism. We propose that, as a consequence of this functional impairment, high-functioning autistic children may understand the intentions of others cognitively but lack the mechanism for understanding them experientially.mirror neurons ͉ motor chains ͉ motor acts ͉ goal understanding ͉ motor intention H umans and monkeys possess a neural system, called the mirror neuron system, that maps visual descriptions of actions done by others onto the observer's motor representations of the same actions (1). In humans, the mirror neuron system has two major components. One is formed by the inferior parietal lobule and the ventral premotor cortex plus the caudal part of Broca's area, the other by the insula and anterior cingulate gyrus (1, 2).The mirror neuron system does not possess a unique function. Besides its originally proposed role in action understanding, its parieto-frontal component appears to mediate the understanding of intentions of others (3) and imitation (4-7), whereas its insular-cingulate component appears to play a fundamental role in emotion recognition (2,8).Recent data obtained in the monkey revealed that the mirror neuron mechanism underlying intention understanding relies on the activation of a specific set of ''action-constrained'' parietal neurons (9). These neurons discharge in association with specific motor acts but become maximally activated when the coded motor act is embedded into a specific motor action. Thus, for example, action-constrained grasping neurons strongly discharge when grasping a piece of food is followed by bringing it to the mouth, but not when it is followed by placing it into a container. Most interestingly, many action-constrained neurons have mirror properties. These neurons selectively discharge when the observation of motor acts is part of a given action (e.g., grasping for eating but not grasping for placing) (9). Their activation provides, therefore, information on the fact that an individual is grasping, but most importantly also gives clues on why the individual is doing it. Through this mechanism the observer, besides recognizing the observed motor act, is also able to predict what will be the final goal of the action. In other words, the ob...