19 Jun 2004 14:34 AR AR217-NE27-07.tex AR217-NE27-07.sgm LaTeX2e(2002/01/18) P1: IKH10.1146/annurev.neuro.27.070203.144230Annu. Rev. Neurosci. 2004. 27:169–92doi: 10.1146/annurev.neuro.27.070203.144230Copyrightc 2004 by Annual Reviews. All rights reservedFirst published online as a Review in Advance on March 5, 2004THE MIRROR-NEURONSYSTEMGiacomo Rizzolatti1and Laila Craighero21Dipartimento di Neuroscienze, Sezione di Fisiologia, via Volturno, 3, Universit`adiParma, 43100, Parma, Italy; email: [email protected];2Dipartimento SBTA, Sezione di Fisiologia Umana, via Fossato di Mortara, 17/19,Universit`a di Ferrara, 44100 Ferrara, Italy; email: [email protected] Wordsmirror neurons, action understanding, imitation, language, motorcognition■ AbstractA category of stimuli of great importance for primates, humans inparticular, is that formed by actions done by other individuals. If we want to survive,we must understand the actions of others. Furthermore, without action understanding,social organization is impossible. In the case of humans, there is another faculty thatdepends on the observation of others’ actions: imitation learning. Unlike most species,we are able to learn by imitation, and this faculty is at the basis of human culture. Inthis review we present data on a neurophysiological mechanism—the mirror-neuronmechanism—that appears to play a fundamental role in both action understanding andimitation. We describe first the functional properties of mirror neurons in monkeys.We review next the characteristics of the mirror-neuron system in humans. We stress,in particular, those properties specific to the human mirror-neuron system that mightexplain the human capacity to learn by imitation. We conclude by discussing therelationship between the mirror-neuron system and language.INTRODUCTIONMirror neurons are a particular class of visuomotor neurons, originally discoveredin area F5 of the monkey premotor cortex, that discharge both when the monkeydoes a particular action and when it observes another individual (monkey or human)doing a similar action (Di Pellegrino et al. 1992, Gallese et al. 1996, Rizzolattiet al. 1996a). A lateral view of the monkey brain showing the location of area F5is presented in Figure 1.The aim of this review is to provide an updated account of the functionalproperties of the system formed by mirror neurons. The review is divided intofour sections. In the first section we present the basic functional properties ofmirror neurons in the monkey, and we discuss their functional roles in actionunderstanding. In the second section, we present evidence that a mirror-neuronsystem similar to that of the monkey exists in humans. The third section showsthat in humans, in addition to action understanding, the mirror-neuron systemplays a fundamental role in action imitation. The last section is more speculative.0147-006X/04/0721-0169$14.00 169Annu. Rev. Neurosci. 2004.27:169-192. Downloaded from arjournals.annualreviews.orgby Princeton University Library on 03/26/08. For personal use only.19 Jun 2004 14:34 AR AR217-NE27-07.tex AR217-NE27-07.sgm LaTeX2e(2002/01/18) P1: IKH170 RIZZOLATTICRAIGHEROWe present there a theory of language evolution, and we discuss a series of datasupporting the notion of a strict link between language and the mirror-neuronsystem (Rizzolatti & Arbib 1998).THE MIRROR-NEURON SYSTEM IN MONKEYSF5 Mirror Neurons: Basic PropertiesThere are two classes of visuomotor neurons in monkey area F5: canonical neurons,which respond to the presentation of an object, and mirror neurons, which respondwhen the monkey sees object-directed action (Rizzolatti & Luppino 2001). In orderto be triggered by visual stimuli, mirror neurons require an interaction between abiological effector (hand or mouth) and an object. The sight of an object alone, ofan agent mimicking an action, or of an individual making intransitive (nonobject-directed) gestures are all ineffective. The object significance for the monkey hasno obvious influence on the mirror-neuron response. Grasping a piece of food ora geometric solid produces responses of the same intensity.Mirror neurons show a large degree of generalization. Presenting widely differ-ent visual stimuli, but which all represent the same action, is equally effective. Forexample, the same grasping mirror neuron that responds to a human hand graspingan object responds also when the grasping hand is that of a monkey. Similarly, theresponse is typically not affected if the action is done near or far from the monkey,in spite of the fact that the size of the observed hand is obviously different in thetwo conditions.It is also of little importance for neuron activation if the observed action is even-tually rewarded. The discharge is of the same intensity if the experimenter graspsthe food and gives it to the recorded monkey or to another monkey introduced inthe experimental room.An important functional aspect of mirror neurons is the relation between theirvisual and motor properties. Virtually all mirror neurons show congruence betweenthe visual actions they respond to and the motor responses they code. Accordingto the type of congruence they exhibit, mirror neurons have been subdivided into“strictly congruent” and “broadly congruent” neurons (Gallese et al. 1996).Mirror neurons in which the effective observed and effective executed actionscorrespond in terms of goal (e.g., grasping) and means for reaching the goal (e.g.,precision grip) have been classed as “strictly congruent.” They represent aboutone third of F5 mirror neurons. Mirror neurons that, in order to be triggered, donot require the observation of exactly the same action that they code motoricallyhave been classed as “broadly congruent.” They represent about two thirds of F5mirror neurons.F5 Mouth Mirror NeuronsThe early studies of mirror neurons concerned essentially the upper sector of F5where hand actions are mostly represented. Recently, a study was carried out onAnnu. Rev. Neurosci. 2004.27:169-192. Downloaded from arjournals.annualreviews.orgby Princeton University Library on 03/26/08. For personal use only.19 Jun 2004 14:34 AR AR217-NE27-07.tex AR217-NE27-07.sgm LaTeX2e(2002/01/18) P1: IKHMIRROR NEURONS 171the properties of neurons located in the lateral part of F5 (Ferrari et al. 2003),where, in contrast, most neurons are related to mouth actions.The results showed that about 25% of studied neurons have mirror properties.According to