Language and mirror neurons - Semantic Scholar€¦ · mouth mirror neurons could be distinguished: ingestive and communicative mirror neurons. Ingestive mirror neurons, which represent

47.1 IntroductionCommunication is a process of exchanginginformation via a common system. There aremany natural ways in which individuals maycommunicate. Besides linguistic communication,which is at the core of human communication,humans communicate using arm gestures, bodypostures, facial expressions, eye contact, andhead and body movements.

Communication may be intentional and non-intentional. In both cases, the sender and thereceiver of the messages must have a commoncode. The difference is that in the case of inten-tional communication the sender plays the lead-ing role and imposes the communication on thereceiver, while in the case of non-intentionalcommunication, the sender sends the messagewithout having any intention to do so. The mes-sage is just there. If sender and receiver have acommon code, the message reaches the receiver,regardless of the will of the sender.

Of these two types of communication, thenon-intentional one is the most basic and prim-itive. It is evolutionarily necessary because, insocial life, individuals have to understand whatothers are doing, whether or not those othersintend to be understood. As will be argued later,it is very plausible that intentional communica-tion is an evolutionarily late development ofnon-intentional communication.

47.2 Neurophysiological basisof non-intentionalcommunication

47.2.1 General considerationsActions done by others are probably the mostimportant stimuli of our lives. Most of others’actions do not convey intentional informationto the observer. From them, however, we under-stand what others are doing and we can inferwhy they are doing it. This involuntary commu-nication is fundamental for interpersonal relations, and is at the basis of social life.

What is the mechanism underlying ourcapacity to understand others’ actions? The tra-ditional view is that actions done by others areunderstood in the same way as other visualstimuli. Thus, action understanding is based onthe visual analysis of the different elements thatform an action. For example, when we observe agirl picking up a flower, the analyzed elementswould be her hand, the flower, and the move-ment of the hand towards the flower. The asso-ciation of these elements and inferences abouttheir interaction enables the observer to under-stand the witnessed action. The discovery ofneurons that code selectively biological motionhas better specified the neural basis of thisrecognition mechanism (Perrett et al., et al.1989). Actions done by others are coded in a

CHAPTER 47

Language and mirror neuronsGiacomo Rizzolatti and Laila Craighero

47-Gaskell-Chap47 3/10/07 8:19 PM Page 771

specific part of the visual system devoted to thistask, which in humans includes some extrastri-ate visual areas and the region of the superiortemporal sulcus (STS) (see Allison et al., 2000;Puce and Perrett, 2003).

It has been recently argued that visual informa-tion alone does not provide a full understandingof the observed action (Rizzolatti et al., 2001;Jeannerod, 2004). According to this view, a fullaction understanding can be achieved onlywhen the observed action activates the corre-sponding motor representation in the observer.In this way, the observed action enters into theobserver’s motor network, and enables him orher to relate it to other similar actions and toactions that usually follow the observed one.Without the involvement of the motor system,the visual representation of the action may leadto its recognition, but critical elements for under-standing what the action is about (e.g. how itrefers to other actions and how to reproduce it)are lacking.

These theoretical considerations receivedstrong support from the discovery that in themotor cortex of the macaque monkey there is aparticular set of neurons that discharge bothwhen the monkey observes a given motor actand when it does the same act. These neurons,

called “mirror neurons,” represent a system thatdirectly matches observed and executed actions.Their properties will be described in the nextsection.

47.2.2 Mirror system in monkeysMirror neurons were originally discovered inthe rostral part of the ventral premotor cortex(area F5, see Figure 47.1) of the macaque mon-key. Like all neurons of this area, mirror neuronshave motor properties. They code mostly distalhand actions such as grasping, holding, tearing,and manipulating. Their defining functionalcharacteristic is that they become active notonly when the monkey does a particular action(like grasping an object) but also when itobserves another individual (monkey orhuman) performing a similar action. Mirrorneurons do not respond to the sight of a handmimicking an action or to meaningless intransi-tive movements. Similarly, they do not respondto the observation of an object alone, even whenit is of interest to the monkey (Gallese et al.,1996; Rizzolatti, Fadiga, Fogassi, and Gallese,1996).

The vast majority of F5 mirror neurons show a marked similarity between the observed

772 · CHAPTER 47 Language and Mirror Neurons

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Figure 47.1 Lateral view of the monkey cerebral cortex. The motor areas (F1–F7) are classifiedaccording to Rizzolatti et al. (1998), while the parietal areas are named according to the nomenclatureof Von Economo (1929). Mirror neurons have been described in area F5 and in the rostral part of theinferior parietal lobule (areas PF and PFG). Abbreviations: AI, inferior arcuate sulcus; AS, superiorarcuate sulcus; C, central sulcus. (From Rizzolatti and Craighero, 2004.)


action effective in triggering them and the exe-cuted action effective in triggering them. This sen-sory–motor congruence is occasionally extremelystrict. In these cases the effective motor action andthe effective observed action coincide both interms of goal (e.g. grasping) and in terms of howthe goal is achieved (e.g. precision grip). For mostmirror neurons, however, the congruence isbroader and is confined to the goal of the action.

Early studies on mirror neurons examined theupper sector of F5, where hand actions aremostly represented. Recently, a study was car-ried out on the properties of neurons located inthe lower part of F5 where neuron activity ismostly related to mouth actions (Ferrari et al.,2003). The results showed that two classes ofmouth mirror neurons could be distinguished:ingestive and communicative mirror neurons.

Ingestive mirror neurons, which represent themajority of mouth mirror neurons, respond tothe observation of actions related to ingestivefunctions (e.g. grasping food with the mouth).Virtually all of them show a good correspon-dence between the effective observed and theeffective executed action. More intriguing arethe properties of the communicative mirrorneurons. For them, the most effective observedaction is a communicative gesture, such as lipsmacking. However, most of them strongly dis-charge when the monkey actively performs aningestive action (Figure 47.2, neuron 76). A feware also active during monkey communicativegestures (Figure 47.2, neuron 33).

This discrepancy between the effective visualinput (communicative) and the effective activeaction (ingestive) is rather intriguing. There is,

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Experimenter lip-smacking Experimenter protrudes his lips

Experimenter sucks from a syringe Monkey protrudes its lips to take food

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Figure 47.2 Examples of two mouth mirror neurons. Neuron 76 responds to the observation of acommunicative gesture (lip smacking). Its motor activity is related, however, to monkey’s food ingestion.Neuron 33 responds to the observation of a communicative gesture (lip protrusion) and dischargeswhen the monkey does a communicative gesture (lip smacking). (Modified from Ferrari et al., 2003.)


however, evidence suggesting that in evolution,monkey communicative gestures—or at leastsome of them—derived from ingestive actions(see below). From this perspective, one mayargue that the communicative mouth mirrorneurons found in F5 reflect a process of cortical-ization of communicative functions not yetfreed from their original ingestive basis.

An issue recently addressed was whether mir-ror neurons are able to recognize actions fromtheir sound. Kohler et al. (2002) recorded F5mirror neuron activity while the monkey wasobserving a “noisy” action (e.g. ripping a pieceof paper), or was presented with the sound ofthe action without seeing it. The results showedthat about 15 per cent of mirror neuronsresponsive to presentation of actions accompa-nied by sounds also responded to the presenta-tion of the sound alone. Most of themdischarged specifically to the sound typical ofthe observed action. These neurons were dubbed“audiovisual” mirror neurons. The properties ofaudiovisual neurons strongly suggest that thtmirror-neuron system is involved in actionrecognition whatever the modality throughwhich the action is presented.

47.2.3 Monkey mirror neuron systemand intention recognitionThe motoric coding of the actions of others hasan important consequence: It enables theobserver to infer future acts of the observedagent by using links among motor acts in his orher motor repertoire. The existence of such aninference mechanism was recently tested bystudying the activity of neurons of the inferiorparietal lobule (IPL).

As shown by Mountcastle (Mountcastle et al.,1975) and Hyvarinen (1982), many IPL neuronsdischarge in association with specific motor acts.In a recent study, single neurons were recordedfrom IPL and tested for their motor properties.Neurons discharging selectively in associationwith grasping movements were tested in twomain conditions. In one, the monkey grasped apiece of food located in front of it and brought itto its mouth, while in the other, the monkeygrasped the same object and then placed it intoa container (Fogassi et al., 2005). The resultsshowed that for most recorded neurons (about75 per cent), the discharge during graspingdepended on the motor act which followedgrasping. A series of control experiments

demonstrated that this selectivity was notdependent on force, movement kinematics, ortype of grasped stimulus. This selectivity of IPLgrasping mirror neurons may appear, at firstglance, as uneconomical. In fact, fewer neuronsare needed if a single set of grasping neurons areused for all possible actions in which grasping isrequired. However, the cost in number of neu-rons is offset by the benefits of efficiency: theIPL organization just described is extremely wellsuited for providing fluidity in action execution.Each neuron codes a specific motor act andsimultaneously, being embedded in a specificchain of motor acts, facilitates execution of thenext act. This enables a continuous sequence ofmotor acts without any pause between them.

In the same experiment, a series of IPL grasping mirror neurons were tested for theirvisual responses in the two conditions used in the motor task. The actions were performedby one of the experimenters in front of themonkey. The first motor act was always thesame—grasping—but there were cues (e.g.presence or absence of the container) thatallowed the monkey to predict which action waslikely to follow. The results were analogous tothose found on the motor side. Namely, for themajority of IPL mirror neurons, the dischargeduring grasping done by the experimenterdepended on whether the following act wasbringing the food to the experimenter’s mouthor placing it into the container.

These data suggest a simple mechanism thatmay mediate the capacity of individuals to readthe intentions of others. When an individualstarts the first motor act of an action, he or shehas clear what is the goal for the entire action.Action intention is set before the beginning ofthe movements, and, as shown by IPL motorproperties, is reflected immediately in the neuron discharge. Because the neurons codingmotor acts in different action chains have mirror properties, their activation signals notonly grasping but, more specifically, grasping-for-eating, or grasping-for-placing. Thus, bytheir activation, the monkey is able to predictthe goal of the observed action, and in this wayto “read” the intention of the acting individual.

47.2.4 Mirror system in humans:neurophysiological evidenceA large amount of data indicate that in humans,the observation of actions done by others



activates cortical areas involved in motor control.Evidence for this effect comes from electroen-cephalographic (EEG) and magnetoencephalo-graphic (MEG) recordings from motor areas,transcranial magnetic stimulation experiments(TMS), and brain imaging studies.

It is well known from EEG studies that activemovements of the recorded individual, such aswrist opening/closure, desynchronize the EEGrhythms of the areas around the central sulcus.EEG and MEG studies showed a similar desyn-chronization, although less marked, during theobservation of action by others (Altschuler etal., 1997; Altschuler et al., 2000; Hari et al., 1998;Cochin et al., 1998; Cochin et al., 1999).A desynchronization of cortical rhythms duringobservation and execution of finger movementswas recently also shown using implanted sub-dural electrodes, a technique that allows a bettersignal localization than surface EEG. Most inter-estingly, the desynchronization during theobservation and execution of these movementswas present in functionally delimited hand andlanguage motor areas (Tremblay et al., 2004).

Fadiga et al. (1995) recorded motor evokedpotentials (MEPs) from the right arm and handmuscles (in normal volunteers), elicited by tran-scranial magnetic stimulation (TMS) of thehand field of the left primary motor cortex. Byusing this technique, they were able to assess themotor cortex excitability in various conditions,and thus to establish whether the motor systemwas activated during action observation.Stimulation was performed while volunteerswere observing an experimenter grasping objects(transitive hand actions) or performing mean-ingless arm gestures (intransitive arm move-ments). Detection of the dimming of a smallspot of light and presentation of objects wereused as control conditions. The results showedthat the observation of both transitive andintransitive actions produced an increase in theMEPs. For both hand and arm movements, theincrease selectively concerned those musclesthat the volunteers used when producing theobserved movements.

Similar results were also found by othergroups (Brighina et al., 2000; Gangitano et al.,2001; Clark et al., 2004; see Fadiga et al., 2005).Among these studies, a particularly interestingone is Gangitano et al. (2001), which showed thepresence of a strict temporal coupling betweenthe changes in cortical excitability and thedynamics of the observed action. MEPs, which

were recorded from hand muscles at differenttime intervals during passive observation of grasping, matched the time-course of thekinematics of the observed action.

The TMS results are particularly importantfor two reasons. First, they show that the obser-vation of actions performed by others specifi-cally activates the sets of neurons that are usedto replicate the observed action. Second,they indicate that, unlike in the monkey, thehuman mirror neuron system also resonates for intransitive actions. Thus, the human mirrorsystem appears to have the capacity, absent in monkeys, to replicate internally actions per-formed by others even when these actions haveno apparent goal.

47.2.5 Mirror system in humans:brain imaging studiesObservation of actions done by others activatesa complex network formed, in humans, byoccipital and temporal areas and by two corticalregions whose function is predominantly orfundamentally motor (e.g. Rizzolatti et al., 1996;Grafton et al., 1996; Grèzes, Costes, and Decety,1998; Iacoboni et al., 1999; 2001, Nishitani andHari, 2000; 2002; Grèzes et al., 2001; Buccino etal., 2001; Perani et al., 2001; Decety et al., 2002;Grèzes et al., 2003; Koski et al., 2003; Koski et al.,2002; Manthey et al., 2003). These two regionsare the rostral part of the inferior parietal lobuleand the lower part of the precentral gyrus plusthe posterior part of the inferior frontal gyrus(IFG) which basically corresponds to area 44.These regions form the core of the human mir-ror neuron system (Figure 47.3). Occasionally,other regions are also found to be active duringthe observation of others’ actions. These regionsare the dorsal premotor cortex and the sector ofIFG most likely corresponding to area 45.

The areas forming the human mirror neuronsystem show a somatotopic organization. In anfMRI study, normal volunteers were presentedwith video clips showing actions done with themouth, hand/arm and foot/leg. Action observa-tion was contrasted with the observation of astatic face, hand, and foot respectively (Buccinoet al., 2001). Observations of object-relatedmouth actions determined signal increase in thelower part of the precentral gyrus and of thepars opercularis of IFG, bilaterally. In addition,two activation foci were found in the parietallobe. One was located in the rostral part of the

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inferior parietal lobule (most likely area PF),while the other was located in the posterior partof the same lobule. Observations of object-related hand/arm actions determined signalincrease in two sectors of the frontal lobe, onelocated in the pars opercularis of IFG, and theother located in the precentral gyrus. The latteractivation was more dorsally located than thatfound during the observation of mouth move-ments. As in the case of mouth actions, two acti-vation foci were found in the parietal lobe. Therostral focus was in the rostral part of the inferiorparietal lobule, but more posteriorly locatedthan that observed during mouth actions, whilethe caudal focus was essentially in the samelocation as that for mouth actions. Finally,observations of object-related foot/leg actionsdetermined an activation of a dorsal sector ofthe precentral gyrus and an activation of theposterior parietal lobe.

In the second part of the same experiment,volunteers were presented with a mimed versionof the object-directed actions. Unlike in the

monkey, in which mimed actions do not excitemirror neurons, in humans, mimed actions produced a signal increase in the frontal sector ofthe mirror neuron system. In contrast, no activa-tion was found in the inferior parietal lobule.This finding is in accord with lack of inferiorparietal lobule activation found also in otherstudies in which intransitive actions were used(e.g. finger movements; Iacoboni et al., 1999;2001; Koski et al., 2002; 2003). Considering thatvisual information reaches premotor cortexthrough the inferior parietal lobule, it is hard tobelieve that the observation of intransitiveactions does not produce any activation of thisstructure. Instead, activation may be more diffi-cult to detect for intransitive actions comparedto transitive actions for the following reason.The inferior parietal lobe receives direct infor-mation from the inferotemporal cortex, in addi-tion to that from visual areas responsive tobiological motion. Thus it is likely that the pres-ence of objects in transitive actions produces asource of activation in addition to that due to


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Figure 47.3 Lateral view of human cerebral cortex. The parietal and frontal lobe regions that areconsistently activated during the observation of actions by others are shown in red and yellow,respectively. They form the core of the mirror neuron system. The areas shown in blue (area 45 anddorsal area 6) have been reported to be active during the observation of others’ action in some studies.It is likely that they also contain mirror neurons.


action observation, thereby resulting in a stati-cally significant activation of inferior parietallobule for the transitive actions. In mimedactions, where there is no object, the activationdoes not reach statistical significance.

47.2.6 Which actions activate themirror neuron system?Humans recognize actions done not only byhumans but also by animals. Are animal actionsrecognized by the mirror neuron system or isthe visual system alone sufficient? Recently,Buccino et al. (2004) addressed this question inan fMRI experiment. Video clips showing silentmouth actions performed by humans, monkeys,and dogs were presented to normal volunteers.Two types of action were shown: biting food,and oral communicative actions (speech reading,lip smacking, barking). As a control, staticimages of the same actions were presented. Theresults showed that the observation of biting,regardless of whether performed by a man, amonkey, or a dog, produced a signal increase inthe inferior parietal lobule and in the pars oper-cularis of the IFG plus the adjacent precentralgyrus. The left parietal and the left frontal acti-vations were virtually identical for observationof all three species, whereas the right-side fociwere stronger during the observation of actionsmade by a human. Different results wereobtained with communicative actions.Observing a human speak activated the left parsopercularis of IFG, whereas the observation oflip smacking—a monkey communicative ges-ture—activated a small focus in the right andleft pars opercularis of IFG; finally, the observationof barking did not produce any frontal lobe acti-vation. In this case the activation was only present in visual areas.

These results show that actions done by others can be recognized through differentmechanisms. Actions belonging to the motorrepertoire of the observer, regardless of whetherthey are done by a conspecific, are mapped ontohis or her motor system. Actions that do notbelong to this repertoire do not excite the motorsystem of the observer and are recognized on avisual basis. It is likely that these two differentways of recognizing actions have two differentpsychological counterparts. In the first case themotor “resonance” translates the visual experi-ence into an internal “first person knowledge,”whereas this is lacking in the second case.

Further evidence that the mirror neuron system is strongly activated when the observersees an action that is part of his or her motorrepertoire has been recently provided by Calvo-Merino et al. (2005). They used fMRIimaging to study differences in brain activationbetween watching an action that one has learntto do and an action that one has not. Experts inclassic ballet, experts in the Brazilian martial artcapoeira, and inexpert controls viewed videosshowing dancers performing classical ballet orcapoeira. The results showed greater bilateralactivations in premotor cortex, intraparietal sulcus region, right superior parietal lobe, andleft posterior STS when expert dancers viewedmovements that they have been trained to per-form compared to movements that they hadnot. These results show that the mirror system isparticularly active when it integrates observedactions of others with an individual’s personalmotor repertoire.

The presence of activations during actionobservation in an area classically considered tobe exclusively related to speech might be sur-prising. However, in recent years, clear evidencehas been accumulated that human area 44 contains (as does monkey area F5, the monkeyhomolog of area 44) a motor representation of hand movements (Krams et al., 1998; Binkofskiet al., 1999; Iacoboni et al., 1999; Gerardin et al.,2000; Ehrsson et al., 2000; Tremblay et al.,2004), in addition to that of orolaryngeal move-ments. It appears, therefore, that the posteriorsector of IFG is involved in both non-intentionalcommunication (i.e. recognition of object-related actions) and intentional (verbal) communication—a finding that suggests, as wewill see in the next section, a link between thesetwo forms of communication.

47.3 Mirror neurons andlanguageHumans mostly communicate by sounds.Sound-based languages, however, do not repre-sent the only natural way for communicating.Languages based on gestures (signed languages)represent another form of complex, fully struc-tured communication system. By using sign lan-guage, people express abstract concepts, learnmathematics, physics, philosophy, and even createpoetry (see Corballis, 2002). Nonetheless, thefact that signed languages represent a fully

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structured communication system has notchanged the view, which many share, thatspeech is the only natural human communica-tion system, and that the evolutionary precursorof human speech consists of animal calls.The argument goes as follows: humans emitsound to communicate, animals emit sounds tocommunicate, therefore human speech evolvedfrom animal calls.

The logic of this syllogism is rather shaky. Itsweakness becomes apparent when one examinesanimal calls and human speech more closely.First, the anatomical structures underlying pri-mate calls and human speech are different.Primate calls are mostly mediated by the cingu-late cortex and by deep, diencephalic and brainstem structures (see Jürgens, 2002). In contrast,the circuits underlying human speech areformed by areas located around the Sylvian fis-sure, including the posterior part of IFG. It ishard to imagine how in primate evolution, thecall system shifted from its deep position foundin non-human primates to the lateral convexityof the cortex where human speech is housed.

Second, speech in humans is not, or is notnecessarily, linked to emotional behavior,whereas animal calls are. Third, speech is mostlya dyadic, person-to-person communication system. In contrast, animal calls are typically emitted without a well-identified receiver.Fourth, speech is endowed with combinatorialproperties that are absent in animal communi-cation. As Chomsky (1966) rightly stressed,human language is “based on an entirely differ-ent principle” from all other forms of animalcommunication. Finally, humans do possess a“call” communication system like that of non-human primates and its anatomical location issimilar. This system mediates the utterances thathumans emit when in particular emotionalstates (cries, yelling, etc.). These utterances,which are preserved in patients with globalaphasia, lack the referential character and thecombinatorial properties that characterizehuman speech.

The advocates of the sound-based theory oflanguage origin consider a strong argument infavor of this theory to be the presence of refer-ential information in some animal calls (e.g.Pinker, 1994). The famous study of the alarmcalls of vervet monkeys (Cheney and Seyfarth,1990), as well as other studies that extendedthese observations to other species and othercommunicative contexts (social relationship,

food, inter-group aggression), showed that evolution tried this pathway. The reason whythis attempt did not succeed is the lack of flexi-bility inherent in any communicative systembased on emotions. In a non-emotional com-munication system the same word, for examplethe word fire, which is basically an alarm mes-sage (“escape”), may assume a completely differ-ent meaning. It may indicate, for example, thatthe fire is ready and we can start to cook ourmeal (“approach message”), as well as conveyingother positive messages. This flexibility cannotoccur in an emotional communicative systembecause a referential meaning cannot indicate abehavior that is in contrast with the emotionthat generated it. Thus the same utterance orcall cannot convey, in different contexts, anescape and an approach message.

If not animal calls, what could be the origin ofhuman speech? An alternative hypothesis is thatthe path leading to speech started with gesturalcommunication. This hypothesis, first proposedby the French philosopher Condillac, hasrecently found several defenders (e.g Armstronget al., 1995; Corballis, 2002). According to thistheory, the initial communicative system in primate precursors of modern humans wasbased on very simple, elementary gesturing.Sounds were then associated with the gesturesand became progressively the dominant way ofcommunication.

The discovery of mirror neurons providedstrong support for the gestural theory of speechorigin. Mirror neurons create a direct linkbetween the sender of a message and its receiver.Thanks to the mirror mechanism, actions doneby one individual become messages that areunderstood by an observer without any cogni-tive mediation. The observation of an individualgrasping an apple is immediately understoodbecause it evokes the same motor representationin the parieto-frontal mirror system of theobserver. Similarly, the observation of a facialexpression of disgust is immediately understoodbecause it evokes the same representation in theamygdala of the individual observing it (Galleseet al., 2004).

On the basis of this fundamental property ofmirror neurons, and the fact that the observa-tion of actions like hand grasping activates thecaudal part of IFG (Broca’s area), Rizzolatti andArbib (1998) proposed that the mirror mecha-nism is the basic mechanism from which lan-guage evolved. In fact, the mirror mechanism



solved, at a initial stage of language evolution,two fundamental communication problems:parity and direct comprehension. Thanks to themirror neurons, what counted for the sender ofthe message also counted for the receiver. Noarbitrary symbols were required. The compre-hension was inherent in the neural organizationof the two individuals.

A criticism of this view is based on the factthat the monkey mirror neuron system is con-stituted of neurons coding object-directedactions. Thus, the monkey mirror neuron sys-tem forms a closed system, which by definitiondoes not appear to be particularly suitable forintentional communication. Yet, if this is truefor the monkey, it is not the case for the humanmirror system. As reviewed above, TMS andbrain imaging studies have shown that activa-tion of the human mirror system is achieved by presentation of intransitive actions (Fadiga et al., 1995; Maeda et al., 2002) as well as duringpantomime observation (Buccino et al., 2001,Grèzes et al., 2003).

It is difficult to specify how the shift from aclosed system of monkeys to an open, intention-ally communicative system, in humans mighthave occurred. The view, however, that commu-nicative actions derived from a more ancientsystem of non-communicative gestures is notnew. Van Hoof (1967), for example, proposedthat many of the most common communicativegestures of the monkey, such as lip smacking,are ritualizations of ingestive actions that mon-keys use for affiliative purposes. The fact thatmouth mirror neurons respond both to theobservation of communicative actions and during the execution of ingestive actionsappears to give a neurophysiological basis to thisidea (Ferrari et al., 2003; see also above, mirrorsystem in monkeys).

Similarly, Vygotsky (1934) suggested thatintransitive actions derive in children fromobject-directed transitive actions. For example,when objects are located close to a child, thechild grasps them. When they are located farfrom the child, the child extends his or herhands towards the objects. Because the motherunderstands this gesture, the child uses it againand again and, eventually, attempts to reachobjects become communicative gestures. Thus,the transition from object-directed to inten-tional communicative gesture can be accommo-dated by the mirror neuron hypothesis oflanguage evolution.

47.3.1 From gestures to soundIt is rather unlikely that the gestural communi-cations, which appeared in the ancestors ofHomo sapiens, reached the sophisticated com-plexity shown by modern sign languages. It ismuch more plausible, as also suggested by Arbib(2005), that gestures emitted for communicativepurposes were soon accompanied by sounds,and that the speech development prevented theoccurrence of a fully-fledged sign language.

The major problem in this evolutionary sce-nario is to understand how sounds, initiallymeaningless, became associated with gestureswhich conveyed specific meanings. Onomatopeia,that is, the similarity between the sound of aword and the noise produced by a correspondingnatural event, is one of the suggested possibilities.Another possibility is represented by interjec-tional utterances emitted by individuals in cer-tain conditions. The problem with both thesehypotheses is that they can account for only avery limited number of words. Thus, althoughthey explain the origin of some words, they lackthe generality necessary to explain most of thelinks between sound and meaning.

An interesting theory attempting to explainthis link was advanced many years ago by Paget(1930). This theory, called “schematopoeia”,posits that human communication started withmanual gestures. These gestures were accompa-nied by unintentional, but analogous, move-ments of tongue, lips, and jaw. Later, thegesticulating individuals discovered that theexpiration of air through the oral cavities pro-duced audible gestures. This was the beginningof voiced speech.

Paget gives many examples of parallelismbetween sound and meaning from a variety oflanguages. For vowels, he suggests that a (as inlarge) often refers to anything that is large, wideopen, or spacious; i, especially in its narrow orthin variety (as in mini), often refers to some-thing that is small or pointed; and aw (as inyawn) generally indicates a cavity. For conso-nants, he suggests that m implies a continuousclosure, while dr or tr denotes running or walk-ing. According to this explanation, the greatmajority of words were originally pantomimic(Paget, 1930: 159). This pantomimic origin isnot readily evident in modern languagesbecause, according to Paget, words are built byaddition of separate significant elements in themanner of Chinese ideographs.



The schematopoeia theory is obviously veryspeculative. Its central tenet, however—thatthere is a basic, natural link between hand/bodygestures and speech gestures—is very ingenious.It suggests a clue as to how an opaque gesturalsystem, like the orolaryngeal system, could con-vey understandable messages, thanks to theclose correspondence between hand/body ges-tures, intrinsically known to the observers, andoro/laryngeal gestures Furthermore, it has aclear neurophysiological prediction: if hand-arm and speech gestures are strictly linked, theymust have a common neural substrate.

A series of recent studies support this predic-tion. TMS experiments have shown that right-hand motor excitability increases duringreading and during spontaneous speech,whereas no language-related effects are foundunder these conditions in the left-hand motorarea or in the leg representations of either hemi-sphere (Tokimura et al., 1996; Seyal et al., 1999;Meister et al., 2003). Meister et al. (2003)stressed that the increase of hand motor cortexexcitability cannot be due to word articulationbecause word articulation recruits motor cortexbilaterally, but the observed activation wasstrictly limited to the left hemisphere. The facil-itation appears, therefore, to result from a

co-activation of the right hand motor area withthe language network.

Similar conclusions were also reached byGentilucci and his co-workers (2001). In a seriesof behavioral experiments, they showed normalvolunteers one of two 3-D objects, one large andthe other small, randomly presented. On the vis-ible face of both objects, either two Xs or a seriesof dots could appear. The volunteers wererequired to grasp the objects and, regardless ofthe symbols written on the object, to open theirmouth in all conditions. The kinematics ofhand, arm, and mouth movements wererecorded. The data showed that in spite of theinstruction to open the mouth in the same wayin all conditions, lip aperture and the peakvelocity of lip aperture increased when the handmovement was directed to the large object(Figure 47.4).

In another experiment, the authors employeda similar experimental procedure, but asked theparticipants to pronounce a syllable (e.g. gu orga) instead of simply opening their mouths. Thesyllables were written on the objects in the samelocations as the symbols in the previous experi-ment. The results showed that lip aperture waslarger when the participants grasped a largerobject. Furthermore, the maximal power level


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Figure 47.4 Influence of hand grasping on mouth aperture. Upper row: mean values of graspparameters; lower row: mean values of lip aperture parameters. Open bar: movements directed towardsthe small object; black bars: movements directred Left: size variation, center: peack velocity, right: timeto maximal hand and mouth aperture. (Modified from Gentilucci et al., 2001.)


recorded during syllable emission was alsohigher when volunteers grasped the large object(Gentilucci et al., 2001).

These experiments indicate that both buccalmovements and the orolaryngeal synergies necessary for syllable emission are linked tomanual gestures. Most importantly, hand actionsproducing large movements share a neural substrate with large mouth actions, precisely asproposed by Paget’s theory.

Finally, evidence for a link between gesturingand speech also comes from clinical studies.Hanlon et al. (1990) showed that, in aphasics,pointing with the right hand to a screen whereobjects are presented facilitates object naming.Similarly, Hadar et al. (1998) found that wordretrieval is facilitated through gesturing inbrain-damaged patients.

Clearly, the reviewed experiments do notprove the schematopoeia theory. Nonetheless,they indicate that the theory is not as bizarre as one may think initially: a link between handgestures and the speech system is present inmodern Homo sapiens.

47.3.2 The appearance of echo-mirror neuronsThe association between specific sounds andcommunicative gestures has obvious advan-tages, such as the possibility of communicatingin the dark or when hands are busy with tools or weapons. Nonetheless, to achieve effectivesound communication, the sounds conveyingmessages previously expressed by gesture (“gesture-related sounds”) ought to be clearlydistinguishable and, most importantly, shouldmaintain constant features; they must be pro-nounced in a precise, consistent way. This requiresa sophisticated organization of the motor systemrelated to sound production, and a rich connec-tivity between the cortical motor areas control-ling voluntary actions and the centers controllingthe oro-laryngeal tract. The large expansion ofthe posterior part of the inferior frontal gyrusculminating in the appearance of Broca’s area inthe human left hemisphere is, most likely, theresults of the evolutionary pressure to achievethis voluntarily control.

In parallel with these modifications occurringin the motor cortex, a system for understandingthem should have evolved. We know that inmonkey area F5, the homolog of human area 44,there are neurons—the so called “audiovisual

neurons” (Kohler et al., 2002; see also above,mirror system in monkeys), that respond to theobservation of actions done by others as well asto the sounds of those actions. This system,however, is tuned for recognition of the soundof physical events and not of sounds done byindividuals. In order to understand the proto-speech sounds, a variant of the audiovisual mirrorneuron system tuned to resonate in response tosounds emitted by the orolaryngeal tract shouldhave evolved. A more sophisticated acoustic sys-tem, enabling a better discrimination of the ges-ture-associated sounds, has also probablyevolved. Note, however, that an improvement inauditory discrimination would be of little use ifthe gesture-related sounds did not activate theorolaryngeal gesture representation in the brainof the listener.

47.3.3 Echo-mirror neuron systemand the problem of its functional roleIs there evidence that modern humans have amirror neuron system that responds to soundsproduced by the orolaryngeal tract and that nat-urally accompanies gesticulation? Consideringthe sophistication of the speech system in ourspecies, it is difficult to prove this point. There isclear evidence, however, that humans areendowed with a motor system that resonatesselectively in response to speech sounds (the“echo mirror neuron system”).

Fadiga et al. (Fadiga et al., 2002) recordedMEPs from the tongue muscles in normal volunteers instructed to listen carefully to acousti-cally presented verbal and non-verbal stimuli. Thestimuli were words, regular pseudo-words, andbitonal sounds. In the middle of words andpseudo-words there was either a double f or adouble r. F is a labio-dental fricative consonantthat, when pronounced, requires virtually notongue movements, whereas r is a linguo-palatalfricative consonant that, in contrast, requiresmarked tongue muscle involvement to be pro-nounced. During the stimulus presentation, theleft motor cortex of the participants was stimu-lated with single pulse TMS. The results showedthat listening to words and pseudo-words con-taining the double r produced a significantincrease of MEP amplitude recorded fromtongue muscles compared to listening to bitonalsounds and words and pseudo-words containingthe double f (Figure 47.5).



Results congruent with those of Fadiga et al.(2002) were obtained by Watkins et al. (2003).Using the single pulse TMS technique, theyrecorded MEPs from a lip muscle (orbicularisoris) and a hand muscle (first interosseus) in fourconditions: listening to continuous prose, view-ing speech-related lip movements, listening tonon-verbal sounds, and viewing eye and browmovements. Compared to viewing eye and browmovements, listening to and viewing speechenhanced the MEP amplitude recorded fromthe orbicularis oris muscle. Intriguingly, theMEPs obtained when subjects listened to non-verbal sounds also increased compared to view-ing eye and brow movement. This increase wasvery likely due to activation of a human mirrorsystem similar to that of the monkey audio-visualmirror neurons (Kohler et al. 2002).

All of these effects were seen only in responseto stimulation of the left hemisphere. Nochanges of MEPs in any condition wereobserved following stimulation of the righthemisphere. Finally, the size of MEPs elicited inthe first interosseus muscle did not differ in anycondition.

Taken together, these data suggest that a mirror neuron system for speech sound—anecho-mirror neuron system—exists in humans:When an individual listens to verbal stimuli,there is an automatic activation of his speech-related motor centers. Did this system evolvefrom the hypothetical gesture-related soundsmirror system discussed in the previous section?There is no doubt that speech is not purely a sys-tem based on sounds as such. As shown byLiberman (Liberman et al., 1967; Liberman andMattingly, 1985; Liberman and Whalen, 2000),an efficient communication system cannot bebuilt by substituting tones or combinations oftones for speech. There is something specialabout speech sounds that distinguish them fromother auditory material, and this is their capac-ity to evoke the motor representation of theheard sounds in the listener’s motor cortex.Note that this property, postulated by Libermanon the basis of indirect evidence, is now demon-strated by the existence of the echo-mirror neuron system. One may argue, however, thatthis property is only for translating heardsounds into pronounced sounds. In other


Figure 47.5 Total areas of the normalized motor evoked potentials (MEPs) recorded from tonguemuscles during listening to words and bitonal sounds. “rr” and “ff” refer to verbal stimuli containing adouble lingua-palatal fricative consonant “r”, and a double verbal labio-dental fricative consonant “f”,respectively. (Modified from Fadiga et al., 2002.)

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words, the basic function of mirror neurons—understanding—will be lost here and only theimitation function, developed on the top offormer (see Rizzolatti and Craighero, 2004)would be present.

It is possible that an echo-mirror neuron sys-tem would evolve solely for the purpose oftranslating heard sounds into pronouncedsounds. We are strongly inclined, however, tothink that the motor link that this system pro-vides to speech sounds has a more profoundevolutionary significance. First, as discussedabove there is a consistent link in humansbetween hand actions and orolaryngeal ges-tures, similar to the one present in the monkeyfor hand and mouth action. Thus, if these neu-rons acquired mirror properties, as other typesof F5/Broca’s area neuron did, a category of neurons evolve coding orolaryngeal tract gestures simultaneously with body-action ges-tures. In other words, neurons appeared whichcoded phonetics simultaneously with semantics.In this way, heard speech sounds produced notonly a tendency to imitate the sound but also anunderstanding of the accompanying body-action gestures (much as audio-visual mirrorneurons allow understanding of the actions thatproduce the sounds). Second, once a primitivesound-to-meaning linkage was established, itserved as the base for the development of addi-tional, increasingly arbitrary, links betweensounds and actions—i.e. the development ofwords. These arbitrary links greatly extendedthe possibilities for rich communication whilerequiring a lengthy, culturally bound learningperiod. Finally, given the necessity of distin-guishing among more speech sounds in morecombinations, the links between heard speechsounds and orolaryngeal gestures becamestronger (as in the modern echo-mirror sys-tem), whereas there was little pressure to further develop the link between sound andmeaning given the success of the learnt systemof arbitrary linkages.

47. 4 ConclusionsThe aim of the present chapter has been todescribe the functional properties of mirrorneurons systems, and to show how a mechanismoriginally devoted to non-intentional commu-nication becomes in evolution the mechanismat the basis of intentional communication.The reconstruction of how speech appeared

in evolution can only be hypothetical.Nonetheless, evidence collected in the last few years strongly supports the hypothesis thathuman speech evolved from gestural communi-cation rather than from animal calls.

We have not addressed the problem of theorigin of grammar. We recognize the impor-tance of this issue—for many scholars the most important issue—but at present there are not sufficient neurophysiological data to discuss its origin, even at a very hypotheticallevel. Nor have we discussed the growing behav-ioral and neurophysiological evidence showingthat during speech processing there is activationof non-speech areas and of the motor system in particular. These findings and their implica-tions are dealt with in detail in the excellentchapters in this volume by Glenberg (21) andPulvermüller (8).

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AcknowledgementsWe are grateful to Arthur Glenberg for his comment on the manuscript. The study wassupported by EU Contract IST 2001-35282,Mirrorbot; EU Contract NEST 012738,Neurocom; Cofin 2004 (GR); OMLL, EURO-CORES Programme; EU Contract NEST 5010,“Learning and development of ContextualAction.”

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Language and mirror neurons - Semantic Scholar€¦ · mouth mirror neurons could be distinguished: ingestive and communicative mirror neurons. Ingestive mirror neurons, which represent