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Neuropsychologia 47 (2009) 3190–3202

Contents lists available at ScienceDirect

Neuropsychologia

journa l homepage: www.e lsev ier .com/ locate /neuropsychologia

he observation of manual grasp actions affects the control of speech:combined behavioral and Transcranial Magnetic Stimulation study

aurizio Gentiluccia,b,∗, Giovanna Cristina Campionea,b, Riccardo Dalla Voltaa,b, Paolo Bernardisa

Dipartimento di Neuroscienze, Università di Parma, via Volturno 39, 43100 Parma, ItalyIstituto Italiano di Tecnologia (IIT), Unità di Parma, 43100 Parma, Italy

r t i c l e i n f o

rticle history:eceived 29 April 2009eceived in revised form 20 July 2009ccepted 28 July 2009vailable online 3 August 2009

eywords:irror system

peechouth kinematics

oice spectra

a b s t r a c t

Does the mirror system affect the control of speech? This issue was addressed in behavioral and Transcra-nial Magnetic Stimulation (TMS) experiments. In behavioral experiment 1, participants pronounced thesyllable /da/ while observing (1) a hand grasping large and small objects with power and precision grasps,respectively, (2) a foot interacting with large and small objects and (3) differently sized objects presentedalone. Voice formant 1 was higher when observing power as compared to precision grasp, whereas itremained unaffected by observation of the different types of foot interaction and objects alone. In TMSexperiment 2, we stimulated hand motor cortex, while participants observed the two types of grasp.Motor Evoked Potentials (MEPs) of hand muscles active during the two types of grasp were greater whenobserving power than precision grasp. In experiments 3–5, TMS was applied to tongue motor cortex ofparticipants silently pronouncing the syllable /da/ and simultaneously observing power and precision

ranscranial Magnetic Stimulation grasps, pantomimes of the two types of grasps, and differently sized objects presented alone. TongueMEPs were greater when observing power than precision grasp either executed or pantomimed. Finally,in TMS experiment 6, the observation of foot interaction with large and small objects did not modulatetongue MEPs. We hypothesized that grasp observation activated motor commands to the mouth as wellas to the hand that were congruent with the hand kinematics implemented in the observed type of grasp.The commands to the mouth selectively affected postures of phonation organs and consequently basic

nits.

features of phonological u

. Introduction

It has been suggested that manual gestures share the same rep-esentational system with speech (Armstrong, Stokoe, & Wilcox,995; Corballis, 1992, 2002; Donald, 1991; Gentilucci & Corballis,006; Givòn, 1995; Hewes, 1973; Rizzolatti & Arbib, 1998; Ruben,005). This is supported by evidence that speech itself may begestural system rather than an acoustic system, an idea cap-

ured by the motor theory of speech perception (Liberman, Cooper,hankweiler, & Studdert-Kennedy, 1967) and articulatory phonol-gy (Browman & Goldstein, 1995). According to this view speechs regarded, not as a system for producing sounds, but rather oneor producing articulatory gestures. Recent evidence suggests notnly that speech is gestural, but also that there are intimate func-

ional connections between hand and mouth, in monkeys as wells in humans. This is primarily shown by the properties of twolasses of neurons recorded from monkey premotor cortex F5. Therst class – the mirror neurons – discharges when the same grasp

∗ Corresponding author. Tel.: +39 0521903899; fax: +39 0521903900.E-mail address: gentiluc@unipr.it (M. Gentilucci).

028-3932/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.neuropsychologia.2009.07.020

© 2009 Elsevier Ltd. All rights reserved.

action is executed as well as observed (Gallese, Fadiga, Fogassi, &Rizzolatti, 1996). Usually, the discharge of these neurons is selec-tive for the type of grasp. For example, neurons discharging whenthe monkey grasps a small object with a precision grasp, are poorlyactivated by the grasp of a large object with a power grasp. Con-versely, other neurons active during grasping a large object witha power grasp, are poorly activated during grasping a small objectwith a precision grasp. Rizzolatti and Craighero (2004) proposedthat during evolution the mirror system might have acquired thefunction of receiving (i.e. interpreting) and sending messages bymeans of arm gestures. The second class of neurons dischargeswhen the same action (for example the grasp of a small object)is performed with the hand or the mouth (Rizzolatti et al., 1988).Gentilucci and Corballis (2006) proposed that this system was pri-marily established in the context of acts of grasping that requirea coordinated activity of both hand and mouth, as it occurs inthe case of food ingestion or object exploration. Later on, it was

adapted for communication. In other words, it formed the basisfor the transfer of a communication system from hand to mouth.According to this view, speech is directly grounded on arm gesturessince mouth actions are specifically linked to manual actions by thishand–mouth motor system. In support of this view neuroimaging

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tudies in humans show that the hand mirror system is putativelyocated in Broca’s area (Buccino et al., 2001; Grafton, Arbib, Fadiga,

Rizzolatti, 1996; Iacoboni et al., 1999; for a review see RizzolattiCraighero, 2004). Broca’s area is traditionally considered to be

lso involved in encoding phonological representations in termsf mouth articulation gestures (Buccino et al., 2004; Demonet etl., 1992; Grafton et al., 1996; Paulesu, Frith, & Frackowiak, 1993;atorre, Evans, Meyer, & Gjedde, 1992).

The joint activity of these two systems has been observedn humans: indeed, both the execution and observation of man-al grasps influence the simultaneous pronunciation of syllablesGentilucci, Benuzzi, Gangitano, & Grimaldi, 2001; Gentilucci,antunione, Roy, & Stefanini, 2004). Specifically, when executingr observing the grasp of large as compared to small objects lipinematics (especially when pronouncing a labial consonant) andoice spectra parameters (in particular formant 1, F1) increased,orresponding to kinematic variation in finger opening/closing dur-ng these grasps. These findings were interpreted by the authors aseflecting the existence also in humans of a double hand–mouthotor command system that would transfer to the mouth some

spects of the meaning of hand actions either executed or observedGentilucci & Corballis, 2006). During action observation the acti-ation of the mirror system would provide the motor aspects to beransferred.

The frontal areas activated by action observation are differen-ially distributed according to the acting effector. Buccino et al.2001) found bilateral activation of area 6 and area 44 for obser-ation of object-related mouth actions. Similar results were foundor object-related hand actions even if the activated portion of area

was located dorsally with respect to that found during mouthovement observation. In addition, a partial overlapping of theouth and hand representations was observed in area 44. The

bservation of object-related foot actions activated the dorsal por-ion of area 6 only. Summing up, in overlapping regions of area 44ransitive (i.e. object-related) actions of hand and mouth, but notf foot are represented. The common neural substrate coding theseepresentations may be involved in interactions between hand andouth. In contrast, since the system involved in the observation of

ransitive (object-related) foot actions is likely located outside area4 (Buccino et al., 2001), a likelihood of this system being directlyelated to mouth control, and more in general to speech is low. Theresent study firstly aimed at verifying whether the observation ofoot actions directed to differently sized object may affect syllableronunciation like the observation of manual actions does.

The presentation of graspable objects alone can activate affor-ances (Gentilucci, 2002, 2003b) and specifically grasp motorrograms (Barbieri, Buonocore, Bernardis, Dalla Volta, & Gentilucci,007; Tucker & Ellis, 1998), which in turn may affect speech. Arevious study (Gentilucci, 2003b) did not find any effect on vocalarameters of syllables pronounced when solids alone were pre-ented. However, familiar objects and in particular edible objectsore easily may activate affordances (Gentilucci, 2003a) and more-

ver they may activate direct interactions with the mouth. Theecond aim of the present study was to verify whether the pre-entation of edible objects alone affects pronunciation of syllables.n addition, we compared the observation of grasp of edible objects

ith that of non-edible objects. Previous studies found that thebservation of the grasp of non-edible objects (Gentilucci, 2003a)ffects speech like the observation of the grasp of edible objectsoes (Gentilucci, Stefanini, Roy, & Santunione, 2004). However, airect comparison between the two presentations was not per-

ormed in order to verify stronger effect of edible objects.

The present study consists of one behavioral experiment (exper-ment 1) and a second series of experiments (experiments 2–6)n which we used the technique of Transcranial Magnetic Stimu-ation (TMS). In behavioral experiment 1, in which we analyzed

ogia 47 (2009) 3190–3202 3191

lip kinematics and voice spectra, we compared (1) the effects ofthe observation of the grasp of differently sized objects with thoseof foot interactions with differently sized objects on the syllableDA pronounced during observation; (2) the effects of the observa-tion of edible and non-edible objects either grasped or presentedalone on the syllable DA pronounced during observation. We choseto present the syllable DA (/da/) because in previous experiments(Gentilucci, 2003a; Gentilucci, Stefanini, et al., 2004) the syllable BA(/ba/) was pronounced and the grasp observation induced effectson both F1 and lip opening. If a coupling exists between F1 and lipopening during pronunciation of a labial consonant (/ba/) becausethe lips play a direct role in the release of occlusion, it should disap-pear or at least be weaker, when pronouncing a dental consonant(/da/) in which the lips play a marginal role in the release of theocclusion.

Previous TMS studies support the existence of a mirror system inhumans (Aziz-Zadeh, Maeda, Zaidel, Mazziotta, & Iacoboni, 2002;Fadiga, Fogassi, Pavesi, & Rizzolatti, 1995; Gangitano, Mottaghy, &Pascual-Leone, 2001; Maeda, Keenan, Tormos, Topka, & Pascual-Leone, 2000). These studies suggested that when the hand mirrorsystem is activated by grasp observation, it sends commands ofgrasp to the hand motor cortex. The mirror neurons are fre-quently selective for type of grasp of differently sized objects.Moreover, it is well known that during precision grasps of smallobjects, peak velocity of finger opening and maximal finger aper-ture are smaller and duration of finger closing is longer comparedto the grasp of large objects (Chieffi & Gentilucci, 1993; Gentilucciet al., 1991; Gentilucci, Toni, Chieffi, & Pavesi, 1994). Conse-quently, the commands for activation of the finger muscles thatare directly responsible for the different kinematics of the open-ing/closing phases of precision and power grasps should be weakerwhen observing precision compared to power grasps. These sub-threshold activations could be revealed by stimulating hand motorcortex. In TMS experiment 2, in order to verify whether the obser-vation of different types of grasp differentially modulates the handmotor cortex excitability, we applied single pulse TMS to handmotor cortex of individuals observing power and precision grasps.We recorded Motor Evoked Potentials (MEPs) from the OpponensPollicis (OP) muscle because it is active during finger closure of bothprecision and power grasps. In a separate session we confirmedthat OP activation is greater while executing a power rather than aprecision grasp.

The results of the behavioral experiment 1 showed that theobservation of different types of manual grasp modulated voicespectra of simultaneously pronounced syllables, in accordance withprevious data (Gentilucci, 2003a; Gentilucci, Santunione, et al.,2004). Specifically, the observation of precision grasp of smallobjects induced a decrease in F1 when compared to the effectsof the observation of the power grasp of large objects. Conse-quently, we hypothesized that motor commands modulated bythe observation of different types of grasps were sent both to thehand and mouth motor cortex. Since a dental consonant (syllable/da/) was pronounced during observation, the commands to themouth probably modulated movements of tongue, which is directlyinvolved in release of occlusion. In other words, we hypothesizedthat the grasp observation mainly affects the phonation organdirectly involved in pronouncing that syllable. This hypothesis waspartially supported by comparing the results of previous experi-ments (Gentilucci, 2003b; Gentilucci, Santunione, et al., 2004) withthose of the present behavioral experiment showing that whenpronouncing a labial consonant the lip opening was affected by

the type-of-grasp observation whereas similar effects on lips werenot observed when pronouncing a dental consonant. Since tonguemodulation in response to grasp observation was not easily testableby using behavioral techniques, we applied TMS to tongue motorcortex during the observation of the two different types of grasp,

3192 M. Gentilucci et al. / Neuropsychologia 47 (2009) 3190–3202

Fig. 1. Variation in F1 of the syllable /da/ pronounced during observation of power and precision grasps of fruits and solids and the same objects presented alone (experiment1 fruit( one. Mg E. (Fort

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). Upper row: frames of the videos, showing the final phase of power grasp of largered cube). Lower row: frames of the videos, presenting the same fruits and solids alrasps (circles) of objects and the same objects presented alone (squares). Bars are So the web version of the article.)

he pantomimes of the same grasps, and the objects presentedlone, respectively. Simultaneously, the syllable /da/ was covertlyronounced (TMS experiments 3–5). Finally, TMS experiment 6howed that the observation of foot movements interacting withifferently sized objects did not modulate the excitability of tongueotor cortex in accordance with the results of behavioral experi-ent 1.

. Experiment 1

In behavioral experiment 1 first of all we compared the effectsf the observation of hand grasp of differently sized objects withhose of foot interactions with differently sized objects on the lipinematics and voice spectra of the syllable/da/pronounced duringbservation; then we compared the effects of the observation ofdible and non-edible objects either grasped or presented alone onhe lip kinematics and voice spectra of the syllable/da/pronounceduring observation.

.1. Methods

.1.1. ParticipantsFifteen Italian volunteers (age 22–27 years, 11 females and 4 males) participated

n the experiment. All of them were classified as right-handed according to thedinburgh Inventory (Oldfield, 1971) and were naïve to the purpose of the study.he Ethics Committee of the Medical Faculty of the University of Parma approvedhe study.

.1.2. Apparatus, stimuli and procedureThe participants sat in a dimly lit room in front of a table on which a 19 in. PC

onitor was placed, at a distance of 100 cm from the observer. By means of the PConitor, three series of video-clips (sampling rate: 25 frames per second, duration:

720 ms) were presented to the participants. They showed the following stimuli. Therst series showed a right hand reaching and grasping fruits (Fig. 1) as well as theame fruits alone (Fig. 1). The fruits consisted of the following: green apple, yelloweach, strawberry and cherry. The second series presented a right hand reaching and

(apple) or solid (green cube) and precision grasp of small fruit (strawberry) or solididdle row: F1 of the syllable /da/ pronounced while observing power and precisioninterpretation of the references to color in this figure legend, the reader is referred

grasping solids (Fig. 1) as well as the same solids alone (Fig. 1). The solids consistedof the following: green cube, yellow cylinder, red cube and purple cylinder. The sizesof the green cube and of the yellow cylinder were approximately the same as thoseof the green apple and of the yellow peach (i.e. large objects), whereas the sizes ofthe red cube and of the purple cylinder were approximately the same as those ofthe strawberry and of the cherry (i.e. small objects). The large objects were graspedwith a power grasp, whereas the small objects were grasped with a precision grasp.At the beginning of the video the actor’s hand was presented with the fingers inpinch position on the right of the scene and once the movement started, the timecourse of the grasp consisted of a finger opening to a maximum (maximal fingeraperture) followed by a finger closing phase on the object. The actor’s hand velocitywas higher when using the power grasp than the precision grasp. Indeed, the meanreach-to-grasp times were 340 and 1200 ms, respectively. In the third series, thevideos presented a right foot approaching and interacting with a large (football) ora small (tennis-ball) object (Fig. 2) as well as the football or the tennis-ball alone(Fig. 2). The football was touched with the foot arch, whereas the tennis-ball wastouched with the foot toes. The approaching velocity was higher when the footinteracted with the football than with the tennis-ball (movement time: 900 ms vs440 ms). In all the videos presenting objects alone, a small white circle appeared onthe object center at a time corresponding to contact time for the effector and thatobject in the videos showing interactions.

The participants were required to observe the videos carefully and to pronouncethe syllable /da/ when the effector touched the object or when the white circleappeared on the object. The following four blocks of trials were presented: in block 1,the videos presented the reaching–grasping actions, in block 2, the videos presentedthe foot actions, in block 3, the videos presented the targets of the reaching–graspingactions, and in block 4, the videos presented the targets of the foot interactions. Thelarge objects (fruit, solid, or football), either involving interaction or presented aloneand the small objects (fruit, solid, or tennis-ball), either involving interaction or pre-sented alone, were quasi-randomly presented 10 times. Consequently, the blocks 1(hand grasping small and large fruits and solids) and 3 (small and large fruits andsolids presented alone) consisted of 40 trials, whereas blocks 2 (foot interacting withfootball and tennis-ball) and 4 (football and tennis-ball presented alone) consistedof 20 trials. Each participant was tested on two sessions run in two consecutive

days. In each session two blocks were presented. The blocks were counterbalancedacross participants except that interaction block and objects alone block were neverpresented in the same session. This constraint prevented participants from evokingmotor representations of interaction with the effector in the object alone block whenthese interactions had been presented in the previous block. At the end of each ses-sion the participants completed a questionnaire on features of the stimuli, both static

M. Gentilucci et al. / Neuropsychologia 47 (2009) 3190–3202 3193

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ig. 2. Variation in F1 of the syllable /da/ pronounced during observation of handexperiment 1). Upper row: frames of the videos, showing the final phase of pownteractions with large (football) and small (tennis-ball) object. Lower row: frameronounced while observing hand and foot interaction with objects (circles) and th

nd moving, presented in the videos. The questionnaire required them to report cat-gory (for example cube, apple), size (large vs small), color of the static stimuli, andype of interaction (for example power grasp, precision grasp) and velocity (highs low) of the effector interacting with the object. All the participants were able tonswer correctly.

.1.3. Data recording and analysisLip movements were recorded using the 3D-optoelectronic SMART system (BTS

ioengineering, Milano, Italy). For technical reasons the performance of only ninearticipants was recorded. The SMART system consists of six video cameras, whichetect infrared reflecting markers (spheres of 5-mm diameter) at a sampling rate of20 Hz. Spatial resolution of the system is 0.3 mm. The PC presenting the video-clipsriggered the beginning of movement acquisition. Recorded data were filtered usingmoving average filter, i.e. a low pass filter where each value was the average com-uted over five samples (window duration 33.3 ms). We used two markers placed onhe upper and lower lip in order to study lip opening during syllable pronunciation.he analyzed parameters were the following: time to beginning of lip opening withespect to time of either the interaction between object and effector or the presen-ation of the white circle on the object (time to lip opening beginning), peak velocityf lip opening, maximal lip aperture and time to maximal lip aperture. Beginningf lip aperture was considered to be the first frame in which the variation in theistance between the two markers was greater than 0.3 mm (spatial resolution ofhe system).

All the 15 participants wore a light-weight dynamic headset microphone (Shure,odel WH20). The frequency response of the microphone was 50–15,000 Hz. Theicrophone was connected to a PC by a sound card (16 PCI Sound Blaster; CRE-TIVE Technology Ltd., Singapore). We acquired voice data of the participants duringyllable pronunciation using the Avisoft SAS Lab professional software (Avisoft Bioa-oustics, Germany), and calculated the participants’ voice parameters using theRAAT software (www.praat.org). In particular, we analyzed the time course of for-ant (F) 1 and 2 of the syllable vowel. It is well known that F1 and F2 exactly define

owels from an acoustical point of view (Leoni & Maturi, 2002). Both formant tran-ition and pure vowel pronunciation were included in the analysis. Mean F1, F2,itch, and intensity were calculated.

The lip kinematic parameters and the voice parameters were submitted to twoeries of ANOVAs. In the first series the grasp and the foot interaction as well ashe corresponding objects presented alone were compared with each other. Theithin-subjects factors were the following: effector-related object (hand–object vs

oot–object), object–effector interaction (effector-interacted object vs object alone)

nd object size (large vs small). In the second series the grasp of fruits and therasp of solids as well as the corresponding objects presented alone were comparedith each other. The within-subjects factors were the following: object–effector

nteraction (grasped object vs object alone), object category (fruit vs solid) and objectize (large vs small). In all analyses, paired comparisons were performed using theewman–Keuls procedure. The significance level was fixed at P = 0.05.

ot interactions with large and small objects and the same objects presented alonesp of large object and precision grasp of small object and the final phase of foot

he videos, presenting the same objects alone. Middle row: F1 of the syllable /da/e objects presented alone (squares). Bars are SE.

2.2. Results

2.2.1. Comparison between interactions of hand and foot withobjects2.2.1.1. Voice analysis. For formant 1 (F1) there was a signif-icant interaction among effector-related object, object–effectorinteraction and object size (F(1,14) = 22.2, P < 0.0005). Post hoc com-parisons showed that F1 was modulated by the observation of thetype of grasp being significantly lower during presentation of theprecision grasp as compared to the power grasp (Fig. 2). In contrast,the type of foot interaction and the size of the objects did not mod-ulate F1. The observation of the power grasp, the objects alone andthe foot interactions induced the same effect on F1 (Fig. 2).

Pitch was affected by the interaction between effector-relatedobject and object–effector interaction (F(1,14) = 4.8, P < 0.05). Posthoc comparisons showed that the foot interactions induced a sig-nificant decrease in pitch (176.8 Hz vs 171.3 Hz), whereas those ofthe hand did not (P = 0.08, 175.3 Hz vs 173.0 Hz). The type of footinteraction did not affect pitch.

Intensity was only affected by the interaction betweenobject–effector interaction and object size (F(1,14) = 5.4, P < 0.05).Post hoc comparisons showed that intensity decreased withdecreasing object size (63.5 db vs 62.8 db) and the decrease wasgreater in the interaction conditions independently of the interact-ing effector. F2 was affected neither by any factor nor interactionbetween factors.

2.2.1.2. Lip kinematics analysis. Time to lip opening begin-ning was affected by the interaction among effector-relatedobject, object–effector interaction and object size (F(1,8) = 20.2,P < 0.0005). The results of the post hoc comparisons were the fol-lowing. When observing interactions of both hand and foot the

participants responded in advance as compared to when observingthe objects presented alone (Fig. 3). The type of grasp modulatedthis parameter whereas the type of foot interaction did not, despitethe fact that the kinematic differences between the two types offoot interactions were comparable with those between the two

3194 M. Gentilucci et al. / Neuropsychologia 47 (2009) 3190–3202

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ig. 3. Variation in time to lip opening beginning when pronouncing the syllable /bjects presented alone (experiment 1). Middle row: time to lip opening beginningcircles) and the same objects presented alone (squares). Other conventions as in Fi

ypes of grasp. Time to lip opening beginning was shorter whenbserving precision grasp than power grasp. Moreover, the time toip opening beginning during observation of the foot interactions

as significantly shorter than that during observation of the powerrasp and significantly greater than that during observation of therecision grasp (Fig. 3).

Peak velocity of lip opening was affected by object sizeF(1,8) = 7.2, P < 0.05). It decreased with decreasing object size108.6 mm/s vs 98.8 mm/s). Moreover, peak velocity of lip openingas affected by the interaction between effector-related object and

bject–effector interaction (F(1,8) = 5.0, P = 0.05). It significantlyecreased when the foot (113.1 mm/s vs 90.5 mm/s), but not theand (109.3 mm/s vs 102.0 mm/s) interacted with objects (post hocest P < 0.05).

Time to maximal lip aperture increased (F(1,8) = 6.1, P < 0.05,.196 s vs 0.243 s) and maximal lip aperture decreased (F(1,8) = 6.6,< 0.05, 46.8 mm vs 44.0 mm) in the condition of object–effector

nteraction.

.2.2. Comparison between manual interactions with fruits andolids.2.2.1. Voice analysis. F1 was affected by the interaction betweenbject–effector interaction and object size (F(1,14) = 25.1,< 0.0005). It significantly decreased during the observationf the precision grasp in comparison with the power grasp (Fig. 1).

o significant difference was observed between fruits and solids

post hoc test P < 0.05, Fig. 1).Pitch (F(1,14) = 6.7, P < 0.05, 174.7 Hz vs 173.4 Hz) and inten-

ity (F(1,14) = 12.1, P < 0.005, 63.7 db vs 62.9 db) decreased withecreasing object size.

d observing hand and foot interactions with large and small objects and the samepronouncing the syllable /da/ and observing hand and foot interaction with objects

2.2.2.2. Lip kinematics analysis. Time to lip opening beginning wasaffected by the interaction between object–effector interaction andobject size (F(1,8) = 23.1, P < 0.0005). Post hoc comparisons showedthat it decreased when the hand interacted with objects and in addi-tion the response was faster when the precision grasp was observedin the comparison with the power grasp (Fig. 4). It was also affectedby the interaction between object–effector interaction and objectcategory (F(1,8) = 21.9, P < 0.0005). The response was significantlyfaster when the participants observed the interactions with fruits(post hoc test P < 0.05, Fig. 4).

Peak velocity of lip opening decreased with decreasing objectsize (F(1,8) = 8.4, P < 0.05, 110.7 mm/s vs 100.5 mm/s) and whensolids were presented (object category factor; F(1,8) = 8.6, P < 0.05,107.9 mm/s vs 103.4 mm/s).

As regards to maximal lip aperture the interaction betweenobject–effector interaction and object size was significant(F(1,8) = 18.7, P < 0.005). Post hoc analyses showed that maximal lipaperture significantly decreased with decreasing object size in thecondition of hand interaction with objects (grasped object: powergrasp, 44.5 mm vs precision grasp, 43.7 mm; object alone: largeobject, 46.9 mm vs small object, 46.6 mm).

3. Experiment 2

Experiment 2 was the TMS baseline experiment aimed at veri-

fying whether grasp observation modified OP MEPs evoked by TMSof hand motor cortex, in a way congruent with the observed type ofgrasp and hand kinematics. During TMS participants observed thepower grasp of large fruits and the precision grasp of small fruitsas well as the same fruits presented alone (Fig. 5).

M. Gentilucci et al. / Neuropsychologia 47 (2009) 3190–3202 3195

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ig. 4. Variation in time to lip opening beginning when pronouncing the syllable /resented alone (experiment 1). Middle row: time to lip opening beginning while prnd the same objects presented alone (squares). Other conventions as in Fig. 1.

.1. Methods

.1.1. ParticipantsSixteen (age 21–25 years, 7 females and 9 males) Italian, naïve, volunteers par-

icipated in the study. All of them were classified as right-handed according to thedinburgh Inventory (Oldfield, 1971) and, as all the other participants in the TMStudy (experiments 2–6), were screened to rule out any history of neurological, psy-hiatric, or medical problems, and to check for possible contraindications to TMSWassermann, 1998). Moreover, before the experiments, all signed consent formspproved by the Ethics Committee of the Medical Faculty of the University of Parma.

.1.2. Transcranial Magnetic StimulationLeft hemisphere motor cortex was magnetically stimulated by means of

onophasic single pulse TMS delivered through a 90 mm figure-of-eight coilESAOTE, Biomedica, Italy). We stimulated the left hand area of the motor cor-ex, which was localized by identifying the area where application of TMS elicited

EPs in the OP (Opponens Pollicis) muscle of the right hand. OP muscle was cho-en because it is involved in finger closure of both precision and power grasps (seeelow). The coil was placed on the head tangentially to the scalp, with the handleointing backward and laterally 45◦ away from the midline, approximately perpen-icular to the line of the central sulcus. The resting motor threshold (rMT), defineds the lowest intensity able to evoke 5 out of 10 MEPs with amplitude of at least0 �V, was determined by holding the stimulation coil over optimal scalp positioni.e., the motor cortex contralateral to the right hand producing the largest MEPs) forhe relaxed OP muscle. rMT was on average 47.2 ± 6.3% of the maximum stimulatorutput. The location of the OP muscle representation in the left motor cortex was, onverage, 6.0 ± 0.2 cm lateral and 0 ± 0.2 cm anterior with respect to Cz (vertex). In allf the TMS experiments the intensity of the stimulator was then set at 120% of theesting motor threshold for stimulations applied during the experimental session.

.1.3. Apparatus, stimuli and procedureIn a dimly lit room the participants sat on a dentist chair in front of a 19 in. PC

onitor placed at a distance of 110 cm from the observer. By means of the PC mon-

tor, video-clips (sampling rate: 25 frames per second, duration: 2680 ms) used inxperiment 1 were presented to the participants. They showed a right hand reachingnd grasping the fruits (grasped objects) as well as the same fruits alone (objectslone, Fig. 5). When the fingers were touching the object a single TMS pulse waspplied to the participants’ hand motor cortex. The same temporal sequence ofvents was followed when the objects were presented alone.

d observing power and precision grasps of fruits and solids and the same objectsncing the syllable /da/ and observing power and precision grasps (circles) of objects

In all the TMS experiments a baseline rest condition was not included in thedesign. This decision was made due to the large MEP variability that is observedwhen participants are not involved in cognitive or motor activity (see Kiers, Cros,Chiappa, & Fang, 1993).

Each fruit, presented either alone or with the interacting hand, was randomlypresented eight times. Consequently, the experiment consisted of 32 trials. The par-ticipants were required to observe carefully the stimuli, both static and moving,presented in the videos because they had to complete that part of the question-naire used in experiment 1 concerning the presentation of grasped fruits and fruitsalone. All participants were able to answer correctly the questionnaire. The taskrequired the observation of objects either grasped or presented alone. The repet-itive presentation of the same objects could decrease the attention to the stimuliwhen observing the objects as well as the object–effector interactions. This inducedus to present each object not more than eight times.

After the TMS session, the participants were required to reach and grasp theapple with their right hand by a power grasp or to reach and grasp the strawberryby a precision grasp, i.e. two of the fruits presented in the videos. Each fruit wasreached and grasped five times while electromyography (EMG) of the right OP mus-cle was simultaneously recorded with the finger movements by means of a digitalcamcorder.

3.1.4. Data recording and analysisContinuous EMG recordings from OP were acquired with a CED Micro 1401

(Cambridge Electronic Design, Cambridge, U.K.). The EMG signal was amplified(1000×), digitized (sampling rate: 8 kHz) and band-pass filtered (5–4000 Hz). TheOP muscle was recorded with a pair of Ag–AgCl surface electrodes (diameter 3 mm).The active and reference electrodes to record OP EMG were placed on the belly andthe distal tendon of the muscle, respectively.

For each trial, the EMG trace was rectified and the areas under the curve cor-responding to the MEP (the duration of the analyzed EMG trace ranged from 20 to42 ms across participants) and to the baseline EMG activity (100 ms preceding thepulse) were calculated. Because MEP size is known to be related to the amount of

baseline EMG activity, an analysis of covariance was used to adjust the MEP valuefor the corresponding baseline EMG activity in each trial (Watkins, Strafella, & Paus,2003). For each participant the adjusted MEPs were then converted to Z-scores bysubtracting the population mean from individual raw scores and then dividing thedifference by the population standard deviation. Z-scores conversion was neces-sary because the variability of MEPs across participants was high and consequently

3196 M. Gentilucci et al. / Neuropsycho

Fig. 5. Variation in OP (Opponens Pollicis) MEPs (Motor Evoked Potentials) evokedby TMS on hand motor cortex during observation of power and precision grasps oflarge and small fruits and the same fruits presented alone (experiment 2). Upperrow: frames of the videos, showing the final phase of power grasp of large (apple)fruit and precision grasp of small (strawberry) fruit. Lower row: frames of the videos,pha

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resenting the same fruits alone. Middle row: OP MEPs after single pulse TMS onand motor cortex while observing power and precision grasps (circles) of fruitsnd the same fruits presented alone (squares). Bars are SE.

-scores, by reducing variability, could reveal significances hidden by analyses onon-normalized MEP data.

Non-normalized MEPs and Z-scores of MEPs were submitted to ANOVAs whoseithin-subjects factors were fruit size (large vs small) and object–effector interac-

ion (grasped object vs object alone). In the ANOVAs performed on Z-scores of TMSxperiments 2–6 the error degrees of freedom were reduced by 1 (Montgomery,984) because the transformation in Z-scores removed a source of error. In all anal-ses paired comparisons were performed using the Newman–Keuls procedure. Theignificance level was fixed at P = 0.05.

.2. Results

MEPs of OP were greater when observing the power grasps compared to the precision grasp. Comparable effects wereot found when observing large and small fruits alone (interac-ion between object–effector interaction and fruit size, Z-scores:(1,14) = 8.96, P < 0.01, Fig. 5, non-normalized data: F(1,15) = 5.0,= 0.05, power grasp: 143.7 mV, precision grasp: 129.0 mV, large

ruit alone: 133.5 mV, small fruit alone: 140.0 mV, post hoc test< 0.05). In the final control session in which the participantsctively grasped fruits, inspection of the OP EMG traces confirmed

hat in all of the participants the muscle activity was greater dur-ng both the finger closure and holding phases of the apple grasppower grasp) than of the strawberry grasp (precision grasp). Exam-les of the OP EMGs are reported in Fig. 6.

logia 47 (2009) 3190–3202

4. Experiment 3

Experiment 3 aimed at determining whether effects similar tothose observed on hand muscle in experiment 2, could be foundon tongue muscle when stimulating mouth motor cortex. Theobjects either grasped or presented alone were fruits and solids(Fig. 7). Solids and fruits were presented as in behavioral experi-ment 1 in order to study the effects of the observation of edible andnon-edible objects either grasped or presented alone on tongueexcitability.

4.1. Methods

4.1.1. ParticipantsTen (age 22–30 years, 5 females and 5 males) Italian, naïve, and right-handed

(according to the Edinburgh Inventory, Oldfield, 1971) volunteers participated inthe experiment.

4.1.2. Transcranial Magnetic StimulationWe stimulated the tongue area of left motor cortex. It was localized by firstly

identifying the hand area where OP MEPs were evoked (see experiment 1) and thenmoving the coil ventrally and slightly anteriorly until the optimal scalp position forthe tongue muscles was found. The rMT averaged across participants in all the TMSexperiments in which the tongue motor cortex was stimulated (experiments 3–6),was 49.3 ± 6.4% of the maximum stimulator output. The location of the tongue mus-cle motor area, averaged across participants in experiments 3–6 was, 11.4 ± 0.6 cmlateral and 0.2 ± 0.4 cm anterior with respect to Cz.

4.1.3. Apparatus, stimuli and procedureThe apparatus was the same as in experiment 2. By means of the PC moni-

tor, video-clips (sampling rate: 25 frames per second, duration: 2680 ms) used inexperiment 1 were presented to the participants. The video-clips presented thehand reaching and grasping either the solids or the fruits as well as the sameobjects alone (Fig. 7). The syllable /da/ was presented on the objects during thefinger closing phase, or at a corresponding time during the presentation of theobjects alone (on which no white circle appeared). When the fingers were touchingthe object, that is 200 ms after /da/ appearance, a single TMS pulse was appliedto the participants’ tongue motor cortex. Note that an increase in excitabilityof left tongue area was observed 100 ms after consonant presentation (Fadiga,Craighero, Buccino, & Rizzolatti, 2002), whereas automatic reading of words occurswithin 300 ms after word presentation (Rayner, 1984). Consequently, the syllablecould be silently pronounced approximately 200 ms after visual presentation. Thesame temporal sequence of events was followed when the objects were presentedalone.

Each object (fruits and solids), presented either alone or with the interactingeffector, was randomly presented eight times. Consequently, the experiment con-sisted of 64 trials. We avoided presenting all the conditions of experiments 3–6 toa unique sample of participants in order to avoid the lack of TMS effects due to adecay of attention to the stimuli because of the long total duration of the trials.

The participants were required to observe carefully the stimuli, both static andmoving, presented in the videos because they had to complete the part of thequestionnaire used in experiment 1 concerning the solids and fruits grasped andpresented alone. All participants were able to answer correctly the questionnaire.

4.1.4. Data recording and analysisThe EMG of tongue muscles was recorded with a pair of Ag–AgCl surface elec-

trodes (diameter 3 mm). The electrodes used to record tongue muscles were fixedon a non-magnetic metal clip device. The active and reference electrodes to recordtongue EMG were placed on the dorsal surface and the ventral aspect of the tongue,respectively, approximately 2 cm caudal to the tongue apex.

Non-normalized MEPs and Z-scores of MEPs of tongue were submitted toANOVAs whose within-subjects factors were object category (solid vs fruit), objectsize (large vs small) and effector–object interaction (grasped object vs object alone).In all analyses paired comparisons were performed using the Newman–Keuls pro-cedure. The significance level was fixed at P = 0.05.

4.2. Results

MEPs of the tongue were affected by the size of the objects (Z-scores: F(1,8) = 10.82, P < 0.01, non-normalized data: F(1,9) = 9.36,

objects, either grasped or presented alone (Fig. 7, non-normalizeddata, large object: 37.8 mV, small object: 35.7 mV). In addition, thepresentation of fruits induced an increase in MEPs of the tongue, incomparison with the effects of the presentation of solids (Z-scores:

M. Gentilucci et al. / Neuropsychologia 47 (2009) 3190–3202 3197

Fig. 6. Examples of the rectified activity of the right OP muscle during the grasp of the apple (large object) with a power grasp and the strawberry (small object) with aprecision grasp.

Fig. 7. Variation in tongue muscle MEPs evoked by TMS on tongue motor cortex during observation of power and precision grasps of objects and the same objects presentedalone (experiment 3). Upper row: frames of the videos, showing the final phase of power grasps of large fruit and solid (green apple and cube) and precision grasps of smallfruit and solid (strawberry and red cylinder). Lower row: frames of the videos, presenting the same fruits and solids alone. Middle row: tongue muscle MEPs after singlepulse TMS on tongue motor cortex while observing the power and precision grasps (circles) of objects and the same objects presented alone (squares). Bars are SE. (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

3 sychologia 47 (2009) 3190–3202

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198 M. Gentilucci et al. / Neurop

(1,8) = 9.23, P < 0.01, Fig. 7, non-normalized data: F(1,9) = 4.10,= 0.07, fruit: 37.3 mV, solid: 35.7 mV).

. Experiment 4

The finding that the size of the observed objects affected tongueEPs, independently of manual interactions, has two explanations.

he first is that the objects directly activated movements of mouthanipulation with tongue. The finding that the TMS effect was

reater when observing fruits (i.e. edible objects) as compared toolids (i.e. non-edible objects) might support this explanation. Theecond stems from the fact that trials in which objects were pre-ented alone were randomly intermingled with trials presentingbjects being grasped. Thus, even when observing the objects alone,articipants could activate hand grasp motor representations ofhat object on the basis of the previously observed interactionsBarbieri et al., 2007; Tucker & Ellis, 1998). In other words thectivation of affordances was easier. These grasp motor represen-ation influenced mouth movements. In order to establish whichxplanation was correct, we carried out experiments 4 and 5. Inxperiment 4 we presented pantomimes of power and precisionrasps, whereas in experiment 5 we presented the same objects asn experiment 3 without presenting manual grasp of the objects.n both the experiments, we stimulated tongue motor cortex. If therst explanation were correct, we should find an effect of objectize in experiment 5 and no effect in experiment 4. In contrast, ifhe second explanation were correct, we should find an effect ofhe type of grasp in experiment 4 and no effect of object size inxperiment 5.

.1. Methods

.1.1. ParticipantsTwelve (age 25–27 years, 5 females and 7 males) Italian, naïve, and right-handed

according to the Edinburgh Inventory, Oldfield, 1971) volunteers participated in thexperiment.

.1.2. Apparatus, stimuli and procedureThe apparatus was the same as in experiment 3. The video-clips presented pan-

omimes of power grasp (movement time: 440 ms) and precision grasp executedith the same right hand (movement time: 940 ms, Fig. 8). Hand movement direc-

ion and temporal sequence of the events (presentation of the syllable /da/ anduccessive stimulation) were the same as for experiment 3. In catch trials the syl-able /da/ alone was presented. Each of the two types of pantomimed grasp wasandomly presented eight times. Consequently, experiment 4 consisted of 24 (16rials showing pantomimed grasps and 8 catch trials) trials. The participants wereequired to observe carefully the stimuli, both static and moving, presented in theideos because they had to complete a questionnaire requiring them to report typef the pantomimed interaction (power grasp vs precision grasp) and velocity (high vsow) of the effector. All participants were able to answer correctly the questionnaire.

.1.3. Data recording and analysisContinuous EMG recordings from tongue muscles were acquired as in exper-

ment 3. Non-normalized MEPs and Z-scores of MEPs were submitted to ANOVAshose within-subjects factor was type of grasp (power grasp vs precision grasp).

he significance level was fixed at P = 0.05.

.2. Results

The observation of pantomimes of power grasp induced a sig-ificant increase in tongue MEPs as compared to the observation ofantomimes of precision grasp (Z-scores: F(1,10) = 11.24, P < 0.01,ig. 8, non-normalized data: F(1,11) = 10.84, P < 0.01, power grasp:7.9 mV, precision grasp: 25.6 mV).

. Experiment 5

.1. Methods

.1.1. ParticipantsFourteen (age 24–35 years, 4 females and 10 males) Italian, naïve, and

ight-handed (according to the Edinburgh Inventory, Oldfield, 1971) volunteersarticipated in the experiment.

and precision grasps. Upper row: tongue muscle MEPs after single pulse TMS onmouth motor cortex while observing pantomimes of the two types of grasp (circles).Bars are SE.

6.1.2. Apparatus, stimuli and procedureThe apparatus was the same as in experiments 2, 3, and 4. The videos showed

the same fruits and solids presented in experiment 3 (Fig. 9), but no grasp actionwas shown. Tongue motor cortex was stimulated 200 ms after presentation of thesyllable /da/, as in experiment 3.

Each object (fruits and solids) was presented eight times. Consequently, theexperiment consisted of 32 trials. The participants were required to observecarefully the stimuli presented in the videos because they had to complete a ques-tionnaire requiring them to report category (for example cube, apple), size (large vssmall), and color of the static stimuli. All participants were able to answer correctlythe questionnaire.

6.1.3. Data recording and analysisEMG activity of tongue muscles was recorded as in experiments 3 and 4. Non-

normalized MEPs and Z-scores of MEPs of tongue were submitted to ANOVA whosewithin-subjects factors were object category (solid vs fruit) and object size (large vssmall). In all analyses paired comparisons were performed using the Newman–Keulsprocedure. The significance level was fixed at P = 0.05.

6.2. Results

Neither size (Z-scores: F(1,12) = 0.09, P = 0.8, Fig. 9, non-normalized data: F(1,13) = 0.22, P = 0.6, large object: 36.0 mV, smallobject: 35.1 mV) nor category (Z-scores: F(1,12) = 0.001, P = 1.0,Fig. 9, non-normalized data: F(1,13) = 0.02, P = 0.9, fruit: 35.5 mV,solid: 35.4 mV) of the objects presented alone had any effect ontongue MEPs. Neither the interaction between object size andcategory was significant (Z-scores: F(1,12) = 1.26, P = 0.3, Fig. 9, non-normalized data: F(1,13) = 0.58, P = 0.5, large fruit: 35.5 mV, smallfruit: 35.6 mV, large solid: 36.4 mV, small solid: 34.6 mV).

7. Experiment 6

Experiment 6 aimed at determining whether the observationof foot movements differently interacting with objects varying insize, differentially affected tongue MEPs after stimulation of tonguemotor cortex.

M. Gentilucci et al. / Neuropsychologia 47 (2009) 3190–3202 3199

Fig. 9. Variation in tongue muscle MEPs evoked by TMS on tongue motor cortexduring observation of fruits and solids (experiment 5). Upper and lower rows: framesof the videos, presenting large and small fruits (green apple and strawberry) andsolids (green cube and red cylinder). Middle row: tongue muscle MEPs after singlepulse TMS on tongue motor cortex while observing the fruits (circles) and the solids(l

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Fig. 10. Variation in tongue muscle MEPs evoked by TMS on tongue motor cortexduring observation of a foot interacting with objects and the same objects presentedalone (experiment 6). Upper row: frames of the videos, showing the final phase offoot approaching and touching a large (football) and a small object (tennis-ball).Lower row: frames of the videos, showing the same objects presented alone. Middle

squares). Bars are SE. (For interpretation of the references to color in this figureegend, the reader is referred to the web version of the article.)

.1. Methods

.1.1. ParticipantsNine (age 26–33 years, 4 females and 5 males) Italian, naïve, and right-handed

according to the Edinburgh Inventory, Oldfield, 1971) volunteers participated inhe experiment.

.1.2. Apparatus, stimuli and procedureThe apparatus was the same as in experiments 3–5. The videos presented the

oot interactions presented in experiment 1 as well as the targets of the foot inter-ctions alone (football and tennis-ball, Fig. 10). The syllable /da/ appeared on thebject 200 ms before touching of the object or at a corresponding time during theresentation of the objects alone. As for the other experiments, tongue motor cortexas stimulated 200 ms after presentation of the syllable /da/.

Each object (football or tennis-ball), presented either alone or with the inter-cting effector, was randomly presented eight times. Consequently, the experimentonsisted of 32 trials. The participants were required to observe carefully the stim-li, both static and moving, presented in the videos because they had to completehe same questionnaire used in experiment 1. All participants were able to answerorrectly the questionnaire.

.1.3. Data recording and analysisEMG recordings of tongue muscle were acquired as in experiments 3–5.

on-normalized MEPs and Z-scores of MEPs were submitted to ANOVAs whose

ithin-subjects factors were object size (large vs small) and effector–object inter-

ction (foot-interacting-with-the-object vs object alone). In all analyses pairedomparisons were performed using the Newman–Keuls procedure. The significanceevel was fixed at P = 0.05.

row: tongue muscle MEPs after single pulse TMS on tongue motor cortex whileobserving a foot interacting with the objects (circles) and the same objects presentedalone (squares). Bars are SE.

7.2. Results

Object size (Z-scores: F(1,7) = 1.47, P = 0.3, non-normalized data:F(1,8) = 2.20, P = 0.2, large object: 37.2 mV, small object: 42.4 mV),effector–object interaction (Z-scores: F(1,7) = 1.62, P = 0.2, non-normalized data: F(1,8) = 2.7, P = 0.1, interaction with the object:42.6 mV, object alone: 36.9 mV) and the interaction betweenthe two factors (Z-scores: F(1,7) = 0.01, P = 1.0, Fig. 10, non-normalized data: F(1,8) = 0.48, P = 0.5, interaction with the largeobject: 39.5 mV, interaction with the small object: 45.7 mV, largeobject alone: 34.9 mV, small object alone: 39.0 mV) were not sig-nificant (see Fig. 10).

8. Discussion

8.1. Behavioral study

The observation of the two types of grasp modulated F1, whereasthe observation of the two types of foot interaction and of thedifferently sized objects presented alone did not. F1 was lowerwhen observing precision grasp than power grasp. Moreover, theobservation of the power grasp had the same effects on F1 as the

observation of the foot interactions and the objects alone (Fig. 2).We have hypothesized that the type-of-grasp observation activatescongruent programs of hand grasp, which in turn are transmitted tothe mouth inducing variation in internal mouth aperture and con-

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equently in F1, when syllables are simultaneously pronounced.pecifically, the power grasp, during which the finger shaping isarger and finger opening/closing is quicker, induces larger mouthperture and an increase in F1. Conversely, the precision grasp,uring which the finger shaping is smaller and the finger open-

ng/closing is slower, induces smaller mouth aperture and decreasen F1. It is well known that F1 depends on internal mouth aper-ure (Leoni & Maturi, 2002). We have also hypothesized that thebservation of different types of foot interactions and of differentlyized objects alone does not induce comparable effects. The resultsalidated our hypothesis. However, F1 of the syllable pronounceduring grasp observation was on average lower than F1 recorded

n the conditions of foot interactions and objects alone. This cane explained considering that the actual pronunciation of the syl-

able was temporally coordinated with the simultaneous covertmitation of the hand grasp. The release of occlusion during syl-able pronunciation is usually quicker than finger opening/closing.onsequently tongue movement slowed down during covert imita-ion of the grasp (and especially the precision grasp). This producedmaller internal mouth aperture and lower F1.

The type of object, i.e. the target of the observed grasp (edible vson-edible object) affected neither F1 nor other voice parameters.owever, the finding that lip opening was faster when fruits wereresented suggests that the edible objects have more powerful rela-ion with mouth activation. Indeed, the presence of fruit activatedommands to eat, which in turn facilitates mouth movements evenf not those specifically related to spoken language.

Pitch and intensity were affected by the size of the objectsven when they were presented alone. These parameters increasedhen observing large objects as compared to small objects. Inprevious study (Gentilucci, Santunione, et al., 2004), we found

n increase in pitch and intensity when presenting power graspss compared to precision grasps. Since in that study we didot present objects alone, also those previous effects might beue to the size of the object. Parameters of the lip kinematicsere greater when observing large objects than small objectsresented alone. The finding that the type of grasp did not con-istently affect the lip kinematics suggests that modification inhe internal mouth was more responsible for variation in F1 as

function of type of grasp. We hypothesized that tongue con-raction was modulated, because tongue is mainly involved inhe pronunciation of /da/. This was confirmed by the TMS studysee below). In a previous study (Gentilucci, Santunione, et al.,004), we observed that the type of grasp consistently affectedhe lip kinematics. This can be explained by considering that theyllable /ba/ was pronounced and the command to the mouthould be planned in order to modulate the release of lip occlu-ion. In contrast, this did not occur during /da/ pronunciationecause lip movements are poorly related to consonant pronun-iation.

The results concerning time to lip opening beginning sug-est that the visual information about the moving effector wassed to predict the contact with the object. Moreover, the activa-ion of programs of covert imitation of the observed movementsould be used for this purpose. In fact, the findings that the pre-iction was not modulated by the type of foot interactions andas delayed for hand interactions with less familiar objects (i.e.

olids) in spite of comparable arm velocities, suggest that pre-iction of the contact with the object was based not only onimple visual analysis of stimulus velocity. Conversely, the findinghat the familiarity of observed hand–object interactions played a

ole suggests that the activation of covert imitation was respon-ible for the prediction because this was easier for commonlyctivated grasp programs. The finding that the type of foot inter-ction did not modulate the prediction of the contact may bexplained as due to the fact that probably, covert imitation of foot

logia 47 (2009) 3190–3202

movements is poorly temporally coordinated with mouth move-ments.

8.2. TMS study

Experiment 2 confirmed that the observation of grasp move-ments affects the excitability of hand motor cortex (Aziz-Zadeh etal., 2002; Fadiga et al., 1995; Gangitano et al., 2001). In addition, theexcitability was modulated by the observed type of grasp and handkinematics. Specifically, MEPs of OP muscles were greater whenobserving power grasps than precision grasps. This finding furthersupports the hypothesis that grasp observation activated the handmirror system. Indeed, mirror neurons are usually selective for thetype of grasp; for example, some mirror neurons discharge whenobserving and executing a precision grasp, but not a power graspand conversely other mirror neurons discharge when observing andexecuting a power grasp, but not a precision grasp (Gallese et al.,1996). Kinematics studies showed that during precision grasps ofsmall objects, peak velocity of finger opening and maximal fingeraperture are smaller and duration of finger closing is longer in com-parison with the power grasp of large objects (Chieffi & Gentilucci,1993; Gentilucci et al., 1991, 1994). Consequently, the motor sys-tem develops different strength for those finger muscles activeduring execution of both types of grasp and this is directly related totheir different kinematics. In our study, the mirror system, differen-tially activated by the observation of power and precision grasps,sent to motor cortex commands of finger opening and closing tothe hand which differed in strength of activation according to theobserved type of grasp and hand kinematics. The finding that OPactivation was greater during execution of power than precisiongrasp confirmed that this muscle is related to the differences in thekinematics of the two types of grasp and explains the differencesin MEPs observed after TMS on hand motor cortex.

Taken together, the results of experiments 3–5 confirmed thedata of the behavioral experiment and specifically the hypoth-esis that covert imitation of manual grasp activated by graspobservation influenced the control of tongue movements duringsimultaneous, covert pronunciation of the syllable /da/. However,in experiment 3 the observation of object features such as size,rather than interactions of the hand with objects, affected MEPsof the tongue. This suggested that the observation of graspableobjects could directly participate in the activation of commands ofmouth manipulation. The finding that edible objects, such as fruits,induced a greater effect than non-edible objects such as solids,might support this explanation. In addition, voice parameters aswell as lip kinematic parameters were directly influenced by thesize. However, in experiment 4 the observation of pantomimesof power and precision grasps (i.e. acted upon a virtual object)induced the same effects as the observation of the correspond-ing transitive actions (i.e. acted upon a real object). Conversely, inexperiment 5, in which objects were presented without intermin-gling presentations of grasp actions (as in experiment 3), effects ofthe objects alone on tongue MEPs were not observed. This suggeststhat in experiment 3 the presentation of grasp actions increased theprobability of activation of hand grasp motor programs when sub-sequently the same objects were presented alone (Barbieri et al.,2007; Tucker & Ellis, 1998). These grasp commands sent also to themouth were probably amplified by direct commands to the mouthand reached the threshold for modulating tongue contraction whenTMS was applied.

The observation of the type of foot interactions with objects of

different size did not affect tongue MEPs after TMS of mouth motorcortex. This is in accordance with data from neuroimaging stud-ies, which have shown that the observation of manual and mouthactions directed to an object (transitive actions) activates sectors ofBroca’s area and premotor areas, whereas the observation of tran-

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itive foot actions activates only a dorsal sector of premotor areaBuccino et al., 2001). Because of the proximity and the partial over-apping in Broca’s area of the hand mirror system sector with bothhe mouth mirror system and speech sectors (Demonet et al., 1992;aulesu et al., 1993; Zatorre et al., 1992), it is more plausible to sup-ose an effect of the hand mirror system, rather than of the footirror system, on mouth movements and speech.However, we assume that the foot may be linked to the

outh, but only through a common representation with the hand.ur reasoning was that foot actions affect arm/hand gestures

Baldissera, Borroni, Cavallari, & Cerri, 2002). These arm/hand ges-ures might affect mouth movements by means of the doubleand–mouth command system. Consequently, an activation of footction representation, triggered by observation, might affect theand kinematics, but was unlikely to affect directly mouth kinemat-

cs and voice. This suggests that the interaction between the motornd speech systems may be stronger and more specific for hands compared to foot/leg motor system (Floel, Ellger, Breitenstein, &necht, 2003; Liuzzi et al., 2008). In particular, Liuzzi et al. (2008)

ound unspecific increase in the excitability of the leg motor cor-ex when participants read aloud or silently words, which coulde related to foot/leg actions. On the basis of these results, we doot exclude that also the observation of foot actions can activateshe mouth motor cortex in a unspecific way. Behavioral data con-erning pitch and lip opening support this possibility. However, wexcluded that the different types of foot interaction with objectsodulated these changes like the different types of hand interac-

ion with objects did on voice formants and tongue excitability.ndeed, we demonstrated that only specific motor commands acti-ated by the hand mirror system are directly sent to the mouth asell as to the hand.

.3. Conclusions

The results of the present study are in accordance with dataf previous studies (Gentilucci, 2003a; Gentilucci et al., 2001;entilucci, Santunione, et al., 2004; Gentilucci, Stefanini, et al.,004) showing that the observation and execution of grasp actionsnd, in general, of hand/arm transitive actions guided by differentlyized objects, influence voice spectra parameters of syllables pro-ounced simultaneously with action observation and execution.

n other words, the data support the existence, as for monkeysGallese et al., 1996; Rizzolatti et al., 1988), of a hand mirrorystem and a system of double motor commands to hand andouth. They work synergistically when transitive actions involve

oth hand and mouth. A remaining open question is whetherhe mirror system and the dual hand–mouth command systemre specific for speech or rather are general purpose systems.n favor of the first hypothesis neuroimaging data have shownocalization of the hand mirror system in Broca’s area (Buccinot al., 2001; Grafton et al., 1996; Iacoboni et al., 1999). More-ver, higher levels of integration between execution/observationf gestures and words have been observed as compared to execu-ion/observation of transitive actions and syllable pronunciation:hen congruent communicative words and symbolic gestures are

imultaneously produced gestures are slowed down and wordocal parameters are enhanced. Moreover, the vocal parametersf verbal responses to the observation of gestures while simulta-eously listening to congruent words are enhanced if comparedo the observation/listening of the sole gesture/word (Bernardis

Gentilucci, 2006). These effects are transitorily abolished after

rief inactivation of Broca’s area by means of repetitive Transcranialagnetic Stimulation (Gentilucci, Bernardis, Crisi, & Dalla Volta,

006). In favor of the second hypothesis, behavioral data havehown the existence of double hand–mouth commands effectiveor simple mouth opening as well as for syllable pronunciation

ogia 47 (2009) 3190–3202 3201

(Gentilucci et al., 2001). Arising from the fact that the two sys-tems were discovered in monkey premotor cortex for activities ofhand–mouth grasp we have previously suggested that the two sys-tems may have evolved initially in the context of ingestion andobject exploration, and later formed a platform for combined man-ual and vocal communication (Gentilucci et al., 2006; Gentilucci &Corballis, 2006). A consistent hypothesis is that in modern humansthe systems have two different functions: the first is involved iningestion activities, the other is involved in relating speech to ges-tures (Bernardis & Gentilucci, 2006; Gentilucci et al., 2006). The twofunctions work synergistically and may be concurrently involvedin language development in children (Bernardis, Bello, Pettenati,Stefanini, & Gentilucci, 2008; Gentilucci, Santunione, et al., 2004).

Acknowledgments

We wish to thank Antimo Buonocore and Marc Sato for theirhelp in carrying out the TMS experiments and Patricia Gough forher comments on the manuscript. The work was supported bygrant from MIUR (Ministero dell’Istruzione, dell’Università e dellaRicerca) to M.G.

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