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finger movements has been shown to activate Brodmann

T, fMRI, and MEG

Cognitive Brain Research 25 (2* Corresponding author. Moss Rehabilitation Research Institute, Korman1. Introduction

According to the direct matching hypothesis, actions

performed by others are recognized by activating the same

spatiomotor representations used for performing the action

oneself. Numerous recent investigations in infant develop-

ment (e.g., [37]) and adult cognitive psychology (e.g.,

[4,47]) suggest that there is a common coding between

perception and action. A possible physiological foundation

for at least some aspects of this common coding is provided

by the recent discovery of so-called mirror neurons in the

inferior prefrontal cortex (area F5, the putative homologue

of human Brocas area) and inferior parietal lobule (area PF)

in the monkey. These cell units discharge both when the

monkey produces an object-related action and when a

comparable action is performed by an experimenter. The

neurons respond best to specific types of prehensile actions

upon objects (e.g., grasping), and are silent when a hand

alone or object alone is viewed [20,49].

Functional neuroimaging evidence suggests that inferior

prefrontal cortex may be involved in both action observation

and production in humans as well as in monkeys. Obser-

vation and execution of grasping movements or simplecoding handobject interactions. Forty-four patients with left-hemisphere stroke, 21 of whom exhibited ideomotor apraxia, performed a number

of pantomime imitation and recognition tasks, and performance was scored with respect to hand posture, arm posture, amplitude, and timing.

Consistent with predictions, there were strong relationships between object-related pantomime imitation and object-related pantomime

recognition, and between imitation and recognition of the hand posture component of object-related actions. Skilled object-related gesture

representations are likely to be closely tied to evolutionarily more primitive systems controlling object grasping, to emerge from a mapping

between object and action information coded by ventral and dorsal streams, and to be lateralized to the left hemisphere in humans.

D 2005 Elsevier B.V. All rights reserved.

Theme: Neural basis of behavior

Topic: Cognition

Keywords: Apraxia; Action; Gesture; Imitation; RecognitionAbstract

A considerable recent literature argues that the same representations, encoded by inferior prefrontal and parietal cells known as mirror

neurons, may be activated in both production and recognition of object-related actions. Here, we test several predictions derived from the

contemporary literature on the parity between production and recognition and the putative emergence of the mirror neuron system from a systemResear

On beyond mirror neurons: Interna

and recognition of skilled ob

Laurel J. Buxbauma,b,*, Kathl

aMoss Rehabilitation Research Institute, Korman 2bThomas Jefferson Universi

Accepted

Available on0926-6410/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cogbrainres.2005.05.014

213, 1200 W. Ta

5926.

E-mail address: Lbuxbaum@einstein.edu (L.J. Buxbaum).eport

presentations subserving imitation

t-related actions in humans

M. Kylea, Rukmini Menona

00 W. Tabor Road, Philadelphia, PA 19141, USA

iladelphia, PA 19107, USA

ay 2005

July 2005

005) 226 239

www.elsevier.com/locate/cogbrainresarea (BA) 44 or 45 in a number of PEbor Road, Philadelphia, PA 19141, USA. Fax: +1 215 456studies [3,22,28,30,40]. These findings are consistent with

the possibility that human prefrontal cortex contains a

metric three-dimensional shapes [8]. Patients exhibiting this

pattern often have lesions involving the left inferior parietal

lobe (IPL) and intraparietal sulcus (IPS). Critically, as will

be discussed further below, there is also evidence that the

Brain Research 25 (2005) 226239 227mirror neuron system similar to that described in the

monkey [20].

In addition to simple object prehension and simple finger

movements, humans produce and recognize a considerable

repertoire of skilled object-related (so-called transitive)

movements and pantomimes. Unlike simple grasping based

on object structure, recognition and production of learned

gestures and pantomimes entail a declarative semantic

component [32]. Skilled gestures also have strong require-

ments for decoding the particular spatial configuration of the

hand and fingers that distinguishes one skilled gesture from

another. Thus, the evidence that production and observation

of simple grasping and finger movements involve the same

or overlapping neural structures does not compel the

conclusion that imitation and recognition of complex,

skilled, meaningful behavior share the same underlying

cognitive representations. Additional support for the latter

hypothesis must be gleaned from studying the relationship

of imitation and recognition of complex skilled actions. An

important source for such data may be found in patients with

ideomotor apraxia (IM).

Individuals with IM are deficient in producing transitive

(familiar object-related) gesture in gesture pantomime (to

command or sight of object) and gesture imitation tasks.

Errors may be postural and/or may involved deficits in

amplitude and timing [4446]. Deficits persist with actual

object use but are more subtle [44], presumably in part

because of the feedback from object structure helps to

constrain degrees of freedom of the movement (see [8]). IM

occurs in nearly 60% of left-hemisphere cerebral vascular

accident patients (LCVA) [1], and involves the non-paretic

left hand of approximately 50% of these patients [34]. In

contrast to their deficits in pantomiming or imitating familiar

object-related actions, IM patients are frequently relatively

unimpaired in the production or imitation of intransitive (non-

object related, symbolic) gestures such as waving goodbye,

signaling stop, or beckoning come here [38].

Evidence from several laboratories, including our own,

indicates that patients with IM may have particular

difficulties producing and recognizing the hand postures

appropriate for skilled object-related actions. In fact, there is

growing evidence that representations for skilled, object-

related hand posture may be particularly vulnerable to loss

in IM, and encoded distinctly from on line programming

of hand posture for prehensile manipulations of objects.

Several reports indicate that apraxics impaired hand posture

may be in contrast to their substantially better arm posture

and trajectory for skilled actions [55], particularly in

pantomime tasks [44]. Hand posture errors are the most

frequent error type of IM patients on skilled sequencing

tasks [56] and pantomimed prehension tasks [33]. IM

patients are deficient in recognizing the appropriate hand

posture for interacting with familiar objects, but perform

normally in selecting hand postures for prehensile inter-

L.J. Buxbaum et al. / Cognitiveactions with novel objects in response to their structure [7]

and relatively normally in reaching to and grasping geo-hand posture component of skilled-object related panto-

mime is not deficient in these patients simply because it is

more difficult than other gesture components [9], or more

difficult than on-line reaching and grasping. For example,

the observed pattern of deficient skilled object-related

pantomime in the face of preserved object grasping doubly

dissociates from the pattern observed in patients with optic

ataxia due to superior parietal damage, who frequently

exhibit deficient grasping of visually-presented objects with

spared pantomime [43], and, in at least some cases, spared

ability to shape the hand when required to grasp familiar,

meaningful objects (cf. patient AT, [29]).

IM has been characterized variously as a disorder of

learned skilled movements [26], a difficulty making volun-

tary gestures [35], an impairment in gesturing to command

[58], and a deficit in the imitation of meaningless movements

[12]. We have suggested that these characterizations may not

capture some of the key features of the disorder [5]. The

marked disparity between performance of transitive and

intransitive actions across a range of production (e.g., to

command, to sight of object) and imitation tasks, and the

relative preservation of on line aspects of motor control

responsive to the structural features of objects, has influenced

our characterization of IM as a deficit in stored representa-

tions of the position and movements of the limb (and

particularly, the hand) subserving skilled object-related

actions. Further, these representations are likely to be

mediated by the left inferior parietal lobe [42].

The hypothesis that the inferior parietal lobe contains

gesture representations critical both for production and

recognition of actions with objects predicts that the two

should be impaired in parallel in patients with IM due to

parietal damage. Consistent with this [27], left parietal lesions

have been associated with both production and recognition

deficits, and frontal lesions only with production impair-

ments. Note that this is potentially inconsistent with the

notion of a mirror neuron system in the frontal lobe that is

critical for gesture recognition. Several more recent studies

have suggested more generically that IM patients are

impaired in the recognition of gesture (e.g., [14,61]). On

the other hand, other recent studies have failed to find an

association between gesture production and recognition

[15,24,31]. It should be noted that these studies have used

small samples ranging from 1 to 14 patients, and it remains

possible that there was insufficient power to detect an

association.1

1 One exception is a study by Bell [2] with 38 subjects; however, the

pantomime recognition task used in that study required subjects to

understand the association between an observed gesture and an associatedobject, and it is possible that this requires more extensive semantic

knowledge than that tapped by gesture recognition alone.

Forty-four left-hemisphere stroke patients participated in

the study. All patients had suffered a single left-hemisphere

basis of the average of scores on (1) novel gesture imitation

and (2) gesture to sight of objects.2 Appendix A provides

details of the administration and scoring of the meaningless

gesture imitation and gesture to command tasks, as well as

transitive and intransitive gesture imitation tasks to be

described below.

Table 1 shows subject demographics as well as scores on

the IM apraxia composite score. Scores on the two tests

comprising the composite score (gesture to sight of object

and meaningless gesture imitation) were highly correlated

(r = 0.74, P < 0.0001). Subjects were characterized as

Brain Research 25 (2005) 226239cerebral vascular accident; one subject had an additional

small asymptomatic right occipital infarct pre-dating the

left-hemisphere stroke. All subjects gave informed consent

to participate in accordance with guidelines of Albert

Einstein Healthcare Network and were paid for their

participation. Subjects were referred to the study from a

large database of potential research subjects in the Phila-

delphia area maintained by Moss Rehabilitation Research

Institute. Subjects were excluded if database records

indicated language comprehension deficits of sufficient

severity to preclude comprehension of task instructions.

Subjects over the age of 80 and/or with histories of co-

morbid neurologic disorders, alcohol or drug abuse, or

psychosis were also excluded. All subjects gave informed

consent to participate in accordance with the guidelines of

the IRB of Albert Einstein Healthcare Network, and were

paid for their participation.

We pursued two complimentary strategies in analyzing

the data. The first strategy was to classify participants as

apraxic or not, and assess whether performance of the two

groups differed on measures of interest. The second strategy

was to treat data as continuous variables to assess the

strength of relationships of scores on the measures of

interest.We derive a number of predictions from the hypothesis

that IM reflects deficient inferior parietal representations

subserving the production, imitation, and recognition of

skilled object-related gesture pantomime and skilled hand

object interactions, six of which we test here. The first

prediction is that patients with IM should be disproportion-

ately impaired in the imitation of transitive as compared to

intransitive gestures. The second is that they should be

particularly impaired in the hand posture component of

gesture imitation. The third is that we should observe an

association between performance of transitive imitation and

transitive recognition tasks. The fourth is that there should

be a similar and more specific association between

imitation and recognition of the hand posture component

of transitive actions. The fifth is that the association

between transitive recognition and intransitive gesture

imitation should be considerably weaker. The sixth is that

patients with deficient hand posture production and

recognition should have lesions that include the left inferior

parietal lobe. In the study that follows, we test these

predictions with a group of 44 left-hemisphere stroke

patients. We then discuss the implications of the data for

informing the likely characteristics of skilled object-related

action representations.

2. Subjects

L.J. Buxbaum et al. / Cognitive228Subjects were characterized as exhibiting ideomotor

apraxia (hereafter, IM) or not (hereafter, LCVA) on the4. Lesion analysis

Clinical T-1 or T-2-weighted MRI scans were available

for 36 of the 44 subjects (18 LCVA and 18 IM). Lesions

were segmented and interpreted by an experienced neuro-

logist. Subtractions of the lesioned regions of the IM versus

LCVA groups were performed by one of the authors using

the MRIcro image analysis program developed by Dr. Chris

Rorden (see http://www.psychology.nottingham.ac.uk/staff/

cr1/mricro.html). Fig. 1 shows the result of this subtraction

2 The use of a composite of novel gesture imitation and gesture to sight of

objects to define apraxia was based on several considerations. First, the

study reported here is one of several studies ongoing in our laboratory

assessing various aspects of IM, and the designation of IM in these related

studies is inclusive, as it is designed to detect IM due either to loss of

knowledge of the gestures associated with objects and/or deficits in praxis

production. Second, although these aspects of praxis can dissociate, in the

great majority of patients they tend to co-occur (see text for evidence of

strong correlation in the present study). Third, as described earlier, there is

considerable disagreement in the literature regarding the most appropriate3. Language comprehension

Participants performed the Comprehension subtest of the

Western Aphasia Battery (Kertesz Ref). Maximum possible

score was 200 points. Apraxics exhibited somewhat greater

deficits in comprehension (mean = 166, range = 117196)

than did LCVA (mean = 187, range = 111200), t(42) = 3.1,

P < 0.01.exhibiting IM if they obtained a composite score more than

2 standard deviations below the mean of age matched right-

handed control subjects who performed the tasks with their

left hands (control n = 10; 5 females; mean age 64.7, range

4377; mean education 14 years, range 1018 years; mean

score = 92.5, SD = 4.5, range = 8598). Subjects

characterized as IM (n = 21) and LCVA (n = 23) by this

criterion did not differ in age (t = 0.50, P = 0.70), education

(t = 0.65, P = 0.63), or months elapsed between lesion and

test (t = 0.82, P = 0.25).test for IM. The combined score captures patients who would be defined as

having IM by many published criteria.

ure co

A7 IM 38 50 44

A8 IM 73 58 65

BrainA9 IM 63 58 60

A10 IM 65 75 70

A11 IM 63 63 63Table 1

Subject demographics and scores on gesture production and imitation

Subject Group Gesture to sight of object Meaningless imitation Gest

A1 IM 64 60 62

A2 IM 75 58 66

A3 IM 60 40 50

A4 IM 45 73 59

A5 IM 75 75 75

A6 IM 55 25 40

L.J. Buxbaum et al. / Cognitiveanalysis. The lesion loci in the IM patients is consistent with

previous reports (e.g., [23]).

5. Experimental tasks

5.1. Study 1: imitation of transitive and intransitive gesture

In the first study, we sought to replicate previously

reported findings that patients with IM are more likely to

have difficulties with imitation of transitive as compared to

intransitive gesture pantomimes. We also assessed the

prediction that IM patients would have disproportionate

difficulty in producing the hand posture component of

A12 IM 75 58 66

A13 IM 70 68 69

A14 IM 50 78 64

A15 IM 65 58 61

A16 IM 60 50 55

A17 IM 55 90 73

A18 IM 75 58 66

A19 IM 50 53 51

A20 IM 65 50 58

A21 IM 58 65 61

L1 LCVA 90 90 90

L2 LCVA 85 93 89

L3 LCVA 88 90 89

L4 LCVA 88 93 90

L5 LCVA 90 93 91

L6 LCVA 90 93 91

L7 LCVA 97 85 91

L8 LCVA 85 90 88

L9 LCVA 93 93 93

L10 LCVA 88 93 90

L11 LCVA 88 95 91

L12 LCVA 90 93 91

L13 LCVA 90 95 93

L14 LCVA 88 100 94

L15 LCVA 95 95 95

L16 LCVA 100 93 96

L17 LCVA 88 93 90

L18 LCVA 93 93 93

L19 LCVA 83 90 86

L20 LCVA 93 88 90

L21 LCVA 78 95 86

L22 LCVA 85 85 85

L23 LCVA 90 98 94mposite Age Education Gender Handedness Lesion volume (cm3)

55 18 F R 208.4

49 16 F R 110.5

79 12 F R 58.2

79 11 F R 8.5

50 12 M R 64.3

59 16 M R 68.7

63 12 F R 253.5

67 12 F R 7.2

56 14 M R 44.9

79 16 F R NA

64 12 M R NA

Research 25 (2005) 226239 229transitive gestures. In addition to the IM and LCVA patients,

also tested were the same 10 age-matched healthy control

subjects described earlier.

5.1.1. Methods

Participants watched videotapes of an examiner perform-

ing 10 transitive and 5 intransitive pantomimes with the

right hand, and were asked to imitate the gesture as precisely

as possible with the unimpaired left hand. Transitive

gestures were hammering, cutting with scissors, sawing,

using a screwdriver, writing with a pencil, using a comb,

winding a watch, brushing teeth, flipping a coin, and eating

with a fork. Intransitive gestures were saluting, waving

goodbye, hitch-hiking, signaling stop, and beckoning

50 10 F R 151.9

49 8 M R 49.0

42 16 F R 143.6

60 12 M R 161.5

78 11 M R 51.1

42 16 F R 131.3

67 14 M R 119.7

39 10 M R 180.8

58 10 F R 45.3

41 10 F R 46.1

35 12 F R 41.8

56 20 M R 96.8

51 14 F R 56.7

64 12 M R 77.4

77 12 M R NA

42 15 F R 16.8

55 12 M R 69.3

51 12 F R 41.4

58 12 M R 58.6

58 3 M R 0.5

50 12 M R NA

65 19 M R 61.6

77 16 F R 13.6

50 18 F R 31.2

51 8 M L 28.0

80 12 F R 0.4

55 16 F R 18.4

56 12 M R 22.4

54 12 F R NA

42 8 M R 178.0

54 18 M L 95.1

40 16 F R 142.2

64 16 M R 25.6

Four KruskalWallis ANOVAs were performed, one with

each gesture component; therefore, a Bonferroni-corrected

P value of .0125 was required for significance. Again, we

tested for between-group differences in transitive versus

e indicate the difference in the proportion of patients in the two groups having

ate relatively more lesion in IM group in increments of 20%. Left and right are

er, there were several regions in which lesions were more likely. These include

39 and 40 (inferior parietal).

L.J. Buxbaum et al. / Cognitive Brain Research 25 (2005) 226239230come here. Participants were permitted to begin imitation

while watching the videos. Gestures were scored according

to the detailed error taxonomy described in Buxbaum,

Giovannetti, and Libon [6] (and see [8,9]) and detailed in

Appendix A.3

5.1.2. Results

Scores are reported here in terms of percent correct: IM

transitive mean = 63%, intransitive mean = 89%, mean

transitive intransitive difference = 25.6%; LCVA tran-

sitive mean = 93%, intransitive mean = 99%, mean

difference = 4.7%; CTL transitive mean = 93%, intransitive

mean = 100%, mean difference = 5.8%. In this and all

subsequent analyses, the difference between transitive and

intransitive performance was calculated for each subject,

Fig. 1. Subtractions of lesioned regions of IM versus LCVA groups. Thes

involvement in a given region. Colors further to right on the color bar indic

reversed. There were no regions uniquely damaged in the IM group; howev

Brodmann areas 6 and 44 (dorsolateral frontal), 22 and 37 (temporal), andand between-group comparisons of the difference scores

were performed with KruskalWallis nonparametric one-

way ANOVAs. Post hoc testing was performed with Mann

Whitney tests. There was an effect of group, H = 28.36, P

0.36). These data replicate previous reports that object-

related gestures are more difficult for IM patients than are

symbolic gestures.

In the next analysis, we assessed whether IM patients

were equally impaired in all components of transitive

gesture. Fig. 2 shows the data entered into these analyses.

3 To assess scoring reliability, gestures for 6 of the participants were

scored by 2 independent coders. Percent agreement between the coders

ranged from 78% to 100% across the 6 subjects (mean 88% agreement;

Cohens kappa = 0.60).Fig. 2. Performance of IM, LCVA, and CTL groups in imitation of

intransitive and transitive gesture pantomimes, scored for hand posture

(HP), arm posture (AP), amplitude (AMP), and timing (TIM) components.

Brainintransitive performance. There were significant between-

group differences in hand posture, H = 35.2, P < 0.0001;

arm posture, H = 10.5, P = 0.003; and amplitude, H = 12.3,

P = 0.002. Post hoc testing of the 3 significant ANOVAs

with MannWhitney tests using a Bonferroni-corrected P

value of .0055 (i.e., 0.05/9) indicated that there were

significant differences between the IM patients and the

other two groups for hand posture and amplitude (P 0.27). Thus, the

disproportionate disparity between transitive and intransi-

tive gesture in the IM group was reliably observed in several

gesture components.

In a final analysis, we addressed the concern that the

disproportionate impairment in transitive as compared to

intransitive hand postures in the IM group may be related

to the greater complexity of the former. We examined the

data from the IM and LCVA groups for a subset of 5

transitive gestures having a simple, stable hand posture

denoting grasp of a tool (hammering, sawing, combing

hair, brushing teeth, and eating with a fork) and all 5 of the

intransitive gestures examined previously (waving good-

bye, beckoning come here, hitch-hiking, signaling stop,

and saluting). Within-group comparisons were performed

with Wilcoxon Signed Ranks Tests (with a Bonferroni-

corrected P value of .008, i.e., .05/6, required for signi-

ficance). For transitive movements, IM patients hand

posture (mean 50.4% correct) tended to be more deficient

than arm posture (66.7%; P = 0.02), amplitude (69%; P =

0.01), or timing (82%; P = 0.009). For intransitive

imitation, IM patients hand posture (mean 91% correct)

tended to be better than arm posture (84%; P = 0.01), and

was equal to amplitude (91%) and timing (89.5%). In

between-group comparisons, MannWhitney U tests (with

a Bonferroni-corrected P value of 0.016 required for

significance) confirmed that the disparity between transitive

and intransitive hand postures was more pronounced for the

IM patients than the other two groups (Ps < 0.008), who

did not differ from one another (P = 0.5).

5.1.3. Discussion

In this study, we replicated previous findings indicating

that transitive gesture imitation is more impaired in patients

with IM than is intransitive imitation. Moreover, consistent

with the possibility that the system that is damaged in IM

patients is specialized for handobject interactions, the hand

posture component of gesture imitation for transitive

(object-related) gestures proved to be the most impaired

aspect of IM patients performance.

One possible objection to the proposed interpretation is

that transitive hand postures may simply be more difficult

than other components of transitive gesture, and more

difficult than intransitive hand postures, and thus more

L.J. Buxbaum et al. / Cognitivesensitive to any type of impairment. There are two lines of

evidence against this interpretation. First, when we com-pared IM and LCVA patients performance in imitation of

intransitive hand postures and transitive postures having a

simple, stable grasp configuration, the intransitive hand

postures were still strikingly superior. Another line of

evidence comes from data we have reported from 4 patients

with corticobasal degeneration (CBD), a degenerative

disorder affecting primarily the superior parietal lobes (areas

5 and 7 bilaterally) in early stages of disease progression [9].

The disorder has been known as primary progressive

apraxia because of its devastating effects on action

production. Although the CBD patients were more impaired

than the IM patients overall, they were less impaired in the

hand posture component of transitive gesture imitation

(mean 88% correct) than arm posture (46%), amplitude

(54%), or timing (58%). The data are consistent across

subjects: hand posture was the least impaired gesture

component in all 4 CBD subjects. Additionally, unlike the

patients with IM due to stroke reported here, the transitive

hand postures of the CBD patients (mean 88% correct) were

slightly better than their intransitive hand postures (mean

80% correct). These data strongly suggest that transitive

hand posture is not simply more sensitive to brain damage

than other aspects of gesture imitation. The system damaged

in the IM patients appears to be particularly strongly

involved in coding information about the position of the

hand for object-related gestures.

It is also of note that Mozaz et al. [38] recently argued

that the frequently observed difference between transitive

and intransitive gestures cannot be reduced to differences in

movement complexity. Healthy subjects were asked to

produce both gesture types as well as to discriminate static

photographs of transitive and intransitive gestures. They

performed more poorly with transitive gestures, both on the

production and picture discrimination tasks. The investi-

gators argued that the difficulty with the latter is not likely

attributable to the differential complexity of transitive versus

intransitive movements, but instead reflects differences in

the underlying representations of the two gesture types.

One additional point of interest in the present study is that

the deficit in transitive gestures, and in transitive hand posture

in particular, was observed on an imitation task. According to

2-route models of gesture production, gesture imitation can

be accomplished via a direct route that bypasses repre-

sentational knowledge, but permits calculation of the current

position of the actors body parts in space, and transformation

of these coordinates into a body-centered system of coor-

dinates appropriate for the observers action [5,21]. Presum-

ably, such a route would be engaged regardless of whether a

gesture was meaningful or not, and transitive or not. The use

of such a direct route would not explain the difference

between transitive and intransitive gestures, unless again the

former were simply harder in terms of spatiomotor trans-

coding, and as we have discussed, the data from the CBD

patients speak against this possibility. Instead, these data

Research 25 (2005) 226239 231suggest that a representational (or indirect) system that is

sensitive to gesture transitivity is recruited for imitation

Data from the Spatial condition of the gesture recognition

task are shown in Fig. 3. It can be seen that IM patients are

more impaired in recognition of the hand posture compo-

nent of gestures than in the other components.

The data from the Spatial recognition condition were

subjected to a repeated measures ANOVA with group as a

between-subjects factor and gesture component (hand

posture, arm posture, amplitude/timing) as a within-subjects

factor. There was a significant main effect of group,

F(2,51) = 28.1, P < 0.0001, and of gesture component,

F(2,102) = 18.1, P < 0.0001, and a significant interaction

of group gesture component, F(4,102) = 6.6, P 0.1]. Thus, taking into account

the poorer comprehension of the IM group does not explain

their particularly strong predilection toward errors in hand

posture recognition.

5.2.3. Discussion

Although previous literature led us to expect that IM

patients would be impaired in both semantic and spatial

aspects of gesture recognition ([19,27,59], the findings with

regard to semantic gesture recognition were not strongly

consistent with this expectation. Although IM patients were

indeed more impaired than non-IM patients in matching

auditory and spoken words to gestures presented with

semantic foils, that difference disappeared when we

controlled for comprehension severity. This may be an

artifact of the recognition test requirements, which had a

strong language comprehension component. Another possi-

bility is that the same underlying semantic processes are

required both for the semantic aspects of gesture recognition

and language comprehension. We are currently constructing

a pre-test to assess comprehension of the test items that will

help to distinguish these possibilities.

The results from the spatial version of the gesture

recognition task indicate that not all aspects of IM patients

recognition problems are confounded with language impair-

ment. IM patients were significantly worse than non-apraxics

in distinguishing correct gestures from spatially-similar foils,

and the difference between groups in recognition of the hand

posture component of the gesture persisted even when

comprehension was controlled. In the remaining analyses,

therefore, we focus on the spatial aspects of gesture re-

cognition and in particular the hand posture component, and

the relationship of spatial gesture recognition to gesture

production.

5.3. Additional analyses of the relationship of production

and spatial gesture recognition

On the hypothesis that the same representations subserve

production and recognition of transitive (but not intransi-

L.J. Buxbaum et al. / Cognitivetive) gesture, we expect the former relationship to be

stronger than the latter.To assess the prediction that components of transitive

gesture recognition (recognition of hand posture, arm

posture, amplitude/timing) should correlate more strongly

with production of these same gesture components in

transitive than in intransitive gesture, we performed non-

parametric (Spearman) correlational analyses of the compo-

nents of gesture recognition (hand posture, arm posture and

amplitude/timing) and these same components in the

transitive and intransitive imitation tasks. As 18 correlations

were performed, a Bonferroni-corrected P value of

scores for the Low Recognition and High Recognition

Groups using a MannWhitney Test. As shown in Fig. 4,

there was greater relative impairment of the Low Recog-

nition group in transitive as compared to intransitive

gestures (U = 62.0, P < 0.0001).

5.3.1. Discussion

The data from three analyses of the relationship between

gesture production and recognition tell a consistent story.

The first analysis showed that transitive gesture imitation is

a strong and unique predictor of transitive gesture recog-

nition. The second analysis showed that individual compo-

nents of transitive gesture recognition (hand posture, arm

posture, and amplitude/timing) are more strongly related to

these same components instantiated in transitive as com-

pared to intransitive gesture imitation. The third analysis

L.J. Buxbaum et al. / Cognitive Brain234showed that patients who fare poorly in transitive gesture

recognition are disproportionately impaired in transitive (as

compared to intransitive) imitation, whereas patients who

are better at transitive gesture recognition show less

disparity between the two types of imitation. Together,

these data argue that the same representations subserve the

recognition and imitation of transitive gesture. Furthermore,

they suggest that these representations may be componen-

tial, with hand posture representations for transitive actions

particularly vulnerable to disruption in left-hemisphere

stroke (but not in CBD, as noted earlier). In the General

discussion section, we will present a model that addresses

what may be special about these representations for

transitive gesture and how they may differ from intransitive

gesture representations.

5.4. Neuroanatomy

A final question regards the neuroanatomic basis of

transitive gesture representations, and particularly the

component of those representations that is specialized for

handobject interactions. As noted, mirror neurons active

Fig. 4. Transitive and intransitive gesture imitation performance of all

patients, divided into High Gesture Recognition and Low GestureRecognition groups on the basis of their performance of the Spatial

Gesture Recognition task.during observation of grasping actions have been identified

in monkey F5, the putative homologue of BA 45 in humans,

as well as in the inferior parietal lobe. In the present study,

we approached lesion analyses in two ways to be described

below.

5.4.1. Methods

Patients lesions were segmented by a neurologist (H. B.

Coslett) and drawn into MRIcro software by an experienced

physician (R. Menon). Brodmann areas were identified by

the first author and Dr. Coslett, who were blinded to

subjects identities, using templates from Damasio and

Damasio [11] and Mai and Assheuer [36].

Patients were ranked in terms of their performance on

the Spatial gesture recognition task, the hand posture score

from the gesture recognition task, the total score from the

transitive gesture imitation task, and the hand posture score

from the transitive gesture imitation task (4 separate

rankings). For each measure, we divided the patient group

into thirds (insofar as permitted due to ties) and discarded

the patients whose performance was in the central third of

the distribution. The lesion data from the high and low

performing groups on the recognition task are shown in

Fig. 5.

We used MRIcro software to identify whether a lesion

was present or absent in 5 Brodmann areas of interest: areas

44 and 45 (inferior prefrontal) and areas 39, 40, and 7

(posterior parietal lobe and intraparietal sulcus) in the high

versus low performing group. Lesions in BA 39 (angular

gyrus) and the inferior portion of area 7, including the

superior bank of the intraparietal sulcus, were more

frequently associated with low Spatial gesture recognition

scores than high Spatial recognition scores v2 > 4.6, P 4.3,P < 0.02 for both), and with low total gesture imitation

scores than high total gesture imitation scores (v2 > 5.0, P 0.2).

5.4.2. Discussion

Lesion analyses suggest that in stroke patients, the

lesion(s) significantly associated with deficits in the

recognition of transitive gesture, and the hand posture

component of transitive gesture, are located in the inferior

parietal lobe and intraparietal sulcus. For prefrontal cortex,

in contrast, the association with deficits in gesture recog-

nition or imitation did not even approach significance. This

is at least partially consistent with earlier findings of Varney

and Damasio [60] indicating that patients with deficits in

Research 25 (2005) 226239matching pantomimes to associated objects were likely to

have lesions in area 40 (supramarginal gyrus), areas 22 and

BrainL.J. Buxbaum et al. / Cognitive37 (posterior superior temporal lobe), and the basal ganglia,

but not the prefrontal cortex. They are also consistent with

data from Ferro, Martins, Mariano, and Caldas [18],

indicating that parietal lobe involvement was frequent in

post-acute (>3 months post-stroke) and chronic patients

with gesture recognition impairments. Together with the

present data, this suggests that in humans, Brocas area may

play a smaller role than the inferior parietal lobe in

recognizing complex familiar actions and hand postures.

6. General discussion

The hypothesis that IM reflects a deficit in the

representations underlying skilled object-related gestures,

with particular degradation of the hand posture component

of skilled object-related gestures, enabled us to generate a

number of predictions that were tested in the present study.

We demonstrated that patients with IM were disproportion-

ately impaired in the imitation of transitive as compared to

intransitive gestures, and were particularly impaired in the

Fig. 5. Subtracted lesioned regions in high versus low performing groups on the

Spatial Recognition task (bottom left and right). See Fig. 1 for additional explanaResearch 25 (2005) 226239 235hand posture component of transitive gesture imitation. This

pattern persisted even when we examined only the transitive

gestures having the simplest hand postures. We also

demonstrated a strong association between performance of

transitive imitation and recognition tasks (compared to a

much weaker association between transitive recognition and

intransitive gesture imitation), as well as a particular

association between imitation and recognition of transitive

hand postures. Importantly, the deficits in object-related

hand posture recognition were not associated with aphasia

severity. Finally, we showed that deficiencies in the spatial

aspects of object-related gesture recognition and hand

posture recognition were associated with lesions to the left

inferior parietal lobe and intraparietal sulcus.

These data suggest that the same representations sub-

serve the imitation and recognition of complex, skilled,

object-related movements, and thus provide support for the

likelihood that the direct matching hypothesis applies to

these complex, meaningful movements as well as to the

simple grasping and finger movements previously examined

in monkey and fMRI studies. In addition, the present data

Spatial Recognition task (top left and right) and Hand Posture score of the

tion.

Brainsuggest that the representations for skilled object-related

arm and hand actions may differ in important ways from the

representations subserving symbolic, non-object-related

gesture. In the following section, we discuss possible

reasons for this disparity.

One possible basis for the distinction between transitive

and intransitive gesture representations is that the former is

more closely related to an evolutionarily pre-existing system

specialized for manual grasping of objects. Fagg and Arbib

[17] take into account previous findings by Sakata and

colleagues (e.g., [52]) and Rizzolatti et al. (e.g., [50]) to

develop a model of the interactions between the anterior

intraparietal sulcus (AIP) and prefrontal F5 in programming

grasping movements. They suggested that AIP represents

the grasps afforded by objects, whereas F5 selects and

drives grasp execution. Thus, F5 aids in selecting from

among the multiple affordances that may be present in an

object based on the relevance of these affordances given

task constraints. AIP is hypothesized to provide an active

memory of the affordance selected by F5, and to

participate in updating this memory to reflect the grasp that

is actually executed. This knowledge can then be used to

inform subsequent interactions with objects (see also

[41,51,63,64]). A related conceptualization is offered by

Schettino, Adamovich, and Poizner [53], who suggest that

hand preshaping in early phases of reaching movements

reflects selection of a family of grasps based on the basic

geometry of the target object as well as developmental

experiences with handobject interactions. They contrast

this with adjustments occurring later in the reach that

modulate the basic grasp based on current information about

the object and hand.

Oztop and Arbib [41] develop this area of work further to

address the relationship between action execution and action

recognition. They offer a model that incorporates another

subclass of neurons in F5, called canonical neurons [39],

that discharge when a suitably graspable object is viewed. In

brief, the model illustrates how the mirror neurons may have

evolved to augment canonical neurons by providing visual

feedback on hand-state, i.e., the relationship of the shape

of the hand to the shape of an object. The hand-state

representation forms the basis for the ability to generalize

from ones hand to anothers hand, which in turn is

hypothesized to undergird the understanding of others

actions.

A number of investigators, then, hypothesize that the brain

computes and stores representations of hand posture based on

previous experiences with prehensile handobject interac-

tions. These representations of hand posture are postulated to

inform (1) subsequent interactions with objects, and (2)

recognition of object-related prehensile actions, and (3)

arguably, to be present in both monkeys and humans. How

does the mirror system that stores memories of prehensile

actions differ from the skilled transitive gesture system of

L.J. Buxbaum et al. / Cognitive236humans? The extensive range of complex, skilled postures

and movements exhibited by humans (consider typing,fingering a stringed instrument, grasping and re-grasping a

screwdriver as it is turned) requires knowledge regarding

functional uses of objects, the functional portions of the

object that are to be grasped, and the specific positions of the

fingers, hand, and arm for particular objects. In some

instances, the functional hand posture may be at odds with,

or an elaboration of, the prehensile hand posture called for by

object structure. The fact that the object-specific representa-

tions can be evoked even in pantomime tasks, without an

object present, suggests that they differ from the representa-

tions encoded by mirror neurons.

In humans, such an indirect route is likely to have

evolved to map between stored representations of objects in

the ventral stream, and representations of the body and

objects in space mediated by the dorsal stream. Even in the

monkey, the IPL is richly interconnected with both dorsal

and ventral stream structures. Regions within monkey IPL

project to distinct subdivisions of the dorsolateral frontal

cortex [10]. Reciprocal connections also exist between the

parietal lobe (area LIP) and the inferior temporal cortex

(areas TE and TEO), an area known to be involved in both

humans and monkeys in object recognition [13,62]. But

when overall brain volume is controlled, human IPL is

significantly larger than that of the rhesus monkey or

chimpanzee [16]. It has been suggested that human left IPL

has undergone evolutionary expansion paralleling the

development of language [16].

We suggest that left IPL/IPS system in humans mediates

between representations of object identity in the ventral

pathway and object/action structure in the dorsal pathway.

By conveying information about object identity to the dorsal

stream, we propose that the left IPL allows movements to be

selected that are appropriate to an objects category

membership, rather than only to its structural attributes.

Furthermore, we conjecture that the representations are

shaped through learning to map between identity and

structure information, taking the form of abstract movement

and posture representations that capture only those aspects

of the target action that are invariant with respect to the

initial body posture and the shape and location of any target

object. We can speculate that these transitive skilled action

representations are uniquely human, and are an elaboration

of the representations of handobject interactions encoded

by mirror neurons, both in terms of their complexity and

their requirements for long-term memory capacity. Their

abstract nature i.e., their independence from the details of

current constraints of objects and the environment renders

them potentially useful for recognizing transitive actions of

others even when the actions are pantomimes performed

without objects.

Unlike object-related actions, intransitive, symbolic

actions are not likely to retain such close ties to evolutio-

narily more primitive systems for object grasping, and are

not likely to be as strongly lateralized to the left hemisphere.

Research 25 (2005) 226239Rapcsak et al. [48] reported a right-handed man who,

consequent to a stroke resulting in virtually complete

same representations underlie perception and action for

abstracted (pantomimed) versions of complex functional

ment of Health. The Department of Health specifically

disclaims responsibility for any analyses, interpretations, or

Brainconclusions. We are grateful to Branch Coslett for perform-

ing lesion analyses and Michael Arbib for his thoughtful

comments on an earlier draft of the manuscript.

Appendix A. Details of praxis screening tasks and

scoring

A.1. Meaningless movement imitation

Participants imitated 10 meaningless movements that

were spatial and temporal analogues of transitive gestures in

terms of plane of gesture (horizontal or vertical) and the

joints around which the movement occurred. The hand

posture was also modified to be unlike the meaningful

gestures (see Buxbaum, Giovannetti, and Libon [6] for

details). For example, the meaningless movement analogous

to hammering was a vertical movement performed by the

side of the body, with movements of the elbow and shoulder

joints, and the hand in a claw shape. Participants were

permitted to begin imitation (with the unimpaired left hand)

while still observing the target gesture. Performance was

videotaped and later scored by an experienced coder inactions, just as for prehensile interactions with physical

objects, suggests that the mirror property may be a basic

organizing principle of the brain.

Acknowledgments

Supported by NIH RO1-NS36387 and NIDRR H133G0

30169 to the first author, and by the Pennsylvania Depart-destruction of the left hemisphere, was severely impaired in

transitive pantomime but relatively unimpaired with intran-

sitive actions. Consistent with these data, left and right stroke

patients demonstrate equivalent impairments in intransitive

gesture pantomime and imitation [25]. Finally, as noted,

normal subjects perform more accurately in discriminating

intransitive gestures than transitive gestures, suggesting that

the former representations may be more widely distributed

and/or more readily activated than the latter [38].

In summary, we have presented evidence that the same

representations, mediated by the left inferior parietal lobe

and intraparietal sulcus, are likely to be evoked in pro-

duction and recognition of pantomimed object-related

actions. The skilled gesture representations are active even

without the physical presence of objects, and unlike mirror

neurons, they appear to encode body and hand postures that

are specific to the functional use of particular objects.

Despite their differences, however, the evidence that the

L.J. Buxbaum et al. / Cognitiveaccordance with guidelines published in Buxbaum, Giova-

netti, and Libon [6] with respect to the components handposture, arm posture, amplitude, and timing, for a maximum

score per gesture of 4 points. Normative data for the

meaningless imitation test are presented in Buxbaum,

Johnson-Frey, and Bartlett-Williams [8].

A.2. Gesture to sight of objects

Participants were instructed to show how they would

hold and use a common object (e.g., hammer, scissors,

screwdriver, saw, toothbrush) displayed on the tabletop,

pretending they had it in their hand. They were not

permitted to touch the object. The first instance of body

part as object error (e.g., for toothbrushing, forefinger

extended and moved over teeth) was corrected with a

repetition of the task instructions and an additional

instruction to show me how you would hold the object

as if it were in your hand. The second instance of such

errors was not corrected. There were 10 trials.

A.3. Error scoring

Both the apraxia screening tests and the gesture imitation

tests were scored according to the taxonomy reported in

Buxbaum, Giovannetti, and Libon [6] and reproduced

below.

1. Hand Posture

Score as 0 if hand posture/grasp is unrecognizable,

flagrantly incorrect, or only transiently correct (small

fragment of total gesture with correct posture or

grasp). Score 0 for Fbody part as object_ (BPO)errors.

Score as 1 if posture is correct or subtly incorrect

(e.g., hand aperture slightly too big or small; wrist

angle slightly incorrect).

2. Arm Posture/Trajectory

Score as 0 if arm posture and/or trajectory (e.g.,

joint angles, plane of movement relative to body/

environment (e.g., side to side instead of back and

forth), shape of movement (e.g., circular instead of

linear) are flagrantly incorrect or only transiently

correct (small fragment of total gesture with correct

posture).

Score as 1 if both arm posture and trajectory are

correct; or if arm posture and/or trajectory are subtly

incorrect (e.g., elbow slightly too bent, trajectory at

slight angle relative to what is appropriate, shape of

movement slightly distorted).

3. Amplitude

Score as 0 if size of movement is clearly too large or

too small (e.g., sawing with small scratching

movement), or if size is only transiently correct (small

fragment of total gesture with correct amplitude).

Score as 1 if size is correct or subtly too large or too

Research 25 (2005) 226239 237small (e.g., slight Fovershoot_ or Fundershoot_ inmovement amplitude).

Brain4. Timing/Frequency

Score as 0 if speed of movement is flagrantly too

fast or slow; and/or if number of cycles of movement

is flagrantly too few or many (e.g., Fflipping_ coin 4times in succession; Fscissoring_ only once).

Score as 1 if speed of movement is subtly too fast or

slow; and/or if frequency is subtly inappropriate (e.g.,

flipping coin twice; scissoring only twice).

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L.J. Buxbaum et al. / Cognitive Brain Research 25 (2005) 226239 239

On beyond mirror neurons: Internal representations subserving imitation and recognition of skilled object-related actions in humansIntroductionSubjectsLanguage comprehensionLesion analysisExperimental tasksStudy 1: imitation of transitive and intransitive gestureMethodsResultsDiscussion

Study 2: recognition of transitive gestureMethodsResultsDiscussion

Additional analyses of the relationship of production and spatial gesture recognitionDiscussion

NeuroanatomyMethodsDiscussion

General discussionAcknowledgmentsDetails of praxis screening tasks and scoringMeaningless movement imitationGesture to sight of objectsError scoring

References