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mirror neurons
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finger movements has been shown to activate Brodmann
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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
Recommended