-
Brain (1996), 119, 933-949
Location of lesions in stroke patients with deficitsin syntactic
processing in sentence comprehensionDavid Caplan, Nancy Hildebrandt
and Nikos Makris
Neuropsychology Laboratory, Massachusetts GeneralHospital,
Boston, USA
Correspondence to: David Caplan, MD, NeuropsychologyLaboratory;
Vincent Burnham 827, Fruit Street, Boston,MA 02114, USA
SummarySixty patients, 46 with left-hemisphere strokes and 14
withright-hemisphere strokes, and 21 normal control subjectswere
tested for the ability to use syntactic structures todetermine the
meaning of sentences. Patients enactedthematic roles (the agent,
recipient and goal of an action) in12 examples of each of 25
sentence types, which weredesigned to test a wide variety of
syntactic operations. Bothright- and left-hemisphere damaged
patients performed worsethan control subjects on syntactically
complex sentences,and left-hemisphere patients performed worse than
right-hemisphere patients. Eighteen patients with
left-hemispherestrokes underwent CT scanning to image the
perisylvian
association cortex. There was no difference between
theperformance of patients with anterior and posterior lesions,and
no correlation between the degree of impairment andthe size of
lesions in different regions of the perisylviancortex. These
results are consistent with the view that syntacticprocessing
involves an extensive neural system, whose mostimportant region is
the left perisylvian cortex. When theseresults are combined with
those of other studies, the picturethat emerges is one in which,
within this cortical region, thissystem manifests features of both
distributed and localizedprocessing.
Keywords: localization of language functions; syntactic
comprehension deficits; localization of syntactic
processing;syntactic comprehension in stroke patients
Abbreviations: CVA = cerebrovascular accident (stroke); rCBF =
regional cerebral blood flow
IntroductionSentences are the level of the language code at
which themeanings of individual words are related to each other
toexpress information about events and states in the world(Jackson,
1874). This information indicates who is doing whatto whom
(thematic roles), which adjectives are associated withwhich nouns
(attribution of modification), what pronounsand other
'referentially dependent' items are related to(coreference), and
other similar semantic information. Theability to express this
propositional information contributesin an important way to the
power that human language hasas a vehicle for thought and
communication.
The propositional content of a sentence is determined bythe
syntactic structure of that sentence (Chomsky, 1986).Individual
words are assigned to different syntactic categories(e.g. noun,
verb, preposition). These categories are organizedinto hierarchical
structures (e.g. noun phrase, verb phrase) inwhich particular
phrases stand in specific relationships toone another (e.g. subject
of the verb, object of a preposition).
© Oxford University Press 1996
Propositional meaning is determined by these relationships.For
instance, in the sentence 'The dog that scratched the catchased the
bird', readers understand that 'the dog' is theagent of 'chased',
despite a sequence of words—'the catchased the bird'—that in other
circumstances could be takento express a proposition. The sentence
is understood this waybecause of the position of the words 'the
cat' and 'the dog'in the syntactic structure of the sentence: 'the
dog' is thesubject of 'chased' and 'the cat' is the object of
'scratched'and has no syntactic relationship to 'chased' (Fig.
1).
Different aspects of syntactic structure determine
differentaspects of meaning. In the sentence discussed above,
thesyntactic relationships of subject and object determinethe
thematic roles played by noun phrases. In other sentences,different
syntactic relationships determine other aspects ofmeaning, such as
what a pronoun (e.g. 'him') or a reflexive(e.g. 'himself') refers
to, which items are modified by anadjective, etc. (Chomsky, 1986).
For instance, in the sentence
-
934 D. Caplan et al.
NP COMP/ \
Det NII
The dog that 0 scratched the cat chased birdFig. 1 Diagram of
the syntactic structure of the sentence Thedog that scratched the
cat chased the bird', illustrating thehierarchical organization of
categories that determines the factthat 'the dog' is the agent of
'chased'.
'Mary's picture of her intrigued Susan', 'her' cannot refer
to'Mary', because of the syntactic relationship between 'her'and
'Mary', while in the sentence 'Mary's picture of herselfintrigued
Susan', 'herself can only refer to 'Mary', becauseof this syntactic
relationship. The syntactic relationshipbetween 'her' or 'herself
and 'Mary' that determines whetherthey can be related is known as
'c-command' (Reinhart,1983) and is different from that which
relates 'the dog' and'chased' in 'The dog that scratched the cat
chased the bird'.
Most researchers believe that determining the meaning ofa
sentence requires the assignment of a syntactic structure(parsing)
and the use of that syntactic structure in conjunctionwith the
meanings of the words in the sentence to determinethe meaning of
the sentence (sentence interpretation). Parsingand sentence
interpretation are thought to involve a numberof processes and
operations that are specific to theconstruction of the particular
syntactic relationships thatdetermine different aspects of meaning
(Frazier, 1987a, b,1989, 1990; for an alternative view, see
MacDonald, 1994).In addition, parsing and sentence interpretation
are thoughtto require a processing resource system, whose size
affectsthe efficiency and even the feasibility of assigning a
syntacticstructure and understanding a sentence (Just and
Carpenter,1992). As an illustration of this resource system,
considerthe sentence 'The man that the woman that the child
huggedkissed laughed'. Most readers cannot assign the thematicroles
in this sentence, though they can do so relatively easilyin the two
sentences that combine to form it—'The man thatthe woman kissed
laughed', and 'The woman that thechild hugged kissed the man'. The
trouble subjects haveunderstanding the sentence 'The man that the
woman thatthe child hugged kissed laughed' is thought to arise
becausethey do not have sufficient working memory capacity
tomaintain the intermediate products of computation theygenerate in
mind while processing the incoming words inthis complex
structure.
Syntactic processing is an important candidate for a
distinctly human cognitive function, whose neural basis
istherefore of considerable neurobiological significance(Pinker.
1994). However, the location of the syntacticprocessors that
operate during sentence comprehensionremains unclear. It has been
suggested that the ability toprocess syntactic structure in
sentence comprehension iscarried out in a neural net based in the
left-hemisphere,whose most active portion is Broca's area and
adjacent partsof the frontal language zone(Mesulam, 1990; Damasio,
1992;Zurif et al., 1993). Evidence supporting this
localizationcomes from the fact that a significant number of
patientswith Broca's aphasia have difficulty understanding
sentencesin which syntactic structure must be used to
determinemeaning (Caramazza and Zurif, 1976; Schwartz et al.,
1980;Caplan and Futter, 1986). One study of regional cerebralblood
flow (rCBF) using I5O-PET has shown a localizedincrease in rCBF in
part of Broca's area (the pars opercularis)when subjects made
acceptability judgements for syntacticallymore complex compared
with syntactically less complexsentences (Stromswold et al., 1996).
Studies of event-relatedpotentials have also identified an early
negative wave inthe left frontal region associated with aspects of
syntacticprocessing (Neville et al., 1991; Kluender and Kutas,
1993).
However, this evidence does not clearly settle the issue ofhow
the brain is organized for syntactic processing for severalreasons.
First, other data imply that Broca's area is not theonly brain
region in which syntactic processing occurs. Manypatients with
lesions outside Broca's area have been describedwith syntactic
comprehension disorders (Seines et al., 1983;Caplan et al., 1985;
Caplan and Hildebrandt, 1988; Tramoet al., 1988). Secondly, there
is evidence that Broca's areais not needed for syntactic
processing. Many agrammaticpatients with Broca's aphasia
demonstrate sensitivity togrammatical structure in grammaticality
judgement and othertasks (Linebarger et al., 1983; Tyler, 1985),
suggesting thatthey can assign the syntactic structure of a
sentence even ifthey cannot use it to determine sentence meaning.
Otheragrammatic patients with Broca's aphasia have shown
intactsyntactic processing in sentence-picture matching
andenactment tasks (Miceli et al., 1983; Caplan et al.,
1985;Nespoulous et al., 1988; R. Berndt, C. Mitchum and AHaendiges,
unpublished data). Thirdly, there are limitationsto the database
supporting localization of syntactic processingin Broca's area.
Most of the investigators who havedocumented syntactic processing
impairments in agrammaticpatients have not reported specific
aspects of lesions, and itis known that lesions in patients with
Broca's aphasia oftenextend well beyond Broca's area (Mohr et al.,
1978; Vanierand Caplan, 1990). The results of another, less
well-controlledPET study implicated regions other than Broca's area
insyntactic processing (Mazoyer et al., 1993). Results of
event-related potential studies have also suggested that a
moreposterior wave (the P600 or SPS) is related to aspects
ofsyntactic processing (Neville et al., 1991; Hagoort et al.,1993).
Some studies that report this wave have found it tobe maximal in
amplitude over the right hemisphere (Osterhout
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Lesions in syntactic comprehension deficits 935
and Holcomb, 1992, 1993), raising the question of
whetherevent-related potentials have the necessary spatial
resolutionto be definitive in determining the neural sites of
languageprocessing. For these reasons, the functional
neuroanatomyof syntactic processing and the role that Broca's area
playsin this function remain unsettled areas.
In this study, we report on 60 stroke patients in whomsyntactic
comprehension deficits were well-characterizedbehaviourally. In 18
patients, left-hemisphere lesions werevisualized by CT scanning.
The results provide data relevantto the functional neuroanatomy of
syntactic processing duringsentence comprehension.
MethodPatientsForty-six patients (29 English subjects and 17
French) withleft-hemisphere vascular lesions and 14 patients (nine
Englishand five French) with right-hemisphere vascular
lesionsparticipated in this study. Patients were recruited from
hospitaland rehabilitation facilities in the Montreal area.
The 29 English left-hemisphere patients consisted of 15males and
14 females, aged 20-88 (mean 63) years. The 17French
left-hemisphere patients included 10 males and sevenfemales aged
38-70 (mean 57) years. The level of educationfor both the English
and French subjects ranged from gradeschool through college.
Subjects were classified as beingright-handed or having anomalous
dominance (Geschwindand Galaburda, 1985) based on questions drawn
from theEdinburgh Handedness Scale that were answered by
thepatient, the patient's spouse, or another close informant.
Allexcept three English subjects and one French subject
wereright-handed.
CT scans were obtained for nine English and nine Frenchpatients.
The nine English patients for whom CT scans wereobtained included
three male and six female patients, eightright-handed and one
ambidextrous, with a mean age of 60years. The nine French patients
for whom CT scans wereobtained included seven male and two female
patients, all ofthem right-handed, with a mean age of 52 years.
The nine English right-hemisphere patients consisted offour
males and five females, aged 48-86 (mean 65) years.The five French
right-hemisphere patients included two malesand three females aged
27-79 (mean 54) years. Level ofeducation for the both English and
French subjects rangedfrom grade school through college. All except
one Englishand two French subjects were right-handed.
Normal subjectsTwenty-one normal subjects (11 English and 10
French) werealso tested on a subset of 22 sentences types. The 11
Englishcontrol subjects were aged 16-76 (mean 59) years. The
10French control subjects were aged 52-73 (mean 64)
years.Handedness was not recorded in the control subjects, who
were included as a benchmark for the patients' performanceon the
comprehension task.
All patients or their spouses and the normal subjects gavetheir
informed consent to participate in the study, which hadthe approval
of the local ethical committee.
MaterialsTwelve sentences of each of twenty-five sentence types
werepresented (see Appendix). Subsets of these sentences
havepreviously been used to test comprehension in stroke
patients(Caplan et al., 1985; Caplan and Hildebrandt, 1988),
patientswith closed head injury (Butler-Hinz et al., 1990),
andpatients with dementia of the Alzheimer's type (Rochonet al.,
1994). Six sentence types contained only 'full' nounphrases, which
are noun phrases like 'the dog' or 'the cat'that refer directly to
items in the real world, and six sentencetypes contained pronouns
or reflexives ('himself or 'him'),which have to be related to
another noun phrase in order tomake reference to an item in the
world. The remaining 13sentence types contained what are known as
'empty nounphrases' (Chomsky, 1986)—items such as the
understoodsubject of 'to jump' in the sentence 'John promised Bill
tojump' or the object of 'scratched' in the sentence 'The dogthat
the cat scratched chased the mouse'. To require thatsubjects
structure these sentences syntactically and not simplyrely on
real-world knowledge to determine the correctmeaning of these
sentences, all sentences were constructedsuch that any noun could
have accomplished or been therecipient of the action of any verb
and could have beenreferred to by any pronoun, reflexive or empty
noun phrasein the sentence. Thus, the sentences were structured so
as toassess a subject's ability to process a wide range of
syntacticstructures in sentence comprehension.
ProcedureSentences were divided into three batteries. The first
batterycontained active, passive, dative and relative-clause
sentencetypes. The second battery contained sentence types with
oneproposition and a reflexive or a pronoun, and matchedsentences
with full noun phrases. The third battery containedsentence types
with two propositions and either full, reflexive,pronoun or empty
noun phrases. Nouns in the first batterywere animal names, and
nouns in the second and thirdbatteries were either a definite
concrete noun phrase ('theold man', 'the boy') or a relational noun
phrase ('the father','his friend'). The three batteries were given
in the samesequential order, with the first battery first and the
third last,with a training period before each.
Subjects were tested individually in testing rooms at
thehospitals or rehabilitation facilities or in their homes. At
theonset of each session, the experimenter indicated the namesof
the objects (animals or dolls) to each patient and thentested the
patient's ability to identify these objects one at a
-
936 D. Caplan et al.
AC-PC
Fig. 2 Lateral view of the left-cerebral hemisphere in the human
showing the cortical regions ofinterest considered in this study,
and cerebral sulci. The shadowed area with the asterisk in it is
notincluded in region of interest Tl. F3t = inferior frontal
gyrus/pars triangularis; F3o = inferior frontalgyrus/pars
opercularis; SG = supramarginal gyrus; AG = angular gyrus; Tl =
superior temporal gyrus;ce = central sulcus; prc = precentral
sulcus; sf = superior frontal sulcus; if = inferior frontal
sulcus;aar = anterior ascending ramus of the sylvian fissure; ahr =
anterior horizontal ramus of the sylvianfissure; phr = posterior
horizontal ramus of the sylvian fissure; par = posterior ascending
ramus of thesylvian fissure; st = superior temporal sulcus; it =
inferior temporal sulcus; poc = postcentral sulcus;ip =
intraparietal sulcus; im = intermediate sulcus of Jensen; ag =
angular sulcus; ao = anterioroccipital sulcus; lo = lateral
occipital sulcus. (Modified from Rademacher et al., 1992.)
time and in series. Patients who could not reliably point toall
objects in the set were excluded from further testing.
Subjects were told that the purpose of the experiment wasto test
their abilities to understand 'who did what to whom'in the
sentences. Subjects were instructed to indicate 'whodid what to
whom' by acting out the sentence using theitems provided. The
experimenter emphasized that subjectsdid not need to show details
of the action of the verb, buthad to clearly demonstrate which item
was accomplishingthe action and which receiving it. Practice
sessions weregiven for each battery, during which some easy and
somedifficult sentence types were presented. During these
practicesessions, the experimenter did not correct errors that a
patientmade, but did ask for repetitions and revisions of
responsesin which it was not clear which item initiated and
whichitem received an action. Practice continued until the
patient'sactions could be clearly interpreted. The experimenter
thenread each experimental sentence with a normal,
neutralintonational contour and recorded the subject's response
(fordetails of the task, see Caplan et al., 1985; Caplan
andHildebrandt, 1988).
NeuroimagingEighteen patients with left-hemisphere strokes
underwent CTscanning. Scans were performed from 7 days to 7 years
afterthe onset of the stroke (one on day seven, two on day
eight,and the remainder from 3 months to 7 years after the onsetof
the stroke). In 17 subjects, a special protocol was usedto obtain
CT images. Scans were supervised by aneuroradiologist. The subject
was carefully positioned so thatthe imaging plane was parallel to
the canthomeatal (CM)line, which runs almost parallel to the
bicomissural (AC-PC) line (Tokunaga et al., 1977; Fox et al.,
1986). A Scoutfilm was obtained with a radio-opaque marker on the
skullperpendicular to the canthomeatal line at the point of the
external auditory meatus, which corresponds to the positionof
the posterior commissure. This marker was visible as awhite dot on
the left side of the head in all CT images, andserved to help
verify the angle of the scan (Vanier et al.,1985). The brain in its
entire width was imaged in series ofsingle slices of 5 mm width in
14 patients and of 10 mmwidth in four patients.
These 17 scans were mapped onto the Talairach andTournoux (1988)
atlas, whose templates are parallel to thebicomissural line. The
anatomical regions defined in theTalairach and Tournoux atlas
correspond roughly tocytoarchitectonic fields (Sanides, 1964;
Rademacher et al.,1993), and the atlas has been the basis for
localization ofchanges in rCBF and regional cerebral blood volume
inactivation studies (Fox et al., 1985; Belliveau et al., 1991;Fox
and Lancaster, 1993). The remaining scan, which wasobtained at a
different angle, was matched to the templatescorresponding to its
angle of imaging in the Damasio andDamasio (1989) atlas. The
templates of the Damasio andDamasio atlas were normalized to the
Talairach dimensionsso that volumes of lesions and regions of
interest would becomparable across the 18 scans.
Film images were traced on transparencies and magnifiedto match
templates in the appropriate atlas. One magnificationfactor for
each brain was used, which was the ratio of themaximum longitudinal
axis of a selected CT scan slice tothe longitudinal axis of its
corresponding template. On aslice per slice basis, guided by key
landmarks (mainly thesylvian fissure and the hemispheric margins),
the surface ofthe lesion was matched to the surface of the atlas
template.This matching required small amounts of spatial
stretching,rotation and translation. Volumetric analysis of each
lesionwas performed by measuring the surface occupied by thelesion
in each normalized CT scan slice, multiplying it bythe thickness of
the corresponding template, and summing
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Lesions in syntactic comprehension deficits 937
Table 1 Mean percentage correct for each sentence type and each
subject group
LCVA (n = 46) RCVA (n = 14) CTRL (n = 23)
Baseline sentences without referentially dependent noun
phrasesTwo-place activeThree-place activeConjoinedActive conjoined
themeThree reflexive expressionsSimple active reflexive
expressions
Sentences with overt referentially dependent noun
phrasesReflexives, simple noun phrasePronouns, simple noun
phrasesSimple active reflexSimple active reflex (friend of X
subj)Simple active pronoun (friend of X subj)Simple active
pronoun
Sentences with empty referentially dependent noun
phrasesTwo-place passiveTruncated passiveTwo-place cleft
objectThree-place passiveThree-place cleft objectSubject-object
relativeObject-subject relativeObject-object
relativeSubject-subject relativePassive conjoined agentObject
controlSubject controlNoun phrase-raising/pass object control
See Appendix for descriptions of sentence types. LCVA = left CVA
patient; RCVA = right CVA patient; CTRL = control subject;* = not
tested on this sentence type.
927769816862
746388636657
70677046543749426065694952
958391978887
888696889186
89908873675570698391776861
10010099*9778
999878777878
100*99969688959794*989396
these values across the templates in which the
lesionappeared.
Five anatomical regions of interest that correspond to
brainstructures within the left-hemisphere language zone
weredefined on the Talairach and Tournoux atlas. The regionswere
defined following the criteria described by Rademacheret al.
(1992), which rely primarily on the morphology of thecerebral
sulci. Lesion size in each region of interest wascalculated as
described above, and expressed as a percentageof the volume of each
region of interest. The five regions ofinterest in which we
performed volumetric analyses areshown in Fig. 2, and were defined
as follows.
Region of interest 1. This is a region in the superiortemporal
gyms, which corresponds to Brodmann's cyto-architectonic area 22.
It is defined posteriorly by a coronalplane that passes through the
dorsal end of the posteriorascending ramus of the sylvian fissure,
ventrally by thesuperior temporal sulcus, anteriorly by a coronal
plane passingthrough the posterior end of the temporal pole, and
dorsallyby the posterior horizontal ramus of the sylvian fissure.
Thecortex corresponding to Heschl's gyrus and the central portionof
the planum temporale (areas 41 and 42) are excluded fromthis region
of interest.
Region of interest 2. This region corresponds to pars
triangularis and the portion of frontal operculum that
underliesthe pars triangularis. It is part of the inferior frontal
gyrusand represents Brodmann's cytoarchitectonic area 45. It
isdefined posteriorly by the anterior ascending ramus of thesylvian
fissure, ventrally by the anterior horizontal ramus ofthe sylvian
fissure, anteriorly by the coronal plane that passesthrough the
rostral end of the anterior horizontal ramus, anddorsally by the
inferior frontal sulcus.
Region of interest 3. This includes the pars opercularisand the
part of frontal operculum that underlies the parsopercularis. It is
is also part of the inferior frontal gyrus. Itcorresponds to
Brodmann's cytoarchitectonic area 44. It isdefined posteriorly by
the precentral sulcus, ventrally by theposterior horizontal ramus
of the sylvian fissure, anteriorlyby the anterior ascending ramus
of the sylvian fissure, anddorsally by the inferior frontal
sulcus.
Region of interest 4. This refers to the angular gyrusand
corresponds to Brodmann's cytoarchitectonic area 39. Acoronal plane
that passes through the caudal end of theanterior occipital sulcus
is its posterior border, and theanterior occipital sulcus together
with the superior temporalsulcus form its ventral border. A coronal
plane that passesthrough the inferior tip of the intermediate
sulcus of Jensen
-
938 D. Caplan et al.
constitutes its anterior limit, and the intraparietal
sulcusdefines the dorsal border of the angular gyrus.
Region of interest 5. This corresponds to the supra-marginal
gyrus and the parietal operculum, and representsBrodmann's
cytoarchitectonic area 40. Its caudal border is acoronal plane
passing through the inferior end of theintermediate sulcus of
Jensen, its ventral limit is defined bythe posterior horizontal and
posterior ascending rami of thesylvian fissure, and the superior
temporal sulcus. Anteriorly,the postcentral sulcus is its border,
and dorsally it is delimitedby the intraparietal sulcus.
Because of concerns related to the accuracy of theparcellation
of the Talairach and Tournoux atlas accordingto the Rademacher et
al. (1992) criteria, a grosser parcellationsystem was also used.
This parcellation grouped togetherregions anterior to the
pre-central sulcus (regions of interest2 and 3) into a single
'anterior' region of interest. This regioncorresponded to the
traditional Broca's area (Brodmann'sareas 44 and 45). A second
region of interest was formed bycombining regions of interest 1, 4
and 5 into a single'posterior' region that included perisylvian
association cortexposterior to the post-central gyrus. Finally, all
five regionsof interest were combined into a single region of
interest thatreflected the entire perisylvian association
cortex.
ResultsPerformance on sentence comprehension taskPerformance of
patients and control subjects on the sentencecomprehension task is
shown in Table I.
The mean overall accuracy on the 25 sentences of the 21normal
subjects was compared with that of the group of 46patients with
cerebrovascular accident (CVA) to the lefthemisphere (L) and the
group of 14 patients with CVA tothe right hemishere (R) in a 3X2
between-subjects ANOVAwith subject type [normal controls, (L)CVA,
(R)CVA] andlanguage (English, French) as orthogonal factors. There
wasa main effect of group \F{2.11) = 20.56, P < 0.001]. Therewas
no main effect of language. There was. however, aninteraction
between group and language type 1/(2,77) =3.48, P = 0.036]. Simple
effects showed that patients with
both left and right CVAs were less accurate overall thannormal
controls. For the French patients, the (R)CVA group(overall
accuracy = 83%) was significantly better than the(L)CVA group
(overall accuracy = 48%). For Englishpatients, (R)CVA patients
(overall accuracy = 82%) did notdiffer from the (L)CVA group
(overall accuracy = 73%).The English (L)CVA patients performed
significantly betterthan the French (L)CVA patients. The difference
betweenthe (R)CVA patients and the English (L)CVA patients wasnot
statistically significant. This pattern of results indicatesthat
damage to both hemispheres affects sentence comprehen-sion, with
greater effects following left-hemisphere damagethan
right-hemisphere damage.
A point to note is that the English patients with
(L)CVAsperformed better than their French counterparts, and
thedifference between them and the English (R)CVA patientsdid not
reach statistical significance. However, in other studiesusing the
same task, English and French aphasic patientshave performed at the
same levels (Caplan et al., 1985), andthe English (L)CVA patients'
level of performance was lowerthan that of the (R)CVA patients in
the present study. Furtheranalyses {see below) indicate that the
English (L)CVA patientsshowed the same impairment in syntactic
processing as theFrench (L)CVA patients. The somewhat better than
expectedoverall performance of the English (L)CVA patients whowere
tested in this study is therefore probably an atypicalfeature of
the (L)CVA patients in this sample. We cautionagainst concluding
from this feature of this group's perform-ance that (L)CVA patients
in general perform at the samelevel as (R)CVA patients on this
task.
There are many factors that enter into the performanceof this
task, and that could have contributed to loweredperformance in the
patient groups. To investigate whetherspecifically syntactic
aspects of sentence processing wereaffected by lesions in either
hemisphere, we comparedperformance on sentences that were more
syntacticallycomplex with that on matched sentence types that
weresyntactically less complex.
Syntactic complexity was determined in the followingmanner. In
most English sentences, the subject noun phraseprecedes the verb,
and the verb is followed by an object andthen by one or more
prepositional phrases. The noun phrasesin subject and object
position are usually assigned thethematic roles of agent (the
perpetrator of an action) andtheme (the person or item upon whom
the action is enacted),respectively. The noun phrases in the
prepositional phrasesplay other thematic roles, such as goal,
beneficiary, etc.,depending upon the preposition that is present.
This order ofthematic roles—agent-theme-goal (or other)—is known
asthe canonical thematic role order for English. It has beenshown
that sentences with empty noun phrases are moredifficult than
sentences with either full noun phrases orpronouns or reflexives
when the order of thematic roles inthe sentence deviates from the
canonical agent-theme-goalorder (Caplan et al., 1985; Schwartz et
al., 1987; Caplanand Hildebrandt, 1988). This complexity is
attributable toprocessing the syntactic structure of these
sentences, asopposed to their length or other factors. Thus, to
testspecifically syntactic aspects of sentence processing,
sevensentence types with empty noun phrases in which thematicroles
occurred in non-canonical order were matched fornumber of nouns,
thematic roles and propositions with foursentences in which
thematic roles appeared in a canonicalorder (see Appendix). For
example, the sentence 'The monkeythat the cow hit pushed the goat'
was compared with thesentence 'The cow hit the monkey and pushed
the goat'.Comparison of performance on these sentences with
non-canonical thematic role order to performance on these
-
Lesions in syntactic comprehension deficits 939
1.0-1
0.9-
CD
6CJCCD
CDQ.
0.8-
0.7-
0.5-
0.4
Controls(R)CVA(L)CVA
Simple ComplexSentence type
Fig. 3 Performance of control subjects and patients with
(L)CVAand (R)CVA on matched sets of syntactically simple
andsyntactically complex sentences.
sentences with canonical thematic role order tests the
integrityof syntactic processing.
The results are shown in Fig. 3. The data were analysedin a
3X2X2 ANOVA with subject type [normal controls,(L)CVA, (R)CVA] and
language (English, French) asbetween-subject factors and syntactic
type (complex, simple)as a within-subject factor. There were main
effects of group[F(2,77) = 29.9, P < 0.001] and syntactic type
[F(l,77) =66.9, P< 0.001]. There was a groupXlanguage
typeinteraction [F(2,77) = 3.3, P = 0.04] and a groupXsyntactictype
interaction [F(2,77) = 18.7, P < 0.001]. No other effectswere
significant. The effect of group and the interaction ofgroup and
language showed the same patterns as the resultsreported above for
performance on all sentence types.
The effect of syntactic type and the interaction of groupand
syntactic type are relevant to the question of whetherpatients
showed deficits in syntactic processing. Simpleeffects showed that
performance of all groups was better onthe simple sentences than on
the complex sentences. Forsimple sentences, control subjects
performed better than bothpatient groups, and the two patient
groups did not differ intheir performance. For complex sentences,
control subjectsperformed better than both patient groups, and
patients withright-hemisphere lesions performed better than those
withleft-hemisphere lesions. These results indicate that both
left-and right-hemisphere damaged patients have more
difficultycomprehending sentences that require more complex
syntacticoperations to be understood than sentences that do not.
Thisdeficit is greater for patients with left-hemisphere
lesionsthan for patients with right-hemisphere lesions, but it
ariseswith damage to either hemisphere. The English (L)CVApatients
showed the same disturbances of syntactic processingas were seen in
the French (L)CVA patients (the three-wayinteraction of
languageXsentence typeXhemisphere was notsignificant).
CDCOCOoCD.a
6 -
2 -
-0.2 0.0 0.2 0.4Syntactic complexity score
0.6
Fig. 4 Graph of the number of subjects with left and right
CVAsshowing different magnitudes of syntactic complexity
effects.
Effects of right-hemisphere lesionsThe presence of a syntactic
complexity effect is expectedfollowing damage to the perisylvian
region of the lefthemisphere. However, it is somewhat surprising
that aneffect of syntactic complexity would arise following
right-hemisphere stroke. Its presence would be readily
explained,however, if it were due to the performance of one ortwo
patients, who might be right-hemisphere dominant forlanguage. To
determine whether this was the case, wecalculated a syntactic
complexity score for each patient,consisting of the average of his
or her performance(expressed as percent correct) on the seven
syntacticallycomplex sentences subtracted from his or her
performanceon the matched syntactically simple baseline sentences.
Forthe (R)CVA patients, these scores ranged from close to
zero(-0.044), indicating the absence of a syntactic
complexityeffect, to 0.514, indicating a considerable effect.
Thirteen ofthe 14 scores of the (R)CVA patients were positive,
indicatingthat 13 of the 14 patients contributed to the
syntacticcomplexity effect. As shown in Fig. 4. only one
right-hemisphere patient had a complexity score that
wassubstantially above the mean score of the left-hemispheregroup
and that indicated a major syntactic complexity effect.With the
exception of this one patient, the scores of the right-hemisphere
patients approximate a normal distribution. Thispattern suggests
that the syntactic complexity effect in theright-hemisphere patient
group is not due to the performanceof one or two patients, but is
due to small complexity effectsappearing in most patients.
Effects of left-hemisphere lesionsWe analysed the performance of
patients with left-hemispherelesions to gain clues as to the
determinants of their sentence
-
940 D. Caplan et al
Table 2 Factor loadingseach factor
Sentencetype
ST19ST4ST6ST14ST22ST5ST2ST15ST8ST11ST7ST10ST9ST13ST3ST12ST24ST23ST18ST21ST16ST20ST25ST17ST1Variance
(%)
Factor 1
0.859550.859160.858970.855160.853810.850070.842460.84040.837430.831550.831280.824290.818780.797440.797370.795390.794850.784220.756630.747450.723150.700030.69730.670320.54815
63.1
and variance accounted for by
Factor 2
0.31956-0.28987-0.36908-0.27535
0.010360.06886
-0.32420.23936
-0.03096-0.27093-0.16539-0.07572-0.33878
0.23839-0.09189-0.01246
0.195940.27470.07758
-0.192580.351820.52692
-0.051510.119180.364646.2
Factor 3
0.145890.019890.052290.207010.16013
-0.181020.048050.0569
-0.28533-0.2347
0.02504-0.15093-0.28031-0.37192
0.12815-0.42088
0.18158-0.1178
0.117460.24770.17586
-0.038020.312780.50633
-0.22485.0
Factor 4
-0.06984-0.10883
0.15141-0.04501-0.03516
0.0709-0.24518-0.31975-0.21175
0.130090.231090.30870.07827
-0.03572-0.04886-0.04885
0.147210.35834
-0.44259-0.26493-0.18863
0.116730.26760.28947
-0.035164.2
comprehension impairments. A factor analysis with
varimaxrotation was carried out on the 25 sentence types in order
toexplore the nature of the relationship between performanceon
these sentence types. Four factors were extracted. Loadingsof
sentence types on factors and proportionate variancecontribution
are shown in Table 2, which shows that the firstfactor accounted
for almost two-thirds of the variance. Witha cut of 0.5 for
inclusion of a variable in interpretation of afactor, all variables
loaded on the first factor, with only onesentence type (simple
active reflexive) loading on the secondfactor, and one sentence
type (subject control) loading onthe third. This analysis, which
replicates the results of Caplanet al. (1985) and extends those
results to a much larger setof sentences, shows that patient
performance is largelyaffected by a single factor. This factor has
been thought ofas the availability of the processing resource
discussed in theintroduction to this paper that constrains overall
sentenceprocessing ability (Caplan et al., 1985). This analysis
thussuggests that a useful approach to the study of the
localizationof syntactic processing would be to see whether
overallperformance and the syntactic complexity score
describedabove, which would be principally determined by
theavailability of this resource, differ as a function of
lesionsite and/or correlate with lesion size in a particular
site.
As indicated above, CT scans were obtained on 18 (L)CVApatients.
To determine whether these patients were similar toother aphasic
patients with (L)CVAs, the performance of
these 18 (L)CVA patients was compared with that of theremaining
(L)CVA patients in a 2X2 [group (patients withand without scans)
Xsentence type (syntactically complexversus baseline)] ANOVA. There
were main effects of group[F( 1,44) = 5.9, />
-
NC EG
Lesions in syntactic comprehension deficits 941
AD ^~—T~^ SD_—-T-^ CD,
Fig. 5 Diagrams of the left lateral hemisphere of the human
brain depicting the approximate extent of the lesion in each of the
18subjects with (L)CVAs whose CT images were analysed. Using a
stylized hemisphere as a template, each case was reconstructed
fromaxial views of the CT scans. Shaded areas include both cortical
and white matter lesions. Anglophone patients are presented in the
leftpanel and francophone patients in the right.
Table 3 Individual lesion volumes as a percentage of normalized
volume of regions of interest in perisylvian cortex, andperformance
on sentence comprehension task
Patient
EnglishW.B.N.C.E.G.A.L.L.M.E.M.D.S.G.T.L.Z.
FrenchA.D.S.D.CD.H.D.R.M.F.R.F.S.G.S.J.V.
Normalizedtotal lesionvolume (cm3)
1487927
1013
1021
1415
1302332
11518
1037
2394
Percentage
ROI1(STG)
887826251900
857
740
29168
49000
of normalized
ROI2(F3t)
8200
180
210
1000
10043
1000
611800
ROI occupied
ROI3(F3o)
6600
560
210
1000
892318
1000
93611217
by lesion
ROM(AG)
33952424130000
00
1741
00001
ROI5(SG)
4043217400
-
942 D. Caplan et al.
Table 4 Measures of specific syntactic operations
Baseline Matched sentence Operation
1 Two-place active (ST1)2 Two-place active (ST1)3 Three-place
active (ST5)4 Active conjoined theme (ST13)5 Object control (ST16)6
Object control (ST16)
7 Two-place active (ST1)8 Three-place active (ST5)9 Conjoined
(ST8)
10 Object-subject (ST10)11 Subject-subject (ST 12)12
Object-object (ST11)13 Object-subject (ST 10)14 Three reflexive
expressions (ST15)15 Two-place active (ST1)16 Simple active
relexive-expression,
complex noun phrase (ST22)17 Three reflexive expressions
(ST15)18 Two-place active (ST1)19 Simple active
relexive-expression,
complex noun phrase (ST22)
P2 (ST2)PI (ST3)P3 (ST6)Passive conjoined agent (ST14)Subject
control (ST17)Noun phrase-raising (English ST25)Passive object
control (French ST25)Two-place cleft object (ST4)Three-place cleft
object (ST7)Object-subject (STIO)Object-object (ST11)Subject-object
(ST9)Subject-object (ST9)Subject-subject (ST12)Reflexives, simple
(ST18)Reflexive, simple (ST20)Reflexive, complex NP (ST21)
Pronouns, simple (ST19)Pronoun, simple (ST24)Pronoun, complex
noun phrase (ST23)
PassivePassivePassivePassiveAntecedent of pronounAntecedent of
nounphrase-traceObject relativizationObject relativizationObject
relativizationObject relativizationObject relativizationCentre
embeddingCentre embeddingAntecedent of reflexiveAntecedent of
reflexiveAntecedent of reflexive
Antecedent of pronounAntecedent of pronounAntecedent of
pronoun
measures are listed in Table 4. None of these comparisonswas
significant.
We also explored the effect of lesion size within a regionon
performance through correlational analyses. We separatelycorrelated
(i) overall accuracy on the entire set of 25 sentencetypes, (ii)
the overall syntactic complexity score describedabove, and (iii)
the 19 separate measures that correspond toparticular syntactic
operations, with (a) normalized lesionvolume in the language zone,
(b) normalized lesion volumein each of the five regions of
interest, and (c) normalizedlesion volume in the anterior and
posterior regions of interest.None of these 168 correlations were
significant. To look forany non-linear relationships that might
have obscured acorrelation, we created separate plots for each of
these 168correlations. We found no evidence of non-linear
relationsin any of these plots.
Finally, correlational analyses were performed to
identifywhether lesion size in a particular region of interest
affectedperformance, once the effect of overall lesion size had
beentaken into account. The percentages of each region of
interestoccupied by a lesion were correlated against the residuals
oftwo regression analyses—one in which the normalized lesionvolume
in the language zone was regressed against the overallaccuracy
scores and one in which this value was regressedagainst the overall
syntactic complexity scores. (NB In thisanalysis, the five regions
of interest were reduced to threeby combining the two frontal and
the two parietal regions ofinterest, resulting in regions of
interest that represent thefrontal, parietal and temporal portions
of the language zone.This reduction was undertaken to reduce the
ratio ofindependent variables to cases, thereby allowing for
theanalysis of these data by regression analyses.) There were
no significant correlations of lesion extent in any of
theregions of interest with the residuals of either of
theseregressions. This indicates that, when the effect of
totalperisylvian lesion volume is removed, there is still
noparticular area within this region in which lesion sizecorrelates
with the degree of syntactic processing impairment.
These analyses indicate that there was no difference betweenthe
patients with and without anterior perisylvian lesions withrespect
to their overall level of performance, the magnitude ofa syntactic
processing deficit and the presence of impair-ments of specific
parsing operations. They also indicate thatthere was no
relationship between lesion size in either theanterior or posterior
language area and the overall magnitudeof a sentence comprehension
deficit, the overall magnitude ofa syntactic processing deficit, or
the magnitude of deficits inspecific syntactic operations. This
suggests that lesions that areconfined to the posterior perisylvian
cortex have essentiallythe same effect on syntactic processing as
lesions that affectboth the posterior and anterior perisylvian
regions.
However, these analyses could be misleading because thepatients
were studied and scanned at different times inrelation to their
lesions. We undertook four analyses todetermine whether this was
likely to be the case. First, wedetermined that the mean interval
from stroke to testing wasthe same in the patients with and without
anterior lesions.For the 12 patients with anterior lesions, time
since strokeranged from 4 to 84 months with a mean of 25 months(SD
= 25); for the six patients without anterior lesions, timesince
stroke ranged from 8 to 65 months with a mean of 28months (SD = 8).
Secondly, we correlated the duration ofillness, measured in months
between stroke and time oftesting, with the overall syntactic
complexity score. Because
-
Lesions in syntactic comprehension deficits 943
Table S Performance (number correct of 12 trials) of five
patients on different sentence types
Sentence types
Sentences with full noun phraseTwo-place activeThree-place
activeConjoinedActive conjoined themeThree
reflexive-expressionsSimple active reflexive-expression
Sentences with pronouns or reflexivesReflexives, simple noun
phrasePronouns, simple noun phraseSimple active reflexiveSimple
active reflexive ('friend of X' subj)Simple active pronoun ('friend
of X' subj)Simple active pronoun
Sentences with empty noun phrasesTwo-place passiveTruncated
passiveTwo-place cleft objectThree-place passiveThree-place cleft
objectSubject-object relativeObject-subject relativeObject-object
relativeSubject-subject relativePassive conjoined agentObject
controlSubject controlNoun phrase-raising/passivized object
control
Total correct
Patients
E.M.
12119
108
10
89
12111112
12129925779
101165
227
F.S.
1257941
75
12069
2830104041522
109
G.S.
10127
1125
82
10568
1011116769567900
173
L.M.
121212121212
121212121212
12121212127
109
121212126
284
L.Z.
10739
1112
81012967
978782233
12952
181
See Appendix for descriptions of sentence types.
the values for illness duration were not normally distributed,we
also correlated the log of illness duration with theoverall
syntactic complexity score. Neither correlation wassignificant.
Thirdly, we repeated all correlational analyses ina subset of 10
patients, who were tested between 3 and 24months after stroke.
These patients were also selected so asto exclude the patient whose
scan was not taken along thecanthomeatal plane, and to exclude the
three patients whosescans were obtained within 2 weeks of stroke.
None of thecorrelations were significant.
Fourthly, though there were insufficient numbers of patientsin
this smaller selected set to compare those with lesionsthat only
affected Broca's area with those with lesions thatspared this
region, we were able to select five patients withroughly equal-size
small lesions, three of which primarilyaffected Broca's area, and
two of which spared it. Weanalysed their patterns of performance,
which are shownin Table 5.
Three patients (E.M, F.S. and G.S.) had lesions that
affectedBroca's area. Overall performance did not correspond to
thepercentage of Broca's area that was affected in these
patients.G.S., a 48-year-old French patient, had a lesion that
occupied
-
944 D. Caplan et al.
sentence that was affected by lesions in either location.
Allfive of the patients had problems comprehending at leastsome
relative clauses. L.M., with the second largest lesionin the group
of five patients, but the best overall performance,only had
problems with sentences of this type and one othersentence type
with an empty noun phrase. L.Z. (with aprimarily temporal lesion)
had difficulty with a set ofsentences with pronouns and reflexives
and full noun phrases,and with all the sentences with empty noun
phrases exceptone type of passive sentence. The patients with
primarilyanterior lesions had a variety of impairments other
thanthose affecting relative clauses. E.M. showed impairments
insentences in which the subject of the main clause was relatedto
the subject of an embedded infinitive (subject control andnoun
phrase-raising sentences), in sentences with three fullnoun
phrases, and in some sentences with reflexives orpronouns. F.S.
showed impairments in all but the simplestsentence types. G.S.
showed impairments in longer sentenceswith full noun phrases
(conjoined sentences and sentenceswith three full noun phrases),
all sentences with reflexivesor pronouns except the most simple
type, and several passivesentence types. Most of these patterns are
interpretable inpsycholinguistic terms, but present no particular
patternacross the patients with lesions in particular
locations.
Overall, this more detailed analysis of single cases withsmall
lesions of roughly comparable size, who were testedat about the
same time after their strokes, illustrates that thedegree of
variability found in quantitative and qualitativeaspects of
patients' performances are not easily related tolesion location or
the size of lesions in the anterior orposterior portion of the
perisylvian association cortex.
DiscussionThe results of this study provide information about
the neuralstructures that are involved in sentence comprehension.
Theyshow that sentence comprehension is affected by lesions inboth
the left and the right hemisphere, more so by the former.This
finding is consistent with other reports in the literature(De Renzi
and Fagiolini, 1978). The more specific resultsof this study
pertain to syntactic processing in sentencecomprehension. They add
to the evidence that lesionsthroughout the left perisylvian
association cortex areassociated with disorders affecting this
process. They alsoraise the question of a possible contribution of
the righthemisphere to this aspect of sentence processing. We
shalldiscuss the results for the right and the left
hemisphereseparately.
The performance of the right-hemisphere population
onsyntactically complex sentences was significantly lower thanthat
of the normal control subjects. The analysis of theperformance of
individual patients makes it unlikely that thepoorer performance of
right-hemisphere patients than controlsubjects was due to a few
patients in this group whowere right-hemisphere dominant for this
aspect of languageprocessing. The finding that there were
significant effects of
syntactic complexity—independent of sentence length—onthe
performance of the right-hemisphere patients, providessupport for
the view that the right hemisphere plays somerole related to
assigning sentence structure and/or using it todetermine sentence
meaning.
It is not yet clear what the role of the right hemisphere
insyntactic processing is. The magnitude of syntactic
processingimpairments was roughly normally distributed in both
theright- and left-hemisphere lesioned patients, with a
greaterdegree of impairment in the left-hemisphere group.
Thissuggests that there might be a reduction in resources
availablefor syntactic processing that varies in its extent in
bothpopulations. The greater effect of left-hemisphere lesionscould
reflect a specialization of this resource capacity forsyntactic
processing within the left-perisylvian cortex; therole of the right
hemisphere might be to provide a lessspecialized working memory
capacity that makes a lessercontribution to syntactic processing
(Caplan and Hildebrandt,1988; Waters et al., 1995). More specific
protocols will haveto be used to determine exactly what aspects of
the totalsentence comprehension process are accomplished by
theright hemisphere. In addition, to determine the specificity
ofany deficit in sentence comprehension for lesions in theterritory
of the middle cerebral artery of the right hemi-sphere, it will be
necessary to study patients with frontallesions and other lesions
outside the perisylvian cortex andto compare their performances
with those of patients withright-hemisphere perisylvian
lesions.
The left-hemisphere damaged patients performed morepoorly and
showed greater effects of syntactic processingthan those with
right-hemisphere lesions. It is possible thatthe poorer performance
of the left-hemisphere patients andthe presence of greater
syntactic complexity effects in thispopulation compared with the
right-hemisphere patients isdue to larger lesions in the left- than
in the right-hemispheregroups. However, though this possibility
cannot be ruled outwithout additional data, it seems unlikely that
the left-hemisphere lesions were on average twice to three times
aslarge as those in the right hemisphere. The most
likelyexplanation for the poorer performance and greater
syntacticcomplexity effects in the left-hemisphere patients is that
theirlesions affected neural structures that are more
cruciallyinvolved in syntactic processing.
The data from the 18 patients in whom CT scanning wasavailable
indicate that deficits in syntactic processingfollow lesions in all
parts of the perisylvian association cortexof the left hemisphere.
Before considering the implicationsof this pattern for the
functional neuroanatomy of language,we must ask whether the
observed pattern of deficit-lesionrelationships might reflect
deficiencies in the methodologyof this study.
The most obvious limitation of this study is that only
18subjects were scanned. However, this limitation should beseen
within the context of previous research on this topic:the present
study is the largest study of patients in whomradiological data
have been obtained and who have been
-
Lesions in syntactic comprehension deficits 945
tested for syntactic processing in sentence comprehension.In
addition, the comprehension data available on each patientconsists
of performance on 12 examples of each of 25sentence types, while
most previous studies provide resultsfor at most three or four
sentence types. Thus, though thelimited number of patients requires
that caution be exercisedin accepting these results, the data
constitute the largestdataset presently available on this topic and
can provide atentative basis for theory construction.
A second concern is that differences in the time fromstroke to
testing could have affected the results. Severalanalyses speak
against this possibility. The magnitude of thefunctional impairment
was not correlated with time sincestroke. Analyses of a subset of
subjects whose strokes werebetween 3 months and 2 years of testing
were identical tothe analyses of the larger group. Though neither
of thesefindings rules out the possibility that patients with
longerperiods of recovery could have lesser deficits, or that
therecould be a non-linear relationship between time since
lesionand degree of recovery, they combine to make
thesepossibilities less likely to have obscured the relationship
oflesion location and size to impairments on this task. Moreover,it
should be born in mind that cortical re-organization post-stroke
cannot explain the presence of deficits followinglesions to brain
regions not premorbidly involved in theexercise of the function.
Therefore, the finding that patientswith lesions in many parts of
the perisylvian cortex havesyntactic processing deficits has
implications for thefunctional neuroanatomy of this aspect of
languageprocessing.
A third concern is that in this study we tested
sentencecomprehension through the use of a single task,
objectmanipulation and that the results thus reflect the demands
ofthis task as well as those of sentence comprehension. Toaddress
the concern that the results of an object-manipulationtask might
not generalize to other tasks, in a separate study,we correlated
the performance of 17 aphasic patients on 10sentence types on
object manipulation and sentence-picturematching tasks (Caplan et
al., 1995). The Spearman rankorder coefficient (p) for performance
on the sentence typesacross the two tasks was 0.66 (P = 0.04).
Thus, at leastfor the overall measure of sentence processing,
patients'performance on the object manipulation task correlates
witha very different task, and can be taken as an externally
validmeasure of their sentence processing capacities.
A fourth issue is that this study tested sentence comprehen-sion
in an 'off-line' task—one that reflects the end-point ofthe
comprehension process, rather than examine 'on-line'syntactic
processing—i.e. the time course of constructingsyntactic
representations. It has been claimed that on-linemeasures reveal
deficits in aspects of syntactic processing inBroca's aphasia, and
this has been taken to implicate Broca'sarea as the locus of
certain syntactic operations in sentencecomprehension (Zurif et
al., 1993). If this claim is correct,we must identify the source of
the qualitatively similar off-line impairments in patients with and
without Broca's area
lesions in this study. One possibility is that certain
on-lineoperations are affected by lesions in Broca's area,
whileothers that interfere with the same final product of
thecomprehension process are affected by lesions elsewhere.More
detailed on-line studies of syntactic processing inaphasia and of
the consequences of different disturbances ofon-line processing for
final comprehension are needed beforethese possibilities can be
settled.
Fifthly, in the present study we have not attempted todefine the
white matter tracts that are lesioned in thesepatients. Damage to
these tracts can cause de-efferentationand de-afferentation of
cortical areas, with functional con-sequences that may be similar
to those caused by lesions tothe regions themselves (Klippel, 1908;
Geschwind, 1965;Kosslyn et al., 1993). The possible importance of
whitematter lesions is highlighted by case D.S., who had a
verysmall cortical lesion, but who only responded correctly to65%
of the sentences and who had a syntactic complexityindex of 0.25.
It is possible that analyses that took whitematter tracts into
account might reveal a higher degree oflocalization of syntactic
processing.
A final concern is that CT scanning primarily identifiesareas of
necrosis, and is not very sensitive to the presenceof hypoperfusion
or hypometabolism in cerebral tissue.Several studies in which
investigators used [l8F]fluoro-deoxyglucose PET, have demonstrated
larger areas of hypo-perfusion than those shown to be necrotic by
CT scanningin aphasic patients (Metter et al., 1983, 1984, 1986,
1987,1989, 1990). It has been suggested that cortical areas thatare
hypometabolic may not sustain normal cognitive functions(Kosslyn et
al., 1993). Therefore, it is possible that a greaterdegree of
localization might be observed if measurements ofmetabolism, blood
flow and/or oxygen extraction were usedto assess CNS damage. One
can hope that future work willbring together different measures of
CNS function withextensive and detailed cognitive analyses of
patients' deficits,to address these issues. Until such time, we are
limited tothe currently available data.
Accepting these data, provisionally, we can ask what
theirimplications are for the functional neuroanatomy of the
leftperisylvian association cortex for syntactic processing.
Thefact that lesions in all parts of the perisylvian cortex
affectedsyntactic processing is consistent with one of two
modelsthat have been proposed for the functional neuroanatomy
ofcognitive processes. One is a model according to which thereare
significant individual differences in the localization ofsyntactic
processing across the population (Caplan, 1987a,b). The second is a
neural net model, in which representationsare distributed
throughout a region of the brain (McClellandand Rumelhart, 1986;
McClelland and Kawamoto, 1986;McClelland et al., 1989).
Evidence against individual differences in localization
ofsyntactic processing comes from the recent PET study,referred to
above (Stromswold et al., 1996), that showed anincrease in rCBF
during syntactic processing in the parsopercularis of Broca's area
in all subjects studied. However,
-
946 D. Caplan et al.
only strongly right-handed college-educated males, betweenthe
ages of 20 and 30 years, with no left-handed familymembers were
studied in that experiment. If there areindividual differences in
the localization of parts of the neuralsystem that is responsible
for syntactic processing, thesedifferences may be related to sex,
handedness, familialhandedness, age, educational level or other
factors. Thepresent study did not include enough patients to
determinewhether correlations between the degree of impairment
insyntactic processing and the size of lesions in
particularlocations would be greater if the correlations were
confinedto subjects of a certain age, sex, educational level
orhandedness profile. Larger studies, both involving deficit-lesion
correlational analyses and functional activation inneurologically
normal subjects, with more subjects in eachof these groups are
needed to explore this issue.
The distributed neural net model maintains that the neuralsystem
that is responsible for syntactic processing includesa cortical
region that extends along the sylvian fissure. Thismodel would
predict impairments in syntactic processingafter lesions throughout
this region, and thus is compatiblewith the results of the present
study. Distributed modelscould possibly also be compatible with the
evidence forlocalization found in the PET study of Stromswold et
al.(1996). It has been shown that neural net models can achievesome
degree of internal structure; i.e. neural nets that aretrained to
accomplish a function frequently develop in sucha way that a
particular stimulus maximally activates aparticular subset of the
units in the net (Plaut and Shallice,1993). This could correspond
to a distributed system in whichthere is a local increase in
activity, observable as an increasein rCBF, when a particular
stimulus is processed. There isone aspect of the data that poses a
challenge to the distri-buted neural net model; namely, the finding
that therewas no correlation between total lesion size and
severityof impairment. Most neural net models obey the principleof
mass action (Lashley, 1950), such that the larger the lossof
computational elements, the greater the overall decrementin
performance (McClelland and Rumelhart, 1986; Pattersonet al.,
1989). Possible areas of research thus include an effortto see if a
neural net model that develops an internal structurecan be lesioned
in such a way that there are similar effectsof lesions throughout
the net but no effect of overall lesionvolume on performance.
In summary, the data presented here are consistent withthe
conclusion that several regions of the left perisylviancortex form
critical parts of a neural system responsible forsyntactic
processing. Other data suggest some degree oflocalization of this
function within the pars opercularis. Thecomplete picture is
consistent with the models proposed byMesulam (1990) and Damasio
(1992). which involve bothdistributed and localized features. Many
aspects of thesemodels remain to be developed to account for the
entirepattern of results seen in both deficit-lesion
correlationalstudies and functional neuroimaging studies with
normalsubjects.
AcknowledgementsWe wish to thank Pierre Delplas, David Kennedy,
RandallBenson, Jeremy Schmahmann and David Gow for
assistanceobtaining, registering and interpreting the CT scans
andperforming the regression analyses. The work reported herewas
partially supported by a grant from the National Instituteof
Deafness and other Communication Disorders (DC00942).
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Lesions in syntactic comprehension deficits 949
Appendix: Sentence typesBaseline (simple) sentences: no
referentially dependentnoun phrases(1)* Two-place active
The frog hit the monkey.(5)* Three-place active
The rabbit passed the cow to the goat.(8)* Conjoined
The monkey scratched the rabbit and patted theelephant.
(13) Active conjoined themeThe frog patted the monkey and the
elephant.
(15) Three reflexive-expressionsThe old man knew that his friend
scratched theboy.
(22) Simple active reflexive-expression ('friend of
X'subject)
The father of the boy pointed to the old man.Overt referential
dependencies ('himself, 'him')(18) Reflexives, simple noun
phrase
The old man said that the father hit himself.(19) Pronouns,
simple noun phrase
The old man believed that the father tickled him.(20) Simple
active reflexive
The old man kicked himself.(21) Simple active reflexive ('friend
of X' subject)
The father of the boy scratched himself.(23) Simple active
pronoun ('friend of X' subject)
The father of the boy kicked him.(24) Simple active pronoun
The old man tickled him.Empty referential dependencies(2)f
Two-place passive
The monkey was hit by the frog.(3) Truncated passive
The rabbit was patted .
(4)^ Two-place cleft objectIt was the cow that the rabbit kissed
.
(6^ Three-place passiveThe elephant was given to the monkeyby
the frog.
(7)1 Three-place cleft objectIt was the goat that the rabbit
gavecow.
to the
(9)f Subject-object relativeThe monkey that the rabbit grabbed
shook thegoat.
(10) Object-subject relativeThe goat hit the rabbit that grabbed
the cow.
(11) Object-object relativeThe monkey tickled the frog that the
goatshook .
(12) Subject-subject relativeThe frog that held the cow caught
theelephant.
(14) Passive conjoined agentThe elephant was hitfrog.
(16)* Object control
by the monkey and the
The old man told the father(17) Subject control
to pray.
to sleep.The old man swore to the father _(25)f Noun
phrase-raising (English)
The old man seems to the father to bebending over.
Passivized object control (French)Le vieillard a t incit par lep
re manger.
*Baseline sentence type used in syntactic complexity
analysis;Sentence types with empty referential dependencies
andnoncanonical word order.