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Abstract:
The construction and decoding of sentences with syntactically
licensed dependencies have been associated with the left inferior
frontal gyrus, i.e., Broca's area. Lesion-deficit correlational
studies, for example, have shown that patients with Broca's aphasia
present with difficulty comprehending sentences such as passives or
object relative clauses in which noun phrases (NPs) have been moved
out of the canonical S-V-O (in English) position (Grodzinsky, l990,
2000; Schwartz et al. l987). On-line sentence processing studies
also have shown that patients with Brocas aphasia show impairments
in sentence processing operations. In contrast to normal subjects
and those with some forms of fluent aphasia, these patients do not
link elements of dependency relations such as moved noun phrases
(direct objects) with their trace sites (Swinney & Zurif l995;
Zurif et al. l993). Some recent neuroimaging studies also have
shown Brocas area activation during complex sentence processing
(Stromswold et al. 1996; Caplan et al. 1998, 1999); others
implicate Wernickes area and right-hemisphere homologues of Brocas
and Wernickes areas as well (Just et al. 1996).
In the present study whole-brain echoplanar fMRI was used to
examine neural correlates of auditory sentence processing as a
function of syntactic complexity in eight normal, monolingual,
English speaking volunteers. Participants matched auditory
sentences of two types, either simple subject-clefts (e.g., It was
the thief who chased the artist) or complex object-clefts (e.g., It
was the artist who the thief chased), to pictures. Subjects
responded by button press to matches and reaction times were
recorded. Thirty-two contiguous 4-mm axial slices were obtained
relative to the AC-PC line. Images were motion-corrected and
normalized onto a common stereotactic space using SPM96. Results
showed that both simple and complex sentences (as compared to a
single words control condition) invoked activation of Brocas area
(BA 44) and the supplementary motor area (BA 6) in the left
hemisphere. Components of Wernickes area in the posterior superior
and middle temporal gyri (BA 21 and 22), and the inferior parietal
lobule (supramarginal (BA 40) and the angular gyri (BA 39)) were
activated, bilaterally. Significant activation also was noted in
the dorsal lateral prefrontal cortex (BA 46, 9) and the
intraparietal sulcus in both the left and right hemispheres, sites
concerned with verbal working memory and visual spatial attention.
When complex sentences were contrasted with simple sentences,
significant activation was seen only in the left hemisphere in
Broca's (BA 44) and Wernicke's areas, including posterior superior
and middle temporal gyri (BA 22, 21), and the angular gyrus (BA
39). These findings suggest that processing both simple and complex
sentences involves traditional anterior and posterior language
areas and that complex syntactic operations engage not only Brocas
area, but also Wernickes area. Processing sentences also engaged
neural networks involved in verbal working memory, but without
clear-cut differences between the simple and complex sentences.
We also studied activation patterns under identical fMRI
sentence processing conditions in five individuals with aphasia and
compared them with our normal subjects data. All subjects had
suffered a single, left hemisphere stroke in the distribution of
the middle cerebral artery and were at least one year post-stroke
at the time of the study. All were right-handed, monolingual,
English speakers with years of education ranging from 14 16 years.
Extensive language and neuropsychological testing showed patterns
of performance consistent with a diagnosis of nonfluent, Brocas
aphasia. Results showed that four of the five subjects recruited
right hemisphere brain sites under sentence (as compared to single
word) conditions.
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