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Abstract:
We monitored regional cerebral activity with BOLD fMRI while
healthy subjects were presented written sentences differing in
their grammatical structure (subject-relative or object-relative
center-embedded clauses) and their short-term memory demands (short
(3-word) or long (7-word) antecedent-gap linkages). Patients were
exposed to eight 40-second blocks of each type of sentence
presented in a word-by-word fashion. Scans included a baseline task
involving pseudofont target detection. Patients were imaged on a
1.5T GE Echospeed scanner using a gradient echo echoplanar
technique (5mm slice thickness x 18 slices, effective TE 50msec, 64
x 40 matrix, voxel size of 3.75 x 3.75 x 5mm). The images were
registered, aligned to Talairach space, smoothed with 8mm and
2.8sec Gaussian kernels, and analyzed with SPM96 using appropriate
protection for multiple comparisons. A core region of left
posterior superior temporal cortex was recruited during all
sentence conditions in comparison to a pseudofont baseline,
suggesting that this area plays a central role in sustaining
comprehension that is common to all sentences. The homologue of
this area in the right hemisphere was recruited during sentences
with long antecedent-gap linkages regardless of grammatical
structure, suggesting that this brain region supports passive
short-term memory during sentence comprehension. Recruitment of
left inferior frontal cortex was associated with sentences that
featured both an object-relative clause and a long antecedent-gap
linkage. We hypothesize that left inferior frontal cortex supports
the cognitive resources required to maintain syntactic dependency
relations during the comprehension of grammatically complex
sentences. Patients with Frontotemporal degeneration (FTD) have
sentence comprehension difficulty, and their poor sentence
comprehension has been attributed in part to an impairment
understanding grammatical aspects of sentences. For example, FTD
patients have a deficit understanding sentences with
object-relative clauses compared to subject-relative clauses, and
their pattern of impaired sentence comprehension differs from that
seen among patients with Alzheimers disease (AD). fMRI obtained at
rest with arterial spin labeling in FTD demonstrated reduced
activity in a frontal distribution. We found a significant
correlation between impaired sentence comprehension and reduced
activity in left frontal cortex in FTD, a correlation that was not
evident in AD. Preliminary fMRI BOLD studies during sentence
comprehension in 8 FTD patients demonstrated that they recruit left
temporal cortex to support sentence comprehension, similar to
healthy subjects. FTD patients also recruited right temporal cortex
during several sentence conditions, similar to healthy subjects.
During presentation of OR long sentences, FTD patients differed
from healthy subjects since they recruited left anterior temporal
cortex but not left inferior frontal cortex. FTD patients also
recruited right parietal and left medial frontal cortex during
several types of sentences. These observations provide converging
evidence from an independent source consistent with the hypothesis
that left inferior frontal cortex plays a critical role in
grammatical aspects of sentence comprehension. The additional
recruitment of left anterior temporal, left medial frontal, and
right parietal regions in FTD may represent cortical reorganization
in the form of collateral sprouting to functional regions adjacent
to compromised brain regions. Alternately, this may represent an
attempt to develop other, albeit less effective, cognitive
strategies to support comprehension of grammatically complex
sentences.
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