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Abstract:
We present several studies investigating the
functional neuroanatomy of syntactic processing in sentence
comprehension. In all studies, PET activity associated
with processing syntactically more complex subject object (SO)
sentences (e.g., The juice that the child spilled stained the
rug) and synonymous syntactically simpler object subject (OS)
sentences (e.g., The child spilled the juice that stained the
rug) was compared. Previous studies have shown activation
in Broca's area in this task in young subjects.
In the first series of studies, we tested young
(18-30 year old) and old (70-80 year old) subjects. Both
young and elderly participants who were efficient at syntactic
processing, as measured by plausibility judgment RTs,
replicated the pattern of increased rCBF in Broca's area when
making plausibility judgments about the more complex SO
sentences compared to the simpler OS sentences. In
contrast, both young and elderly participants who were less
proficient at the task activated posterior structures in the
left parietal lobe.
To investigate the role of working memory in
determining these different rCBF patterns, we then carried out
an additional study of young participants (n = 9 in each group)
who were matched in terms of language processing efficiency,
but who differed in terms of working memory capacity as
measured by standard tests. High- and low-span
participants both activated the same areas in the left and
right inferior frontal cortex, as well as midline
structures. We then regrouped these subjects to form two
groups of subjects (n = 8 in each group) who were matched for
working memory capacity but who differed in processing
efficiency. High-proficient participants activated the
inferior frontal cortex, and low-proficient participants
activated posterior structures in the left superior temporal
lobe.
The results demonstrate that there are
individual differences in the rCBF effects of syntactic
processing. These differences do not appear to be related
to age or to working memory capacity, as measured by standard
tests of working memory. They appear to be related to the
proficiency with which subjects process syntactic
structures. The determinants of individual differences in
processing proficiency and of the rCBF effects seen in these
two sets of subjects remain to be determined.
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