This target article discusses the verbal working memory system
used in sentence comprehension. We review the concept of working
memory as a short-duration system in which small amounts of
information are simultaneously stored and manipulated in the service
of accomplishing a task. We summarize the argument that syntactic
processing in sentence comprehension requires such a storage and
computational system. We then ask whether the working memory
system used in syntactic processing is the same as that used in
verbally mediated tasks that involve conscious controlled processing.
Evidence is brought to bear from various sources: the relationship
between individual differences in working memory and individual
differences in the efficiency of syntactic processing; the effect of
concurrent verbal memory load on syntactic processing; and syntactic
processing in patients with poor short-term memory, patients with poor
working memory, and patients with aphasia. Experimental results from
these normal subjects and patients with various brain lesions converge
on the conclusion that there is a specialization in the verbal working
memory system for assigning the syntactic structure of a sentence and
using that structure in determining sentence meaning that is separate
from the working memory system underlying the use of sentence meaning
to accomplish other functions. We present a theory of the divisions of
the verbal working memory system and suggestions regarding its neural
basis.
Positron Emission Tomography (PET) was used to determine regional cerebral blood flow (rCBF) when eight normal right-handed males read and made acceptability judgments about sentences. rCBF was greater in Broca's area (particularly in the pars opercularis) when subjects judged the semantic plausibility of syntactically more complex sentences as compared to syntactically less complex sentences. rCBF was greater in left perisylvian language areas when subjects had to decide whether sentences were semantically plausible than when subjects had to decide whether syntactically identical sentences contained a nonsense word. The results of this experiment suggest that overall sentence processing occurs in regions of the left perisylvian association cortex. The results also provide evidence that one particular aspect of sentence processing (the process that corresponds to the greater difficulty of comprehending center-embedded than right-branching relative clause sentences) is centered in the pars opercularis of Broca's area. This process is likely to be related to the greater memory load associated with processing center-embedded sentences.
Positron emission tomography (PET) was used to determine regional cerebral blood flow (rCBF) as a function of the syntactic form and propositional density of sentences. rCBF increased in the left pars opercularis, part of Broca's area, when subjects processed syntactically more complex sentences. There were no differences in rCBF in the perisylvian association cortex traditionally associated with language processing when subjects made plausibility judgments about sentences with two propositions as compared to sentences with one proposition, but rCBF increased in infero-posterior brain regions. These results suggest that there is a specialization of neural tissue in Broca's area for constructing aspects of the syntactic form of sentences to determine sentence meaning. They also suggest that this specialization is separate from the brain systems that are involved in utilizing the meaning of a sentence that has been understood to accomplish a task.
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