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Abstract:
When attempting to account for complexity and garden pathing
effects in sentence comprehension, appeals are frequently made to
the limits of working memory (WM). For example, if the
comprehender cannot carry both analyses of an ambiguous sentence
in working memory, the parser must be serial. However, to provide
substance to such explanations, a fully specified model of WM is
necessary. What is encoded in working memory, are different sorts
of information stored separately or together, and how do these
functions relate to the brain?
First, what is stored and where? Recent neuroimaging
experiments suggest that both maintenance of words in memory
(Fiez et al., 1996) and (working memory for) processing of
complex sentences (Stromswold et al., 1996) activate the left
inferior frontal lobe. These data are compatible with two
independent functions which are located near each other or in the
same area; alternatively, both types of task may appeal to one
working memory function in which both words and sentential
structure are stored.
In one study, we made positron emission tomography (PET) scans
under four conditions (word lists, simple sentences, complex
sentences, and syntactically ambiguous sentences). Word lists
load only lexical memory, while, complex and ambiguous sentences
load structure memory more than simple sentences, and under
certain reasonable assumptions, lexical memory as well. A complex
combination of lexical and syntactic load best predicted
activation in the left inferior frontal lobe. This suggests that
both words and higher level sentence representations are
maintained in this area, providing a partial answer to the what
and where questions. In another study external lexical memory
load and sentential complexity showed an interactive effect on
blood flow in this area, rather than a (near) additive effect.
This suggests that the inferior frontal lobe has the cognitive
function of a single complex working memory store in which both
lexical and structural information are available during sentence
processing.
While increasing sentence complexity leads to inferior frontal
activation (see above), lateral anterior temporal lobe is
activated more for sentence processing than word lists (Mazoyer
et al, 1993). Evidence from a number of sources suggests that
lateral anterior temporal lobe is involved in encoding lexical
information in memory; we suggest that this encoding is used for
sentence comprehension and is not sensitive to the complexity of
later processing. We suggest that it is necessary to distinguish
between encoding and storage functions in working memory in order
to adequately explain the full range of areas involved in
sentence processing.
Fiez, J.A., Raife, E.A., Balota, D.A., Schwarz, J.P., and
Raichle, M.E. (1996). 'A positron emission tomography study of
the short-term maintenance of verbal information'.
Journal of Neuroscience,
16, 808-822.
Just, M.A., Carpenter, P.A., Keller, T.A., Eddy, W.F., and
Thulborn, K.R. (1996). Brain activation modulated by sentence
comprehension.
Science,
274, 114-116.
Mazoyer, B.M., Tzourio, N., Frak, V., Syrota, A., Murayama, N.,
Levrier, O., Salamon, G., Dehaene, S., Cohen, L., and Mehler, J.
(1993). The cortical representation of speech.
Journal of Cognitive Neuroscience,
5, 467-479.
Stromswold, K., Caplan, D., Alpert, N., and Rauch, S. (1996).
Localization of syntactic comprehension by positron emission
tomography.
Brain and Language,
52, 452-473.
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