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Abstract:
While regional cerebral blood flow (rCBF) correlates of
syntactic comprehension processes have been reported in or near
Broca's area (Stromswold, Caplan, Alpert, & Rauch, 1996; Just,
Carpenter, Keller, Eddy, & Thulborn, 1996), there are, to date,
no data on neural correlates of syntactic production. In order to
investigate the influence of syntactic encoding on cortical
activity, we developed an experimental paradigm for the controlled
elicitation of naturally produced responses with different degrees
of syntactic encoding ('restrictive scene description').
Experimental design
Subjects described animated visual scenes involving a fixed set of
three geometrical objects of different grammatical gender in
German, three colors, and two actions ('go next to', 'push away').
The actions were performed by one or two agents. Beginning with the
agent(s), all action participants, their respective colors, and the
action had to be named in all conditions, ensuring constant
conceptual processing. Subjects were instructed to describe the
scenes either in a full sentence (S), with a sequence of noun
phrases (NP), or with a sequence of single words (W).
Subjects and Procedure
Twelve (6 female, 6 male) right-handed native speakers of German
were trained on the task one week before PET measurement. During
PET measurement, the stimuli were presented on a PC monitor,
responses were recorded on DAT tape and subsequently analyzed for
voice onset time (VOT) and duration. Twelve PET scans (4
repetitions per condition) were performed. Data were analyzed with
the Statistical Parametric Mapping (SPM96) program package
(Friston, Holmes, Worsley, Poline, Frith, & Frackowiak,
1995).
Results
Behavioural data:
The three conditions did not differ in onset and duration of
verbal responses.
Regional CBF data:
The maximal contrast (S-W) yielded the left frontal operculum as
the only area with a highly significant (p<.02 corr., .000
uncorrected) rCBF increase. At a more liberal statistical threshold
(p<.01 uncorr.) this area was also found to be activated in the
comparison S-NP. A Conjunction Analysis, testing for common
activations across contrasts, showed that not only the contrasts
S-W and S-NP, but also S-W and NP-W shared activation of the left
frontal operculum (p<.001, uncorr.). The signal intensity of the
maximally activated voxel showed a graded response with the lowest
value for W, intermediate value for NP, and highest value for
S.
Conclusions
The left frontal operculum subserves syntactic encoding in
language production. The obtained response pattern suggests that
this area is not selectively sensitive to syntactic encoding at the
sentence level but also at the noun-phrase level.
References
Friston, K. J., Holmes, A. P., Worsley, K. J., Poline, J. P.,
Frith, C. D., & Frackowiak, R. S. J. (1995). Statistical
parametric maps in functional imaging: A linear approach.
Human Brain Mapping,
2, 189-210.
Just, M. A., Carpenter, P. A., Keller, T. A., Eddy, W. F., &
Thulborn, K. R. (1996). Brain activation modulated by sentence
comprehension.
Science,
274, 114-116.
Stromswold, K., Caplan, D., Alpert, N., & Rauch, S. (1996).
Localization of syntactic comprehension by positron emission
tomography.
Brain and Language,
52, 452-473.
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