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Abstract:
Event Related Potential (ERP) studies have linked syntactic
and semantic violations to distinct electrophysiological
correlates. Violations of syntactic phrase-structure are associated
with an early left anterior negativity (LAN) and a central
positivity (P600), whereas conceptual-semantic violations lead to a
central negativity (N400) (e.g., Neville, Nicol, Barss, Forster,
& Garrett, 1991). Intracranial electrode and
magnetoencephalographic studies suggest left temporal-lobe
generator(s) for the N400 (Nobre, Allison, & McCarthy, 1994;
Simos, Basile, & Papanicolaou, 1997). However, these techniques
have not yet succeeded in identifying sources for the LAN or P600.
We examined the temporal and spatial neural correlates of
phrase-structure and conceptual-semantic violations (after Neville
et al., 1991) in an Event-Related fMRI study. Eight right-handed
male subjects were scanned using a 1.5T Siemens Vision MR scanner
(TR = 3 sec; TE = 40 msec; voxel size = 5x5x5 mm; 27 axial slices)
while they read randomly-ordered sentences with and without
conceptual-semantic violations (32 sentences with and 32 without
violations; e.g., "The Queen rewarded Hubert's {harmony/loyalty} to
the Crown.") or phrase-structure violations (32 sentences with and
32 without violations; e.g., "The students heard Daniel's {about
lecture/lecture about} the planets."). Sentences were presented
sequentially, one word at a time (SOA = 500 msec). The (violation
or control) target word occurred in a consistent position within
each type of sentence. One second after the end of each sentence,
subjects were prompted to indicate with a button-press whether it
was a good or bad English sentence. The time lag between sentences
was 12 seconds, to minimize the overlap of hemodynamic responses to
violations in consecutive sentences. The fMRI time series data were
analyzed using multiple regression. The contrast examined was
designed to be similar to the "difference wave" technique
traditionally used in analyses of ERP sentence violation
experiments. The time course of the canonical sentences was
subtracted from that of the violation condition. These differences
were examined at two different time lags (6 and 9 seconds),
relative to the target words of each sentence.
Initial analyses focused on left prefrontal and left
temporal-lobe regions. In left prefrontal regions, the hemodynamic
response associated with phrase-structure violations peaked and
diminished earlier than the activation pattern associated with
semantic violations. Left temporal-lobe regions showed similar time
courses for both conditions, but higher activation levels for the
semantic condition.
The data indicate that distinct spatio-temporal patterns of
neural activation are associated with syntactic phrase-structure
and conceptual-semantic violations. The finding that activation in
left prefrontal cortex peaked earlier in the syntactic than the
semantic condition is consistent both with the hypothesis that
syntactic processing is fast and automatic (Fodor, 1983), and with
the finding that the LAN associated with phrase-structure
violations temporally precedes the N400 associated with
conceptual-semantic violations (Neville et al., 1991). In addition,
the greater left temporal-lobe activation levels in the semantic
than the syntactic condition are consistent with N400 localization
claims of previous intracranial electrode and
magnetoencephalographic investigations. The preliminary findings
thus support the hypothesis that syntactic and conceptual-semantic
processes have distinct temporal and spatial neural correlates.
Fodor, J. A. (1983). The modularity of mind: An essay on faculty
psychology. Cambridge, MA: The MIT Press.
Neville, H., Nicol, J. L., Barss, A., Forster, K. I., &
Garrett, M. F. (1991). Syntactically based sentence processing
classes: Evidence from event-related brain potentials. Journal of
Cognitive Neuroscience, 3(2), 151-165.
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