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Abstract:
Abstract: N400 topographic distributions of responses to
congruous and incongruous targets occurring either mid-sentence or
in sentence-final position were examined using a 128-channel dense
electrode array. Average-referenced ERPs (n=60) reveal a clear
left-temporal development for all targets, starting with a strongly
left-lateralized N1 for both congruous and incongruous words and
tracking a continuous spread of negative activity along the left
temporal lobe, leading to differential effects for congruous and
incongruous targets in the N400 window ("the N400 effect"). For
mid-sentence targets, congruous and incongruous words display
identical topographies at 400ms after word onset, with larger
overall amplitudes (increased activation) for incongruous words.
Source modeling indicates that the pattern of results is consistent
with left anterior and mid-posterior generators in medial limbic
cortex, in or near cingulate gyrus. For sentence-final targets,
distributions are qualitatively different for the two conditions,
suggesting a more complex, nonoverlapping distribution of sources,
in addition to the sources identified for the mid-sentence
condition. Ongoing analyses seek to identify alternative sets of
neural generators consistent with the distribution of responses to
sentence-final targets. The observed mid vs. final topographic
differences underscore the importance of separating effects due to
semantic processing (the N400) from other end-of-sentence context
effects, including increased processing negativities, CNV release,
and response preparation.
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