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Abstract:
A word preceeded by related semantic context elicits shorter
reaction times, shorter gaze durations, and smaller N400
components in the event-related brain potential (ERP). To explain
these effects, psycholinguists have proposed two distinct
mechanisms: a quick, automatic, "lexical" one which relies on
pre-stored associative relationships, and a slower, controlled
process that presumably underlies sentential context effects.
However, because effects of lexical association and sentential
context have typically been studied in isolation, the relative
contribution of each to on-line sentence processing has been
difficult to disentangle.
To establish the relevance of both lexical and sentential
mechanisms for sentence comprehension, we created sentences which
pitted lexical processing mechanisms against sentential ones, and
utilized an on-line measure of brain activity associated with
comprehension, the amplitude of the N400 component of the ERP.
Stimuli consisted of four sentence conditions formed by crossing
two factors, the plausibility of the final word as a sentence
completion (congruent or incongruent), and the occurrence of a
lexical associate of the final word earlier in the sentence
(associated or unassociated). Examples can be seen below:
Congruent Associated: When someone has a heart attack a few
minutes can make the difference between LIFE and DEATH. Congruent
Unassociated: The gory details of what he had done convinced
everyone that he deserved LIFE in PRISON. Incongruent Associated:
The gory details of what he had done convinced everyone that he
deserved LIFE in DEATH. Incongruent Unassociated: When someone
has a heart attack a few minutes can make the difference between
LIFE and PRISON.
The critical word pairs (associated and unassociated) were
tested in two preliminary experiments to ensure that associated
(and not unassociated) word pairs would elicit N400 context
effects. The first experiment utilized the lexical decision task,
and revealed an effect of association on both reaction times
[associated=681ms, unassociated 721 ms, F (1,11) = 21.2, p <
.001], and the N400 [mean amplitude 300-600 ms post-word onset:
F(1,11) = 6.62, p < .05]. To avoid potential confounds from
decision-related effects on the ERP (viz. P300), the second
experiment utilized a letter probe task which was completed after
the presentation of both words in a pair. This experiment also
revealed an effect of association on the N400 component [F (1,7)
= 15.2, p < .01].
In the final experiment, ERPs were recorded as a different set
of 24 participants read sentences like those in the examples
above, and N400 amplitudes were measured. The four conditions
were subjected to an ANOVA with sentence congruity (congruent vs.
incongruent), lexical association (associated vs. unassociated),
and electrode site (13 levels) as factors. Remarkably, while
these analyses revealed an effect of sentence-level congruity
[F(1,23) = 66.0, p < .0001), neither lexical association nor
the congruity by association interaction approached significance
(F's < 1.9). These results indicate that lexical context
mechanisms evidenced by priming effects in word pairs may have
neglible influence on the processing of sentences. Moreover,
these data present the possibility that lexical context
mechanisms are not automatic.
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