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Abstract
The effects of working memory (WM) on the semantic N400 response were studied using
high-resolution event-related brain potentials (ERPs). Participants were presented
with semantically related sentence pairs and the terminal word congruence was varied
in the second sentence. WM load was varied for the sentence pairs using a modified
fan procedure [J.R. Anderson, Retrieval of propositional information from long-term
memory. Cogn. Psychol., 6 (1974) 451-474; J.R. Anderson, A spreading activation theory
of memory. J. Verbal Learn. Verbal Behav., 22 (1983) 261-295]. ERPs were recorded
to the onset of the terminal word (Congruent and Incongruent) for sentence pairs at
two levels of WM load (WML1 and WML2). Behavioral data analysis revealed that reaction
times (RTs) increased as a function of WM load (i.e., the fan effect). Semantically
incongruent words elicited an N400 response. Increased WM load reduced the congruency
effect and, importantly, resulted in a significant delay in N400 peak latency (approximately
50 ms). Moreover, the latency delay was correlated with a neuropsychological measure
of individual WM capacity. WM load effects on the N400 were interpreted as a result
of competing activation in WM, both modulating semantic expectancies and delaying
semantic integration. Exploratory source analysis revealed activation in occipital,
temporal, and parietal regions. Consistent with prior reports on the N400 and semantic
processing, equivalent current dipoles were modeled in the left perisylvian region.
Increased WM load led to novel source activation in the left inferior parietal region
as well as increased activation levels in anterior temporal sources. The source modeling
results were in agreement with the functional roles typically ascribed to these areas
and confirmed that the scalp-recorded WM load effects on the N400 were present within
the intracranial generators.