Source Localization of Strategies: N400 During Lexical Decision Tasks

Abstract:
Abstract: We have reported differences in the N400 component during two lexical decision task experiments with different levels of controlled processes. We found a significant N400 effect in both high- and low-proportion of related word-pairs tasks in the experiment where directly-related pairs were used. During the mediated-related word-pairs experiment, however, the effect was only significant for the low-proportion task. In order to explore whether these differences were due to the activation of different brain structures, the following analyses were performed: Average peak amplitude values of the N400 in each subject for each condition and task were measured and used to compute the source distributed inverse solutions with the Variable Resolution ElectromagneticTomography method (VARETA). VARETA solutions were projected over the average brain of the Montreal Neurological Institute. The results showed that all solutions involved prefrontal areas and bilateral temporal and occipital regions, independently of task and condition. Similar results have been observed by other groups using fMRI and PET. By comparing the amount of activation in each task, however, whilst the strategies decreased across tasks, the activation over prefrontal areas decreased. In conclusion, we claim that, during a lexical decision task, prefrontal areas are involved in controlled processes and temporal and occipital regions with automatic processes.