Source analysis of electrical and magnetic brain responses associated with selective attention and language processing

For 8 subjects we determined the field distributions of the electrical and magnetic brain responses (in two separate sessions) in selective search tasks and semantic priming tasks. In the selective search paradigm, letters were presented either to the left or to the right of fixation, while the subjects were instructed to attend to one side of fixation only. Subjects searched at the attended side for the presentation of a variable number of pre-memorized target letters. In the semantic priming tasks subjects were presented with word-pairs, which were either associated or non-associated. The results of the selective search tasks and the semantic priming tasks will be presented in two separate posters. In the selective search tasks, early components of both the electrical and magnetic brain responses were modulated by spatial attention. The field-distributions were indicative of localized patterns of brain activity in extrastriate visual regions. Both the electrical and magnetic brain responses showed effects of memory load. In the semantic priming tasks, the ERPs showed a late negativity of the non-associated word-pairs as compared to the associated word-pairs (N400). This effect was maximal at central regions of the head. In the magnetic responses a similar phenomenon was observed; the effect could be observed over both the right and left temporal regions were it had the same field direction. This suggested that brain areas in both hemispheres are involved in generating the N400 component.