Lay summary
This research project focused on mechanisms underlying hemispheric specialization in human. It was evoked by the fact that the traditional views of hemispheric specialization only holds true for cerebral lesions and for split brain patients, but that it is difficult if not impossible to reliably show it in individual healthy subjects. We hypothesized that intra- and inter-individual differences in brain lateralization might be sustained by differences in the electric neural state before the incoming of the stimulus. Our second hypothesis was that this momentary functional state of the brain varies over time, and also between men and women. Finally, we hypothesized that it varies over the different phases of the menstrual cycle within women. To test these hypotheses, we performed several behavioral, electrophysiological and clinical studies. Our main aim was to further elucidate and understand hemispheric specialization in the healthy and pathological brain. We developed a bilateral lexical decision task with simultaneously presented words and non-words. Subjects had to indicate by button press on which side a word appeared. Words are presented very briefly and subjects were not aware that some of the words were of emotional connotation. Behavioral results showed a general better detection of emotional words. Most striking was the difference for words presented to the left visual field, i.e. to the non-dominant hemisphere, where neutral words were detected at chance level. The analysis of the electrophysiological data (high density EEG) showed that this emotional word advantage depended on the momentary state of the brain just before stimulus presentation. This state dependency was more pronounced in men. Women showed most man-like behavior during the menstrual phase. The analysis of the event-related potential showed very early (at ~100 ms) differences between emotional and neutral words. Again, this was more pronounced in men than women. In general, women should more bilateral activation after word presentation than men in the time period where language processing takes place.