EP Differences Based on Prestimulus EEG

The hypothesis that momentary functional microstate at stimulus arrival influences the way the stimulus is perceived and processed was tested directly by determining the most dominant microstate just before stimulus presentation and then calculating the stimulus-evoked potentials (EP) separately for these pre-stimulus classes. As anticipated, experiments with different types of visual or auditory stimuli revealed drastic EP differences at different latencies (Lehmann et al. 1994; Kondakor et al. 1995, 1997). They were not dependent on a particular cognitive task or a particular cognitive load, since state-dependent EP variations were recorded in a simple auditory odd-ball paradigm as well as in complex visual attention tasks. Recently, Braeutigam and Swithenby (2003) have described state-dependent evoked magnetic fields that were specific for the perception of faces. In a recent experiment, Mohr et al. (2005) found that the functional state at the moment of stimulus entry influences functional hemispheric specialization for emotional word processing that differs between men and women. In a general manner, these findings suggest that subtle variations of ongoing functional state of the brain prior to input access provide an endogenous context that influences subsequent event-related information processing by the brain, which follows commonrules over subjects (Lehmann et al. 1994).

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