This study compares a visual Eriksen flanker task and a flanker task with auditory distractors with respect to differences in electrophysiological and hemodynamic activation measured with EEG and fMRI. In the flanker task observers are asked to selectively attend to a central target and signal the direction it points to, while ignoring flanking stimuli. Depending on condition the flanker stimuli point in the same direction as the target (congruent), in the opposite direction (incongruent) or they offer no direction information (neutral). In the crossmodal flanker task only neutral visual flankers are presented, congruency is manipulated by beeping sounds originating from the left/right or center.
In the unimodal as well as in the crossmodal version a lateralized readiness potential is observed in the EEG at electrodes C3 and C4 - like expected for the flanker task. Reaction times were shorter in congruent trials than in incongruent trials, reactions were faster in the crossmodal task compared to the unimodal task. Contrasting fMRI-data (region of interest analyses) of congruent and incongruent trials with neutral trials shows higher brain activation in primary sensory cortices according to the modality of distractors, i.e. in the primary visual cortex in the unimodal task and in the primary auditory cortex in the crossmodal task. In the unimodal task a broad frontoparietal network is found activated when contrasting incongruent trials with congruent trials. The same contrast reveals a more specific activation in the inferior parietal gyrus in the crossmodal task. Both flanker tasks evoke similiar behavioral and electrophysiological responses, the unimodal visual task shows a more distributed brain activation than the crossmodal task.