In the competition for limited processing resources, top-down attention and cognitive control processes are needed to separate relevant from irrelevant sensory information and to interact with the environment in a meaningful way. The demands for the recruitment of top-down control processes depend on the relative salience of the competing stimuli. In the present event-related functional magnetic resonance imaging (fMRI) study we investigated the dynamics of neuronal networks during varying degrees of top-down control demands. We tested 20 participants with a dichotic auditory discrimination task in which the relative perceptual salience of two simultaneously presented syllables was parametrically varied by manipulating the inter-aural intensity differences (IIDs) and instructing the subjects to selectively attend to either the louder or weaker of the two stimuli. A significant interaction of IID manipulation and attentional instruction was detected bilaterally in the inferior parietal lobe and pre-supplementary motor area, and in the precentral gyrus, anterior cingulate cortex, and inferior frontal gyrus of the right hemisphere. The post hoc analysis of the interaction pattern allowed for an assignment of these regions to either of two sets of regions which can be interpreted to constitute two different brain networks: a fronto-parietal attention control network, involved in the integration of saliency-based and instruction-based processing preferences, and a medial-lateral frontal cognitive control network, involved in the processing of the conflicts arising in the attempt to follow the attentional instruction in face of the varying inter-aural stimulus salience.
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