Attentional bias of competitive interactions in neuronal networks of early visual processing in the human brain.

Multiple objects in a visual scene compete for neuronal representation. We investigated competitive neuronal dynamics in cortical networks of early visual processing in the human brain. Coloured picture streams flickered at 7.42 Hz, evoking the steady-state visual evoked potential (SSVEP), an electrophysiological response of neuronal populations in early visual areas synchronised by the external pacemaker. While these picture streams were at a fixed location in the upper left and right quadrant, respectively, additional competing picture streams flickering at a different frequency were continuously changing the distance to the stationary streams by slow motion. Analysis of the 7.42 Hz SSVEP amplitude revealed significant amplitude decreases when the competing stimulus was closer than about 4.5 degrees of visual angle. Sources of the SSVEP suppression effect were found in early visual areas of the ventral and dorsal processing streams. Attending the stationary stimulus resulted in no difference in 7.42 Hz SSVEP amplitude regardless of spatial separation to the competing stimulus. Contrary to the predictions of the model, we found co-amplification of the competing stimulus at close spatial proximity accompanied by an increase of an intermodulation frequency, suggesting integrated neuronal processing of target and competing stimuli when both streams are close together.