An event-related potential examination of attended and unattended stimuli in visual selection using bilateral stimulus presentation.

The experiment reported here examined an interference paradigm using a bilateral stimulus presentation in which stimuli were presented simultaneously in the left and right visual fields. The lateralization of the early visual components allowed an ERP examination of material presented in each field. Attention was directed to one field or the other on each trial by a 100% valid cue. Two letters were nominated as targets and the simultaneous presentation allowed presentation of material compatible, incompatible or neutral with reference to the target. A negative peak was observed at 230 ms post stimulus at occipital and temporal sites. There was a variation in this peak for unattended stimuli, with compatible and incompatible target letters being significantly different to non-target letters. Contrary to previous research, this finding suggests that material is not filtered out at an early stage as proposed by early selection. It was found that the response to unattended target material also varied according to the type of item presented at the attended location. These findings were discussed in relation to previous studies which found no difference in the processing of unattended target and non-target material, and also in relation to suggestions that automatic processing of unattended material occurs only when there is controlled processing occurring simultaneously.