Suppression of salient stimuli inside the focus of attention.

We investigated how attention is distributed when one of two attended stimuli stands out from the visual context. Participants judged whether the line orientations within two geometric shapes at two predictable locations were same or different, which induced a wide focus of attention around the two locations. One of the geometric shapes surrounding the lines could be a salient color or shape singleton but was irrelevant for the task. In Experiment 1, the salient and non-salient items were both placed on the horizontal midline. Electrophysiological recordings at posterior electrode locations PO7/8 revealed a positivity between 200 and 300 ms contralateral to the singleton, consistent with the occurrence of the PD. The PD is thought to reflect attentional suppression. In Experiment 2, one attended item was placed on the vertical meridian and the other one on a lateral position. Lateral line targets triggered robust N2pc components when there was no singleton present, reflecting attentional selection. However, this N2pc to lateralized line targets was abolished when a singleton was presented at the same lateral position, and conversely, was increased when a singleton was presented on the vertical position. This suggests that salient elements inside the focus of attention are suppressed and attention is enhanced at the other location. It can be concluded that salient elements inside the focus of attention do not capture attention, as bottom-up control of attention would propose, but that salient elements are suppressed, possibly to assure unbiased processing of equally relevant stimuli.