Dimension-based attention modulates early visual processing

Target selection can be based on spatial or dimensional/featural mechanisms operating in a location-independent manner. We investigated whether dimension-based attention affects processing in early visual stages. Subjects searched for a singleton target among an 8-item array, with the search display preceded by an identical cue array with a dimensionally non-predictive, but spatially predictive singleton. Reaction times (RTs) were increased for changes in the target-defining dimension but not for featural changes within a dimension. This RT effect was mirrored by modulations of the P1 and anterior transition N2 (tN2). Current density reconstructions revealed increased activity in dorsal occipital cortex and decreased activity in left frontopolar cortex owing to repeated dimensional pop-out identities. These findings strengthen dimension-based theories of visual attention by indicating dimension-, rather than feature-, specific influences within the first 110 ms of visual processing.