Selection of features within and without objects: Effects of gestalt appearance and object-based instruction on behavior and event-related brain potentials

We studied how physical and instructed embedding of features in gestalts affects perceptual selection. Four ovals on the horizontal midline were either unconnected or pairwise connected by circles, forming ears of left and right heads (gestalts). Relevant to responding was the position of one colored oval, either within its pair or relative to fixation ("object-based" or "fixation-based" instruction). Responses were faster under fixation- than object-based instruction, less so with gestalts. Previously reported increases of N1 when evoked by features within objects were replicated for fixation-based instruction only. There was no effect of instruction on N2pc. However P1 increased under the adequate instruction, object-based for gestalts, fixation-based for unconnected items, which presumably indicated how foci of attention were set by expecting specific stimuli under instructions that specified how to bind these stimuli to objects.