[Role of connectedness in early object-based attentional selection].

It has been suggested that uniform connectedness is the most fundamental factor in forming units of attentional selection, while there are evidences that attention can select a perceptual group that consists of separate elements with similar features. The present study examined the effects of connectedness and a boundary-feature similarity on early spatial-selection processes using a sustained-focal-attention paradigm of event-related potentials (ERPs). Bilateral stimuli were manipulated to have an orthogonal combination of connectedness (C-, C+) and a similarity in boundary feature (S-, S+). ERPs were recorded from 15 participants who were instructed to pay attention to the left or the right visual field and to respond to a target shape that appeared infrequently in the attended field. The ERP attention effect in the N1 latency range (125-185 ms) was decreased for stimuli with connectedness and/or boundary-feature similarity, and the effects of the two grouping factors were independent of each other. The present result suggests that multiple grouping factors, including connectedness, operate in parallel in early processes of object-based attention-spreading.