Shifts of object-based attention (OBA) are affected by object closure in the double-rectangle cueing paradigm (Marino & Scholl, 2005). The effect of closure on object-based selection and attentional filtering strength have not previously been investigated. Using a modified flanker paradigm (Eriksen & Eriksen, 1974), we presented subjects either a set of vertically oriented rectangles (rectangle condition) or those same rectangles with the horizontal top/bottom of each rectangle removed (line condition). In Experiments 1 & 2, a centrally presented object was flanked by four identical objects. One end of the central (target containing) object was then exogenously cued. Subjects performed a letter discrimination task on a color singleton target letter appearing on the central object in the presence of flanker letters (on flanking objects) that were either compatible or incompatible with the target response. Experiment 1 (homogeneously rectangle or line objects) showed that OBA selection is strong when objects are closed, preventing flankers from influencing performance. Experiment 2 (spatial attention control) showed that closure does not affect performance without OBA selection. Experiments 3 (flanking line objects) and 4 (flanking rectangle objects) showed that both target & flanking objects play a role in attentional filtering of distracters. During object-based attentional selection, flanking (non-selected) perceptual objects may serve to confine the effects of distracters while target (selected) perceptual objects may serve to shield the target from the effects of distracters.