Two additional remarks need to be made here. First, our manipulations in different feature dimensions did not answer why a collinear column slowed down target search, but very likely that effect is not originated by the salience computation, for two reasons: (1) Salience in the color and luminance dimensions did not produce the same impairment as orientation, and (2) salience usually guides rather than masks attention. It is likely to originate from a site where information of both collinearity and orientation contrast is available. Second, the unique impairment effect by the salient collinear column was observed under the condition that participants were required to search for a local target. When the task was to pay attention to the global structure, as in work by Jingling and Zhaoping (
2008), then the salient collinear column facilitated performance. The exact mechanism of how a salient collinear column directs selective attention to hinder local target discrimination, however, is not yet clear. Several possibilities require further studies to test them. First, it is possible that collinear columns produce stronger crowding effects than noncollinear columns and thus mask target discrimination in visual search. The crowding effect, though, usually demonstrated in a simple rapid display, might be able to account for visual-search performance under the assumption that several saccadic eye movements are required in a visual-search display (Gheri, Morgan, & Solomon,
2007; Rosenholtz, Huang, Raj, Balas, & Ilie,
2012). Collinear contour can produce a larger crowding effect than orthogonal contour (Chakravarthi & Pelli,
2011; May & Hess,
2007; Yeotikar, Khuu, Asper, & Suttle,
2011); thus it is possible that a target on a collinear contour suffers a stronger crowding effect and becomes more difficult to discriminate in our display. However, as the collinear columns are more salient and should attract initial gaze, the targets are more likely to be in the fovea rather than in the periphery. Also, crowding is negatively correlated with perceptual salience (Gheri et al.,
2007; Livne & Sagi,
2007,
2010; Saarela, Sayim, Westheimer, & Herzog,
2009; Whitney & Levi,
2011), and our overlapping targets should be in a more salient region than nonoverlapping targets. Thus whether the crowding effect can account for the search impairment requires further testing.