Sensitivity to attachment, alignment, and contrast polarity variation in local perceptual grouping.

A number of leading theories (e.g., Grossberg & Mingolla, 1985; Kellman & Shipley, 1991; Rensink & Enns, 1995) commonly assume that perceptual grouping by contour alignment occurs preattentively across reversing contrast polarity elements. We examined this notion in seven visual search experiments. We found that only grouping by attachment supported preattentive visual search and that grouping by contour alignment required attention in order to operate. Both attachment grouping and grouping by contour alignment were sensitive to contrast reversals. Further results showed that contour alignment was a strong grouping cue only among elements with the same contrast sign but that it did not facilitate grouping across reversing contrast. These results suggest that grouping by contour alignment operates only on inputs of consistent contrast polarity.