Asymmetries in preattentive line detection.

We tested the relative contribution of absolute orientation and collinearity to asymmetries in preattentive line detection. Subjects reported the location of a single line that was tilted by 16 deg against the orientation of the background lines. Perceptual pop-out strength was quantified as the threshold presentation time to correctly locate the salient element. We found that (1) threshold times were significantly shorter in vertical than in oblique background lines and that (2) threshold times were significantly shorter in collinear than in non-collinear background lines. Our data suggest that the sensitivity of preattentive processes for orientation gradients is modulated by both collinearity and absolute orientation.