Orientation contrast vs orientation in line-target detection.

This study concerns the roles of absolute and relative orientation in determining detectability of a line-element target in a background field of uniformly oriented line elements. Target detectability was determined as a function of background-field orientation, sampled at 5 deg intervals, for three levels of orientation contrast--the difference between target and background orientations--sampled at 10, 20 and 30 deg. Stimulus displays were presented briefly and followed by a mask. There were 10 observers, whose detection performance was quantified by the discrimination index d' from signal detection theory. Target detectability was found to depend both on absolute orientation, represented by background-field orientation, and on orientation contrast. At each level of orientation contrast, performance was best when the background field, not the target element, was vertical or horizontal. These data are difficult to explain by general models of orientation discrimination based on simple orientation opponency between local line-sensitive filter units; three other models specifically concerned with target detection are briefly considered.