Anisotropy in Oriented-Line-Target Detection at High and Low Luminance Contrast

Observers can detect a uniquely oriented line segment (‘target’) in a background field of uniformly oriented line segments (‘nontargets’) even if viewing time is brief. When the lines have high luminance contrast, the variation of orientation increment threshold with nontarget orientation is periodic, generally with a period of about 90° although smaller periods have been found. Do the orientation-sensitive mechanisms giving rise to periodicities function only at high contrast?

This question was addressed in a line-target detection experiment. Twenty white line segments of length 1 deg visual angle were presented in a circular field of diameter 20 deg visual angle. Nontarget orientations were in the range 0°, 5°, …, 175°, and the difference between nontarget and target orientations was varied adaptively. Stimulus displays lasting 40 ms were followed by a blank interstimulus interval lasting 60 ms and then a random-line mask. A view-tunnel provided a grey background of luminance 35 cd m⁻² and stimulus contrast was 0.1, 0.3, 0.5, 0.7, or 0.9 log unit above the observer's luminance detection threshold for one line segment. When contrast was 0.1 log unit above this threshold, performance was near chance level. As contrast increased from 0.3 to 0.9 log unit above luminance detection threshold, performance improved and orientation increment thresholds decreased, showing that early orientation-processing is most effective at high luminance contrast. Nonetheless, periodicities were found in all conditions where performance was better than chance which suggests that the orientation-sensitive mechanisms associated with periodicities operate at both high and low luminance contrast.