Spatial interactions with real and gap-induced illusory lines in vernier acuity

Vernier acuity for illusory line targets induced by gaps in a horizontal grating was measured in the presence of real and illusory flanks. In a 500 msec presentation forced choice task, observers judged the position of a comparison illusory line positioned 3 min arc below the target. The results show that illusory lines are capable of interacting with real lines in spatial localization. Thus, they provide psychophysical evidence for a common localization mechanism that supports real and illusory contour definitions. The results further show a sensitivity of the visual system to the contrast polarity of real lines. This sensitivity was absent for illusory lines. The present findings are discussed in terms of their relationship to physiological findings, and in terms of their potential to constrain computational models that account for illusory contour brightness.