Abstract
A regular array of black squares against a gray background increases the threshold for a light disk in an intersection of the alleys separating the squares, an illusion variously called the extinction illusion, "blanking" phenomenon, or "vanishing disk". Curving the alleys (making the squares into curved "tetragons") further increases the thresholds for both light and dark disks. This raises three questions: (1) Do the edges of the tetragons interact with similarly oriented components of a target? (2) Is blanking a different phenomenon from the obscuring (more moderate increase in threshold in the presence of an array) that affects dark disks, or is blanking simply weaker for dark disks? (3) Is blanking a function of light versus dark targets or dependent upon contrast relative to the polarity of the inducing array? We replaced the target disks with parallel line segments to explore the influence of orientation of the line segments relative to the nearby orientation of the alleys. We show that the blanking phenomenon is sensitive to the orientation of the line segments, while the weaker obscuring of dark targets is not. We also examine these effects with white tetragons. Reversing the polarity of the tetragons exchanged which line segments were orientation-sensitive, although the effectiveness of white tetragons for blanking was weaker than that of black tetragons. We consider possible reasons why white tetragons may be less effective than black tetragonsunder our conditions.
Highlights
The mechanisms of the grid illusions have been the subject of considerable research
Would detection of a target in an intersection be influenced by that target’s orientation? Would any effect of orientation be manifested for dark targets that are presumably not affected by the blanking phenomenon?
A paired t-test for the difference in threshold between cardinal and diagonal line segments was significant for light line segments at 0% curvature (t [11] = 5.52, p < 0.001); no other difference between cardinal and diagonal line segments was significant
Summary
The mechanisms of the grid illusions have been the subject of considerable research. In their canonical form, the grid illusions consist of a regular array of black squares separated by lighter alleys. The Hermann grid has white alleys, and illusory dark “smudges” are seen in intersections of the alleys peripheral to fixation (Fig. 1A); the scintillating grid has gray alleys with white disks in the intersections, and transient black dots appear centered upon disks away from the center of gaze as the eyes scan the grid (Fig. 1B). These illusions are effective in negative images, with white squares on darker alleys. The explanation for the illusory effects are not entirely understood, as the traditional lateral inhibition explanation for the Hermann illusion, proposed by Hering (1890) and elaborated by Baumgartner (1960), has proven inadequate (Geier et al, 2004; de Lafuente and Ruiz, 2004; Schiller and Carvey, 2005; Geier et al, 2008; Bach, 2009)
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