The shape of luminance increments at the intersection alters the magnitude of the scintillating grid illusion

The scintillating grid illusion refers to an illusory perception of black spots on the luminance increments at the intersections of gray grids on a black background. In this study, we examined how the shape of luminance increments modulated the strength of the illusion. In Experiment 1, we concurrently controlled the size and shape of luminance increments, and found significant reduction of the illusory strength on the square, compared with circle and diamond, in the largest size condition. In Experiment 2, we controlled overall orientation of squared luminance increments, and confirmed the significant reduction of the illusion when the relative edge orientation of luminance increments and the grids was larger than 30 deg. This indicates that not the categorical difference of the shape, but the orientation difference between the grids and the luminance increments determines the strength of the illusion. We discussed about the contribution of orientation processing to scintillating grid illusion. Illusory gray spots are observed on the intersections of white grids against a black background (Fig. 1). The well-known phenomenon is referred to as Hermann Grid Illusion (Brewster, 1844; Hermann, 1870). Previous studies designed several variations of the illusion and showed that the illusion existed even when the grids were sloped (Spillmann, 1994). Lateral inhibition is known as a convincing mechanism for the illusion (Baumgartner, 1960). Figure 1. Hermann Grid Illusion By adding circular luminance increments to the intersections of Figure 1 and reducing the luminance level of the intersections, illusory black spots are observed on the luminance increments (Fig. 2). This illusion is so called, scintillating grid illusion (Schrauf, Lingelbach, & Wist, 1997). Figure 2. Scintillating Grid Illusion The scintillating grid illusion is critically different from the Hermann grid illusion in that the illusory black spots are not constantly perceived, but momently scintillated. Why does the percept of scintillation occur? Schrauf and his colleagues conducted experiments on 3 different conditions, pursuit eye movement on stationary grids, smooth displacement of the grids with steady gaze, and brief exposure of the stationary grids, and found that not the eye movement itself but a transient stimulation caused by the eye movements or brief exposures is essential for generating scintillating grid illusion (Schrauf, Wist & Ehrenstein, 2000). They also showed that high stimulus speed or brief exposure less than 210 ms either reduced the strength of the illusion. This indicates that the spatial and temporal integration of the activity of visual neurons is important for generating the illusion. Recently, VanRullen & Dong (2003) reported that the distance between an attended location and intersections determined the strength of illusory spots at the intersections, and implied that the spatial distribution of covert attention affects the illusion. Perhaps, the slow temporal course of scintillating grid illusion may be related with attention shifts that seem to be required for the scintillation of illusory spots. Furthermore, scintillating grid illusion likely stems from the different mechanism for Hermann grid illusion. As described above, Hermann grid illusion is explained with lateral inhibition (Baumgartner, 1960). However, because of the complexity of the intersections due to the luminance increments, the explanation with Hermann grid illusion cannot perfectly stand for scintillating grid illusion. Moreover, scintillating grid illusion is evidently affected by the diameters of luminance increments that do not exist in Hermann grid illusion. In this study, we explored the underlying mechanism for the scintillating grid illusion by concurrently controlling the shapes and sizes of the luminance increments. Previous studies examined the effects of sizes and luminance on the illusion (Schrauf, Lingelbach, & Wist, 1997). However, the shape of the luminance increments was not examined: Only circle shape was introduced into luminance increments at the intersections. Thus, it was unclear whether the effect of sizes of luminance increments was common among variable shapes. We planned to test diamond and square shapes beside the original circle shape. Although the two new shapes have the same side length and interior angles, they are different in terms of their overall orientation. By using three kinds of shape, we tried to confirm the role of edge orientation of luminance increments in the illusion. By comparing the strength of the illusion among three shapes, we discuss the underlying mechanism for the illusion in terms of orientation processing.