The effect of luminance values of the edge between regions on figure-ground perception.

The present study examined whether luminance values of an edge between regions affect figure-ground perception. A dark-gray or light-gray region was presented on the left (or upper) or right (or lower) side of an edge. Observers reported which region was perceived as a figure. The results in the 0.2° edge width condition showed that dark-gray regions with low luminance edges were perceived as a figure significantly more often than dark-gray regions with high luminance edges. The edge luminance effect occurred, partly occurred, and did not occur when the edge widths were 0.2, 0.5, and 0.8° of visual angle, respectively. These results showed that luminance values of the edge between the regions affected figure-ground perception, and that regions with luminance values closer to that of the edge between the regions were perceived as a figure more often, but only when the edge width was narrow. The edge luminance effect occurred less often as the edge width was increased and would be  attributed to similarity grouping between edge and region.

Evidence for a size underestimation of upright faces.

We quantitatively examined the difference in perceived size between upright and inverted faces using the method of constant stimuli. The stimuli included eight face images modified from two cartoon faces produced by Kitaoka (2007, http://www.psy.ritsumei.ac.jp/-akitaoka/kao-e.html and 2008, Cognitive Psychology 5 177-185) and six photographic faces, including a photographic face used by Thompson (2010, http://illusionncontest.neuralcorrelate.com/2010/the-fat-face-thin-fft-ilusion/). Experiment 1 showed that an upright face and outline were perceived to be significantly smaller than an inverted face and outline, respectively. Moreover, the amount of the size underestimation in the face stimulus condition was significantly larger than that in the outline stimulus condition. Experiment 2 showed that an upright face was perceived to be significantly smaller than 90 degrees and 270 degrees rotated faces, whereas an inverted face was not perceived to be significantly larger than a 90 degrees or 270 degrees rotated face. Experiment 3 showed that upright faces were perceived to be significantly smaller than upright and inverted outlines, whereas inverted faces were not perceived to be significantly larger than upright or inverted outlines. Experiments 4 and 5 showed that upright photographic faces were also perceived to be significantly smaller than inverted photographic faces. These results provide quantitative evidence for a size underestimation of upright faces.

Increment of the extinction illusion by long stimulation.

We quantitatively examined the effect of stimulus duration on the extinction illusion. A white disc was presented or not presented at an intersection of a grey grid (intersection configuration) or on a homogeneous background (background configuration). The extinction illusion was quantified as the subtraction of no-disc responses in the background configuration (ie baseline) from no-disc responses in the intersection configuration, when the disc was presented. Experiment 1 showed a temporal effect: the extinction illusion increased as stimulus duration increased; this temporal effect was observed when the disc was presented at 9 deg from the fixation point and when the stimulus duration was 1000-6000 ms. Experiment 2 showed a visual field anisotropy: the extinction illusion occurred more frequently in the upper visual field than in the lower visual field, when the stimulus duration was 200 ms; the anisotropy was not observed when the stimulus duration was 6000 ms. Experiment 3 showed an alley-length effect: when the grid alley was long, the extinction illusion occurred more frequently in the 6000 ms condition than in the 200 ms condition; the temporal effect was not observed when the grid alley was short. These results suggest that the temporal effect of the extinction illusion might be due to perceptual filling-in of luminance information of the grid alley.

Anisotropies of linear and curvilinear completions at the blind spot.

Previous studies have shown anisotropies of perceptual completion of the line at the blind spot. The present study examined anisotropies of collinear and noncollinear completions at the blind spot. A physically aligned or misaligned pair of horizontal or vertical line segments was presented on opposite sides of the blind spot. Observers reported the appearance of the line segments. In Experiment 1, the range of noncollinear completion in the vertical line segments was significantly larger than that in the horizontal line segments, whereas an anisotropy of the range of collinear completion was not observed. In Experiment 2, linear and curvilinear completions predominantly occurred in the horizontal and vertical line segments, respectively. In Experiment 3, the tolerance of collinear completion was not significantly larger than vernier acuity. These results suggest that linear and curvilinear completions at the blind spot have different anisotropic tendencies. These different anisotropic processes might bring about anisotropies of perceptual completion of the line at the blind spot.

Perceptual filling-in of a line segment presented on only one side of the blind spot.

Two experiments examined whether filling-in occurred at the blind spot when a line segment was presented on only one side of the blind spot. We used static and dynamic stimuli: a static test line segment and a pair of probe line segments were presented in Experiment 1 and a moving test line segment was presented in Experiment 2. We compared the probability that the proximal end was perceived to be on the blind spot side when the test line segment came into contact with the blind spot (blind spot condition) with that when the test line segment was outside the blind spot (control condition). The results of the two experiments showed that the proximal end was perceived to be more on the blind spot side in the blind spot condition than in the control condition. Notably, when a dynamic stimulus was presented below the blind spot, the mean amount of filling-in reached 2.84 degrees. Therefore, filling-in occurred at the blind spot even when a line segment was presented on only one side of the blind spot.

Appearance of an illusory object in the blind spot.

We report a phenomenon showing new aspects of perceptual filling-in at the blind spot. When two and three discs were presented below the blind spot, observers perceived them as three and four discs, respectively, but when they were presented in the nasal visual field as a control, the perceived numbers were veridical. The phenomenon demonstrates that the visual system produces an illusory object in the blind spot even when the inducing objects presented on only one side of the blind spot are clearly discrete and countable.

Anisotropy of tolerance of perceptual completion at the blind spot.

Three experiments examined anisotropies of tolerance of perceptual completion at the blind spot when a pair of line segments was presented on opposite sides of the blind spot. The tolerance of perceptual completion is defined as the maximum difference in a stimulus attribute between the line segments on opposite sides of the blind spot when perceptual completion of a line has occurred. The misalignment, orientation difference, and luminance difference between the line segments were used as the stimulus attributes in Experiments 1, 2, and 3, respectively. The results showed anisotropies of the tolerance of perceptual completion between horizontal and vertical configurations of the line segments. Vertical superiorities, which imply a greater extent of tolerance in the vertical configuration than that in the horizontal configuration, were observed for misalignment and orientation difference, while horizontal superiority, which implies a greater extent of tolerance in the horizontal configuration than that in the vertical configuration, was observed for luminance difference. We discussed possible origins of the anisotropy of the tolerance of perceptual completion at the blind spot.