The role of orientation processing in the scintillating grid illusion

Abstract

In the scintillating grid illusion, illusory dark spots are perceived on white patches at the intersections of gray bars. Previous studies have suggested that processing related to the orientation of the bars plays a role in this illusion, but the specific underlying mechanisms are unclear. In the present study, we investigated the role of orientation processing across the intersection in generating the scintillating grid illusion. The results revealed that the illusion was attenuated when the patch was located at the intersection of short bars (Experiment 1), irrespective of the spatial distance between patches (Experiment 2). The local cruciform patterns determined the strength of the illusion, even when lateral offset of the patches was employed (Experiment 3). The illusion was observed even when a small spatial gap was introduced around the patches. A larger gap produced a weaker illusion (Experiment 4). Spatial offsets of the bars across the gapped intersection greatly reduced the illusion (Experiment 5). We discuss these findings with regard to the activity of S1-type simple cells that respond to the luminance along an oriented edge across the intersection.

The scintillating grid illusion (Fig. 1a) is a perceptual phenomenon in which observers perceive scintillating dark spots on white disks that are located at the intersections of gray grids on a black background (Schrauf, Lingelbach, & Wist, 1997).

Fig. 1
figure1

a Scintillating grid illusion. Scintillating black illusory dots can be perceived within the white circular patches at the intersections of gray bars. b Hermann grid illusion. Dark smudges at the intersections of the white grids can be perceived in the peripheral visual field

The spatial properties of this illusion have been examined from several perspectives. In the first investigation of the scintillating grid illusion, Schrauf et al. (1997) found that the ratio of the width of the grids to the diameter of the luminance patches (white disks) influenced the magnitude of the illusion. This finding indicates that the scintillating grid illusion is tuned to specific spatial relationships between grids and patches. In a later study, VanRullen and Dong (2003) demonstrated that the distribution of attention in space affects the strength of the illusion, suggesting that the change in receptive field size due to attention shifts, as reported in other studies (e.g., Yeshurun & Carrasco, 1998), may modulate the magnitude of the illusion. The finding that the magnitude of the illusion depends on receptive field size change is consistent with the notion that low-level mechanisms may underlie the scintillating grid illusion (Yu & Choe, 2006).

On the other hand, several studies have proposed the importance of orientation processing in the grid illusion. M. W. Levine and McAnany (2008) investigated the effects of the straightness of the edges of grids on two types of the grid illusion (scintillating grid illusion and vanishing-disk illusion), finding that curved grids reduced or even abolished the scintillating grid illusion, while they strengthened the vanishing-disk illusion. On the basis of the results, they suggested that contrast sensitivity is effectively reduced at the intersection of curved grids and that the reduced contrast sensitivity is involved in the lower strength of the scintillating grid illusion. However, it was still unclear how orientation processing was related to the scintillating grid illusion.

We recently reported that orientation information provided by the edges of the luminance patches and the grids altered the magnitude of the illusion (Qian, Yamada, Kawabe, & Miura, 2009), consistent with the notion that the orientation interaction of the patches and grids is important for this illusion. More specifically, Qian et al. utilized square and diamond-shaped luminance patches, in addition to typical circular patches (i.e., disks), and found that both diamond-shaped and circular patches could generate the illusion. However, the illusion was significantly weakened when diamond patches were rotated by 45 °, meaning that square patches were presented at the intersection. Qian et al. reasoned that square patches inhibited orientation processing across the intersection. However, the role of orientation processing at the intersection in the scintillating grid illusion has not been systematically investigated in previous studies.

In the present study, we investigated the role of orientation processing at the intersection in the scintillating grid illusion. We hypothesized that the scintillating grid illusion requires orientation processing across the intersection, on the basis of the following findings from earlier studies. (1) The illusion is substantially weakened when the patch is placed at the intersection of curved grids (M. W. Levine & McAnany, 2008). The data indicate that the grid should be straight across the intersection to generate a strong illusion. (2) The illusion is weakened when the diamond patch is rotated by 45 ° (thus perceptually rendered into the square patch) (Qian et al., 2009), suggesting the involvement of orientation processing in the illusion.

To further investigate the role of orientation processing in the scintillating grid illusion, we manipulated three factors: the length of bars, the lateral offset of bars, and the size of spatial gaps between separated bars (or around patches). Previous studies have indicated that bar length strongly modulates perceptual phenomena other than the scintillating illusion, such as the Ehrenstein illusion (Banton & Levi, 1992; Salvano-Pardieu, 2000; Shipley & Kellman, 1992). We predicted that longer bars would cause a stronger illusion. Previous evidence indicates that lateral offset of bars weakens the orientation interaction across space (Kapadia, Ito, & Gilbert, 1995; Kapadia, Westheimer, & Gilbert, 2000), so we predicted that large lateral offsets would reduce the scintillating grid illusion if orientation processing is critical in the illusion. Regarding spatial gap size, we predicted that the illusion would occur even when a spatial gap was inserted around the patch, as long as the spatial interaction of orientation processing occurred across the patch (or the intersection; see Experiments 4 and 5 for details).

Experiment 1: The effects of bar length

Experiment 1 was conducted to test whether bar length would affect the strength of the illusion. In Experiment 1a, we used a rating method to measure the illusion strength. To test whether this rating method was affected by response bias and/or the use of an arbitrary criterion by each observer, in Experiment 1b we measured the strength of the illusion using a cancellation method in which the illusion strength was measured with an objective scale (i.e., background luminance to cancel the illusion). We compared the results of the rating and cancellation methods to assess the validity of the rating method.

Experiment 1a: Rating the strength of the scintillating grid illusion

Method

Observers

Six observers (4 male and 2 female; age range: 25–31) participated in this experiment. All observers reported that they had normal or corrected-to-normal visual acuity and were naive about the purpose of the experiment.

Apparatus and stimuli

Stimuli were generated with a Macintosh computer (Apple, Mac Pro, MA356J/A) and were displayed on a 19-in. CRT monitor (Mitsubishi, Diamond M2, RDF193H, 1,024 × 768, 100 Hz). The experiments were programmed and conducted using MATLAB and Psychtoolbox (Brainard, 1997; Pelli, 1997). A chin- and headrest was used to stabilize the visual field of the observers, at a viewing distance of 42 cm.

The stimuli consisted of 8 × 6 intersections of vertical and horizontal bars with variable lengths. The luminance values of the background, the grid, and the circular patch were 0.01, 13, and 61.95 cd/m2, respectively. The width of each bar was 0.33 ° of visual angle. The diameter of the circular patch was 0.46 °, and the distance between the centers of neighboring circular patches (the interpatch distance) was 3.26 °. The bar length was selected from one of the following five lengths: 0.65 °, 1.30 °, 1.96 °, 2.61 °, or 3.26 °. In the 3.26 ° condition, the bars were connected with each other, constituting the traditional stimulus configuration used in the scintillating grid illusion. Observers were asked to rate the strength of the illusion while observing the grid stimulus. A digit (0, 1, 2, 3, or 4), which was related to the strength of the illusion, was displayed under the stimulus. The luminance of the digit was 9.72 cd/m2, the height of the digit was 0.90 °, and the distance from the center of the display to the center of the digit was 12.63 °. Observers reported their ratings by selecting a digit using the left and right arrow keys, pressing the down arrow key to indicate their selection. Greater values indicated a stronger illusion. An example of the stimuli is shown in Fig. 2a.

Fig. 2
figure2

a Demonstration of a stimulus used in Experiment 1a with 2.61 ° bar length (not scaled). b Results of Experiments 1a and 1b. The bars denote standard errors of the means (n = 6). c Correlation of mean rated strength and mean threshold of the background luminance. The bars denote standard errors of the means (n = 6)

Procedure

Practice trials were conducted before the experimental trials, to familiarize observers with the rating procedure. Practice trials continued until every stimulus was displayed at least one time. Thus, the number of trials was different among observers and less than 50 trials. The practice session was followed by the experimental trials, in which observers rated the strength of the illusion while observing the stimulus on each trial. Each stimulus was tested 10 times, in a total of 50 trials. The presentation order of the stimuli was randomized. No time limits were imposed for either the observation or the response period.

Results and discussion

The results indicated that longer bars resulted in a significantly stronger scintillating grid illusion (Fig. 2b), revealed by a significant main effect of bar length in a one-way analysis of variance (ANOVA), F(4, 20) = 16.405, p < .0001. Multiple comparison tests (Ryan’s method; Ryan, 1960) of the main effect of bar length showed no significant differences between 0.65 ° – 1.30 °, t(20) = 0.516, p > .61, 1.96 ° – 2.61 °, t(20) = 0.701, p > .49, 1.96 ° – 3.26 °, t(20) = 2.537, p > .02, and 2.61 ° – 3.26 °, t(20) = 1.836, p > .08. The results indicated that the illusion was strong when the bars were long. These findings support the notion that the scintillating grid illusion is related to orientation processing across the patch.

Experiment 1b: Canceling the scintillating grid illusion

Purpose

The results of Experiment 1a revealed that the strength of the illusion changed according to the bar length. However, the rating method employed in Experiment 1a may be prone to response bias and/or the use of an arbitrary criterion for each observer. More objective methods may be more suitable for measuring the illusion. Schrauf et al. (1997) reported that a brighter background weakened the illusion when the luminance of bars and patches was constant. Their findings suggested that a brighter background is necessary to cancel a stronger illusion. Thus, we sought to replicate Experiment 1a using a cancellation method to manipulate the background luminance. This type of cancellation method was used in a previous study investigating the Hermann grid illusion (e.g., Howe & Livingstone, 2007). Specifically, observers adjusted the luminance of the background so as to extinguish the illusion. The cancellation method is more objective than the rating method, because the response criterion is the presence/absence of the illusion, and the strength of the illusion can be measured on a more objective index (i.e., background luminance). We predicted that as the bars became longer, the illusion would become stronger, and hence, a brighter background would be necessary to cancel it.

Method

Observers

To enable a comparative analysis of the results of Experiments 1a and 1b, we employed the same observers as those who had participated in Experiment 1a.

Apparatus, stimuli, and procedure

The apparatus, stimuli, and procedure were identical to those used in Experiment 1a, except for the following changes. The default luminance of the background was 0.01 cd/m2. This was adjustable from 0.01 to 13 cd/m2 (i.e., the luminance of the grid). While observing the stimulus, observers were asked to adjust the luminance of the background to determine the critical luminance at which they could no longer perceive illusory dark spots at the intersections. The right and left arrow keys were used to increase and decrease the background luminance. The step size of the luminance change was approximately 0.26 cd/m2. Observers were instructed to identify the critical luminance as precisely as possible, pressing the down arrow key to indicate their response.

Results and discussion

We calculated the threshold luminance by averaging the critical luminance for each observer. Figure 2b shows the group mean data of the threshold luminance. A one-way ANOVA revealed a significant main effect of bar length, F(4, 20) = 16.405, p < .001. Multiple comparison tests on the main effect of bar length revealed significant differences between 0.65 ° – 1.96 °, t(20) = 4.115, p < .0001, 0.65 ° – 2.61 °, t(20) = 4.816, p < .0002, 0.65 ° – 3.26 °, t(20) = 6.652, p < .0001, 1.30 ° – 1.96 °, t(20) = 3.599, p < .002, 1.30 ° – 2.61 °, t(20) = 4.300, p < .0004, and 1.30 ° – 3.26 °, t(20) = 6.316, p < .0001.

In general, brightening the background canceled the illusion, consistent with Schrauf et al.’s (1997) previous report. The threshold luminance to cancel the illusion depended on the bar length: The illusion was canceled by lower background luminance in the shorter bar condition than in the longer bar condition. The results also indicated that the illusion was weaker when bars were shorter. To quantitatively compare the results in Experiments 1a and 1b, we tested the correlation between the rated strength in Experiment 1a and the threshold of the background luminance and the bar length in Experiment 1b. An R-squared value of .93 indicated a strong correlation between these two variables (Fig. 2c). The results suggested that the dependency of the illusion on the bar length occurred regardless of the measurement method, at least within the stimulus range we employed. Thus, in further experiments, we used the rating method to measure the strength of the illusion.

Experiment 1 demonstrated that sufficiently elongated bars generate a robust scintillating grid illusion. However, it remained unclear whether the distance between neighboring patches would modulate the effects of bar length on the strength of the illusion. We predicted that the effect of the bar length would be constant across the distances between the patches if the absolute length of the bar was critical for generating the illusion. On the other hand, if the illusion was influenced by patch density and/or the relationship between bar length and the interpatch distance, we predicted that the effect of bar length would vary with the distance between the patches.

Experiment 2: The effect of interpatch distance on the illusion

Method

Observers

Seven observers (4 male, 3 female; age range: 20–29) participated in this experiment. Four of the observers had participated in Experiment 1a or 1b. All observers reported that they had normal or corrected-to-normal visual acuity, and all observers were naiveFootnote 1 about the purpose of the experiment.

Apparatus, stimuli, and procedure

The apparatus was identical to that used in Experiment 1. The stimuli and procedure of this experiment were identical to those in Experiment 1a, except for the following. The distance between the centers of neighboring circular patches (the interpatch distance) was set at one of the following five lengths: 1.81 °, 2.17 °, 2.54 °, 2.90 °, or 3.26 °. For each distance, the bar length was set to obtain the ratio of bar length to interpatch distance, at one of four values: 0.25, 0.5, 0.75, or 1. In the 1 condition, the bars were connected to each other, similar to the display traditionally used in the scintillating grid illusion. No bars were presented when the interpatch distance was 1.81 ° and the ratio condition was 0.25, because the length of the bars was 0.226 °, a little shorter than the radius of the patches (0.23 °). Thus, the entire size of the stimuli ranged from 26.08 ° × 19.56 ° to 11.77 ° × 8.15 °. Each stimulus was tested 4 times, in a total of 80 trials.

Results and discussion

Figure 3a shows the mean rated strength in Experiment 2. For each condition of interpatch distance and the ratio between the patch distance and the bar length, we calculated the actual bar length. Note that when the ratio was 1, the actual bar length was equivalent to the interdistance between patches (i.e., the bars were connected to each other). We fitted a cumulative Gaussian function to the mean rated scores for the illusions as a function of the actual bar length (Fig. 3b). The R-squared value of the fitting was .97, indicating that the actual bar length was a critical factor for illusion strength, irrespective of patch distance.

Fig. 3
figure3

Results of Experiment 2. a Mean rated strength of illusion with respect to bar length. b Mean rated strength of illusion with respect to the ratio of bar length to the interpatch distance. The bars denote standard errors of the means (n = 7)

The results of Experiments 1 and 2 indicated that orientation processing across the patch is important for generating the scintillating grid illusion. The illusion still occurred, even though the grids were segmented. If local orientation processing across the patch plays a critical role in the illusion, its strength would not change even with the introduction of lateral offset of the patches, as long as the cruciform patterns that included a patch did not disrupt the orientation processing required for the illusion. We tested this prediction in the following experiment.

Experiment 3: Lateral offset of the patches

Method

Observers

Fourteen observers (7 male, 7 female; age range: 20–30) participated in this experiment. Five of the observers had participated in at least one of the previous experiments. All observers reported they had normal or corrected-to-normal visual acuity and were naive about the purpose of this experiment.

Apparatus, stimuli, and procedure

The apparatus, stimuli, and procedure were identical to those used in Experiment 2, except for the following. Two factors were manipulated in this experiment, bar length and lateral offset of the patches. Lateral offset refers to the offset along the orthogonal axis to each bar. The magnitude of this offset was randomly selected from −1.22 °, −0.82 °, −0.41 °, 0 °, 0.41 °, 0.82 °, and 1.22 °, where a positive value indicates an offset to the right along the orthogonal axis and a negative value refers to an offset to the left. Figure 4a shows a sample of the stimuli. The distance between parallel bars was held constant at 3.26 ° in this experiment. The bar length was set to one of four values: 0.82 °, 1.63 °, 2.45 °, or 3.26 °. When the bar length was 3.26 ° and the offset was 0 °, each bar was connected to the neighboring one, similar to traditional scintillating grid illusion stimuli. Each stimulus was tested 5 times in a total of 140 trials, presented in a randomized order.

Fig. 4
figure4

a Demonstration of a stimulus in Experiment 3 with 0.41 ° lateral offset of the patches and 1 ° bar length (not scaled). b Results of Experiment 3: Mean strength ratings of illusion with respect to the bar length and lateral offset of the patches. The bars denote standard errors of the means (n = 14)

Results and discussion

Figure 4b shows the mean rated strength in Experiment 3. A two-way ANOVA was conducted with bar length and lateral offset as factors. The results revealed a significant main effect of bar length, F(3, 39) = 134.141, p < .0001. The main effect of the lateral offset was not significant, F(6, 78) = 1.279, p > .27. The interaction between the two factors was significant, F(18, 234) = 2.249, p < .004. When the bar length was 3.26 °, the simple main effect of lateral offset was significant, F(6, 312) = 5.298, p < .0001. The results of multiple comparison tests for the simple main effect indicated that the illusion was significantly stronger when there was no lateral offset (i.e., when the bar length was 3.26 °), as compared with when there was lateral offset [−1.22 °–0 °, t(312) = 3.982, p < .0001; −0.82 °–0 °, t(312) = 3.708, p < .0003; −0.41 °–0 °, t(312) = 5.081, p < .0001; 0 °–0.41 °, t(312) = 4.394, p < .0001; 0 °–0.82 °, t(312) = 3.296, p < .0011; 0 °–1.22 °, t(312) = 3.708, p < .0003].

The illusion magnitude was constant as long as the local orientation around a patch was unchanged. On the other hand, the illusion was strengthened when the bars were connected to each other. This may be because the doubled bar length enhanced the orientation processing across the patches, consistent with the notion that local orientation processing is critical for the illusion.

Experiment 4: The effect of gaps around patches

The experiments described above demonstrated that orientation processing across the patch played a critical role in the scintillating grid illusion. The illusion occurred as long as the bar length was preserved to some extent. One possibility is that signal integration along oriented bars across the patch contributed to the illusion. We predicted that the illusion would still occur even when the patch and bars were physically unconnected, as long as the signal integration across the patch was maintained. To confirm this prediction, in Experiment 4a, we employed a spatial gap with variable magnitudes around the patch. It was expected that a larger spatial gap would attenuate the illusion because the signal integration across the patch was likely to be hampered. In addition, we tested whether the illusion with the spatial gap around the patch was mediated by the filling-in of fragmented bars. Ramachandran (1992) reported that fragmented bars are perceived to be connected in the peripheral visual field during prolonged observation. The perceptually connected bars might have contributed to the illusion. In Experiment 4b, we directly asked observers to report whether bars were connected in the stimuli, to examine the relationship between illusion and the filling-in of the fragment bars when stimuli involved a spatial gap around the patch.

Experiment 4a: Rating the illusion with gaps around patches

Method

Observers

Seven observers (4 male, 3 female; age range: 21–30) participated in Experiment 4a. All observers had participated in at least one of the previous experiments but were naive about the purpose of this experiment.

Apparatus, stimuli, and procedure

This experiment was identical to Experiment 2, except for the following. We inserted a spatial gap at the intersection with one of four variable sizes of 0.22 °, 0.46 °, 0.9 °, or 1.36 ° (Fig. 5a). When the gap size was 0.22 °, the gaps became invisible, because they were shorter than the diameter of the circular patches, so the bars were connected to the patches, constituting the traditional scintillating grid stimuli. The bar length reached its minimum length (i.e., 0.9 °) when the gap size was 1.36 ° and the distance between neighboring patches was 3.26 °.

Fig. 5
figure5

a Demonstration of a stimulus in Experiment 4 with 3.26 ° patch distance and 0.45 ° gap size (not scaled). b Mean strength ratings of illusion with respect to the ratio of the bar length to the gap size. c Mean rated strength of illusion with respect to the gap size. The bars denote standard errors of the means (n = 7)

Results and discussion

Figure 5b shows the mean rated strength in Experiment 4a. We calculated the ratio of the bar length to the gap size for each condition and used these ratios as independent variables in the data analysis. We fitted a cumulative Gaussian function to the mean rated strength of the illusions as a function of the ratios (Fig. 5c). The R-squared value of the fitting was .98. These results suggest that the ratios of the bar lengths to the gap sizes were a decisive factor in the strength of the illusion, irrespective of the interpatch distance.

These results indicate that the larger spatial gap disrupted signal integration across the patch, leading to the weakening or absence of the illusion. On the other hand, the small gap did not appear to hamper signal integration across the patch, retaining the illusion to some extent. These findings are in accord with the notion that signal integration along collinear bars across the patch contributes to the illusion.

Experiment 4b: Can perceptual filling-in explain the illusion when the stimulus involves a gap around patch?

Method

Observers

Five observers (2 male, 3 female; age range: 25–30) who had participated in Experiment 4a took part in this experiment.

Apparatus, stimuli, and procedure

This experiment was identical to Experiment 4a, except for the following: Observers were asked to judge whether the bars in the stimuli were connected across patches, by pressing the left arrow key if they perceived any connected bars and the right arrow key if they did not.

Results and discussion

A two-way ANOVA was conducted with patch distance and bar length as factors. The results revealed a significant main effect of gap size, F(3, 12) = 585.545, p < .0001. Multiple comparison tests revealed that the proportion of connected bars with a 0.22 ° gap size was significantly higher than those with other gap sizes [0.22 °–0.46 °, t(12) = 34.343, p < .0001; 0.22 °–0.9 °, t(12) = 34.343, p < .0001; 0.22 °–1.36 °, t(12) = 33.973, p < .0001]. There were no significant differences among the proportions of connected bars with 0.46, 0.9, or 1.36 ° gap size [0.46 °–0.9 °, t(12) = 0.000, p > .99; 0.46 °–1.36 °, t(12) = 0.369, p > .71; 0.9 °–1.36 °, t(12) = 0.369, p > .71]. These results suggested that the connected bars were reported only when the bars were physically connected across patches. The results demonstrated that no filling-in occurred under the conditions of Experiment 4. As is shown in Fig. 6, the correlation between illusion strength and the proportion of connected bars was weak. The results indicated that the illusion became stronger with increasing bar length. However, perceptual filling-in did not occur for any condition of bar length. These results suggest that the effect of bar length on illusion strength was not due to perceptual filling-in of the bars.

Fig. 6
figure6

Correlation of mean rated strength and proportion of trials on which connected bars were reported. The bars denote standard errors of the means (n = 6)

Experiment 5: Lateral offset of the bars

We predicted that if the signal integration along collinear bars across the patch contributes to the illusion, the lateral offset of the bars across the patch would hamper the illusion because the collinearity of the bars would be disrupted. We investigated this prediction by manipulating the lateral offset of the bars across the patch.

Method

Observers

Fifteen observers (9 male, 6 female; age range: 20–31) participated in this experiment. Nine of the observers had participated in at least one of the previous experiments. All observers reported that they had normal or corrected-to-normal visual acuity. All observers were naive about the purpose of this experiment.

Apparatus, stimuli, and procedure

This experiment was identical to Experiment 3, except for the following. Two factors were manipulated in this experiment: the ratio of the bar length to the gap size and the lateral offset of the bars. The lateral offset in this experiment referred to the offset along the orthogonal axis to the orientation of the bars. However, in contrast to Experiment 3, lateral offset was applied to the bars across the patch, while retaining the spatial gap between the bars at the intersection (Fig. 7a). The offsets of the bars were in the same directions and were randomly selected from −0.54 °, −0.36 °, −0.18 °, 0 °, 0.18 °, 0.36 °, and 0.54 °. As is shown in Fig. 7a, the direction of the offsets around one patch was opposite to that around its neighboring patch. The ratio of bar length to gap size was set at one of the following five levels: 6, 3.33, 2, 1.2, or 0.67. The distance between patches was kept at 3.26 °. Each stimulus was tested 4 times in a total of 140 trials, presented in a randomized order.

Fig. 7
figure7

a Demonstration of a stimulus in Experiment 5 with 0.18 ° lateral offset of the bars and 3.33 ratio of bar length to gap size (not scaled). b Results of Experiment 5: Mean strength ratings of illusion with respect to the ratio of bar length to gap size and lateral offset of the bars. The bars denote standard errors of the means (n = 15)

Results and discussion

A two-way ANOVA was conducted with factors of lateral offset of the bars and ratio of the bar length to the gap size. The results showed significant main effects of lateral offset, F(6, 84) = 75.127, p < .0001, and ratio, F(4, 56) = 29.399, p < .0001, as well as a significant interaction between the two factors, F(24, 336) = 19.519, p < .0001. Figure 7b shows the mean strength rating of the illusion in this experiment. In contrast to Experiment 3, lateral offset was found to influence the magnitude of the illusion. The simple main effect of lateral offset was significant when the ratio was 6, F(6, 420) = 95.806, p < .0001, 3.33, F(6, 420) = 65.387, p < .0001, 2, F(6, 420) = 28.432, p < .0001, or 1.2, F(6, 420) = 7.683, p < .0001. Multiple comparison tests revealed that, for these ratios, the illusion was strongest when no lateral offset was applied to the bars and became significantly weaker as the lateral offset increased [e.g., −0.18 °– 0 ° (ratio = 6), t(312) = 11.607, p < .0001; −0.18 °–0 ° (ratio = 3.33), t(312) = 7.539, p < .0001; −0.18 °–0 ° (ratio = 2), t(312) = 6.845, p < .0001; 0 °–0.18 ° (ratio = 1.2), t(312) = 2.877, p < .005]. In addition, the direction of the lateral offset did not influence its effect [e.g., −0.18 °–0.18 ° (ratio = 6), t(312) = 0.694, p > .48; −0.18 °–0.18 ° (ratio = 3.33), t(312) = 0.694, p > .48; −0.18 °–0.18 ° (ratio = 2), t(312) = 0.794, p > .42; −0.18 °–0.18 ° (ratio = 1.2), t(312) = 0.794, p > .42].

Overall, the strength of the illusion decreased as the lateral offset increased, indicating that signal integration along the “collinear” bars across the patch contributed to the illusion.

General discussion

In the present study, we sought to elucidate the way in which orientation processing across the patch contributed to the scintillating grid illusion. First, we confirmed that the strength of the illusion was related to the length of the bars (Experiments 1 and 2). The lateral offset of the patches did not affect the illusion strength as long as the bar length connected to the patch was not changed (Experiment 3). The illusion still occurred when the patch was located at the gapped intersection, as long as the gap size was sufficiently small (Experiment 4). For the illusion with gapped intersection stimuli, the lateral offset of the bars across the patch was detrimental to the illusion strength, indicating that the collinear bars across the patch are necessary for the illusion to occur (Experiment 5).

All of these findings are consistent with the notion that signal integration along the collinear bars across the patch is critical for the scintillating grid illusion to occur. These findings raise questions about the nature of the signal integration that contributes to the scintillating grid illusion. Below, we propose a putative underlying mechanism of the scintillating grid illusion based on our finding of signal integration across the patch, focusing on the involvement of S1-type cells, which are considered a possible neural substrate for the Hermann grid illusion.

Hermann grid illusion and S1-type simple cell theory

The present results should be considered in the context of another classical optical illusion, the Hermann grid illusion. Hermann grids consist of a black background and white grid, without circular patches. In the illusion, gray smudges are perceived at the intersection of the grids (Fig. 1b; Hermann, 1870). The scintillating grid illusion was discovered as a variation of the Hermann grid illusion (Bergen, 1985; Schrauf, Lingelbach, Lingelbach, & Wist, 1995; Schrauf et al., 1997). The mechanism of the Hermann grid illusion has been explained by the spatial interaction of the retinal ganglion cells (Baumgartner, 1960). However, conflicting evidence from recent studies has cast doubt on this explanation (J. Levine, Spillmann, & Wolf, 1980; Lingelbach, Block, Hatzky, & Reisinger, 1985; Lingelbach & Ehrenstein, 2002; Schiller & Carvey, 2005; Spillmann, 1971, 1994; Spillmann & Levine 1971). Recently, Schiller and Carvey proposed a new interpretation, focusing on certain properties of the S1-type simple cells in the primary visual cortex. The simple cell is a type of neuron in area V1, reported to be selective to luminance-defined orientation (Hubel & Wiesel, 1962; Mullikin, Jones, & Palmer, 1984). These cells have been found to possess one or more subfields, which are excited by increments or decrements in luminance. Approximately 27 % of simple cells have only one subfield (Schiller, Finlay, & Volman, 1976a), and this subgroup of cells is termed S1-type simple cells. Possessing only one subfield means that some of these cells can fire when luminance increases, while the rest can fire when luminance decreases. The former type is referred to as an ON S1-type simple cell, while the latter is referred to as an OFF S1-type simple cell. It should be noted that S1-type simple cells are sensitive to oriented luminance blobs. The ON and OFF S1-type simple cells are excited by spatially elongated light edges or dark edges, respectively, falling on their subfields (Malpeli, Schiller, & Colby, 1981; Schiller, Finlay, & Volman, 1976b; Schiller & Malpeli, 1978).

Schiller and Carvey (2005) proposed a possible mechanism for the Hermann grid illusion on the basis of the properties of S1-type simple cells. According to this proposal, because there are no continuous edges at the intersections of the Hermann grids, the responses of ON and OFF S1-type cells are reduced, and the intersection is consequently perceived to be darker than the rest of the grids. Importantly, the activation of S1-type simple cells is increased or decreased even at the intersection. The bars near the intersection stimulate the subfield of S1-type simple cells, causing the cells to respond weakly.

An explanation of the scintillating grid illusion involving S1-type simple cells

We assumed that, similar to Hermann grid stimuli, the bars near the intersection would also stimulate the subfield of S1-type simple cells in the scintillating grid stimuli, even though the patch exists at the intersection. At the intersection of the display, the subfield of the ON S1-type cells is stimulated by the gray bars near the intersection. As compared with the strong response of the ON S1-type cells along the bars, the response of the cells at the intersections is weak. Although weak, the output of ON S1-type simple cells is likely to serve as the neural representation of the gray bars. We propose that the visual system perceives the weak representation of the gray bars as a dark smudge within the patch at the intersection (Fig. 8). That is, signal integration at the intersection, which we have discussed as a putative mechanism of the scintillating grid illusion, may be based on the activity of S1-type simple cells.

Fig. 8
figure8

Possible explanation for the variation in the strength of the scintillating grid illusion in terms of the activity of S1-type simple cells. Upper panel: As the bar length increased, the bars caused more stimulation to the subfield of the S1-type simple cell, leading to a stronger illusion. Middle panel: As the size of the gap at the intersection decreased, the bars caused more stimulation to the subfield of the S1-type simple cell, leading to a stronger illusion. Bottom panel: As the lateral offsets of the bars decreased, the bars caused more stimulation of the subfield of the S1-type simple cell, leading to a stronger illusion. The graph shows a theoretical prediction for the illusion strength as a function of S1-cell activity

However, several other aspects of the scintillating grid illusion must also be explained by any theoretical mechanism. Below, we describe how our proposed mechanism can explain other aspects of the present findings and the results of previous studies.

Longer bars generate stronger illusions

Schiller et al. (1976a) reported that the responses of simple cells increased when the oriented bar in the receptive fields was elongated. The results of the present study indicated that the illusion strengthened as bar lengths increased. This finding is consistent with the notion that elongated bars sufficiently stimulate the subfield of oriented S1-type simple cells centered on the intersections. In contrast, short bars do not stimulate the subfields, resulting in a weak or null response of the cell. The strength of the cell’s response may correspond to the strength of the illusion. We propose that the subfields of S1-type cells are stimulated even when the spatial gap is inserted at the intersection, as long as the spatial gap is small, causing the illusion within patches located at the gapped intersections (Fig. 8).

Lateral offset of the bars suppresses the scintillating grid illusion

The lateral offset of the bars in Experiment 5 gradually reduced the strength of the illusion. An S1-type simple cell is likely to sum the luminance along a straight edge with its subfields. Thus, when the edge is spatially offset, the response of the cell would be expected to weaken. In a similar vein, the response of S1-type cells at the intersection of the scintillating grid illusion display is likely to be reduced as the lateral offset of the bar increases. On the other hand, in Experiment 3, the lateral offset of the patches (i.e., cruciform patterns) did not affect the magnitude of the illusion. We propose that this occurred because the responses of the S1-type simple cells were not affected by the lateral offset of the patches (i.e., the cruciform patterns).

Curved bars reduce the strength of the scintillating grid illusion

Our proposed explanation involving S1-type simple cells is also in accord with the reduction of the illusion in a display with curved bars reported in a previous study (M. W. Levine & McAnany, 2008). The authors ascribed the diminishment of the illusion to the reduced visibility of illusory dark spots caused by the presence of salient features such as curved edges. However, we propose an interpretation based on the activity of S1-type cells, since S1-type cells preferentially respond to straight bars over curved bars. Consequently, a curved bar would not be expected to stimulate the subfields of S1-type cells, consequently diminishing the scintillating grid illusion.

Scintillating grid illusion strengthens when patches become brighter

The luminance relationship among the patches, bars, and background is reported to be critical for the scintillating grid illusion (Schrauf et al., 1997). The illusion occurs when the bars are sufficiently darker than the patches, but brighter than the background. When the luminance of the bars and background are constant, the brighter patches generate stronger illusions. As the patches get brighter, the representation of the gray bars at the intersections may become more distinguishable from the patch, resulting in a stronger illusion. However, as the luminance of the patch approaches the bar luminance, the representation of the gray bar at the intersection may become indistinguishable from the patch, resulting in a weak illusion.

Scintillating grid illusion occurs only in the peripheral visual field

The scintillating grid illusion does not occur in the central visual field. No previous studies have satisfactorily explained why the illusion is limited to the peripheral visual field. Several studies have suggested that the scintillating effect is related to spatial frequency processing. Before the scintillating grid illusion was discovered, Bergen (1985) found the first scintillating illusory effects at the grid intersections by blurring the Hermann grid illusion using low-pass filtering. Hamburger and Shapiro (2009) also reported that a low-pass equiluminant grid created scintillating effects.

How can the dependency of the illusion on spatial frequency be explained by the activity of S1-type cells? As was described above, we assume that S1-type simple cells centered on the intersection produce a representation of the gray bars at the intersection and that the visual system perceives this representation as smudges on the patch. We propose that the illusion occurs when the patch does not completely suppress the representation of the gray bars at the intersection. In the central visual field, the high spatial frequency components of the patch are processed accurately. On the other hand, it is plausible that S1-type simple cells centered at the intersection preferentially respond to the low spatial frequency components of the bar, because the cell would be expected to summate the luminance along the elongated subfields. A previous study demonstrated that high spatial frequency strongly affected the appearance of the low spatial frequency components (Burr & Morrone, 1990). Thus, it is possible that in the central visual field, the high spatial frequency components of the patch suppress the low spatial frequency components of the bars, resulting in the weakening or absence of the illusion. On the other hand, in the periphery, the edge of the patch is not well processed, due to an inherently low resolution. Thus, the patch does not suppress the bar representation at the intersection. This preserves the representation of the gray bar at the intersection, leading to the scintillating grid illusion. In this scenario, the representation of patches would exhibit perceptual rivalry with the representation of the gray bars, and this rivalry produces a “scintillating” illusory percept of a dark spot within the patches at the intersections, besides the attention shift proposed by VanRullen and Dong (2003) as a source of the scintillation. Our proposed model, described above, requires further empirical validation in the future. To elucidate this mechanism, we plan to conduct further experiments in which the spatial frequency components of the bar and patch are systematically manipulated.

Square patches generate a weaker illusion than do circular or diamond-shaped patches

Qian et al. (2009) demonstrated that the strength of the scintillating grid illusion was substantially reduced when the patch was square, as compared with when it was diamond or disk shaped. We propose that Qian et al.’s results can be explained by the interaction of S1-type simple cells tuned to vertical and horizontal orientations. Two of the four sides of a square patch are orthogonal to the bars connected to the patch. Because the patch is brighter relative to the bars in the scintillating grid illusion display, the responses of S1-type simple cells responding to the square patch would be expected to override the responses of cells responding to the bar. Consequently, the square patch would be expected to suppress the bar representation at the intersection, weakening the illusion. Because the circular and diamond patches have no clear orthogonal orientation to the bar, the orthogonal interaction of S1-type cells is not strong, and thus the representation of the bar would not be suppressed by the patch, resulting in the scintillating grid illusion.

Conclusion

Overall, the present results indicate that the signal integration across the patch is critical for generating the scintillating grid illusion. The signal integration across the patch is likely to be mediated by S1-type simple cells. S1-type simple cells may underlie both the scintillating grid illusion and the Hermann grid illusion, although there are likely to be slight differences in the role of the S1-type simple cells between these two illusions. According to our proposed model, the scintillating grid illusion arises from the interaction of the activity of S1-type simple cells responding to bars and that of cells responding to patches at the intersection. In contrast, the Hermann grid illusion arises from differences in the magnitude of activity of S1-type simple cells at the intersection and those along the bars.

Notes

  1. 1.

    We did not tell the observers the purpose of the study before and after the experiment. Thus, we considered that the observers were naive about the purpose of the study, even though most of them participated in multiple experiments.

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Qian, K., Kawabe, T., Yamada, Y. et al. The role of orientation processing in the scintillating grid illusion. Atten Percept Psychophys 74, 1020–1032 (2012). https://doi.org/10.3758/s13414-012-0295-y

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Keywords

  • Visual perception
  • Neuropsychology
  • Spatial Vision