Effects of binocular disparity on impressions - Wiley Online Library

Japanese Psychological Research 2012, Volume 54, No. 1, 38–53 Special issue: Stereoscopic depth perception

doi: 10.1111/j.1468-5884.2011.00510.x

Effects of binocular disparity on impressions DAISUKE TOYA

Yamaguchi University

MAKOTO ICHIKAWA* Chiba University

Abstract: We examined how the size of binocular disparity, which determines the apparent depth magnitude, affects the impressions in different dimensions in viewing nonrepresentational stereograms and representational stereoscopic pictures by the use of semantic differential method with the scales that are related to the basic dimensions of impression: evaluation, activity, potency, and reality. In viewing nonrepresentational stereograms, which presented a binocular disparity ranging from 0 to approximately 70 min of arc between the central line, or a rectangle with a peripheral frame, the ratings for the scales for the evaluation impression were the highest at approximately 10–20 arc min of disparity, while the ratings for the activity impression increased with the size of the disparity. In viewing the representational and nonrepresentational stereograms, the ratings for the scales, which are related to the evaluation, activity, and reality factors, increased with the size of the disparity if the disparity specified multiple depth layers. These results indicate that the effects of binocular disparity in viewing a stereoscopic picture depend on the disparity distribution and disparity size rather than the representationality of the stereogram. Key words: stereoscopic images, haploscopic observation, crossed and uncrossed disparity, depth layer structure, factor analysis.

This study is concerned with the effects of binocular disparity on apparent depth magnitude and on impression formation in stereogram observation. Since the discovery of binocular disparity (Wheatstone, 1838), we have enjoyed stereoscopic views of static and dynamic pictures. The fact that people have enjoyed viewing the stereogram suggests that the binocular disparity and depth perception obtained when viewing the stereograms have special effects which entertain observers. The problem is then how the semantic qualities of pictures vary with binocular disparity. However, there has been no or little research that has examined this problem. Understanding the relation between binocular disparity and the acquired impression, as well as the perceived depth, when viewing stereograms is important now

that three-dimensional (3-D) picture presentations are becoming popular for entertainment. In this study, we investigated how the disparity size affects the acquired impressions, as well as the apparent depth magnitude in stereogram observation. Based on a previous study which investigated the aesthetic impression and visual illusion, one expects that aesthetic impressions are exaggerated when viewing a stereogram that induces a large magnitude of depth. Noguchi and Rentschler (1999) conducted experiments in which observers measured the extent of the visual illusion and rated the aesthetic impression (beautifulness) for several geometrical illusions (i.e., the Botti, Oppel-Kundt, and Helmholtz illusions). Their results demonstrated that the extent of the illusion and the

*Correspondence concerning this article should be sent to: Makoto Ichikawa, Department of Psychology, Chiba University, Yayoi-cho, Inage-Ku, Chiba 263-8522, Japan. (Email: [email protected]) © 2012 Japanese Psychological Association. Published by Blackwell Publishing Ltd.

Effects of binocular disparity

aesthetic impression peaked at the medium range of the stimulus complexity. Based on these results, they proposed that, when viewing illusory figures, both the extent of the illusion and the aesthetic impression are most enhanced at the same condition. An observer can perceive depth when viewing a stereogram that is actually a flat image. As in several textbooks in general psychology (Smith, NolenHoeksema, Fredrickson, & Loftus, 2003) and perception (Palmer, 1999; Sekuler & Blake, 1994), we consider depth perception in stereogram observation as a consequence of visual illusion, and the apparent depth magnitude as the extent of the illusion because there is a discrepancy between the actual stimulus property (flat picture) and the perception (objects in 3-D space). Therefore, based on Noguchi and Rentschler’s proposal, we might expect that, when viewing a stereogram, the aesthetic impression would increase with the binocular disparity size that corresponds to the magnitude of the illusory depth. Based on the results of another previous study, one might expect a strong aesthetic effect of the illusory depth perception when viewing a stereogram with large binocular disparity. Ramachandran and Hirstein (1999) proposed that isolating a single module and allocating the observer’s attention is a main principle of visual art. That is, one might expect that viewing a stereogram with large binocular disparity, which would isolate the module for depth perception and allocate the observer’s attention to the depth dimension, has a strong effect on impression formation. However, if the disparity size is too large to maintain a single image for the stereogram, then observers see double images, but still see vague depth, although the apparent depth magnitude does not appear to increase with the increment of disparity size. This largest disparity size at which observers can see vague depth is called the “upper limit” of binocular disparity (Howard & Rogers, 2002). Therefore, we examined how the aesthetic impression varies at large disparity. In order to understand the effects of disparity on forming impressions for stereogram, we also

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examined the interaction between the effects of disparity and effect of representationality of the stereogram on the aesthetic impressions. Previous studies have demonstrated that representational pictures can give strong impressions in different dimensions (Markovic, 2011), and that, in particular, observers consistently preferred representational pictures over abstract pictures in rating them for beautifulness (Feist & Brady, 2004). It is known that presenting semantically congruent stimuli at the same time would exaggerate the impression in multisensory studies (Iwamiya, 1994; Ryan, 1940). If we may exaggerate the aesthetic impressions by combining the effects of disparity with those of representationality of the picture, using representational pictures would be a useful technique to manipulate impressions by the use of 3-D pictures.Therefore, in Experiments 2 and 3, we investigated whether the effects of disparity would enhance the impressions of the representational pictures. In addition, we examined how the aesthetic effects of disparity depend on the number of depth layers. It is known that observers can segregate up to six simultaneous depth layers, and that the segregation of the depth layers would be difficulty for small interlayer disparity (Tsirlin, Allison, & Wilcox, 2008). This suggests that the disparity size and the number of depth layers would affect both the perception and the impression formation for the multiple depth layers. Therefore, in Experiment 3, we examined how the disparity and the number of depth layers interact with each other in determining apparent depth and forming the aesthetic impressions. Noguchi and Rentschler (1999) examined the relation between the extent of the illusion and the aesthetic impression. However, our impression formation has three main dimensions: the evaluation, activity, and potency (Osgood, Suci, & Tannenbaum, 1957). The aesthetic impression is related to the evaluation dimension, but the relations between the illusory effects and the other two dimensions have not been examined. In the present study, we examined the relation between the disparity size and the impressions comprehensively. Based on the © Japanese Psychological Association 2012.

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D. Toya and M. Ichikawa

results of the experiment described herein, we aimed to propose a way to manipulate the impressions in different dimensions by the use of stereograms.

Experiment 1 In the first experiment, we examined how the size of the horizontal binocular disparity affects the apparent depth and impressions in different dimensions for nonrepresentational stereograms (Figure 1).We used front parallel-surface stereograms of four types that specify a line or a square as protruding or hollowing in a frame, and slanted-surface stereograms of three types that specify a slant surface by the use of shear disparity or compression disparity (Gillam, Chambers, & Russo, 1988). Ogle (1952) reported the upper limit of the horizontal binocular disparity to be approximately 70 arc min. Therefore, we used horizontal disparities that were at most approximately 70 arc min.

Figure 1 © Japanese Psychological Association 2012.

Methods Apparatus and stimuli. We used a personal computer display (Toshiba DynaBook SSM4/ 260CCH) to present the stereogram in a dark room. The viewing distance to the display was 50 cm. Observers viewed the display through a viewing window (11.6 ¥ 11.6 arc deg), which was set at the middle point of the viewing distance (25 cm) so that the stereogram image for each eye was fused in the frame of the window (Grove, Gillam, & Ono, 2002). A chin-rest was used to fixate the viewing distance and the observer’s eye level at the center of the display. We used stereograms of seven types (Figure 1). Four of them were made by a line (2.3 arc deg) or square (2.3 ¥ 2.3 arc deg) on the display. In those stereograms, binocular disparity existed between the frame (5.8 ¥ 5.8 arc deg) and a vertical line (Figure 1a), a horizontal line (Figure 1b), a white square (Figure 1c), or a black square (Figure 1d). Additionally, we prepared slanted-surface stereograms of three types by the use of compression disparity or

Stereograms used in Experiment 1.


shear disparity. The compression disparity specified a slanted rectangle against the horizontal axis (Figure 1e) for which the right side was on the frame plane. The shear disparity specified a slanted rectangle against the vertical axis (Figure 1f), for which the bottom was on the frame plane, and a folded surface against the vertical axis (Figure 1g) for which the top and bottom were at the frame plane. We prepared five disparity size conditions (4.8, 9.6, 19.2, 38.4, and 76.8 arc min) for both crossed and uncrossed disparity, as well as the 0 arc min condition. Procedure. Presentation of the four front parallel-surface stereograms and the three slanted-surface stereograms was blocked separately. In each block, each stereogram was presented once in random order for an observer. In each trial, observers reported the apparent depth magnitude by pulling a tape measure out of its case.Thereafter, they rated impressions by the use of 11 scales (Table 1), which were selected from the scales presumed to be related to the three basic impression factors (evaluation (beautiful-ugly, good-bad, comfortableuncomfortable, relax-nervous), activity (excitable-claim, alive-dead, active-static), and potency (heavy-light, closed-open) (Osgood et al., 1957)) and impressions that are induced when viewing stereograms (realistic-fantastic, natural-unnatural) by the use of seven-point Likert scales with neutral at the center. Neither eye movement nor viewing time was restricted. Observers. The observers were 20 university students, who were 19–22 years old. Each had normal or corrected-to-normal visual acuity, and was familiar with the equipment to perceive depth specified by binocular disparity.

Results and discussion A factor analysis (principal factor solution, varimax method) for the rated scores in 11 scales extracted three factors (evaluation, activity, and potency) for which the Eigenvalues were larger than 1.0 (Table 1). Figure 2 presents the average of apparent depth magni-

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tudes and the averages of the ratings for each adjective pair for which the factor loading was larger than 0.4 for each impression dimension for each condition. For these data, we conducted two-way repeated measures analyses of variance (ANOVA) separately with disparity size (11) and the types of the front parallelsurface stereograms (4) and slanted-surface stereograms (3). For the apparent depth magnitude when viewing the front parallel-surface stereogram (Figure 2a), the main effect of the disparity size, F(10, 190) = 38.688, p < .001, and the interaction of the two factors, F(30, 570) = 1.546, p < .05, were significant, although the main effect of the stereogram was not significant. For the results of the slanted-surface stereogram (Figure 2b), the main effects of the stereogram, F(2, 38) = 3.919, p < .05, and the disparity size, F(10, 190) = 15.484, p < .001, and interaction of these two factors, F(30, 570) = 4.945, p < .001, were significant. The results of statistical analyses and Figure 2 indicate that the apparent depth magnitude increased with the increment of disparity size when viewing a stereogram of any type. For the main effect of the stereogram, Tukey’s post hoc HSD test revealed that the surface against the horizontal axis induces more protruding depth perception than the folded surface stereogram does. For the evaluation when viewing the front parallel-surface stereograms (Figure 2c), the main effect of disparity size was significant, F(10, 190) = 5.511, p < .001, although the main effect of stereogram or interaction of the two factors was not significant. The HSD test for the main effect of disparity size showed that the impressions for the 76.8 arc min crossed and uncrossed disparities were weaker than those for the other whole disparity size conditions, and that those for the 38.4 arc min uncrossed and the 4.8 arc min crossed disparity were weaker than those for the 19.2 arc min and the 9.6 arc min uncrossed disparities, p < .05. For the slanted-surface stereograms (Figure 2d), the main effect of disparity size was significant, F(10, 190) = 16.636, p < .001, although the main effect of the stereogram or interaction of the two factors was not significant. The HSD test © Japanese Psychological Association 2012.

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Table 1

Factor loading and communality for each experiment Factor

Type Experiment 1 I

II

III II·III Experiment 2

II III I·III II·III Experiment 3 I

II

III II·III

Factor loading

Scales (adjective pairs)

Communality

Good-Bad Comfortable-Uncomfortable Natural-Unnatural Beautiful-Ugly Relax-Nervous Realistic-Fantastic Active-Static Excitable-Calm Alive-Dead Heavy-Light Closed-Open

0.630 0.659 0.589 0.521 0.394 0.357 0.624 0.623 0.499 0.128 0.314

Beautiful-Ugly Comfortable-Uncomfortable Good-Bad Relax-Nervous Closed-Open Excitable-Calm Active-Static Realistic-Fantastic Natural-Unnatural Heavy-Light Alive-Dead

0.677 0.639 0.610 0.606 0.511 0.799 0.710 0.424 0.650 0.540 0.604

Active-Static Excitable-Calm Alive-Dead Open-Closed Relax-Nervous Light-Heavy Good-Bad Realistic-Fantastic Natural-Unnatural Comfortable-Uncomfortable Beautiful-Ugly

0.745 0.627 0.712 0.703 0.485 0.428 0.722 0.600 0.601 0.738 0.656

Factor I

Factor II

Factor III

30.81% 0.819 0.803 0.809 0.723 0.585 0.572 -0.198 -0.254 0.304 -0.044 -0.166 30.73% 0.815 0.782 0.743 0.661 -0.591 -0.005 -0.081 0.173 0.662 -0.432 0.323 24.90% 0.856 0.783 0.745 0.693 -0.050 0.085 0.328 -0.086 0.173 0.284 0.351

20.05% 0.089 0.022 -0.066 -0.032 -0.266 -0.014 0.844 0.829 0.709 0.025 -0.467 18.80% 0.104 0.129 0.109 -0.343 -0.308 0.887 0.838 0.048 -0.006 -0.140 0.553 21.31% 0.104 0.074 -0.016 0.459 0.687 0.633 0.586 -0.100 0.342 0.645 0.600

6.00% -0.166 -0.117 0.045 0.203 -0.139 0.238 -0.030 0.025 0.009 0.551 0.440 12.03% 0.052 0.101 0.216 -0.228 0.258 -0.115 -0.027 0.626 0.459 0.578 0.440 17.58% -0.027 -0.096 0.396 0.109 0.106 -0.140 0.216 0.763 0.674 0.491 0.418

Note. Bold and italic numbers show the factor loadings whose absolute values were more than 0.6 and 0.4, respectively. The percentage below each factor’s numeral represents the contribution rate.

showed that the impressions for the 76.8, and the 38.4 arc min disparities were weaker than for any other disparity size conditions, and that those for the 19.2 arc min uncrossed and the 9.6 arc min crossed disparity were stronger than those for the 4.8 and the 19.2 crossed disparity, p < .05. Some observers reported that they had difficulty in fusing the stimulus binocularly for the large disparity sizes, and that this difficulty of fusion might degrade the evaluation. © Japanese Psychological Association 2012.

For the activity when viewing the front parallel-surface stereograms (Figure 2e), the main effect of the disparity size was significant, F(10, 190) = 34.849, p < .001, although the main effect of the stereogram or interaction of the two factors was not significant. For the slantedsurface stereograms (Figure 2f), the main effect of the stereograms, F(2, 38) = 8.392, p < .001, disparity size, F(10, 190) = 52.655, p < .001, and interaction of the two factors, F(20,


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Figure 2 Averages of apparent depth magnitude and ratings for evaluation, activity, and potency in Experiment 1. Apparent depth magnitudes, evaluation, activity, and potency are presented respectively in (a) (c) (e), and (g) for the front parallel-surface stereogram, and in (b) (d) (f), and (h) for the slanted-surface stereogram. Positive and negative disparity sizes denote those for the crossed and the uncrossed disparity, respectively. © Japanese Psychological Association 2012.

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280) = 4.442, p < .001, were significant. The HSD test for the interaction showed that, at 76.8, 38.4, and 96 arc min uncrossed disparity, and 38.4 and 76.8 arc min crossed disparity, the respective impressions for the horizontal slant were weaker than those for the other stereograms. For the main effect of stereogram, the impression for the horizontal slant was weaker than that for the other two stereograms. The results of statistical tests and Figures 2e and 2f suggest that the activity increased with the increment of disparity size, and that the horizontal slant introduces a weaker impression than those introduced by the vertical slant and vertically folded surface. Weak effects of compression disparity would result from the difficulty in extracting depth perception from the compression disparity (Cagenello & Rogers, 1993; Gillam et al., 1988). For the potency when viewing the front parallel-surface stereograms (Figure 2g), the main effect of stereograms was significant, F(3, 57) = 6.018, p < .001, although the main effect of disparity size or interaction of the two factors was not significant. The HSD test for the main effect of stereogram showed that the impression for the black square was weaker than that for either the horizontal line or the white square. This result suggests that the black square introduces a heavy impression. For the slanted-surface stereograms (Figure 2h), however, the respective main effects of stereogram, disparity size, or interaction of the two factors was not significant. These results suggest that the stereogram does not give us a consistently heavy impression at any disparity size without luminance variance.

Experiment 2 In Experiment 1, the depth structure specified by the stereogram was very simple, and the stereograms were nonrepresentational. However, usually, we use representational pictures of the stereogram in entertainment. In the following two experiments, we examined how the effects of disparity interact with the effect of the representationality of pictures. In Experiment 2, © Japanese Psychological Association 2012.

we examined how the effects of disparity affect the impression of the representational pictures. We prepared stereograms with representational pictures, which give us strong or weak impressions in different dimensions. Methods Apparatus and stimuli. Two pairs of mirrors (10.0 cm ¥ 12.5 cm) and 19-in. displays (Samsung Syncmaster 940B LCD monitor) controlled by a personal computer (PCKOUBOU Scenage Value) were arranged as a Wheatstone-type-haploscope to present a binocular disparity cue. The distance from the observer’s eyes to the center of the mirrors was 6.5 cm and that from the mirrors to the displays was 43.5 cm. Therefore, the total viewing distance from the eyes to the displays was approximately 50 cm. The experiments were conducted in a dark room. We conducted a preliminary test to select representational pictures which would introduce strong or weak impressions in the main impression dimensions. For the preliminary test, we prepared 30 colored representational pictures to induce strong or weak impressions in the evaluation, activity, and potency dimensions which were found in Experiment 1. We used computer software (Adobe Illustrator 10) to create those pictures. The picture size was 13.0 ¥ 17.3 arc deg. They were presented to 20 naïve observers (10 male and 10 female) with zero binocular disparity. The observers rated their impressions by the use of the same 11 scales as used in Experiment 1. For the results, we conducted a factor analysis (principal factor solution, varimax method), and extracted three factors; the evaluation, reality, and activity for which the Eigenvalues were larger than 1.0. The extracted factors were slightly different from the factors found in Experiment 1. The scale which is related to potency was involved in the evaluation. Then, we found a new “reality impression” as the second factor. This difference in the extracted factor between Experiments 1 and 2 is expected to result from the representationality of the pictures used in Experiment 2.


We selected six pictures to which observers assigned the highest or lowest ratings in those three impression dimensions (Figure 3). For each picture ((Figure 3a, Alpine Flowers) high evaluation, (Figure 3b, Factory) low evaluation, (Figure 3c, Music Club) high activity, (Figure 3d, Moonlit Night) low activity, (Figure 3e, Rainy Day) high reality, and (Figure 3f, Guitarist) low reality), we introduced the horizontal binocular disparity by displacing the elements in the picture horizontally so that those pictures had four to six depth layers. The image frame has zero disparity with the display frame, and the picture elements were hollowing in the display by the use of

Figure 3

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uncrossed disparity. The disparity size conditions for the deepest part of the picture were 0, 17.2, 34.4, and 68.8 arc min. Procedure. Each of the 24 stimulus conditions (representational pictures (6) ¥ disparity size (4)) was presented once in a random order. The observers reported the apparent depth magnitude and impressions by the use of the same procedures as used in Experiment 1. Observers. In Experiment 2, 20 observers (aged 20–27 years) participated. Each had normal or corrected-to-normal visual acuity and normal binocular stereopsis. Of them, 11 had taken part in the preliminary test.

Representative pictures used in Experiment 2. © Japanese Psychological Association 2012.

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Results and discussion Factor analysis (principal factor solution, varimax method) extracted three factors (evaluation, activity, and reality) for which Eigenvalues were larger than 1.0 (Table 1). Figure 4 shows the average of the rating for the scales for which the factor loads for the three factors were larger than 0.4, and the average apparent depth for each condition. We conducted two-way repeated measures ANOVA with the picture (6) and disparity size (4) separately for the apparent depth magnitude, and ratings for the evaluation, activity, and reality. For the apparent depth magnitude, the main effect of the picture, F(5, 95) = 3.720, p < .005, the main effect of disparity size, F(3, 57) = 17.457, p < .001, and the interaction of the two factors, F(15, 285) = 2.053, p < .05, were significant. For the main effect of disparity size, the

apparent depth magnitudes for nonzero disparity sizes were larger than that for 0 arc min, and that for 68.8 arc min was larger than that for 17.2 arc min, p < .05. These results and Figure 4 show that the apparent depth magnitude increased with the increment of the disparity size in each picture condition. For the evaluation, the main effect of the picture, F(5, 95) = 12.535, p < .001, the main effect of disparity size, F(3, 57) = 17.826, p < .001, and the interaction of the two factors, F(15, 285) = 1.760, p < .05, were significant. The HSD test for the main effect of picture revealed that the impression for “Alpine Flowers” was stronger than those for “Factory,” “Music Club,” “Guitarist,” and “Rainy Day.” That for “Moonlit Night” was stronger than that for “Factory.” These results indicate that the manipulation of the evaluation in terms of the

Figure 4 Averages of apparent depth magnitudes and ratings for the evaluation, reality, and activity dimensions in Experiment 2. In each panel, the open circles, filled circles, open triangles, filled triangles, open diamonds, and gray diamonds respectively present the results for the pictures “Alpine Flowers,” “Factory,” “Rainy Day,” “Guitarist,” “Music Club,” and “Moonlit Night.” © Japanese Psychological Association 2012.


representational picture was successful. For the main effect of disparity size, the impression for 68.8 arc min was stronger than those for 0 and 17.2 arc min. For the simple main effect of disparity size in the interaction, the impression for 68.8 arc min was stronger than that for 0 arc min in any picture. These results of statistical analyses and Figure 4 demonstrate that a large disparity exaggerated the evaluation when viewing any representational picture. For the activity, the main effect of the picture, F(5, 95) = 34.60, p < .005, the main effect of disparity size, F(3, 57) = 20.583, p < .001, and the interaction of the two factors, F(15, 285) = 2.218, p < .05, were significant. The HSD test for the main effect of picture revealed that the impression for “Music Club” was stronger than that for any of the other five pictures. The respective impressions for “Alpine Flowers,” “Moonlit Night,” “Rainy Day,” and Guitarist’ were stronger than that for “Moonlit Night.” The impressions of “Rainy Day” and “Guitarist” were stronger than those for “Alpine Flowers” or “Factory”. These results indicate that the manipulation of the activity in terms of the representational pictures was successful. For the main effect of disparity size, the impression for nonzero disparities was stronger than that for the 0 arc min condition, and the impression for 68.8 arc min was stronger than that for 17.2 arc min. For the simple main effect of disparity size in the interaction, the impression for 68.8 arc min was stronger than that for 0 arc min when viewing any image except for “Moonlit Night,” for which no significant simple main effect of disparity size was found. These results indicate that large disparity exaggerated the activity when viewing representational pictures, although this effect might be limited when viewing the picture that would give us low activity. For the reality, the main effect of picture, F(5, 95) = 19.102, p < .005, and the main effect of disparity size, F(3, 57) = 7.297, p < .001, were significant, although the interaction of the two factors was not significant. The HSD test for the main effect of picture revealed that the impression for “Guitarist” was weaker than that of any of the other five conditions, and that for

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“Moonlit Night” was weaker than that for “Factory” and “Rainy Day.” These results indicate that the manipulation of the reality in terms of the representational picture was successful. The HSD test for the main effect of disparity size revealed that the impressions at 68.4 and 34.4 arc min were stronger than that at 0 arc min, and that at 68.8 arc min was stronger than that at 17.2 arc min. This result suggests that the effects of disparity size on the reality are independent of that of the representationality of pictures.

Experiment 3 In Experiment 2, the stereograms specified more than three depth layers in which the picture elements were located on several depth planes in terms of binocular disparity, although, in the stereograms in Experiment 1, only one line, square, or slanted surface protruded or hollowed from the frame plane. Therefore, the possibility exists that the depth-layer structure, rather than the representationality of the picture, generated the difference of the disparity effect on the evaluation for large disparities. Experiment 3 examined how the number of depth-layers and the representationality of a picture affect the impression when viewing a stereogram. In Experiment 1, we obtained introspective reports from observers that the difficulty of binocular fusion when viewing the large disparity condition degraded the evaluation. Therefore, in Experiment 3, we examine how the ease of binocular fusion is related to the impressions in different dimensions. Methods Apparatus and stimuli. We used the same apparatus as that used in Experiment 2 to present stereograms. We prepared depth-layer structure conditions of two types. First, in the six-depth-layer condition, the elements of the picture were located on six different depth layers. The nearest plane was the zero-disparity plane. The disparity between each plane was equal. Second, in the two-depth-layer condi© Japanese Psychological Association 2012.

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tion, the elements of the pictures were located on the zero-disparity plane or on the deepest plane. We used four disparity size conditions (0, 17.2, 34.4, and 68.8 arc min) between the nearest and deepest planes. We prepared two types of representational and two types of nonrepresentational pictures. We selected Figure 3b (Factory) and Figure 3f (Guitarist) as the representational pictures because they induced strong negative and positive impressions in each of the three impression dimensions (Figure 4). We used random dot stereograms with 24 black dots (1.3 ¥ 1.3 arc deg) on a white background (13.0 ¥ 17.3 arc deg) as the nonrepresentational pictures. There were two conditions for the random dot stereogram. In the random distribution condition, the dots were distributed randomly on each of two or six depth planes. In the staircase distribution condition, the dots were distributed from the nearest plane to the deepest plane in accordance with the vertical position of the dots so that the height depth cue was available from the vertical position of the dots. Procedure. Each of the two types of representational picture and the two types of random dot stereogram (4) with a different disparity size condition and depth-layer structure condition was presented once in random order. Observers reported the apparent depth magnitude and impressions by the use of the same procedures as those used in Experiment 1. Additionally, in each trial, they rated the stability of single vision as the ease of binocular fusion by the use of the seven-point-scale choices method. Observers. In Experiment 3, 20 naïve observers (aged 20–24 years) participated. Each had normal or corrected-to-normal visual acuity and normal binocular stereopsis. Of these observers, 13 had taken part in Experiment 2.

Results and discussion Factor analysis (principal factor solution, varimax method) extracted three factors (the © Japanese Psychological Association 2012.

activity, evaluation, and reality) for which Eigenvalues were larger than 1.0 (Table 1). Figure 5 shows the average and 95% confidence limit of the rating for the scales for which the factor loads for the three factors were larger than 0.4, and the average of the apparent depth magnitude for each condition. For these data, we conducted three-way repeated measures ANOVA with the picture type (4), number of depth-layers (2), and disparity size (3) (we used only nonzero disparities to examine the effects of depth-layer structure). For the apparent depth magnitude, although the main effect of the pictures was not significant, the main effects of the depth-layer condition, F(1, 19) = 10.349, p < .005, and disparity size, F(2, 38) = 39.348, p < .001, were significant. In addition, the interaction of the picture and disparity size, F(6, 114) = 4.515, p < .001, and the depth-layer condition and disparity size, F(2, 38) = 12.542, p < .001, were significant. The main effect of the depth-layer condition indicates that binocular disparity with the sixdepth-layer condition might introduce larger depth magnitude than that with the two-depthlayer condition did, for both representational and nonrepresentational pictures.The HSD test for the main effect of disparity size showed that the apparent depth for each disparity size differed from the others, p < .05. For the simple main effect of disparity size in the interaction of the picture and disparity size, the apparent depth magnitude at each disparity size differed from any other disparity size conditions except for the 34.4 arc min condition shown in Figure 3b. These results of statistical analyses and Figure 5 show that the apparent depth magnitude increased with the increment of disparity size in any picture and depth-layer condition. For the evaluation, the main effects of the picture, F(3, 57) = 18.131, p < .001, depth-layer condition, F(1, 19) = 14.836, p < .001, and disparity size, F(2, 38) = 4.751, p < .05, were significant. The interaction of the depth-layer condition and disparity size, F(3, 57) = 11.617, p < .001, was significant, although the interaction of picture and depth-layer condition, or the interaction of the picture and disparity size was


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Figure 5 Averages of apparent depth magnitude and ratings for the activity, evaluation, and reality impression dimensions in Experiment 3. In each panel, open circles and triangles respectively show the results of the two-depth-layer and the six-depth-layer conditions. In the rightmost panels, a gray solid line and a dotted line represent the apparent depth magnitudes for the two-depth-layer and the six-depth-layer conditions, respectively. © Japanese Psychological Association 2012.

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not significant. The lack of significant interaction related to the picture suggests that the effects of depth-layer condition and disparity size do not vary with the representationality of pictures. For the simple main effect of disparity size in the interaction of the depth-layer condition and disparity size, the impression for 68.8 arc min was significantly stronger than that for 17.2 arc min only in the six-depth-layer condition. The averages and 95% confidence limits (Figure 5) show that, when viewing any picture, the impression for nonzero disparity size might be larger than that for 0 arc min only in the six-depth-layer condition.These results indicate that the binocular disparity exaggerates the evaluation for the six-depth-layer condition, but the effect of disparity on the impression is weak for the two depth-layer condition. For the activity, the main effects of the picture, F(3, 57) = 5.836, p < .001, depth-layer condition, F(1, 19) = 63.075, p < .001, and disparity size, F(2, 38) = 6.424, p < .005. were significant. The significant main effect of depthlayer condition indicates that the six-depthlayer condition exaggerates the impression for both representational and nonrepresentational pictures. The interaction of picture and depthlayer condition, F(3, 57) = 12.848, p < .001, and the interaction of picture and disparity size, F(6, 114) = 3.520, p < .005, were significant, although the interaction of depth-layer condition and disparity size was not significant. The HSD test for the interaction of picture and depth-layer condition revealed a significant simple main effect of the depth-layer condition when viewing “Guitarist,” the random dot stereogram with a staircase, and with random distribution, p < .05. The analysis of the simple main effect of disparity size in the interaction of picture and display size revealed a significant difference between the impression for 17.2 arc min and 68.8 arc min only when viewing “Guitarist.” This result suggests that the large disparity exaggerated the activity of the picture, which introduces an active impression by itself. In addition, the averages and 95% confidence limits (Figure 5) show that, when viewing any picture, the impression for nonzero disparity size might be larger than that for 0 arc min only © Japanese Psychological Association 2012.

in the six-depth-layer condition. These results indicate that binocular disparity exaggerates the activity for the six-depth-layer condition, although the effect of disparity is weak for the two-depth-layer condition. For the reality, significant main effects of picture, F(3, 57) = 16.390, p < .001, and depthlayer condition, F(1, 19) = 27.011, p < .001, were found. The results also showed a tendency of the main effect of disparity size, F(2, 38) = 2.823, p < .10. The interactions of the picture and depth-layer condition, F(3, 57) = 10.385, p < .001, picture and disparity size, F(6, 114) = 3.470, p < .005, depth-layer condition and disparity size, F(2, 38) = 12.940, p < .001, and the three factors, F(6, 114) = 2.306, p < .05, were significant. The simple main effect of disparity size was not significant for either the interactions of picture and disparity size, or of the depth-layer condition and disparity size. For the interaction of the depth-layer condition and disparity size, the simple main effect of depth-layer condition was significant at 34.4 and 68.8 arc min. The averages and 95% confidence limits (Figure 5) show that the impression for nonzero disparity size was larger than that for 0 arc min in the six-depth-layer condition for any picture, and in the two-depth-layer condition for “Factory.” These results indicate that the binocular disparity exaggerated the reality for both the six-depth-layer and twodepth-layer conditions, although that exaggerative effect is more salient for the six-depth-layer condition. We conducted regression analyses to assess the correlation between the ease of binocular fusion and the average rating in the three impression dimensions (N = 560). The analyses revealed a significant negative correlation between the ease of binocular fusion and the evaluation, r = -0.312, p < .001, activity, r = -0.366, p < .001, and reality, r = -0.151, p < .001, impressions.

General discussion In three experiments, the apparent depth magnitude increased with the increment of dispar-


ity size.Although the activity increased with the increment of disparity size, the evaluation did not always increase with the increment of disparity size. If the stereogram presented only two-depth layers, then the evaluation was most exaggerated at the middle range of disparity rather than at the extreme range of disparity, regardless of the representationality of stereogram (Experiment 3). These results indicate that binocular disparity affects the apparent depth magnitude and impression in each dimension in different ways. The evaluation decreased at large disparity when viewing the two-depth-layer structure, even though the apparent depth magnitude increased with the increment of disparity in Experiments 1 and 3. Also, the result of Experiment 3 shows the discrepancy between the apparent depth magnitude and evaluation for the two-depth-layer structure. These results are incompatible with the assumption that aesthetic impression is exaggerated when viewing the illusory figure, which induces strong illusory effects (Noguchi, 2003; Noguchi & Rentschler, 1999). The discrepancy between our results and those of Noguchi and Rentschler (1999) shows that the relation between the extent of the illusion (apparent depth magnitude, in this case) and the aesthetic effect depends on the type of illusion. For illusory depth perception from binocular disparity, which specifies a two-depthlayer structure, our results suggest that the most beautiful condition would correspond to the middle range of the stimulus variable (Berlyne, 1970; Kaplan, Kaplan, & Wendt, 1972). As previous studies have shown (Lambooij, IJsselsteijn, & Heynderickx, 2011; Speranza, Tam, Renaud, & Hur, 2006), a large disparity might induce visual discomfort in terms of stressing accommodation-convergence linkage and unnatural blur. However, beyond the middle range, disparity can improve the evaluation. That is, the results of Experiment 3 showed that, regardless of the representationality of the stereograms, the sixdepth-layer condition can exaggerate the evaluation, even at a large disparity, although the strength of the evaluation was constant for the two-depth-layer condition. This result indicates

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that, when viewing a multiple-depth-layers structure, a large disparity can exaggerate the evaluation, as well as the apparent depth. The basis of the effect of multiple depth layers will be discussed later. The activity impression invariably increased with the increment of the disparity size as well as the apparent depth magnitude. This result implies a strong relation between the apparent depth and activity. Human psychophysics has revealed that disparity processing and motion processing interact with each other (Bradshaw & Rogers, 1996; Cornilleau-Pérès & Droulez, 1993; Ichikawa, Saida, Osa, & Munechika, 2003). These results are compatible with findings in human brain studies, demonstrating that binocular disparity processing (Bridge & Parker, 2007) and motion processing (Smith & Wall, 2008) are conducted in the same visual cortex area (V3, V4, and MT+). Therefore, we are proposing that disparity processing might activate the motion processing and therefore exaggerate the active impression. The activity impression was exaggerated in terms of the interaction between the disparity and the representationality of the pictures used in Experiments 2 and 3. That is, for the activity, we found that the exaggeration of activity in terms of disparity was salient when viewing the picture which introduced an active impression in Experiments 2 and 3 (“Guitarist”), although there was no exaggerating effect of disparity size for the picture which representationally induced a weak activity in Experiment 2 (“Moonlit Night”). Such an exaggerating interaction was not found for the other impressions. These results suggest that the effect of disparity size on activity varies with the impression that each picture representationally induces by itself. The potency impression (Experiment 1) was not consistently affected by disparity size, although stereogram type has an effect on it. This result indicates that the potency is determined by the appearance of the stereogram rather than the apparent depth. The reality impression (Experiments 2 and 3) increased with the increment of disparity size. These results suggest that increasing disparity © Japanese Psychological Association 2012.

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size might exaggerate the reality impression regardless of what the picture depicts. The six-depth-layer structure exaggerates not only apparent depth magnitude but also evaluation and activity. How does the number of depth layers affect those impressions? In Experiment 3, we found that the ease of binocular fusion is correlated with the evaluation, activity, and reality impressions, which suggests that the ease of binocular fusion is an important factor in manipulating impressions in different dimensions by the use of stereograms. However, our results suggest that the ease of binocular fusion is not only a determinant of the impressions: the observers’ rating of the ease of binocular fusion when viewing the stimuli of the six-depth-layer condition was lower (-0.41 on average) than that when viewing the two-depth-layer condition (-0.29 on average). These results indicate that, even if observers had difficulty in binocular fusion, and therefore with visual discomfort, the multipledepth layer itself can exaggerate the evaluation. These effects exist regardless of the representationality of the stereogram. Moreover, the multiple-depth-layer structure can exaggerate not only the evaluation, but also the activity, reality, and apparent depth at a large disparity. In particular, although the effects of disparity size on the reality were not significant in Experiment 2, the multiple-depth-layer structure exaggerated the reality at a large disparity regardless of the representationality of the stimulus in Experiment 3. This result indicates that the depth-layer structure, rather than the disparity size, is more effective in exaggerating the reality. These results suggest that the number of depth layers is important for manipulating these impressions by the use of binocular disparity for both representational and nonrepresentational pictures. A previous study has reported that a small interlayer disparity (less than 7 arc min) impaired the segregation of depth layers for the six-depth-layer structure and the localization of elements in 3-D space (Tsirlin et al., 2008). The results of the present study suggest that the large interlayer disparity would not only facilitate the accurate perception of the depth structure, but © Japanese Psychological Association 2012.

also exaggerate impressions in different dimensions when viewing a stereogram which specifies a multiple-depth-layer structure. Finally, based on the results of the present experiments, we are proposing methods to manipulate observers’ impression by the use of binocular disparity cues for visual images. First, using a multiple-depth-layer structure can exaggerate the apparent depth, as well as the evaluation, activity, and reality impressions, regardless of the representationality of the visual image. This exaggerative effect is expected to increase up to the “upper limit” of binocular disparity. However, if the image gives low activity impression by itself, the exaggeration of activity in terms of binocular disparity can be restricted to be minor. Furthermore, to give a strong evaluation for a picture with a simple depth structure, such as a two-depthlayer structure, we must use the middle range of disparity size, and should avoid using an extremely large disparity, which would elicit a low evaluation.

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