how we look at photographs 1 (jsps&tj 2005)

518

J. Soc. Photogr. Sci. Technol. Japn. (2005) Vol.68 No.6: 518-531

一般論文

"How We Look at Photographs"

As Indicated by the Ability to Discriminate Contrast in Grey-Scale vs. Photographs

コントラスト感度に基づく写真の見方の研究

Sharon GERSHONI*

and Hiroyuki KOBAYASHI*

Sharon GERSHONI*・小林裕幸

*

Abstract In order to examine the roles and interferences of local and global elements in lightness perception and object recognition

processes of photographs with meaningful contents, we examined whether contrast discrimination is a response to spatialconfiguration properties of photographs, or also a function of conceptual contents. In three experiments we compared contrastdiscrimination performances of observers, when presented with contrast increments applied to discrete tonal regions in grey-scales and in several categories of black-and-white photographs of natural scenes. In Experiment 1, observers performedcontrast discrimination in grey-scales by rank-order tasks. In Experiments 2 and 3, trained and novice observers performedcontrast discrimination of photographs by sortingtasks. We found substantial differences in response to contrast increments,depending on the region, but no significant effect of category. Nevertheless, low performances in shadow region of grey-scales,significantly improved in photographs due to complexity of configuration. We also found differences in performance between

photographs of light and night scene.

要旨本研究は白黒写真の輝度対比がもたらす美学的経験と,輝度対比を検知あるいは感ずる能力,すなわち観察者のコントラ

スト感度との関係を調べたものである.コントラスト感度が,写真の物理特性(テーマのない刺激)にだけ依存するのか,

それとも写真の絵柄やテーマ(テーマのある刺激)にも依存するのかを明らかにするために,テーマのある刺激として

Ansel Adamsの写真を,そしてテーマのない刺激としてグレースケール画像を用い,それぞれのコントラスト感度を心理

学的尺度構成法により比較した.Ansel Adamsはゾーンシステムを用い,メリハリがあり,階調豊かな写真を作った写真

家であるが,Adamsが見,そして感じたものが,ローカルコントラストを調節することで強調されていること,そして

ローカルコントラストはアンカーのようにローカルフレームワークを強調し,高い明度恒常性のある観察条件をもたら

し,美的感覚を増大させることがわかった.

Key words: contrast discrimination, lightness perception, black-and-white photography

キーワ― ド:コントラスト感度,白黒写真とグレイスケールの比較,概念的要素と空間配置要素の比較,Anchoring理論,ローカ

ルフレームワークとグローバルフレームワークの比較

1. Introduction

In recent years there is a growing interest in the correlationbetween the physical structure of pictures and perceptual andcognitive functions. Furthermore, analysis of the process ofmaking pictures offers new approaches to understanding themeasured physical properties as technical means of the aestheticexpression as the products of the physiology of the brain and itsconstraints. A theory of aesthetics as a biological function hasshown the assimilation between art and brain based on simi-larities in their aims, strategies and functions (Zeki, 2000) 1).Another theory proposed laws of artistic experience, based onneurobiological strategies with high survival value, such asthe peak shift effect and gestalt grouping principles, as a brain

response-mechanism model that applies to every work of art

(Ramachandran and Hirstein, 1999) 2).Though until recently, researched as been focused mainly on

paintings, and overlooked photography as a means for investi-

gation of the mechanisms mentioned above, the presentedresearch suggests photography as an appropriate case, for the

following reasons:

(1) Photography has roots in both the tradition of art and repro-duction technologies, and its main concern is the relation-

ship between properties of light sensitive material and the

visual phenomena (perceptual and cognitive).

(2) As a medium of expression, photography is composed of theinteraction between two basic motivations: the synthetic

approach that is the creation of a visual statement about the

Received 30th, September 2005, Revised and accepted 7th, October 2005

平成17年9月30日受付平成17年10月7日改訂受付・受理* Department of Information Science, Graduate School of Science and Technology, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

千葉大学工学部〒263-8522千葉市稲毛区弥生町1-33

S. GERSHONI and H. KOBAYASHI " How We Look at Photographs" 519

subject, and an analytical approach- the direct representa-tion of the subject 3).

(3) The argument of transferability thesis 4) i.e. conditions for"pictorial sameness" , under which a reproduction isaesthetically valuable as the original, and the opposingclaim, that pictorial sameness of a reproduction cannotresult in sameness of aesthetic value 5), are solved for

photography, since photography itself is a reproductionsystem.

Black-and-white photographs in particular are an interestingcase, since they represent subject properties as relationshipsbetween correlating luminance values, with only a one-dimensional luminance grey-scale or luminance contrast forinput to the lightness perception processes, and the interactinghigher processes: object recognition, grouping and segmenta-tion, etc. discussion follows.

Earlier approaches to perception of lightness assumed low-level processing: the adaptation level theory (Helson, 1962,1964) 6) 7), presumed the mean luminance of the visual field to bethe reference and that the visual system responds to changesabove that average. The main weakness of the theory was thatin a visual field, composed of a test object and surroundinguniform backgrounds, background alone would determine themean luminance hence it fails to represent real world conditions.Simultaneous contrast theory (Wallach, 1948) 8) suggested acomparison of the amount of light reflected by the object withthat of adjacent objects. The strength of the theory was that itsolved the dependency of object luminance on surrounding areasize (Leibowitz, 1965) 9), and its weakness that different spatialconfigurations result in different perceptual organizations, whichaffect the simultaneous contrasts. Ratio rule theory, based onthe physiological lateral antagonism mechanism, derived thelightness of an object from the ratio of its luminance to that ofits surround. It failed in cases of large differences betweenluminance of object and its surround (Jameson and Hurvich,1964)10). Although Retinex model (Land and McCann, 1971) 11),by integrating edge information over space managed to recon-struct reflectance of complex stimuli such as 'Mondreans', it didnot suffice for real world objects, affects by configuration andmeaningful interpretations (Knill and Kersten, 1991) 12). Afterthe effect of interpretation given to pattern over perception oflightness was shown (Gilchrist, 1977) 13), it became evident thatlightness perception involves three level processes, i.e. low,high and a mid-level, where surface, contour and grouping are

processed. This approach was based on the gestalt perceptualorganization and mid-level mechanisms. More recently, Intrinsic-image theory suggested that the luminance image is constructedby its reflectance and illuminance (Barrow and Tenenbaum,1978; Mend, 1994; Adelson and Petland, 1996) 14-16). Mostcompatible with gestalt emphasis on the role of configurationand perceptual organization (higher-level processes) is theAnchoring Theory (Gilchrist and Cataliotti, 1994; Gilchrist, A.,and Bonato, F, 1995) 17) 18). The anchoring model was initiatedwith the highestluminance rule, according to which, in mappingluminance into a lightness scale, the highest luminance isanchored (assigned) to white, and the rest of the values arescaled relative to it (also Brown, 1994; McCann, 1994; Schirilloand Shevell, 1996) 19-21). Other factors found to influenceanchoring are the area rule, Tjunction and configural attributesof the image, such as articulation, insulation and gestalt grouping

principles; the result is a compromise between the rules thatdetermines the anchor. The anchor can occur within a local

framework, containing a group of patches or a global frameworkthat could include even the entire visual field. Earlier experi-ments in object recognition showed that grouping is not priorto lightness constancy, but a later process, composed of bothlow and high processing (Beck, 1975, Olson and Atteneave,1970) 22) 23), and even influenced by luminance constancy (Rock,Nijhawan, Palmer and Tudor, 1992) 24). In addition, it was alsoshown that grouping is based on similarity of complete shapes

(Palmer, Neff and Beck, 1996) 25). Gestalt approach insists per-ceptual grouping, such as region segmentation, to be a globaland direct process independent of prior processes such asedge-detection (Shi and Malik, 1997) 26), and furthermore, that

previous experience, viewing of the object's stable properties,causes a future perceptual organization and object recognitionto involve attention-based selection (Wertheimer, 1924) 27), suchas figure/ground (Peterson and Gibson, 1991, 1993; Peterson,1994) 28-30). The crucial effect of figural familiarity on the relativerole of low and high level processes of grouping is further sup-

ported by phenomena like visual completion and amodal com-pletion, and the hypothesis of nonaccidentalness, predicting thatthe visual system prefers to process information arising fromaccidental viewpoint, if they violate the structural regularitiesof the general viewpoint (Rock, 1983; Lowe, 1985; Albert andHoffman, 1995) 31-33). In addition, it was found that 40% of V2cells fired when presented with illusory contours (Peterhansand von der Heydt, 1989, 1991) 34-35). Several contradictingfindings are that there is a process occurring on both sides ofthe contour prior to figure/ground recognition (Peterson andGibson, 1991, 1993, 1994; Vecera and O'Reilly, 1998) 28-30) 36)and that internal details of object features are necessary formatching with the object memory (Mitsumatsu and Yokosawa,2002) 37). Other approaches to part/whole hierarchy are: aboundary based region segmentation based on detection ofluminance at edges (Marr and Hildreth, 1980) 38), and the uni-

form connectedness, as a principle for the partition of the visualfield into units of uniform spatial characteristics (Palmer andRock, 1994a, 1994b) 39) 40) or connected regions that was exam-ined successfully with textured pictures Thompson, Chronicleand Collins, 2003) 41).

In summery, either information processing proceeds fromlocal to global level, with a feedback from global level to facilitate

processing of local elements, or global wholes allow consciousaccess to early processing results. In any case, both local and

global processes govern lightness perception and object rec-ognition alike, in what seems to be mutual interference.

The purpose of the present study of lightness perception in

photographs lies in the fact that photography is a language oftones and contrast relationships functioning as letters andwords to communicate impressions and delivers aestheticexperiences, and which all its vocabulary is composed solely by

quantitatively measurable luminance values, hue, chroma andcontrast. Moreover, there is already extensive understanding inlightness processing of simple images, or complex images suchas scales, or Mondrian patterns 42) 43), that can be examined forviewing photographs as aesthetic images with meaningful con-tents.

To gain understanding of the roles and interferences of localand global elements in lightness perception and object recogni-

520 J. Soc. Photogr. Sci. technol. Japan Vol.68 No.66 (2005)

tion processes of meaningful images, we asked whether contrastdiscrimination for meaningless images such as grey-scalesdiffers from photographs, and if so in what ways? and whethercontrast discrimination in photographs is only a response tospatial configuration properties, or also a function of conceptualcontents, such as category? In three experiments we comparedcontrast discrimination performance of observers, when pre-sented with contrast changes, applied to discrete tonal regions in

grey-scales and in several categories of black-and-white photo-graphs of natural scenes.

1.1 A Note On The Stimuli

The stimuli were photographs of Ansel Adams 44) (USA 1902-1984), since they stand out as one of the peaks in the historyof photography, manifesting both synthetical and analyticalapproaches, and are a pioneering conscious attempt to combinethem methodologically into one whole expressive framework.Adams belonged to group "f/64" following Edward Weston'sObjectivism, which opposed subjectivity in pictorial aestheticsdeclaring photography's ultimate goal to be a scientific expres-sion of esthetics, "the absolute impersonal recognition of thesignificance of facts" 45). "f/64" manifesto from 1932 demandedstraight photography to draw solely upon its inherent technical

qualities (i.e. "the microscopic revelation of the lens") and bedefined as an art-form by only a simple and direct presentationthrough purely photographic methods 10). In order to achievesuch representation of subject matters with a wide variety ofluminance ranges, Adams developed the fundamental conceptsof photographic visualization and the Zone System. Zone Systemis based on nineteenth century sensitometry of Harter andDriffield (D-log-E Curve in 1890), and a grey-scale of ten lightintensities, corresponding to a tone-scale from white to black inthe print 46). The system offered maximum range of tones andcontributed to the tonal and contrast control, and enabled thetreatment of discrete subject areas of the image for tonalemphasis. Photographic "Visualization" is a process for envi-sioning the final print 'tonal management', step-by-step fromcamera adjustments, through film development and printing.Adams' photographic visualization and Zone System are equiva-lents to the scaling and anchoring in intrinsic image 47) light-ness perception models (see above). Furthermore, Adam's

printing method for the expression of emotional experiences isnot the mere recreation of the subject, but its enhancement bymeans of local contrast and extraction of features, to grab atten-tion to local regions of change, where processing is "more inter-esting or pleasing" 2).

2. Stimuli Preparation

2.1 Grey-Scales Rank Order Task (Experiment 1)Kodak (Scientific Imaging Systems) Gray Scale, consisting of

20 steps with 0.10 density increments from nominal "white" of0.05 to 1.95 and an 18% neutral grey background. Sample sizewas 20 cm x3 cm, with a 3 cm wide neutral grey masking.

2.2 Photograph- Sorting Task (Experiments 2 and 3)Nine black-and-white photographs by the photographer Ansel

Adams 44) were used as stimuli for the present study:1. "Spanish Peaks, Colorado", 1951, page 182. "Canyon de Chelly, National Monument, Arizona", 1947, page

303. "Moonrise, Hernandez, New-Mexico", 1944, page 55

4. "Navajo Woman, Wide Ruin, Arizona", 1948, page 395. "Maynard Dixon, painter, Tucson, Arizona", 1944, page 166. "Martha Porter, pioneer woman, Ordeville, Utah", 1961, page

817. "Adobe Dwellings, Northern New-Mexico", 1958, page 498. "Santuario de Chimayol, New-Mexico", 1950, page 509. "Arches, North Court, Mission San Xavier del Bac, Tucson,

Arizona", 1968, page 94The photographs belong to three major themes in photography:Landscape (photographs no. 1, 2 and 3), Portrait (no. 4, 5 and 6)and Architecture (no. 7, 8 and 9) and are among Adams' mostknown and reprinted works (Fig. 1, and for histograms pleaserefer to appendix). Sample size was 25 cm x 30 cm (the samesize as the prints in the book), with a 3cm wide neutral, 18%

reflectance grey masking. The prints were laminated, with aneutral, transparent lamination, to avoid strong reflections fromthe print surface and to protect the samples from wearing out-conditions, which are liable to influence participants' judgment.

2.3 Stimuli Reproduction ProcessThe photographs were scanned in an "Epson" scanner GT-

9700F. For each photograph, a sample set composed of two

prints with original tones and three sets of 10 prints, for each ofthe three curves was composed, as shown in Fig. 2:

"OR" is the central straight line corresponding to the repro-

duction of the original,

(1) A curve for "SH" region (toe), where contrast wasincreased in the shadows and compressed the highlights.As a result the visual impression is that the images looklighter.

(2) A curve for "HI" region (shoulder), where contrast wasincreased in highlights and compressed the shadows. Theresulting visual impression is the overall darkening of theimages.

(3) A curve for "MT" region (straight line), where contrast wasincreased in the mid-tones, while compressed both high-lights and shadows.

For all three regions ("HI", "SH" and "MT") contrastincrement ranged between 1% and 10% in increments of 1%

(see Fig. 2), in Photoshop curve function, named 1 to 10accordingly.

The curves were applied to the scanned grey scale and photo-graphs using Photoshop software, and prints were produced byLambda system- in a silver gelatin process, on a photographicblack-and-white paper. Lambda prints are made on a DurstLambda printer, which uses three colored lasers to expose tra-ditional photographic media. These prints have the advantage ofusing the same rich RGB color space employed by computermonitors. In addition, these prints are free of dots since unlikeinkjet printers, the laser outputs are continuously modulatedrather than switched on and off. Their resolution is comparableto 1200 dpi screened output.

For calibration of the reproduction system, the densities ofthe original grey-scale and its reproduction were measured in adensitometer and plotted against each other to yield a linearrelationship.

3. Subjects

Subjects belonged to two groups1. 18 observers who were skilled in image evaluation tasks,


photo #1 photo #2 photo #3

Photo #4 photo #5 photo #6

Photo #7 photo #8 photo #9

Fig. 1 Photographs used as stimuli for the sorting task (Experiment 1)Nine photographs by the photographer Ansel Adams, "Photographs of the Southwest", New-York Graphic Society, Boston Massachusetts thatwere used as stimuli:1. "Spanish Peaks, Colorado", 1951, page 182. "Canyon de Chelly, Nationa Monument, Arizona", 1947, page 303. "Moonrise, Hernandez, New-Mexico", 1944, page 554. "Navajo Woman, Wide Ruin, Arizona", 1948, page 395. "Maynard Dixon, painter, Tucson, Arizona", 1944, page 166. "Martha Porter, pioneer woman, Ordeville, Utah", 1961, page 817. "Adobe Dwellings, Northern New-Mexico", 1958, page 498. "Santuario de Chimayo, New-Mexico", 1950, page 509. "Arches, North Court, Mission San Xavier del Bac, Tucson Arizona, 1968, page 94

named: "trained" group.2. 15 inexperienced observers, named: "novice" group.

Average age of subjects for "trained" and "novice" groups was25 and 27.83% of the trained subjects and 37% of the novicesubjects were practicing photography. 50% of the trained sub-

jects and 10% of the novice were familiar with Ansel Adamswork and 28% of the trained and 5% of novice reported to have

previously seen the photographs used as stimuli.

4. Experimental Conditions

Lighting: D-50 (600 lux)Grey-scale order-rank experiment was 30 minutes per ses-

sion: sorting time for each of the three greyscale sets was5 minutes, and 2 intermissions of 5 minutes between sets. Pho-tographs sorting experiment was 30 minutes per session, with-out intermission- time for sorting each set was 10 minutes.

The task was in three sequencing sessions. Subjects weretrained to discriminate a subset of two samples: with originaltone and maximum contrast increment applied.

522 J. Soc. Photogr. Sci. Technol. Japan Vol.68 No.6 (2005)

Fig. 2a Contrast increment curves applied to SH and HI regions

Fig. 2b Contrast increment curves applied to MT region

5. Procedure

Experiment 1: Grey-Scales Rank Order Task 49)

14 trained subjects were individually presented with grey

scale sets for regions: "HI", "SH", "MT". Participants were

instructed to arrange the grey-scales in each set from the grey-

scale with the lowest contrast at the left end of the row to the

one with the highest contrast in right end (Prototype for the

"Rank Order Scaling" is modeled after Bartleson and F Grum

Eds, 1984) 48).

Experiments 2 and 3: Photographs Sorting Task 49)

For each photograph, a set composed of the sets for regions

"MT", "HI" and "SH" plus "OR" (altogether 3lsamples) were

randomly mixed and arranged in a pile, placed in the center of

the table. Another "OR" sample was set as a reference, to which

each of the samples in the pile was compared. On the right of the

pile was an area marked "harder" (which in Japanese means

"higher contrast") •\for all samples, judged to have higher

contrast than the reference. On the left of the pile was the

area marked "softer"("lower contrast" in Japanese") •\for all

samples, judged to have less contrast than reference or equal to

it. To reduce observer criterion drift (observer internal defini-

tion) and create a typical observer condition, definition and

usage of the terms "softer" and "harder" in relation to the term

"contrast" were clarified before each experiment 49).

6. Results and Discussion

Experiment 1: Grey-Scales Rank Order Task

Subjects generally showed more correct rank order rates for

contrast increments applied to HI and MT region sets of grey-

scales than to SH, and mean correct response rates were 8.57,

1.21 and 8.5, respectively, as shown in Fig. 4. For all three

sets, correct ranking increased systematically as a function of

contrast increments (especially samples 7 to 10). Because indi-

vidual subjects exhibited different rates of correct rank order,


Highlight-GraySCales Set:

Shadows-Grayscales Sets:

Midtone-Grayscales Scts:

Fig. 3 Grey-scale used as stimuli of the ordinal task (Experiments 2and3)

relative responses were averaged across the subjects. This

yielded a comparison between the three region sets, revealingthat correct ranking rates for HI and MT were substantiallyhigher than SH, as shown in Fig. 5. In all results and figuresdescribed 1%-10% contrast increments (see stimuli prepara-tion) are indicated by sample numbers 1 to 10.

The correct ranking ratio for HI ranged between 0.64 and0.71 for samples 5 and 6, and 1.00 for samples 8 to 10. Similarly,for MT set, ratio ranged between 0.64 for sample 2, 0.86 forsamples 8 and 9, and 0.93 for 10. However, the ratio for SH setwas substantially lower, ranging between 0.07 for samples1 to3 contrast increments, and 0.21 for samples 4, 6 and 8 to 10.In order to confirm these observations, a one-way analysis of

variance (ANOVA), with region (HI vs. SH vs. MT) as a variable,

was conducted. In this and all other statistical tests, alpha level

of 0.05 was used. There was a significant main effect of region

[F(2,26)=58.56].Inadditionapairedcomparisonofcontrastsof

HI and MT with SH, revealed the differential control of the

region, to which contrast increments were applied, over dis-

crimination of contrast increments [F(2, 26)=87.85].

Experiment 2: Photographs Sorting Task with TrainedSubjects

(a) Discrimination ratios for categories "portraits", "land-scape" and "Architecture", separately calculated, are shown asafunctionoftheregions"HI","SH","MT"(with1%-10%

contrast increments, indicated by sample numbers 1 to 10) inthe three graphs of Fig. 6.

We expected to find a differential discrimination ratio as aneffect of category, such as, perhaps, a higher response rate for

portraits than for the other categories (Papathomas and Bono,2004; also: Dolan et al., 1997 ) 50) 51), based also on the findingsthat meaningfulness and knowledge of specific object shapeinterfere with attention and visual organization processes (seeintroduction: Peterson and Gibson, 1991, 1993, 1994) 25-30);also that features such as junctions are not solely processed bylow level mechanism but necessitate the use of global processesand are not detected prior scene interpretation (McDermott,2004) 52). However, we observed a general similarity in contrastdiscrimination performance for all categories examined. Further-more, variations in discrimination ratios were more substantialamong region sets within the categories than between catego-ries. This suggests a stronger control of configural propertiesof the stimuli over contrast discrimination performance, thanmeaningfulness of the category.

The category effect interaction is shown as a function of theregions in Fig. 7. To explore the effects of category and region,a two-way analysis of variance (ANOVA), with categories

(portrait vs. landscape vs. architecture) and regions of contrastincrement (HI vs. SH vs. MT) as variables, was conducted.The effect of category was not significant [F(2, 34)=1.93],reflecting that there is no differential control of such conceptualcontent over discrimination of contrast increments in the exam-ined regions. In addition, there were significant main effects ofregion [F(2, 34)=96.22]. Paired comparisons of contrastsrevealed that while discrimination performance did not differmuch between SH and HI regions [F(1, 34)=2.45], responseratio significantly increased at MT region [F(1, 34)=189.99].

(b) An unexpected finding was that discrimination ratio waslower at SH region of "landscape" category compared with theother two categories, shown in Fig. 7. In order to investigate this

524 J. Soc. Photogr. Sci. Technol. Japan Vol. 68 No. 6(2005)

Fig. 4 Mean relative correct responses for rank-order task with region-sets (HI, MT, SH) of grey-scales (Experiment 1)

Fig. 5 Comparison between means correct rank-order for region-sets (HI, SH, MT) of grey-scales (Experiment 1)

difference we compared the discrimination ratios of landscapestimuli #1 and #3; both stimuli are similar in composition,depicting a wide view of a plane in the desert, with distant lowmountains, and proportion between the area of 'sky' and 'land'is approximately 2:1 respectively. The main difference is thatstimuli #1 depicts a light view, while stimuli #3 depicts a nightview (see section 2.2). The "portrait" graph in Fig. 6 showssimilar discrimination ratios at HI and MT regions of #1 and #3.On the other hand, discrimination ratio at SH decreased to zero.It is especially notable, since it did not occur at any other stimuli.According to a two-way analysis of variance (ANOVA), withlandscape stimuli (light #1 vs. night #3) and regions (HI vs. SHvs. MT) as variables, there was a significant main effect ofregion [F(2,34)=104.65]. In addition, the interaction was sig-nificant [F(2,34)=5.17], as shown in Fig. 8. Paired comparisonsof contrasts revealed that discrimination performance wasbetter at SH region for stimulus #1 (light) than #3 (night)

[F(1,34)=11.35]. One way to explain this finding is that when animage is overall dark (night scenes), the increment in contrastapplied to HI region (visually perceived as a further darkening ofthe image) is conceived as an increase in contrast, while, thevisual effect of increment in contrast applied to SH region

(resulting in the impression of the image as becoming less dark),is perceived as a decrease in contrast.

Experiment 3: Photographs Sorting Task with NoviceSubjects

(a) Discrimination ratios of for categories "portraits", "land-scape" or "Architecture" separately calculated, are shown as afunction of the regions "HI", "ST", "MT" in the three graphs ofFig. 9. Novice subjects' performance of contrast discriminationresembled the trained subjects (see Fig. 6) in respect tovariation in discrimination ratios being more substantial amongsamples within the categories than between them. The categoryeffect interaction plot is shown as a function of the region in


A. "Portrait" Category

B. "Landscape" Category

C. "Architecture" Category

Fig. 6 Mean relative contrast increment discrimination for region sets (HI, SH, MT) of photographs (Experiment 2, trained subjects)

526 J. Soc. Photogr. Sci. Tecknol.Japan Vol. 68 No. 6(2005)

Fig. 7 Category effect interaction plot for region-sets of photographs in categories: portrait, landscape and architecture (Experiment 2, trained subjects).

Fig. 8 Light vs. night scenes effect interaction plot for stimuli #1 and #3(Experiment 2, trained subjects)

Fig. 10. To explore the effects of category and region, a two-wayanalysis of variance (ANOVA), with categories (portrait vs. land-scape vs. architecture) and regions of contrast increment (HI vs.SH vs. MT) as variables was conducted. The effect of categorywas not significant [F(2,28)=0.67], reflecting that there is nodifferential control of conceptual category over discriminationof contrast increments in any of the regions. In addition therewere significant main effects of region [F(2,28)=36.24] andP <0.001. Paired comparisons of contrasts revealed thatresponse to contrast increments in the SH and HI regions differless [F(1,28)=11.93] and P=0.018, than the enhanced responsefor MT region [F(1,28)=60.55].

These results are consistent with the previous observations

(for trained subjects) that there is a stronger control of con-figuration attributes over contrast discrimination performance,than of category, since, unlike trained subjects, who are skilledin performing image evaluation tasks, and might have been

intentionally neglecting the conceptual content of the stimuli andconcentrating on configuration attributes, such as edges, etc,novice subjects have no earlier experience in such tasks,therefore their response suggests that the response rate isindependent of acquired skill.

(b) One of the substantial differences in performance betweenthe trained and novice groups of subjects is in the response ratiofor contrast increments applied to SH region in the "Landscape"

category day vs. night, stimuli #1 and #3 (see above results6.2.b.), where, unlike the trained group, novice group's categoryeffect interaction plot revealed that there was no significantdifference between the two stimuli. In addition, while the trained

group showed a mean average response ratio for SH 9.13 andfor HI 6.93, the novice group response ratio for HI was 13.49and for SH only 6.69.This suggests an effect of skill (training)over the interpretation of contrast increments, when applied toHI or SH regions. To confirm these observations a two-way

S. GERSHONI and H. KoBAYAsHI "How We Look at Photographs" 527

A. "Portrait"Category

B. "Landscape"Category

C. "ArchitectuTe"Category

Fig. 9 Mean relative contrast increment discrimination for region-sets (HI, SH, MT)of photographs (Experiment 3, novice subjects)


Fig. 10 Category effect interaction plot for region-sets of photographs in categories: portrait, landscape and architecture (Experiment 3, novice subjects).

Fig. 11 Group effect interaction plot for region-sets HI and SH of photographs (Experiments 2 and 3, novice vs. trained)

analysis of variance (ANOVA), with groups (trained vs. novice)and regions (HI vs. SH) as variables was conducted. The inter-action was significant [F(1, 2)=1293.24], reflecting the differen-tial control of training over the responding to contrast incre-ments in regions HI and SH. Paired comparison of contrastsrevealed that trained subjects responded significantly moreto SH than to HI [F(1, 2)=154.55], and more enhanced wasthe novice subjects differential response to HI than to SH

[F(1,2)=1476.53]. Furthermore, a pair comparison between thegroups revealed that novice subjects responded to HI signifi-cantly more than did the trained subjects [F(1,2)=1375.54],whereas trained responded to SH significantly more than novicesubjects did [F(1, 2)=189.59], as shown in Fig. 11. A suggestedexplanation is that while novice subjects interpret the overalldarkening of an image (for contrast increments applied to HIregion) as an increase in contrast, trained subjects, in contrary ,interpret the overall lightening of an image as an increase in

contrast. Both groups, however, showed no significant differencein response ratio for "MT" region.

(c) Another difference between the two groups of subjects isthe false alarms ratios- when discriminating OR from OR (seesection 2.3). The mean average for false alarms ratio of thenovice group was 0.18, whereas that of the trained group wasonly 0.07, as shown in Figs. 6 and 9 (the unit at the extreme lefton x-axis refers to OR). This observation was confirmed by aone-way analysis of variance (ANOVA) with group (novice vs.trained) as a variable, which revealed that the rate of falsealarms differed between novice and trained groups [F(1,8)=12.46], reflecting the effect of training over false alarms in

performance of contrast discrimination, and suggesting a betterperformance training.

S. GERSHONI and H. KOBAYASHI "How We Look at Photographs" 529

7. General Discussion

The present results can be summarized as follows. In Experi-

ment 1, subject discriminated contrast increments applied tothree regions (highlights, shadows and mid-tones) of separate

grey-scale sets. They generally showed higher correct responserates to increments in highlights and mid-tones than shadows.In Experiment 2 trained subject discriminated contrast incre-ments applied to natural scene photographs of three categories

(portrait, landscape and architecture). Subjects showed a sys-tematic increase in response as a function of contrast incrementsat all regions of all stimuli examined. However, the discrimi-nation ratio was significantly higher at MT than SH and HIwith no significant difference between the last two. Further-more, in Experiment 3, experimentally naïve subjects performedthe same task as in Experiment 2, and generally showed thesame discrimination pattern as the trained subjects. Two maindifferences between subject groups were found. Novice subjects'false alarms rates were more frequent than the trained subjects.And an even more interesting difference was that novice sub-

jects responded to HI more than the trained subjects, whereas trained subjects responded to SH more than the novice subjects.Nevertheless, the findings of Experiments 2 and 3 converge onthe conclusion that the differences in contrast discrimination atHI vs. SH between subject groups was on the basis of inter-

pretation and intentional judgment (see 6.3.c), rather than skill,obtained by training, as both groups showed priority of spatial

configuration properties over category. One can view this differ-ential interpretation of contrast as yet another evidence for theinterference of higher, cognitive processes, such as interpreta-

tion and intention, in lightness perception, although some otherhigher processes, such as grouping, are not prior to lightnessconstancy, but later processes, composed of both low andhigh processing (Beck, 1975; Olson and Atteneave, 1970) 22) 23).

This view is consistent with the approach that although per-ceptual processing proceeds from local to global wholes, peoplemay gain conscious access to the results in the opposite order

(Marcel, 1983) 53), and affect them. The main conclusion of the present study is discussed by

means of the comparison between contrast discrimination for

grey-scales (Experiment 1) and photographs (Experiment 2and 3). This comparison revealed a difference mainly at SHregion. In grey-scales the high contrast discrimination ratios forMT and HI region sets and the extremely low ratios for SH,suggest a better ability to discriminate contrast increments inthe mid-tones (with compressed highlight and shadow tones), orin highlights (with compressed shadow tones), where perceivedimpression is of the darkening of the image, than in shadows

(with compressed highlights), where the impression is theoverall lightening of the image. However, in photographs, con-trast discrimination ratios at SH region sets, were significantlyimproved, that is, the contrast increment in shadows withcompressed highlights, did not result in the same low ratios asin greyscales, despite the perceived lightening of the image.This is in line with "Anchoring Theory of Lightness Perception"

(Gilchrist and Cataliotti, 1994; Gilchrist, A., and Bonato, F,1995) 17) 18) 47). According to anchoring theory, the grey-scales

and photographs can be described in terms of global and localframeworks, which weighing, as stronger or weaker anchorsdepends on factors, such as grouping, configuration, luminance

gradient, articulation and insulation. Thus, it is possible toexplain the significant difference between greyscales and

photographs, in terms of spatial properties and the resultingdifferences in the strength of their local frameworks asanchors, rather than in terms of meaningfulness of conceptualcontent. Local frameworks in grey-scales are much weaker thanin photographs, due to their line configuration, low articulation

(of only 20 patches), and the low luminance gradient-0.1 densityincrements between adjacent patches. These properties suggestthat in grey-scales the compromise between the global and localframeworks resulted in a weak anchoring to the local frameworkthan the global one and a luminance matching with poor light-ness constancy. In photographs though, the high articulation,complex configuration and the abundance of edges with variousluminance gradients, are factors that strengthen anchoring tolocal frameworks and result in better lightness constancy.Thus, the low ability to discriminate contrast increments,

applied to shadow region in grey-scales, may be explained as theresult of weak local frameworks together with enlarged "white"area, which strengthen the global framework luminance assign-ment. That may also explain the perceived impression of thelightening of the image. However, in photographs, although thesame lightening impression occurred, the complex configurationand insulation of the white patches and the increased local con-trast, strengthened local frameworks and improved lightnessconstancy. Hence, discrimination at shadows improved. Thisview is consistent with recent results, showing that lightnessconstancy increases when the contrast at contextual edge islarger than the mediating or background one (Soranzo andAgostiini, 2004) 54); see also introduction.

The present study is the first systematic investigation oflightness perception in art photographs, which greater scope isthe investigation of the connection between visual processingand the aesthetic experience of art. The main conclusion fromthe above discussion is that contrast discrimination improvesas a function of better anchoring to local frameworks. One ofthe main factors that strengthen local frameworks' anchoring isthe abundance of local contrasts. Ansel Adams, in his photo-

graphs, methodologically applied the zone system to createdhighly articulated and tonally rich images and enhanced what is"saw and felt" by controlled local-contrast . We further assume

that images, which offer better lightness constancy, amplify theaesthetic experience, as evident by the intentional choice madeby the photographer. This is in line with recent approachesabout the neural basis of art and aesthetic experience1)2)55),according to which the basis of aesthetic experience is biologi-

cally motivated and is a manifestation of the brain mechanismsand constraints. Additional research, using tasks for preference

and evaluation of photographs with contrast changes, and find-ing the relation between ability to detect contrast changes and

preference, is needed to examine this assumption.

Acknowledgements

The authors wish to express their gratitude to M., Tsukada,Horiuchi Color, Tokyo for printing and laminating the entirestimuli and supporting the research and M., Jitsumori, Depart-ment of Cognitive and Information Sciences, Chiba University,

Japan, for her teaching, much assistance and advice, R.,Yamamoto for her helpful suggestions.


Appendix

Histograms of stimuli with 10%(maximal)

contrast increment in regions: HI, SH, MT

vs. the unaltered stimuli OR(from left to

right accordingly):

Stimuli#1

Stimuli#2

Stimuli#3

Stimuli#4

StimuIi#5

Stimuli#6

Stimuli#7

Stimuli#8

Stimuli#9

* Please note that the Shadows are at the le負 of the histogram and highlights at the right.

S. GERSHONI and H. KOBAYASHI "How We Look at Photographs" 531

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