JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR

PIGEONS' DISCRIMINATION OF PAINTINGS BYMONET AND PICASSO

SHIGERU WATANABE, JUNKO SAKAMOTO, AND MASUMI WAKITA

KEIO UNIVERSITY

Pigeons successfully learned to discriminate color slides of paintings by Monet and Picasso. Followingthis training, they discriminated novel paintings by Monet and Picasso that had never been presentedduring the discrimination training. Furthermore, they showed generalization from Monet's to Ce-zanne's and Renoir's paintings or from Picasso's to Braque's and Matisse's paintings. These resultssuggest that pigeons' behavior can be controlled by complex visual stimuli in ways that suggestcategorization. Upside-down images of Monet's paintings disrupted the discrimination, whereas in-verted images of Picasso's did not. This result may indicate that the pigeons' behavior was controlledby objects depicted in impressionists' paintings but was not controlled by objects in cubists' paintings.Key words: stimulus control, concept, pattern discrimination, vision, key peck, pigeon

When we see paintings by Picasso and Mo-net, we can with some accuracy recognize whichis Picasso's and which is Monet's, even if wehave never seen the particular paintings be-fore. There are many possible cues for thisdiscrimination, such as color, style of brushing,favorite subjects, and so on, but no single fea-ture differentiates each artist. It is also clearthat we have acquired such visual concepts ofpaintings of Picasso and Monet by experience.Can pigeons discriminate paintings of one art-ist from those of another artist? If they can,do they also show generalization to paintingsof other artists belonging to the same group,such as an impressionist or a cubist? Porterand Neuringer (1984) reported successfullearning of musical discrimination of Bach andStravinsky by pigeons. Can pigeons discrimi-nate visual arts also?

Birds have excellent visual ability compa-rable to that of humans, and there have beenmany experimental studies showing acquisi-tion of visual concepts in birds. Since Herrn-stein and Loveland (1964) successfully trainedpigeons to respond to color slides on which ahuman being appeared and not to respond tothose without a human, there have been manystudies demonstrating learning to discriminatenatural concepts (e.g., Cerella, 1979; Herrn-

The authors wish to express their gratitude to R. J.Herrnstein for encouragement and comments on this re-search. This research was supported by Grant-in-Aid forPromotion of Sciences 05206113. Address correspondenceand reprint requests to Shigeru Watanabe, PsychologyDepartment, Keio University, Mita 2-15-45, Minato-ku,Tokyo, Japan.

stein & de Villiers, 1980; Herrnstein, Love-land, & Cable, 1976; Roberts & Mazmanian,1988; Watanabe, Yamasita, & Wakita, 1993),artificial concepts (Bhatt, Wasserman, Reyn-olds, & Knauss, 1988; Watanabe, 1991), andsymmetry of objects (Delius & Habers, 1978).Most of these natural-concept experiments

used a slide projector as the stimulus-presen-tation device, and pigeons showed transfer ofdiscrimination of photographs to real objectsand of real objects to photographs (Watanabe,1993). Representational paintings have fea-tures similar to photographs, but paintingspatterned after impressionism are not precisereflections of the real world. They often areconsidered to be a reflection of the artist's sub-jective world. We can, however, identify "ob-jects" in the paintings by Monet, Renoir, andCezanne. In other words, we find a relationbetween these paintings and real objects. How-ever, such a relation is often weak in the paint-ings by Picasso, Matisse, and Braque. Realismis relevant only for a perceiver who can see apainting as a representation of a three-dimen-sional world. If realism makes a difference toa pigeon, we can presume that it can see apainting as a representation of a three-dimen-sional world.

EXPERIMENT 1In this experiment pigeons were trained on

a discrimination between photographs or vi-deos of paintings by Monet and those of Pi-casso. The paintings differed in their color,sharpness of contour, and objects. Potentialcues for discrimination were examined by as-

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SHIGERU WATANABE et al.

sessing effects of distortion, such as left-rightreversal and upside-down reversal. Keller andSchoenfeld (1950) defined a concept as a gen-eralization within a class of stimuli and a dis-crimination between the classes. The gener-alization of the discrimination of paintings byMonet and Picasso to novel paintings of theseartists and those of other artists was tested.

METHODSubjects

Eight experimentally naive pigeons (Co-lumba livia) were used for the present exper-iment. They were individually housed in stain-less steel cages and were maintained at about80% of their free-feeding body weights.

ApparatusThe experimental chambers were two iden-

tical operant conditioning chambers (30 cm by30 cm by 30 cm) with a rectangular glass peck-ing key (5 cm by 7 cm). The key could beactivated by a force of 0.2 N. A frosted glassscreen (5 cm by 7 cm) was attached 1 cmbehind the key. The key was mounted 20 cmabove the floor and 13 cm above the apertureof a food hopper. Stimuli were projected onthe screen by a slide projector (Super Cabin2) in one chamber and by a video projector(Phillips LCP5000) in another chamber. Amicrocomputer (Sanyo MSX) controlled theexperiment. The projected stimuli provided theonly illumination in the chamber. The cham-ber was not sound isolated, but white noise(70 dB) was continuously broadcast.

StimuliTwo different sets of stimuli, taken from

picture books, were used as training stimuli(Table 1). Each set consisted of 10 differentpaintings by Monet and 10 by Picasso. Set Awas used for training with the slide projector,and Set B was used for training with the videoprojector. "Typical" paintings were selectedas stimuli. "Atypical" paintings (such as thoseof Picasso's blue period) were not used as stim-uli. Table 2 lists stimuli used for the gener-alization test. Three novel paintings by Monetand Picasso and three each by Cezanne,Braque, and Delacroix were used as testingstimuli after training with Set A. Four novelMonet paintings, four new Picasso paintings,and four each by Renoir, Matisse, and De-

lacroix were presented in the generalizationtest after training with Set B.

ProcedureThe pigeons were first trained to peck the

key illuminated by a projector lamp withoutany painting stimulus. Then they were dividedinto two groups of 4 birds each. In the MonetS+ group, responses to paintings of Monetwere reinforced by 4-s access to a feeder con-taining hemp seeds and responses to paintingsof Picasso were extinguished, whereas in thePicasso S+ group paintings of Picasso wereassociated with reinforcement and those ofMonet were not. Two different sets of stimuliwere used. Two birds in each group receivedtraining with Set A, and the other 2 weretrained with Set B.A training session consisted of 20 randomly

ordered presentations of each painting once,each lasting 30 s, separated by a 5-s blackoutperiod. During presentation of S+, reinforce-ment was available on a variable-interval (VI)30-s schedule, whereas no reinforcement wasavailable during S- periods (mult VI 30-sext). The subjects received one training sessionevery day.

This discrimination training continued untilthe subjects showed a discrimination ratio above90%, calculated by dividing the number of re-sponses to S+ by the total number of responses,summed over two successive sessions. Then thefollowing tests were carried out under extinc-tion (i.e., pecking did not activate the foodhopper during the tests). The subjects receivedat least two sessions of discriminative trainingbetween tests. If the subjects did not show atleast 90% correct on the two sessions, addi-tional training was given until the subjectsreached the criterion again.

Test 1. Color of paintings by Monet mightdiffer from those by Picasso. Differences incolor thus might serve as cues for discrimi-nation. In this test, monochromatic pictures ofall training paintings were used to examinethe possibility that color cues controlled thediscrimination. The order and period of stim-ulus presentation were similar to those in dailytraining sessions. Monochromatic slides wereused for Set A, and monochromatic imageswere produced by tuning the video projectorfor Set B.

Test 2. Most of the paintings by Picasso havesharp contours, but most of those by Monet

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Table 2

Paintings used for tests. S and R indicate catalogue numbers as Table 1. M indicates cataloguenumber in Matisse by J. Guillaud and M. Guillaud (Guillaud Edition, 1987, Paris).

Test stimuli after training with Set A Test stimuli after training with Set B

New MonetLa GrenouillereLady with a parasoleWater lily

New PicassoDanceWoman looking at the glassStill life with an ox head

CezanneSitting manStill life with onionsBig water bathing

BraqueFemale musicianStill life with "le Jour"An easel and a woman

DelacroixStill life with a lobsterJuly 28thAtelier

New Monet1869 S8 Le Grenouillere1886 S23 The Thames ant the capital1914 S31 Church

Palazzo da mula in Venezia

IoQ-, Iq1 New Picasso

1937 S251942 S27

1898 S171895 S231898 S27

1917 S131929 S241936 S27

1827 S51830 S81830 S9

Donna con ventaglioNatura morta spagnolaFoglio di musica e chitarraArlecchino

MatisseNotre-dameLa TovagliaBlouse roumaineApricot

RenoirLa senna ad argenteuilCanottieri ad argenteuilDonna alla grenouilereFrutta del meridione

DelacroixStill life with lobsterAtelierJuly 28thShopin

1869 S81871 S1l1883 S221908 S29

1909 R2241912 R4711912 R5771913 R615

1902 S31908 R1071940 M4131948 S30

1883 R961883 R971879 R3731881 R486

1827 S51830 S91830 S81838 S15

do not. In this test all training paintings were

presented out of focus to examine the role ofcontour. Other procedures of the stimulus pre-sentation were identical to those in daily train-ing sessions. In these pictures, two lines (0.43mm wide) separated by 0.43 mm fused com-

pletely on the screen.

Test 3. Training stimuli were used here butthree S+ and three S- pictures were left-rightreversed and another three S+ and three S-pictures were upside down. Six normal stimuliwere also presented. The same testing proce-dure was used here (i.e., each test stimuluswas presented once for 30 s).

Test 4. As shown in Table 2, two differentsets of stimuli appeared in generalization tests.One set, which was used after training withSet A, consisted of three novel paintings byMonet, three novel paintings by Picasso, andthree each by Cezanne, Braque, and Dela-croix. These 15 new stimuli and six old stimuliused for discriminative training (three each byPicasso and Monet) were randomly presented

three times each. The other set, which con-

sisted of four novel paintings each by Monetand Picasso and four each by Renoir, Matisse,and Delacroix, was used after training withSet B. These 20 new stimuli and eight oldstimuli (four each by Picasso and Monet) ran-

domly appeared three times each during thetests.

RESULTS AND DISCUSSIONAll subjects' behavior came under stimulus

control. The number of sessions to reach thecriterion ranged from 6 to 22 for the MonetS+ group and 8 to 24 for the Picasso S+group. There was no statistically significantdifference in speed of acquisition between theMonet S+ and Picasso S+ groups (two-tailedt test, = 0.86, df = 6) or between Set A andSet B (t = 1.64, df= 6).

During the training period, the subjects re-

sponded more often to some paintings than toother paintings. There might be, thus, paint-ings that are easy to discriminate and those

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DISCRIMINATION OF PAINTINGS

that are hard to discriminate. To examine therole of each painting in acquisition of discrim-ination, total number of responses emitted toeach S+ painting until each subject reachedthe criterion were analyzed. Because the num-ber of sessions to the criterion differed amongsubjects, the rank order of the responses to eachS+ painting was used for the analysis. TheSpearman's rank-order correlation coefficient(rho) between 2 subjects in the Monet S+group trained with Set A was -.02, and thatwith Set B was .06. The rho between the 2subjects in the Picasso S+ group trained withSet A was .19, and that with Set B was .37.None of the correlations was statistically sig-nificant. There was thus no systematic bias ofresponding caused by particular stimuli, eventhough individual subjects subjects showed dif-ferences in responding to each stimulus.

Figure 1 presents results of Tests 1 and 2.Because there were individual differences inabsolute number of pecks, percentage of cor-rect responses is shown in the figure. All birdsmaintained their discrimination in the mon-ochromatic-stimulus test, although some birdsshowed less than 90% correct. Thus, color wasnot a crucial cue for the painting discrimina-tion. There was no statistically significant dif-ference between the Monet S+ group and thePicasso S+ group (unpaired two-tailed t =0.176, df = 6).Most of the paintings of impressionists lack

sharp outlines, whereas most of the paintingsof cubists have sharp contours. Sharp contoursthat might be a cue for discrimination wereinvestigated in Test 2. Although D24 and B34in the Picasso S+ group showed less than 90%correct responding in the test of focus, otherbirds showed more than 90% correct respond-ing. D24 saw Set A, and B34 saw Set B. Birdscan show individual differences in selectivestimulus control in discriminations of com-pound stimuli consisting of shape and colorcues (Reynolds, 1961) and in discriminationof more complicated stimuli (Watanabe et al.,1993). There was no statistically significantdifference in percentage correct between theMonet S+ and Picasso S+ groups in the testof focus (unpaired two-tailed t = 0.635, df =6). Results of Tests 1 and 2, together with theanalysis of the categories of subjects of paint-ings, suggest that neither category, color, noredge sharpness uniquely controlled the dis-crimination.

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Figure 2 shows results of Test 3. The re-sponse to each stimulus is shown as percentageof correct responses. The subjects emitted re-sponses to the mirror image and the upsidedown images of both S+s. Discrimination be-tween S+ and S- was clear, even when thestimuli were reversed or inverted. The reversedimages, however, reduced responding in theMonet S+ group relative to responding onnormal pictures, but did not consistently sup-press responding in the Picasso S+ group. Thedifference between responding to the originalS+ and their upside-down images was statis-tically significant in the Monet S+ group (t= 8.69, p < .05).

Figure 3 presents results of Test 4. The

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Fig. 2. Results of Test 3. The vertical axis indicatesrelative response to each category of stimuli as a proportionof the total number of responses emitted by each subjectduring the test. The numbers in parentheses show the totalnumbers of responses. The upper panel shows results fromthe birds that were trained to respond to paintings byMonet, and the lower panel shows those from birds trainedto respond to paintings by Picasso. M indicates paintingsof Monet, and P indicates those of Picasso. (R-L) and (U-D) indicate left-right reversal and upside down, respec-tively.

correct response to each painting was ex-pressed as percentage of total responses in thetest. The birds trained to respond to paintingsby Monet showed relatively high respondingto the novel paintings of Monet and paintingsof other impressionists (Cezanne and Renoir).Thus, they showed generalization to novelpaintings by Monet and generalization to otherimpressionists. The subjects showed some in-dividual differences in response to Delacroix.Although a difference between discriminationof new Monet and Delacroix was statisticallysignificant in the group data (t = 4.86, p <.05, df = 3), D22 responded more often to theDelacroix paintings than to those of Cezanne.The subjects in the Picasso S+ group seemed

to generalize their responses to the novel Pi-

casso paintings and to those of Braque andMatisse. A statistical analysis, however, iden-tified a statistically significant difference be-tween responding to the original Picasso andnovel Picasso paintings (two-tailed paired ttest, t = 3.47, p < .05, df = 3) and betweenresponding to the original Picassos and otherabstract paintings (t = 3.51, p < .05, df = 3).These results suggest that stimulus controlmight have been based partly on each originalS+ stimulus. Nevertheless, the subjects main-tained the discrimination between the abstractpaintings and those of the impressionists. Theseresults are consistent with generalization basedon each original S+ painting. All birds emittedresponses less often to paintings by Delacroixthan to original Picassos (t = 4.67, p < .05,df = 3).

Flatter generalization of the Monet S+group suggests that the subjects in this groupformed a categorical discrimination, whereasthe relatively steeper generalization in the Pi-casso S+ group suggests that the birds in thisgroup learned both categorical and individual-painting discriminations.

EXPERIMENT 2Experiment 1 demonstrated pigeons' ability

to discriminate paintings of Monet and Pi-casso. If pigeons could not learn discriminationamong paintings of Monet or among those ofPicasso, the results of the generalization inExperiment 1 might reflect confusion amongpaintings by each artist. In Experiment 2, pi-geons were trained a pseudoconcept discrim-ination similar to those examined by Wasser-man, Kiedinger, and Bhatt (1988) andWatanabe (1991). The subjects had to dis-criminate two arbitrary groups of paintings,each consisting of paintings by Monet and Pi-casso.

METHODSubjects and ApparatusTwo experimentally naive pigeons (Co-

lumba livia) were used. They were maintainedunder conditions like those in Experiment 1.The experimental chamber with a screen forthe slide projector was used. Details of theapparatus were identical to those of Experi-ment 1.

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Fig. 3. Results of Test 4. The vertical axis indicates relative response to each category of stimuli as a proportionof the total number of responses emitted by each subject during the test. The numbers in parentheses show the totalnumbers of responses. The upper panel shows results of birds in the Monet S+ group, and the lower panel showsthose of birds in the Picasso S+ group. D14, D22, D24, and G34 were trained with Set A, and B22, E33, B34, andE24 were trained with Set B. Each bar indicates proportion of test responses to each category of stimuli during testing.MN: paintings by Monet used for the discriminative training; new MN: paintings by Monet that were never presentedduring training; CZ: paintings by Cezanne; RN: paintings by Renoir; DC: paintings by Delacroix; BQ: paintings byBraque; MS: paintings by Matisse; new PC: paintings by Picasso that were never presented during training; PC:paintings by Picasso used for training.

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correct over two successive sessions after 16,0- o vsessions, and E32 reached criterion after 17

to - sessions. Figure 4 shows learning curves of the-i--O D32 2 subjects. The results clearly demonstrate the-0--* E32 pigeons' ability to acquire the discrimination.

0 The number of sessions required to reach the1 5 l 0 15 20 criterion were comparable to those observed in

SESSIONS Experiment 1. Wasserman et al. (1988) re-g. 4. Learning curves of 2 subjects in Experiment ported that pigeons had difficulty in learningowing percentage of correct pecks over sessions. a pseudoconcept, but Watanabe (1991), using

edible and nonedible objects as discriminativestimuli, did not find a statistically significantdifference between development of a pseudo-

luli concept and a more natural concept. Keepinget A was used, but S+ consisted of four in mind that the number of subjects was smallitings by Monet (S10, S15, S22, and S29 in both Wasserman et al.'s experiments andable 1) and five by Picasso (S 13, S20, S22, ours, and that the pigeons showed substantialand S32 in Table 1), and S- consisted individual differences in speed of acquisition

ie remaining paintings in Set A. of the complicated discrimination tasks, diffi-culty of pseudoconcept discriminations seems

-edure to depend on the kinds of stimuli involved.The Spearman's rho between the rank or-

'he subjects received discrimination train- ders of total numbers of responses emitted tosimilar to that in Experiment 1. After they each stimulus until the subjects reached thehed the criterion of discrimination, a mon- criterion was .16. Thus, no stimuli were es-omatic-stimuli test (Test 1), an out-of-fo- pecially easy to discriminate.stimuli test (Test 2), and rotated-stimuli Figure 5 shows results of Tests 1 and 2.(Test 3) were carried out under extinc- Both subjects maintained their discriminationDetails of these testing procedures were well in the test with out-of-focus stimuli. Al-

i Experiment 1. though E31 showed some decrease of discrim-ination in the test of monochromatic stimuli,its performance was well above chance level.Thus, these single cues did not uniquely con-

100_ trol the discrimination. D32 responded oftento the mirror images and to the upside-down80 a images of S+, whereas E3 1 showed a decreasel l l n 1 in responding to these stimuli (Figure 6).

6060~~~ o GENERAL DISCUSSION40 The present results showed that pigeons'

discriminative performance could be con-20 trolled by paintings of different styles. A pre-

vious study by Porter and Neuringer (1984),0 -~ who reported discrimination by pigeons be-

D32 E31 D32 E31 tween music of Bach and Stravinsky, and the

MONOCHROM TEST OUT OF FOCUS TEST present study suggest that pigeons have abil-ities that enable them to identify both musical

g. 5. Results of Tests 1 and 2, showing percentage and visual artists.rrect responses. Pigeons saw monochromatic pictures Removal of color or sharp contour cues dis-est 1 and out-of-focus pictures in Test 2. The number,ch bar indicates total number of responses emitted by rupted performance in some birds, but we could)ject during testing. not identify any single cue for the discrimi-

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DISCRIMINATION OF PAINTINGS 173

nation. Because paintings of Monet and Pi-casso differ in many aspects, we are suggestinga polymorphous concept (Lea & Harrison,1978; Lea, Lohman, & Ryan, 1993) in thiscase.We observed a decrease in responding to

left-right reversed or upside-down stimuli inthe Monet S+ group but not in the PicassoS+ group. Using geometric figures, Delius andHollard (1987) demonstrated that pigeons caneasily identify rotated patterns when the orig-inal pattern is given. Wasserman et al. (1988)also reported accurate responding to rotatedstimuli after category discrimination, but theirbirds showed some decrement of responding toupside-down stimuli. Using bird faces as stim-uli, Phelps and Roberts (1994) did not findeffects of picture orientation. Pigeons in ourexperiments discriminated well the reversedS+ from the S- and the reversed S -. Thereversed stimuli, however, decreased respond-ing more in the Monet S+ group than in thePicasso S+ group. It is not clear why the birdsshowed this difference in stimulus control.

Pigeons trained to respond to figures of thecartoon character "Charlie Brown" main-tained their responding when a scrambled chi-mera of his body parts was presented (Cerella,1980). Although Wasserman, Kirkpatrick-Steger, Van Humme, and Biederman (1993)confirmed discrimination of component partsof complex visual stimuli in pigeons, they dem-onstrated the importance of spatial organiza-tion in picture perception. These studies em-ployed drawings that should not have anyecological importance for pigeons. In a relatedstudy, we trained pigeons with a pigeon's face(that should have biological importance) andfound that the birds did not maintain the dis-crimination in response to scramblings of partsof the pigeon's face (Watanabe & Ito, 1991).

These results and the present tests with ro-tated paintings suggest that this distortion dis-rupts control when the original stimulus rep-resents a real object (as in Monet's paintings),but less disruption is produced when the stim-ulus has a weaker relation with the real world(as in Picasso's paintings). The fact that thepigeons could learn the category discrimina-tion (Experiment 1) and the pseudocategorydiscrimination (Experiment 2) suggests thatthey can group stimuli into one group and thatthey can also discriminate each stimulus. Hu-mans can not only discriminate between paint-

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ings by Monet and Picasso but can also dis-criminate each picture. Such flexibleclassification is one of the bases of human cog-nitive abilities.

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Received June 15, 1994Final acceptance October 19, 1994