PSYCHOLOGICAL SCIENCE

Research Report

PIGEONS ARE SENSITIVE TO THE SPATIAL ORGANIZATIONOF COMPLEX VISUAL STIMULI

Edward A. Wasserman,' Kim Kirkpatrick-Steger,' Linda J. Van Hamme,' andIrving Biederman"

'7/ie University of Iowa and ^University of Southern California

Abstract—Two experiments investigatedthe role of spatial organization in the dis-crimination and generalization of com-plex visual stimuli hy pigeons. In Exper-iment I, after pigeons had been trainedlo discriminate line drawings of four ob-jects, they were tested with novel pic-tures in which the same component partsofthe objects were spatially rearranged.The -Spatially scrambled pictures led to adramatic drop in recognition accuracy,but responding remained above chance.In Experiment 2, pigeons reached a highlevel of discriminative performancewhen required to choose among four dif-ferent spatial arrangements of the sameobject parts. These results conjirm Cer-ella's (1980) conclusion that pigeonsdiscriminate the component parts ofcomplex visual stimuli, but. unless it isassumed that the scrambling deleted orcreated emergent features, the resultsdisconfirm his conclusion that spatial or-ganization plays no role in pigeons' pic-ture perception.

In an important and widely citedstudy. Cerella (1980, Experiment III)trained two pigeons to discriminate car-toons of Charlie Brown (the positivestimulus class) from cartoons of otherPeanuts characters (the negative stimu-lus class). Cerella then tested the birdswith scrambled versions of CharlieBrown which involved vertical rear-rangements of Charlie's head, torso, andlegs. Both birds responded at the samehigh rate of keypecking to the intact andthe scrambled versions.

The observation that test respondingwas unaffected by spatial rearrangementled Cerella to make several strong con-clusions about the pigeon's analysis ofcomplex visual stimuli: (a) "Insensitivityto scrambling indicates that the absolute

Address correspondence to E.A. Wasser-man. Department of Psychology. The Univer-sity of Iowa, Iowa City. lA 52242.

or relative placement of features withinthe image is unimportant" (p. 5). (b)"The mechanisms of identification mustoperate . . . at the level ofthe local fea-tures which survive the fragmentation ofthe stimulus" (p. 5). (c) "The identifica-tion of [cartoons of Charlie Brown can-not] depend on the computation of globalproperties such as three dimensionalstructure" (p. 5).

What are the implications of Cerella'sresults? It is implausible, for example,that scrambling a scene or an objectwould show no effect on humans' recog-nition of that stimulus. Indeed. Bieder-man and his associates (1972; Bieder-man, Glass, & Stacy, 1973; Biederman,RabinowitE, Glass, & Stacy, 1974)showed deficits in scene perception andin the recognition of objects in suchscenes when the scenes were jumbled.Thus, if Cerella's conclusions were gen-erally true, we would be faced with theintriguing possibility that the pigeon'shighly developed ability to recognize ob-jects by sight (e.g., Bhatt, Wasserman,Reynolds. & Knauss, 1988; Herrnstein,1985) is both computationally and neuro-physiologically different from that of hu-man beings. Unlike humans, pigeonsmight "respond to complex line projec-tions as collections of local featuresrather than as representations of threedimensional objects" (Cerella, 1980, p.1). Before accepting Cerella's thesis, weconducted two experiments designed toexplore further the effect of componentscrambling on the pigeon's discrimina-tion of visual portrayals of complex ob-jects.

Before describing the experiments,we note that there is an asymmetry inwhat can be concluded if there is or isnot an effect of scrambling on discrimi-native performance. If no effect ofscrambling is obtained (i.e.. if pigeonsrespond as well to the scrambled ver-sions of a stimulus as to the original),then it can be concluded, as Cerella did,that these birds are indeed insensitive to

the relations among the parts. Moreover,one can also conclude that whateveroriginal features might have been lost ornew features created as a consequenceof the scrambling, those features are ir-relevant to the pigeons' performance. Incontrast, if generalization pertbrmancesuffers because of scrambling, then,without additional research or assump-tions, it is unclear whether the pigeonsare sensitive to the component relationsor to new or lost features produced bythe scrambling.

This uncertainty concerning the ef-fects of scrambling arises from the ambi-guity in the information that controls thepigeon's visual discrimination. There isevidence (Biederman & Cooper, 1991)that people represent a complex objectas an arrangement of the convex regionsthat correspond to the object's parts. If itis assumed that pigeons do the same,then a decrement in generalization toscrambled stimuli that preserve the ob-ject's parts would be evidence that pi-geons were sensitive to the relationsamong those parts. Above-chance gener-alization to scrambled stimuli could beinterpreted as evidence that pigeons alsorespond to the parts, independent oftherelations among them.

EXPERIMENT 1

In our first experiment, we adaptedthe four-alternative forced-choicemethod of Bhatt et al. (1988) to train pi-geons concurrently to discriminate linedrawings of four common objects. Sub-jects were then tested with novel pic-tures in which the same component partsofthe objects were portrayed, but rear-ranged both vertically and horizontallyfrom their original positions. Would thepigeons continue to discriminate thescrambled drawings as accurately asthey discriminated the original ones? Orwould their accuracy drop as a result ofspatial scrambling?

336 Copyright ® 1993 American Psychological Society VOL. 4. NO. 5. SEPTEMBER 1993

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E.A. Wasserman et al.

MethodSubjectsFour experimentally naive feral pi-

geons reduced to 85% of their free-feeding weights served as subjects.

Fig. I. The four original training stimuliin Experiment 1: desk lamp, iron, water-ing can, and sailboat.

Apparatus

Four Skinner boxes were the experi-mental chambers (for details, see Bhattet al.. 1988). Through an opening in thewall behind the front panel of each box.a rotary projector presented slides on afrosted plastic screen that measured 7 x7 cm. Located near each corner of thescreen was a key 1.9 cm in diameter, andbehind each key was a miniature projec-tor containing an incandescent lamp.When these lamps were lit. the top leftkey was orange, the top right key white,the bottom left key green, and the bot-tom right key red. A microswitch behindeach of these keys and one behind a clearplastic key in front ofthe viewing screenrecorded the pigeon's pecks. An openingin the front panel below the screen al-lowed access to food during reinforce-ment. A personal computer equippedwith the MED-PC programming systemand interface (Tatham & Zurn, 1989)managed experimental control and datacollection.

Stimuli

Original training stimuli were linedrawings of four human-made objects; adesk lamp, an iron, a watering can. and asailboat (Fig. I). Each drawing containedfour components or geons (Biederman,1987). The maximum dimension (heightor width) of each drawing was 5.7 cmon the pigeon's viewing screen. Teststimuli were created by scrambling thecomponent parts both vertically and hor-izontally, with the constraint that theheight;width ratio not be affected. Be-cause some components overlapped withothers in the original drawings, scram-bling sometimes disclosed a hidden con-tour, but the occlusion of already-present contours was not permitted. Thedisclosure of hidden contours was per-mitted in some drawings (leading to"smooth" contours) but not in others(leading to "notched" contours). Fourscramblings of each object in both"smooth" and "notched" versions werecreated (Fig. 2), and all stimuli were pho-tographed and transferred to 35-mmslides. When projected on the pigeon'sviewing screen, the slide images were ofblack lines on a white background.

Fig. 2. All eight spatial scramblings ofthe desk lamp used during testing in Ex-periment 1. The left column depictsnotched versions and the right columndepicts smooth versions. Spatial scram-blings of the other three training stimuliwere created similarly.

ProcedureFollowing training to eat from the

feeder and to peck the blank, white view-ing screen and the differently colored re-port keys, the pigeons began training todiscriminate the four line drawings.

On the four-choice discriminationprocedure, each of 48 daily trials beganwith the display of a line drawing for 15s. The first peck to the viewing screenafter that interval lit all four differentlycolored report keys.' A single choice re-

L Each bird had different line drawing-

VOL. 4. NO. 5. SEPTEMBER 1993 337

PSYCHOLOGICAL SCIENCE

Spatial Organization and Discrimination

sponse was then permitted. If the correctkey for the stimulus on the viewingscreen was pecked, all ofthe visual stim-uli were turned off and the pigeon wasfed mixed grain. If the response was toany of the three incorrect report keys,then all key lights were turned off andthe trial was repeated. Only the firstchoice response of a trial was scored;correction trials were not analyzed. Tri-als were separated by intertrial intervalsaveraging 15 s. Training continued for108 daily sessions.-

Testing was conducted with two newslide trays, each entailing a differentslide order and containing 40 drawings oforiginal objects and 8 drawings of scram-bled objects. Each 48-trial testing ses-sion involved 10 original versions and 2scrambled versions of each object, Iscrambled version with smooth contoursand I with notched contours (Fig. 2). Be-cause there were four scramblings ofeach object in each version, it took foursessions to show all of the test stimuli.We repeated the entire testing set twicefor a total of eight test sessions.' To al-low for continued and unconfoundedtesting with the scrambled stimuli, foodreinforcement was given to birds on ailtest trials regardless of which report keywas pecked; only one choice responsewas permitted to each test stimulus, withno subsequent correction for errors.

Results and Discussion

All of the subjects learned the four-stimulus discrimination, and by the endof training, discrimination performancewas quite good. Levels of accuracyacross all 4 pigeons averaged at least75% correct (chance was 25%) over thelast seven 4-session blocks of training.

Across all eight sessions of testing,mean accuracy to the original drawingsaveraged 80.5%, whereas mean accuracy

report key assignments, so that each drawingwas associated with each key across all 4 pi-geons.

2. Wiihin every block of four sessions,each subjecl received training with four dif-ferent slide trays, each involving different or-ders of the line drawings.

3, Pairs of test sessions were separated by2 to 20 sessions of retraining in order lo main-tain high-level discriminative performance.

Tahle L Percentageduring testby subject

Bird

1234

Mean

sessions

Original

92.762.082.384.980.5

of correct responsesof Experiment I,

Visual stimulus

Notched

71.940.646.950.052.3

Smooth

78.143.856.331.352.3

to each of the two types of scrambleddrawings averaged 52.3% (Tables I and2). Analysis of variance disclosed a sig-nificant difference in accuracy to the dif-ferent versions of the depicted objects,F(2, 72) = 15.01, p < .01. Sheffe posthoc analyses indicated that the two typesof scrambled drawings (notched andsmooth) did not differ from one another{p > .05), but that the two combined dif-fered significantly from the origina!drawings {p < .05).

Binomial tests were conducted to de-termine whether the 52.Wr correct re-sponses shown to the scrambled draw-ings exceeded the chance score of 25%.Across all 4 birds, all four stimuli, and alleight sessions of testing, the obtainedlevels of accuracy surpassed the levelexpected by chance for each of the twotypes of scrambled drawings ip < .01,7V^ 128, for both smooth and notched ver-sions). For each individual subject, ac-curacy across all four stimuli, both formsof scrambled pictures, and all eight ses-sions of testing surpassed that expectedby chance (/? < .05, N - 64).

Our primary finding was that spatialscrambling of a picture's componentparts did lead to a large drop in its dis-

crimination by pigeons. This result failsto replicate Cerella's (1980) widely citedfinding that spatial scrambling of a car-toon character was without effect on pi-geons" discrimination performance andleads us to reappraise his conclusionsconcerning the pigeon's visual analysisof pictures.

First, the absolute or relative place-ment of components within an image canbe quite important to the pigeon's pic-ture perception, as documented by thegeneralization decrement when the com-ponents were scrambled. Second, someprocessing of local features did occur, asscrambled pictures were responded to atabove-chance levels of accuracy. Be-cause the same component pieces werepresent in both the original and thescrambled drawings, these componentsmay have been the local features that af-forded some level of discriminative per-formance.

We are not sure why our results ofpicture scrambling differed from Cerel-la's. Many procedural disparities be-tween the studies can be noted: (a)Cerella's 2 birds were given a go/no godiscrimination, (b) his test stimuli weredivided into thirds, (c) they were verti-

Table 2. Percentage of correct responses duringsessions of Experiment I, by object

Object

Desk lampIronWatering canSailboatMean

Original

8278788180

87785

Visual stimulus

Notched

46.943.871.946.952.3

test

Smooth

4646625352

99513

338 VOL. 4, NO. .S. SEPTEMBER 1993

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E.A. Wasserman et al.

cally scrambled, and (d) they were asso-ciated with nonreinforcement. In ourstudy, (a) the 4 birds were given a four-choice discrimination, (b) the test stimuliwere divided into fourths, (c) they wereboth vertically and horizontally scram-bled, and (d) they were associated withreinforcement. Yet these differencesmay not have been so great as the pos-sibility that Cerella's procedure mayhave encouraged his birds to form a fea-ture-positive discrimination (Hearst,199!) based on one or a small number ofattributes present on positive (rein-forced) trials, but absent on negative(nonreinforced) trials. A good candidateis the zigzagged pattern on CharlieBrown's shirt. If this salient feature waspositively associated with reinforcementand with keypecking. then it is not sur-prising that its vertical relocation in thedisplay had little impact on the pigeons"rate of response.

Such an analysis suggests that if hu-mans were given the same task and train-ing regimen, then their behavior mightresemble that of Cerella's birds. Indeed,under conditions in which the stimuli arevaried from trial to trial, humans readilyemploy limited distinctive features withwhich to make their discriminations(Biederman & Shiffrar, 1987; Gibson,1969). Thus, a feature such as the zigzagon Charlie Brown's shirt could serve as apowerful discriminative feature for hu-mans as well as pigeons.^ As to our ownstudy, scrutiny of Figure I suggests thatthe four object drawings—all comprisingfour components—did not each contain asingle part which was radically differentfrom those in all other drawings andwhich could have been distinctively as-sociated with a particular reportresponse. Thus, it is unlikely that our pi-geons were encouraged to form a fea-ture-positive discrimination on which tobase responding.

Finally, we do not yet know what fea-tures of our drawings controlled our pi-geons" discriminative behavior. Overallspatial arrangement, three-dimensional-ity, or both may participate in the birds'processing of such complex visual stim-

4. One test of the effectiveness of such afeature would be to include it in an otherwisenegative picture and see if the subject re-sponds positively to it.

uli.*̂ And there is another interesting pos-sibility. Some features produced by theconnection between picture parts or be-tween pairs of lines from different partsmight provide information for discrimi-native performance. For example, asseen in Figure I, the connections amongthe shade, fixture, and stem ofthe desklamp form a nooklike feature. However.in the top row of Figure 2. this feature isgone; here, the connections among thefixture, base, and stem produce a newfeature—an unclosed triangle. Whetherthese local emergent features participatein discrimination and, if so. whethertheir participation can be isolated fromthe effects of component location and or-ganization will surely require much fur-ther research and analysis.

EXPERIMENT 2

Because the results of our first exper-iment differed so decidedly from Cerel-la's (1980). our second experimentsought to increase the generality of theseinitial findings by seeing whether pigeonscould use spatial information to discrim-inate different scramblings of the sameobject parts. Individual pigeons weretrained to discriminate four differentscrambled versions of an object. Specif-ically, the four smooth scramblings ofeach ofthe four objects used in Fxperi-ment 1 were employed. Subjects weretrained with the four-alternative forced-choice method of Experiment I, nowwith each key associated with a differentscrambled version of a single object, andwith different objects and their four spa-tial scramblings assigned to differentbirds.

Method

Subjects

Four different feral pigeons reducedto 85% of their free-feeding weights wereused. The subjects had served in an ear-lier project involving the four-alternativechoice procedure, but they had not pre-

5. Although we are disinclined to do so.some people might propose that the birds' be-havior indicates that Ihey perceived the linedrawings as whole entities, or gestalts.

viously seen the stimuli in the presentstudy.

Apparatus

The same apparatus as that in Exper-iment I was used.

Stimuli

The stimuli were the four smooth ver-sions of the scrambled objects usedin the test phase of Experiment I (seeFig. 2).

Procedure

No pretraining was required as thesubjects were accustomed to the appara-tus and general routine from a previousstudy.

Discrimination training was con-ducted as in Experiment I. Subjectswere given 48 daily trials consisting of 12presentations of each of the four scram-blings of an object. Birds 5, 6, 7, and 8were given scramblings of the wateringcan. the iron, the desk lamp, and the sail-boat, respectively. Training continuedfor 108 daily sessions.

Results and Discussion

All 4 birds acquired the four-choicediscrimination among the differentscrambled versions of the images; therate of learning and overall level of dis-crimination differed among birds. Figure3 shows the mean percentage of correctchoices on each of twenty-seven 4-dayblocks for individual subjects. The figureclearly reveals that the 4 birds acquireddiscriminative behavior, an observationconfirmed by an overall analysis of vari-ance. Hlb, 78) - 33.63, p < .001. All ofthe birds reached a high level of accu-racy; the best scores were 75.0% on Day105, 89.6% on Day 106, 93.8% on Day100. and 87.5% on Day 108 for Birds 5 to8, respectively. Such performance isconsistent with the conclusion of Exper-iment I that pigeons are able to discrim-inate the spatial arrangement of objectcomponents, but inconsistent with Cer-ella's (1980) conclusion that they are sen-sitive only to the presence of componentfeatures.

A comparison of the acquisition ratesofthe four-key discriminations in Exper-

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Spatial Organization and Discrimination

BIRDSWATERING CAN

P 1 3 5 7 9 11 13 15 17 19 21 23 25 27 1 3 5 7 9 11 13 IS 17 19 21 23 25 27

13 15 17 19 21 23 25 27 1 3 S 7 9 11 13 15 17 19 21 23 25 27

4-DAY BLOCKS

Fig. 3. The mean percentage of correct responses for Birds 5 to 8 across the 4-dayblocks of training in Experiment 2.

iments I and 2 was also conducted. Themean number of training days requiredto reach a criterion of 75% correct was46.3 in Experiment I and 71.8 in Exper-iment 2. Although the difference in ac-quisition rates between Experiments 1and 2 did not reach the .05 level {/ =- 1.04, p < .38), the group means sug-gest that the discrimination task in thesecond experiment was more difficultthan that in the first. This trend is notsurprising since in Experiment I, pi-geons were able to use both componentfeatures and their spatial arrangement toform the discrimination, whereas in Ex-periment 2, the discrimination operation-ally permitted the use of component spa-tial organization alone.

GENERAL DISCUSSION

The results of the present pair of ex-periments both confirm and contradictCerella's (1980) analysis of picture per-ception by pigeons. Consistent with hisconclusion that pigeons discriminate thecomponent parts of complex visual stim-uli, we found in Experiment I that spa-

tially scrambled drawings did still permitthe occurrence of discriminative choicebehavior. Contrary to his conclusion thatpigeons discriminate the componentparts of pictures but not their spatial or-ganization, we found in Experiment Ithat spatial scrambling led to a dramaticdrop in discriminative responding and inExperiment 2 that pigeons could discrim-inate effectively among different picturescomprising the same component parts,but in different spatial arrays.*'

In accord with our findings, Bieder-man (1987) has proposed that human ob-ject recognition entails a representationspecifying both the parts of an object andthe spatial relations among those parts(Hummel & Biederman, 1992). Bieder-man has asserted that from a singleglance at an object, people can create astructural description that allows view-point invariance. so that the same object

6. We would also expect that new scram-blings of the component parts of the four dif-ferent arrays of our objects used in discrimi-nation training in Experiment 2 would occa-sion lower levels of accuracy, just asscrambling did in Experiment 1.

can be readily recognized if seen fromanother orientation, provided that theoriginal parts and relations still charac-terize the object (Biederman & Gerhard-stein, in press). A structural descriptionthat allows viewpoint invariance wouldbe disrupted by jumbling the parts of anobject even if the parts remained intact,because the original spatial relationswould no longer characterize the object.Based on our findings, it is possible thatpigeons, too, form such structural de-scriptions.

We are, of course, far from under-standing the behavioral and neural mech-anisms of the pigeon's perception ofcomplex visual stimuli. Workers havejust begun to examine the importance ofspatial organization (Van Hamme,Wasserman, & Biederman, 1992) andthree-dimensionality (Cabe. 1980; Cer-ella. 1977: Delius, 1992) to the pigeon'svisual discriminations. The present re-sults indicate that the pigeon's represen-tation of the shape of a complex objectinvolves the individual parts of the ob-ject as well as the spatial arrangementsamong them. In this respect, the pi-geon's processing of shape resembles theprocessing of shape by humans.

Acknowledgments—This research wassupported by National Institute of MentalHealth Grant MH 47313 to E.A. Wasser-man and I. Biederman, The authors grate-fully acknowledge the help of Eric Cooperin preparing some ofthe drawings used inthis research.

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(RECEIVED 11/4/92; ACCEPTED 1/29/93)

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VOL. 4. NO. 5, SEPTEMBER 1993 341