211-223 effects of the number and form of stimuli on visual search in the pigeon

7/31/2019 211-223 Effects of the Number and Form of Stimuli on Visual Search in the Pigeon.

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J ou rna l of Exper imenta l Psychology:A nim al B ehavior ProcessesV O L . 5, No. 3 J U L Y 1979

Effects of the Number and Form of Stimulion V isual Search in the PigeonDonald S. BloughBrown University

Pigeons' pecks at small luminous forms on a large display field w ere pho to-electrically recorded. Pecks to target (S+) forms w e r e reinforced w i t h fo o d ;pecks to distractor ( S ) forms produced a short t ime-out. The speed andaccuracy of search for a target declined as the number of distractors in -creased to a m ax im u m of IS. Search w as further impaired w h e n the dis-tractors were quite similar to the target. H ow e ve r, search w as unaffected bya change in the number of potential targets (memory se t size) from one totw o, nor w as i t clearly affected by the use of several different distractorforms instead of a single repeated form. Search w as sw i f t over a large ( 5 0 )visual field, in the apparent absence of substantial head and eye movements .

The survival of m any higher species m ustdepend in part on a capacity to search th evisual field for significant objects, yet thiscapacity has been little studied except inhumans . Research w i th hu m an subjectsdates at least from Jastrow (1892), butmost has been done in the last 15 years,coinciding w ith recent interest in hu m aninformation processing. The speed and ac-curacy of search is thought to relate to suchmatters as the serial or parallel nature ofsearch, recognition mechanisms, the scan-ning of short- term memory, and the influ-ence of attention.The questions under study in most hu-man search experiments seem open to in-vestigation in animals. Though studies w ithh u man s typically have used letters or nu-merals as stim ulus ma terials, such me aning-f u l symbols are often not essential to the

This research wassupported in part by U.S. PublicHealth Service Grant MH-02456.Requests for reprints should be sent to Donald S.Blough, Department of Psychology, Brow n U ni-versity, Providence, Rhode Island 02912.

questions under s tudy. O ther areas of in-formation processing, su ch as attention andmemory, are already being studied in sev-eral animal laboratories. (Riley and Lei th ,1976, and Hulse, Fow ler , an d Honig, 1978,review ed some recent advances in this area.)Visual search seems a prom ising new direc-tion for information processing researchw i t h animals.T he present report adopts a conventionalinformation-processing conceptual point ofview. V isual search is assumed to involve asequence of processes that each take t ime;reaction time in some m an ne r reflects theseprocesses. Suppose, for example, a subjectis looking for a specific target in a displaythat sometimes has few nontarget formsand sometimes has many. If the time takento find the target w ere to r ise l inearly w ithth e n u m b e r of forms in the display, onemight suspect that the subject w asexamin-ing the forms one by one in serial fashionunti l the target w as found. If, at the otherextreme, th e search time did not vary w i th

. the n u m b e r of forms in the display, on emight suspect that all the forms w ere beingCopyright 1979 by the A me r ic a n Psychological Association, Inc. 0097-7403/79/0503-0211$00.75

211


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212 D O N A L D S . B L O U G H. O B S E R V I N G !..J .:.

B L A N K I N G . .. : I :: ......

Figure 1 . S u m m a r y of the var ious fo rms that ap-peared on the oscilloscope screen. (Each black dotrepresents an i l luminated point . The S+ and S forms ([targets and distractors] w ere approximately4 mm in diameter . )examined at once, in para l le l . L inked tothis serial-parallel distinction is the notionof capacity. If the n u m b e r of f o r m s in a dis-p lay w ere smal l , for example , a parallelsearch process might be possible, but thecapacity of this process might be exceededi f th e n u m b e r of forms became to o large,and a serial process might then become nec-essary. T he form of the func t ion relat ingsearch time to number of forms might reflectsuch a capacity l imi ta t ion . Models of vis-ual search typically make complex assump-tions regarding modes of processing, ca-pacity, and other matters. Shiffrin andSchneider (1977) provided a recent reviewof such models. Because th e present dataare not extensive enough to discr iminateamong current models to an important de-gree, the information-processing ideas areused here in a relatively descriptive fashion.The pigeon seems a part icular ly apt sub-ject for search experiments. It is a highlyvisual animal and in nature spends m u c hof it s time looking for and pecking at smal lobjects. Its visual acuity is quite good (P.Blough, 1971; Hodos, Leibowitz, & Bon-b r igh t , 1976), and other aspects of its visualpsychophys ics and physiology are relativelywe l l k n o w n . Extensive studies of "signtracking" in the pigeon (Hearst & Jenkins,1974) documen t its strong tendency to peckat visual targets that are correlated w iththe presentation of food. This tendency isput to use in the present experiments.Apar t from a short report f rom our labo-ratory (D . Blough, 1977b), search has notbeen described in pigeons. Thus, the presentstudies are exploratory and empirically ori-ented . They assess search by measur ing thespeed and accuracy w ith w hich pigeons findand peck at targets in a visual display. Ex-periment 1 concerns the effect upon search

of the num ber o f nontarget distractor f o r m sin the display and of the sim ilar i ty of thedistractors to the target. Exper imen t 2compares search for dif ferent target f o r m sand for differing n u m b e r s of potential tar-gets (this last variable is often called mem-ory set size). Exper iment 2 also comparessearch w hen one or several di f fe ren t sortsofdistractor fo rms appear at once in a dis-p lay .General Method

SubjectsThree W hite Carneaux pigeons w ere m ainta ine d atapproximately 75% of their free-feeding w eights bysupplementary feeding, if necessary, af te r each ex-per imenta l session. All w e re b e tw e e n 1 and 2 yr oldat the start of their pre tra in ing . Except as noted be-low , the y w ere run da ily in sessions th at lasted ap-proximately 1 hr, and they received most of theirfood (mixed grain) during these experimental ses-sions. With the bird under l ight Nembutal anesthe-sia, a small photocell w as glued to its beak, and a1-mm polyethylene tube w as implanted u nd er itsscalp. Wires from th e cell w ent through th e tube to aplug behind its head. This arrangement did not seemto in te rfe re w i th th e bird's eating or d r i n k i n g .

ApparatusThe apparatus has been described in detail else-w h e r e (D . Blough, 1977a); essentials are noted here.A subject chamber, a display oscilloscope, and aL I N C compute r w e re th e major components .The subject box w as mad e of acryll ic plastic, cov-ered w ith opaque paper except for a n u m b e r of v i e w -ing holes. Its interior dimen sions w ere 35 X 45 X 45cm. V isual displays w ere presented o n a 12-cm-diam.oscilloscope screen positioned behind a round 14-cm

hole in the f ront w a ll of the box. A s tandard LehighVal ley grain feeder w as mounted below and to theleft of the oscilloscope screen. The box w as dark ex-cept for periodic visual displays and the i l luminationprovided by 2-W lamps w hich came on du ring re in-forcement, one over th e display screen and one overth e feeder . The box w as located in a darkened ex-perim ental alcove, w hich w as also provided w ithw hite masking noise.The visual display consisted of one or more smallforms 4 mm in diameter and composed of dots, typi-cally eight in n um ber, arranged to form an O , an X,or some other pattern (Figure 1); their effective lum i-nance was approximately 2 cd/m2. Although thesesmall , l u m i n o u s fo rms appeared to be "on" s imul-taneously and cont inuously dur ing a presentation,they actual ly w ere intensified momenta r i ly in afixed sequence, such that each flickered at about150 Hz.


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VISUAL SEARCH 213A sma ll photocell, glued to the pigeon's beak, sentsignals via a connecting w ire and amplifier to thecomputer w henever th e beak approached to w i thinabout 2 cm in front of one of the l u min o u s forms. Atthe instant w hen the signal from this photocell ex-

ceeded a set threshold, the computer noted the typeand location of the form that had just been displayed.This specific form and no other w as necessarily th esource of the inpu t from th e photocell (apart fromelectrical noise), since the flashes from differentforms were sufficiently separated in time (approxi-mately 40 0 /isec) to avoid confusion. Since inputreceived at the moment a particular form w as in -tensified mean t that th e pigeon's beak w as near thatform, the computer could use such inputs to recorda response of know n t ime, locat ion , and target. Ef-fects of electrical noise w ere virturally elimina ted byrequir ing the c om puter to record five flashes from th esame form before recording a response to that form.At approximately 150 flashes per second, this redun-dancy added a constant of about 34 msec to each re -action time recorded. Further details of the display,input , and central control arrang em ents can be foundin the previous publication devoted to this method(D . Blough , 1977a).Procedure

The experimental method evolved over a 30-moperiod, d u r in g w hich th e details of apparatus andprocedure passed through several variations to theform described in th is report . There w ere changes inth e size and luminance of the display, th e gain andf requency response of the amplif ier, th e input thresh-old and n u m b e r of above-threshold in puts requiredto cause the computer to sense a response, and thew ay in which th e photocell and connec ting leads w ereattached. Such parameters, how ever, remained con-stant dur ing th e experiments reported here, andthroughout their training th e birds' essential taskand reinfo rcem ent contingencies rem ained constant.Trial format. Throughout most of training an dal l experimental sessions, search trials follow ed acommon sequence. Each began w i th the appearanceof an "observing" form, a rectangular array of 12dots, w hich w as centered on the screen (Figure 1) .A peck at this form produced a ,3-sec dark interval ,follow ed by the display of "search" forms alw ays in -cluding an S + target and usually one or more S distractors. W ith one exception (the form), th esearch form s w ere all composed of eight closelyspaced luminous dots, w i th in a 4-mm square area,as sh o wn in Figure 1.A peck at any of these forms produced one of thefo l low ing sequences. If the form pecked w as S+, theform disappeared. O n most such trials (probability= 11/12) a "blanking" form (Figure 1) now ap-peared in all positions on the screen that w ere in usefor search forms at any t ime dur ing the session. Thisblanking form obliterated th e afterglow of any pre-viously presented form. The b lank ing s t imul i stayedon for .7 sec; then the screen w as dark for .3 sec be-fore th e observing s tim ulus appeared to start th e nexttrial. A random 1/12 of pecks at the S+ brought re -

inforcement : The display vanished, a sm all lam pover th e display screen and another over th e foodmagazine w ere l i t , and mixed grain w as presentedfor 3 or 4 sec, depending on the bird being run . Afterreinforcement, the blanking pattern came on, fo l -lowed by the observing stimulus, w hich signaled thebeginning of the next trial.If th e bird pecked at an S form, the screen w entdark for S sec. This blackout w as follow ed by themasking pattern and the reappearance of the ob-serving st imulus . A trial-rerun correction procedurew as always in effect. After a peck at S w i th i n aparticu lar display, exactly th e same display appearedon subsequent tr ials unti l th e bird pecked the S+." N e w " trials follow ed only correct pecks; the 800-tr ial session cou nt included only such new trials.Pretraining procedure. Fol low ing food depriva-t ion, each bird w as induced by a combination ofautoshaping and hand shaping to peck at an S-f-form that appeared in random locations on the dis-play screen. For Birds 1 and 2 , th is S+ w a s a n 0;fo rBird 3 , S+ w as an I (Figure 1). Nex t th e bird w asreinforced a few times for pecking at the observingform, w h i c h appeared in the center of the displayscreen. (D uri ng early trainin g, the O served as theobserving form as wel l as S+ for Birds 1 and 2 . Bird3 w as trained w ith the rectangular form subsequentlyused w i t h all birds.) When th e bird w as peckingreadily at the observing form, food reinforcement forthis response w as discontinued for the remainder ofth e experiment . Instead, th e trial sequence describedabove was initiated. The S+ appeared randomly atone of eight locations w ithin a 7-cm square areaaround the position of the observing stimulus, and apeck at this produced food.Af t e r about 10 0 reinforcements of the observing-peck/target-peck sequence, an S appeared alongw i th the S+ af ter an observing peck. For Birds 1 and2, th is new form w as an X; for Bird 3, i t w as fl(Figure 1). Pecks at these S forms w e n t unrein-forced and produced a S-sec blackout. (D uri ng anearly period in the training of Birds 1 and 2, theblackout lasted 1 sec.)Discrimination training continued, w ith S += O and S- = X for Birds 1 and 2, and S+ = Iand S = fl for Bird 3 . Ov er from S to IS sessionsthe n u m b e r of trials per session increased to 800, theprobability of reinforcement for pecks to S+ fell to1/12, and the number of S form s appearing w ithS+ on a given trial rose to a m a x i m u m of 16. Afterthe first session or t w o , th e birds made relatively fewerrors less than 10% in the 16-form condition andless than 5% in the other conditions.From this point on, the training histories of thethree birds differed considerably. Bird 1 w as run in130 sessions comprising approximately 104,000 in-dividual search trials prior to the experim ental ses-

sions described below . All of these sessions involvedsearch for the O S+ . O n a small percentage of thesetrials, the O appeared alone; in the rest, i t w as se-lected from patterns of 2, 4, 8, or 16 forms, the otherforms being one or another of the S charactersshown in Figure 1. The training of Bird 2 w as similarbu t extended over only 50 sessions.


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214 D O N A L D S. B L O U G HE X P 1

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x x x x \ JL n4.5 CM

EX P 2O X

Figure 2. Sample search displays from the tw o ex-periments, sh ow ing the pattern and relative size ofth e entire configuration presented to the pigeon.(Two of the num erous possible pa tterns in Experi-ment 1 are shown in the top row, and tw o from Ex-periment 2 are shown in the bottom row . All displaysin Experiment 2 had eight forms located as show nin the low er row . The observing form appeared in aposition central to each of these displays, but it dis-appeared .3 sec before the search display appeared.)

Some of the data collected from Birds 1 and 2 dur-ing the above mentioned training period have al-ready been reported (D . Blough, 1977b, as Birds 1and 3). Bird 3 had less extensive training. After theinitial pretraining described above, this bird hadfive sessions in w hich the I S+ was to be foundamong five fl S forms, an d then three sessions inwhich the O S+ w as to be found amon g five fl S forms. Follow ing the trainin g described, the birdswere run as indicated below except that th e actualorder of running w as Experiment 2A, Experiment1A, Experiment 2B, Experiment IB .Data analysis. The reaction time on a given trialw as defined as the time betw een the appearance ofth e display containing a target form and the sensingof a peck to one of the forms in this display. An errorw as recorded if a nontarget form w as pecked, bu tthe reaction times of such errors w ere no t fur theranalyzed. Data from the one or more correction trialsfollow ing an error w ere also rejected. Reaction time so f less than 120 msec w ere assumed to arise onlyw hen the pigeon's beak was near the screen at thet ime of stim ulus onset. The percentage of t r ials w i thsuch reaction times w as computed only in Experi-ment 1A and w as found to be less than 1% of thetotal; such trials w ere eliminated from considerationin al l experiments. Median reaction times fo r correctresponses and percentages of errors w ere computedfor conditions w i th in individual sessions as specifiedbelow. Most of the data appear as means of theseindividual session medians.

Exper iment 1: Array Size andDistractor FormA previously published experiment (D .Blough, 1977b) demonstrated a systematicincrease in me dian reaction time w ith in-creasing numbers of distractor elements ina search display. Experiment 1 used tw o o fth e same birds (Birds 1 and 2 in the presentw o r k ) and one new bird to replicate thatfinding and to provide inform ation on threeadditional matters: th e effect of target-distractor similarity, the interaction of dis-tractor form and array size, and the effectof target location w i th in the display.

MethodSubjects an d apparatus. Subjects an d apparatuswere as described above.Procedure. The gen eral procedu re w as describedabove; Experiment 1 included also the following as-pects. The search display w as composed of 16 orfewer forms. Each form w as located at one of 16positions w i th i n a 4 X 4 matrix 7-cm square (seeFigure 2 ). While observing this display, th e birdsheld their heads about 7 cm from the screen. Thisdistance implies a visual angle for the entire dis-play of about 50.The target form, or S+, w as the O shown in Fig-ure 1. The computer 's pseudorandom generatorplaced this S+ an equal number of time s at each ofthe 16 locations in the matrix. In each block of 80trials, the S+ appeared in each location just fivetimes, once under each of five distractor conditions.For these conditions, 0, 1, 3, 7, or IS distractor formsappeared along w ith the O (S+). These distractorforms w ere positioned rando mly on the IS matr ixlocations rem aining after th e S+ position had beendetermined. The five distractor conditions w ererandomly ordered, each appearing 16 t imes in an

80-trial block. A ltho ug h the positions of the S+ w er eequalized in number across trial blocks, i t w as pos-sible for statistical irregularities to have existed inthe positioning of the S forms, in terms of cluster-ing, left-right imbalance, and so on. Regardless ofth e number of distractors presented on a trial, th estimulus display w as follow ed by masking forms atall 16 ma trix locations.In Part A of Experiment 1, sessions in w hich theS w as alternated w i th sessions in which S w asn (Figure 1). Ten sessions w ere run , five for each ofthe conditions. In Part B of Experiment 1, sessionsin w hich the S w as fl al ternated w i t h sessions inw hich S was a broken O (rightmost S in Figure1). Three sessions w ith each form w ere given toBirds 1 and 2 and on ly tw o sessions w ith each toBird 3 because its photocell became dislodged afterthese sessions.


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V I S U A L SEARCH 215

I-.6trI'UJ AUJ

.3I 2 4 8 1 6

N FORMS PRESENTEDFigure 3. Reaction times (RT) for each bird and con-dition used in the first part of Experiment 1. (Notethat the target [S+]was always O and that it ap-peared alone w h e n the number of fo rms was equal to1.)Results

Median reaction times for correct re-sponses were computed for each conditionw i th i n each subject-session, and means ofthese medians for the first part of Experi-ment 1 appear in Figure 3. (Errors are con-sidered below.) Effects of subject, fo rmtype, and number of fo rms are all evidentin the figure. The effect of number of S fo rms replicates that reported in the previ-ously published study (D. Blough, 1977b),with one new subject (Bird 3, w h ic h re-

placed the Bird 2 of that study) and di f fe r -ent S forms. As predicted, the birds foundthe target 0 more rapidly among the forms than among the seemingly more simi-lar p| forms.Note that the leftmost points in Figure 3and similar figures represent the presenta-tion of the target S+ by itself. Individualtrials for this N = 1 condition are identicalacross S conditions, and any dif ference inthe outcome of these conditions for N = 1w o uld have to be due to variability or toother trials on which S forms did appear.In fact, no s igni f icant context effect of thissort appeared.

Though monotonically increasing foreach bird, the effect of array size shown inFigure 3 is rather small. It is difficult todetermine, for example, whether fo rm andnumber interact to produce diverging func-tions or whether increasing fo rm similaritysimply adds a constant to reaction time forall arrays. Experiment IB used as one noisecondition a fo rm that differed f rom the tar-get O in the position of only one l u mi n o u spoint (Figure 1). It was hoped that such asimilar S might substantially slow downthe search process. Figure 4 shows the re-sults from these sessions.Figure 4 provides the same sort of in-

formation as does Figure 3; results f rom in-dividual sessions are also included here. Itshows that, as in Figure 3, both form and

OLJ< /> .9w J 3P.70.6

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B I R D 1 B I R D 2 B I R D 31 2 4 8 1 6 1 2 4 8 16 1 2 4 8 16TOTAL F O R M S S I M U L T A N E O U S L Y P R E S E N T E D

Figure 4. Reaction times for each bird and condition used in the second part of Experiment 1. (Thepoints represent medians of individual sessions; the lines connect means of these medians.)


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216 D O N A L D S. BL O U G H.7

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M E A N3 B I R D S ( B O T T O M ) - I

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Figure 5. Data combined across both parts of Ex-periment 1 and all birds, show ing the effect of verti-cal placement of the target in the display. (This w asth e only effect of target position that w as clearlypresent in all three birds. A reaction t ime [RT] w asincluded in the data for a "row" if the target on thattr ial w as in one of the four locations defining thatr o w , regardless of w he the r any or all of the other lo -cations in that row w ere f i l led w i th distractors.)n u m b e r of S stimuli affected reactiont ime. The curves for S = p| are similarto those for the same S in Figure 3, as onew o ul d expect for replicated conditions. T hen e w , similar S slowed search consider-ably. All 32 points representing the moresimilar condition (open circles, Figure 4)fall above the corresponding 32 points rep-resent ing the less similar condition. It isnotable that when the S+ appeared alone(N = 1) reaction time was not affected byth e noise condition on other trials in thesame session, as shown by the overlapamong the leftmost open and filled circlesfo r each bird. The separation of the tw o S curves with increasing number of forms in -dicates an interaction between S+/S similarity and the number of forms in thedisplay, the greater number producing thegreater slowing of search. (The interactionis significant, p < .01 by sign test of rankeddifferences.)T he overall level of reaction time differedless among subjects in Part B of Experi-m e n t 1 than in Part A. This change is prob-ably not attributable to the difference in S in some sessions. Shifts in overall level, ap-parently uncorrelated with experimentalcondit ions, occurred several times in theexperimental history of these birds. Such

shifts appear to be due mainly to changes inresponse topography, a matter that re-ceives fu r t h e r attention below.The target stimulus, O, appeared at eachlocation in the display matrix an equal num-ber of times in each noise condition. Thus,reaction time medians at each location maybe compared across conditions. This wasdone for the data from Experiment 1A bycombin ing the reaction time distributionsfo r each location and display-size conditionacross all sessions that used the same S fo rm . A median reaction time w as com-puted for each of these distributions. T hemedians were arranged in 4 X 4 matricesby location and were scanned for patterns.Some patterns of reaction time wererather consistent w i th i n birds regardless ofnoise condition. Figure 5 reflects a patternthat was present in all birds: Responding totargets located in the middle tw o rows ofthe display matrix was on the average 50-100 msec more rapid than responding to thetop and bottom rows. (This difference wass igni f icant by two-tailed sign test withp < .01 for each bird.) It should be recalledthat in order to present the search display,the bird pecked an observing stimulus posi-t ioned in the center of the screen, betweenthe locations occupied by the second andthird rows and the second and third col-u m n s . Horizontal location had much lesseffect, t h o u g h ' B i r d 2 did take significantlylonger to peck at forms on the righthandedge of the display than in the other col-u m n s (sign test, p < .001). The fact thatthe row effect is present even when th e tar-get was presented by itself (N = 1) suggeststhat it may simply have taken the birdlonger to move its head to the top or bottomrows. Figure 5 suggests that large arraysaccentuate the row effect, since th e meancurves spread apart somewhat from left toright. To investigate this matter, I com-pared th e difference in mean median reac-tion times between top/bottom rows andmidd le rows for each column in the N = 1displays with th e difference for the corre-sponding column in the N = 16 displays.Sign tests on these differences showed sig-nif icant accentuation of the row effect withincreasing N for Birds 1 and 2 (p < .01 for


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V I S U A L SEARCH 217each) but an equally significant effect in theopposite direction for Bird 2 (p < .01).Error data were collected for Experiment1, but these data are difficult to interpretbecause they arose from at least two dis-tinct sources. O ne source, of little interestto the present analysis, resulted from re-sponse topographies that brought thepigeon's beak close enough to a nonpeckedform to trigger the response circuit. Sucherrors could be identified only by watchingthe birds, and though their rate appeared tobe low, they clearly differed from "trueerrors" in w h ic h the peck was aimed at adistractor (S ) form rather than a target(S+). Errors increased montonical ly witharray size in Experiment 1, in accordancew ith the reaction time data: In Experiment1A for the H stimulus, mean errors acrossbirds for arrays of 2, 4, 8 and 16 s t imul iwere 3.6%, 5.1%, 6.7%, and 9.3%, respec-tively. Conclusions from this observationm u s t be tentative, however, because thespur ious errors just described would be ex-pected to increase with th e number of dis-tractors near w h ic h the beak might pass.However, errors also increased with dis-tractor similarity, and this effect is not at-tributable to spurious errors. In ExperimentI B , mean errors approximately tripled forfor each bird w i t h the change f rom the lessto the more similar distractor: Bird 1,5.3% to 19%; Bird 2, 1.6% to 4.3%; Bird3, 7.9% to 25.1%.Discussion

The results of Experiment 1 may be sum-marized as follow s. T he time required forthe pigeons to find and peck a target form(S+) increased with the number of dis-tractor forms (S ) simultaneously presentand also with increasing similarity betweenS+ and S . The independent variables in-teracted, with high similarity accentuatingthe effect of array size on reaction time.The effect of target location varied some-what across birds, but all pecked moresw i f tly at forms near the horizontal axis ofthe display than at fo rms near the top orbottom. Errors increased with array size,but this observation is ambiguous due tospuriously recorded errors. Errors unam-

biguously increased with distractor simi-larity.We may profitably compare these ob-servations with existing data from humansubjects , if w e bear in mind procedural dif-ferences and the limitations of the presentstudy, w h i c h include the l imited range ofvariables and possible confounding by prac-tice and order effects. For their review ofvisual search, Teichner and Krebs (1974)chose studies according to criteria thatlargely apply to the present experiments.These include low error rate, simple targets,display visible un t i l response, and lack ofvisual noise. U n l i k e the experiments withh u m a n s , the present study did not useforms famil iar to the subjects before theexper iment . The requirement that the tar-get be touched, and the large visual anglew e r e also u n i q u e to the pigeon study. None-theless, the present results agree rather we l lw ith the data Teichner and Krebs reviewed.Summar iz ing eight studies, they concludedthatat first, search tim e is almost unaffected as the num-ber of st imuli in the array increases: (then) aftersome critical number, search time begins to increase.The inflection point and the slope appear to be re-lated to the ease w ith w h ich th e target can be dis-criminated from it s background. The more d i f f i c u l tth e discrimination, the smaller the nu mbe r of st imuliat the inflection point and the steeper th e rise afterthe inflection point . (Teichner & Krebs, 1974, p. 18)

T he procedures of many additional hu-m an search experiments differ more mark-edly from that employed here; most oft h e m , for example, involved tachistoscopics t imulus presentation. Yet when targetsand distractors are used consistently, theresults generally agree with those describedhere . Reaction times are constant or rises low ly with array size for highly discrimin-able targets, and rise more steeply for tar-gets confusable with background distractors(e.g., Egeth, Jonides, & Wall, 1 9 7 2 ; Estes,1972 ; Schneider & Shif f r in , 1977).Despite the apparent compatibility be-tw een the results of Experiment 1 andavailable human data, the unique proce-dures of the present experiments mighthave had unexpected consequences. There-fore Experiment 1A w as replicated withthree adult human subjects. The subjects


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218 D O N A L D S . B L O U G Hw e r e exposed to the same sequence of dis-plays upon th e same screen as the pigeons.They w ere seated w ith in comfor tab le read-ing distance of the screen, and w hen theyfound the target, they touched it w i t h aphotocell mounted on a short rod. Eachsubject served for several 800-trial sessions.Their data looked remarkably similar tothose show n in Figure 4 for more similardistractors; increasing array size cause re -action tim e to rise w ith a shallow slope forthe dissimilar target and w i th a mu chsteeper slope to the similar target. Therate of reaction times covered by these func-tions w as comparable w ith those for thepigeons in Experiment IB .The results in Exper iment 1, then, giveus no ground to suspect that pigeons andh u m a n s use different strategies for search-ing these sorts of displays. Thus the pigeondata appear open to the same sorts of the-oretical interpretation that have been ap-plied to human search results. Existingmodels are too various and complex forsu m m a r y here . I t is w orth noting, though ,that neith er the prese nt results nor those ofmost human studies conform to either thesimple serial processing model or a simpleparallel processing model. Serial search im-plies hun t ing through th e visible forms oneby one. This strategy w ould generate a re-action time function r is ing l inearly w itharray size. Parallel processing of all visibleforms w ould generate reaction times invari-ant w ith array size.T he data generally are intermediate be-tw een th e serial and parallel predictions.How ever , the curves in Figure 4 containtw o suggestions that serial processing m aybe implicated in this task, at least w h e nsearch becomes especially difficult . First,th e slope of the reaction time function in -creases w ith target-distractor sim ilarity.The serial processing model predicts such anincrease, on the assumption that w h e n th eforms become more similar, the subjectmus t take longer to process each one andthat this increased t ime w ill be multipliedby the number of forms to be processed.Second, the functions for Birds 1 and 3 w i ththe most similar distractor (up per curves,Figure 4) are not far from the linear form

consistent w ith serial processing, as may beseen by rep lotting the m on a linear abscissa.Combinations of serial and parallel strate-gies as well as shifts from one to the otherare of course possible and have been sug-gested. For a recent theoretical analysis, seeSchneider and Shiffrin, 1977, and Shiffrinand Schneider, 1977.The general agreement betw een the out-come of Exper iment 1 and comparable hu-m an data is partic ularly interesting becauseth e pigeons searched a display that w as somuch larger, in te rms of visual angle, thanthat used in human research. Though un-controlled, th e view ing distance of the pi-geons seemed in fact remarkably constant .After pecking the observing spot, the birdskept the display in the ir f rontal fields (beaktow ard th e screen) and their eyes about 7cm from the screen. At this distance, thesquare 16-form m atrix subtended about 50top to bot tom. In contrast, the typical dis-play in human search studies is a fractionas large, from perhaps 2 to 10 of visualangle on its w idest dime nsion.Certain characteristics of the pigeon'svisual system may be related to its per-formance of this w ide-field search. Hodos etal. (1976) found that pigeons held theireyes abou t 6 cm f rom acuity-testing targets,in good agreement w ith th e approximately7 cm reported here. They argued that this isprobably about th e near-point of accom-mo dation for the pigeon's fronta l field. Thevisual acuities reported by Hodos et al . forth e luminances used here are adequate forth e birds to resolve the s ingle lum inou spoints that composed the stimulus forms.How ever, it is probable that the wh o lesearch display w as not equally in focus forth e birds, and this might be one reason thatsearch speed varied w ith target locus (Fig-ure 5). Human search speed is affected bytarget locus even in much smaller displays.Chaiken, Corbin, and V olkmann (1962) re -ported substantial location effects in a vis-ual field of 5, one-tenth th e diameter andone-hundredth th e area of the field used inthe present study.Here again, substantial procedural dif -ferences betw een the hum an and the pigeonexperiments make it difficult to interpret


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V I S U A L S E A R C H 219comparative data. The use of tachistoscopicpresentation by Chaiken et al., as in manyhuman search experiments, prevented eyemovements from aiding search. One mightthink that pigeon eye movements w o uld beimportant in the present study, because thedisplay remained visible unt i l pecked. H o w -ever, eye movements are quite small in thepigeon (Nye, 1969) and probably w o uld notbe m u c h help in search over large visualangles. Frequent observations of the birdssuggested that head movements also werelittle used in search. Head rotations werenot seen; indeed, the speed of most re-sponses left little time for such movements.However, w h e n reaction t i m e was u n u s u -al ly long, slight forward-backward headmotions were sometimes observed, andthese could have played some role in thesearch process. The pigeon has a relativelyuni fo rm, complex, cone-rich retina (Chard& G u n d l a c h , 1938; Galifret , 1968). This ob-servation combines w ith the search resultsto suggest that the bird may "see clearly"over a large portion of its visual field, incontrast to h u m a n s , who must search a largef ie ld by serial f ixat ions . I f true, this couldmake the pigeon a unique subject for f u r -ther comparative studies of visual atten-tion and perception.

Experiment 2: Size of Noiseand Target SetsExperiment 1 involved only manipula-tions of the S forms amid w h ic h the tar-get S+ was to f ound . One may assume thatthe form of the target itself is also a factorin search speed. Less predictable is theeffect of varying th e number of dif ferentforms that may serve as targets and thenumber of dif ferent forms that appear simul-taneously as distractors. Experiment 2 dealswith these matters of target fo rm, targetset size, and distractor set size.

MethodSubjects an d apparatus. Subjects an d apparatuswere as described above.Procedure. The procedure w as the same as thatof Experiment 1, wi th the following exceptions. O neach trial the observing peck produced a display ofeight forms, arranged in the pattern illustrated by

.5

A

z.3U2

B I R D S 1 &2B I R D 3s-' n

-S + C O N S T A N To-o s+ V A R I E S

O I 0I 0IS + STIMULUS FORMS

Figure 6. Reaction time (RT) from Part A of Experi-ment 2. (On a given trial, th e target w as either 0 orI, but in some sessions i t w as alw ays just one of these[S + constantfilled circles] and in other sessionsthe tw o appeared in random sequence [S + variesopen circles]. The distractor [S ] w as a lw ays th esame bu t varied betw een birds as show n. )the sample displays in the low er half of Figure 2.This display pattern w as chosen to minimize posi-t ional differences in errors an d reaction times. TheS+ target could be either 0, as in Experiment 1, orI (see Figure 1). The I produced a slightly greaterinput voltage, w hich w as compensated by an in-crease in the input voltage threshold required fordetection of the I.Sessions w ere organized in to 50 blocks of 16 trialseach (excluding correction trials). If both the O andthe I w ere being used in a session, each appearedonce in each location ran dom ly ordered w ith in eachblock. If only one of the targets w as in use, it ap-peared tw ice in each location w ithin each block.Seven distractors (S ) appeared on each trial, fill-ing th e remaining locations. If more than one typeof S was in use, the location of each type w as ran-domly determined.Part A of Experiment 2 employed the tw o targets,O an d I, on a constant background of either dis-tractor X (Birds 1 and 2) or f l (Bird 3). As previ-ously noted, Part A of Experiment 2 w as the first ofth e studies r un , amon g those reported here. Prior tothis expe riment, Birds 1 and 2 had a long history ofsessions w ith S+ = O . In preparation for Experi-ment 2, Bird 1 was given 10 sessions of training w ithS+ = I and 4 sessions in which O and I both ap -peared as targets. Bird 2 had tw ice this amount oftraining w ith 1: 20 sessions of S+ = 1 and 8 ofmixed I and 0. Bird 3 w a s initially trained w ithS+ = I . I t started Experiment 2 w ith only 3 train-ing sessions of S+ = O and 1 of mixed O and I.Experiment 2A consisted of 12 sessions: 3 sessionsw i t h S+ = O , 3 sessions w ith S+ = I, and 6 ses-sions w ith S+ = 0 on half of the trials and S+ = Ion the other half of the trials. Over the 12 sessions,then, O and I each appeared on 2,400 trials dur ingsessions w ith only one target, and each appeared on2,400 trials during sessions w ith each target appear-ing on a semirandom half of the trials. To put the


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220 D O N A L D S . BL OU G H1086

O ^O 2~0 1 - 4

z io oa /> 3

B I R D IC O N S T A N TV A R I A B L E

BIRD 2

B I R D 3

0 . 1 .2 .3 .4 .5 .6 .7 .8 .9REACTION TIME - SEC.

Figure 7. Distributions of reaction times for Part Aof Experiment 2. ( O n ly reaction times to the 0 tar-get are shown. Constant points [open circles] repre-sent the sum of responses on the 3 days duringw h ic hon ly the O appeared as the target. Variable points[filled circles] represent responses on O target trialsduring the 6 days in which the O and I targets wererandomly mixed.)matter di f ferent ly , th e memory set size w as 1 in halfof th e sessions and 2 in the other hal f . T he sequenceof the three kinds of sessions varied across birds.Experiment 2B used the same two targets, O and1, in combination with three distractors: , fl , X(see Figure 1). The targets w ere mixed w i th in eachsession as in the variable condition of Part A, butseparate sessions were devoted to each distractorcondition. Each bird was given sessions with eachdistractor alone, as in the lower left panel of Figure2 . T he four th type of session used a mixture of allthree distractor types: two X, two fl , and three forms were randomly scattered over the seven vacantlocations in the display, as in the lower right panelof Figure 2. Birds 1 and 3 were tested for 16 sessions,4 with each distractor type, in semirandom sequence.Bird 2 was tested for only eight sessions since thetwo sessions with each distractor condition producedvirtually identical results.Results

Figure 6 shows mean median reactiontimes for the three birds across the targetconditions of Experiment 2A . "S+ Con-stant" refers to those sessions in which onlyone of the targets appeared, "S+ Varies"to those sessions in which both 0 and I ap-peared. It is evident that this difference in

the manner of S+ presentation had noeffect on reaction time; the open and filledcircles fal l virtually on top of one anotherfor each bird and form combination. Birds 1and 2, working with the X distractor, tooksignif icantly longer to find the I than tofind the O; in all instances, individual ses-sion medians for I fell above those for O onassociated sessions. Bird 3, working with adi f fe rent distractor (p|) and less trainingthan the other birds with S+ = 0, showedno significant difference for the tw o forms.Error rates were low, f rom 1.3% to 2.8%,and did not dif fe r s igni f icant ly as a f unc t ionof target f o r m .

Reaction time distributions composed ofall responses for each condition of Experi-ment 2A are shown in Figure 7 . N o t onlywere the median reaction times for the con-stant and variable conditions the same(Figure 6), but it is evident f rom Figure 7that the fo rm of the reaction time distribu-tions was almost identical for sessions rununder the two conditions. The funct ions inFigure 7 also represent quite well the formsof the reaction time distributions that char-acterized each bird throughout the experi-ments. The reaction times of Bird 1 wereconcentrated in a single mode, w h il e thoseof Birds 2 and 3 were distributed in tw omodes, accounting for the consistentlygreater median reaction times of the lattertwo birds. These observations of individualdifferences and multimodal curves replicatethose previously reported (D. Bl o u g h ,1977b) .T he primary purpose of Experiment 2 Bwas to determine whether increasing thedistractor set size would s igni f icant ly de-crease search speed. Figure 8 shows that ifthere is such an effect , it is not p o w e r fu lenough to override other variables in thesituation. Figure 8 shows the mean acrosssessions of the median reaction times foreach condition in Part B of Experiment 2 .Only Bird 1 took longer on the average in

the "mix" condition than in any of the con-stant-distractor conditions, and even in thatbird the medians f rom individual sessions(not shown) overlapped considerably. Thislack of a clear effect from increasing dis-tractor set size is the more s igni f icant , in


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VISUAL SEARCH 221that all birds' extensive practice prior tothis experiment was with constant dis-tractors. Error scores (means of 4.9%, .6%,and 3.1% for Birds 1 , 2 , and 3 respectively)also showed no hint that the mixed condi-tion w as more difficult than the others.Discussion

A number of studies have investigatedthe effect of memory se t size on the speed ofh u m a n visual search. It is appropriate toconsider only those studies in which targetsand distractors were used consistentlythat is, a target in one session would neverbecome a distractor in another session, forexample. Schneider and Shiffrin (1977) re-view ed experiments by themselves andothers in which the subject reported th epresence of any one of several target formsthat might appear in a display. Typical datashow reaction time rising approximately asa linear function of loga of memory se t size.Schneider and Shiffrin suggested that withsufficient practice subjects m ay ultimatelyshow no effect of memory set size, but alltheir functions show at least a slight in-crease in reaction time when th e numberofpotential targets rises above 1.Here again, procedural differences, suchas tachistoscopic presentation, set humansearch studies apart from the present pigeonresearch. For this reason, Experiment 2Aw as replicated with one human subject,using the same display procedure to whichthe pigeons had been exposed and the"touch the target" response previously de-scribed. Blocks of 200 trials, either all O oral l I targets (memory set size of one), al-ternated with 400-trial blocks in w h i c heither O o r I could appear (memory se t sizeof two). For this subject, the "target con-stant" condition consistently yielded re-action times 50-100 msec shorter than the"target varies" condition. This differencedid not diminish after 3,000 trials of prac-tice.H u m a n search, then, appears usuallyto beaffected by memory set size. Search by thethree pigeons, in contrast, w as unaffectedby a change from one to two targets (Fig-ures 6 and 7). Recallingthat th e trainingofthe birds on the variable target condition

H-.cc

ou2-4

'3

B I R D 3

B I R D 2

B I R D

n - x MIXS- (DISTRACTOR) FORMSFigure 8. Reaction times (RT) from Part B of Ex-periment 2. (The two target [S+] forms, 0 or I,appeared in random order in all sessions, but the dis-tractor [S ] forms differed across sessions asindicated.)had not been very extensive, one suspectsthat practice does not account for this re-sult . Such invariance suggests parallelsearch, that is, that the birds searched fort he tw o targets simultaneously. I t wi l l beinteresting to see if memory search inpigeons proves to be parallel with largersets and in other contexts.In interpreting this seemingly interestingmemory-set-size result, however, w e shouldnot be led astray by implicit assumptionsabout th e effects of training and experi-menta l context. Such effects are often con-densed into th e notion of subject"strategy." Two alternative notions aboutthe pigeons' strategies come to mind thatlead to testable predictions. First, it is pos-sible that th e pigeons were always lookingfor either the O or the I in Experiment 2A ,even on sessions in w h i c h only one of thesetargets appeared. That is to say, perhaps asingle session with one target w as insuffic-ent to establish a presumably more efficientstrategy of lookingfor just one target. Evenw e r e this hypothesis correct, one mightexpect to see at least some use of the sup-posed single-target strategy toward theend of an 800-trial session with one target.To check this, one session from each of theS+ constant conditions was randomlychosen for each bird, and mean reactiont imes for successive 32 trial blocks were


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2 2 2 D O N A L D S. BLOUGHcomputed and graphed. No trend appeared,apart from a small "warm-up" drop in re-action time during the first block of trialsin tw o birds.A second strategy assumption leads to adifferent prediction. O ne could argue thatin Experiment 1A the birds did not need tosearch for "either an I or an O"; rather,they might always have looked for "anyform different f rom all other forms." Cate-gory membership of targets and distractors,though usually of a different sort than this,has proven a significant factor in humansearch (e.g., Jonides & Gleitman, 1972).T he results of Experiment 2 B suggest thatBirds 2 and 3, at least, did not use the"peck di f fe rent" strategy, since their searchw a s n ot slowed in the mixed distractor con-dit ion. However, since the birds could havechanged strategy in going from Experi-ment 2A to Experiment 2 B , w e clearly needdata on search with multiple targets andmixed distractors.

General DiscussionWe have seen that the present data re-flect suggestively on a number of issues, butfirm general conclusions clearly must awaitadditional data. We would particularlybenef i t from systematic data on practiceeffects, since th e influence of variables suchas array size and memory set have beenfound to decrease with practice in humansearch experiments. Schneider and Shiffrin(1977) argued that with sufficient practiceu n d e r the sort of consistent target and dis-

tractor conditions used here, human sub-jects switch from "controlled search" to"automatic detection" processes that arelargely parallel in nature and lead to con-stant reaction times. Data relating to thisissue were not presented here because thet ra in ing history of the subjects w as con-founded with methodological changes thataccompanied the development of the tech-nique . There were def ini te indications, how-ever, that array size did have a diminishingeffect as training progressed. If systematicstudy showed this to be the case, it wouldbe yet another aspect of the array-sizeparadigm in which pigeon and human datacorrespond.

Important also is f u r the r development ofthe suggestion from Experiment 2 A thatm e m o r y set size does not affect the pigeon'ssearch speed. If one accepts th e distinctionbetween controlled and automatic search,this finding suggests automatic search inthe pigeon. It may be relevant that pigeonreaction times have proven surprisinglyinvariant in other situations as w e l l (D .Blough, 1978; Heinemann, 1974). Blough(1978) found, fo r example, that pigeons ina wavelength discrimination situation re-sponded with equal speed regardless of thedifference in wavelength between a stimu-lu s and the reinforced S + .T he procedure used in the present experi-ments proved effective, but its drawbacksshould not be minimized.The most import-ant of these is probably the dif f icul ty inel iminat ing spurious "errors" w h e n , as inExper imen t 1, many closely spaced stimuliappear on the display screen. Theconfigura-tion used in Experiment 2 largely el imi-nated this problem, yet many tasks can beimagined in which densely packed stimuluselements would render a photocell sensing

system almost useless. The need for fre-q u e n t replacement of photocells and adjust-ment of the input electronics also suggeststhat the photocell system is a method ofchoice only for large displays with well-separated stimuli, in which it is importantto know the locus of the response.Large individual differences in reactiontime distributions (Figure 7) characterizedthe present data. Such differences are com-m o n , as are multimodal reaction time dis-tributions that vary in shape among birds(e.g., Heinemann, 1974). It seems likelythat the modes in such distributions aretraceable to a periodicity inherent in thepigeon's pecking motion. Despite its prob-able irrelevance to search processes, suchperiodicity could affect search data in un-for tunate ways. For example, if "t$ue searchtime" were to vary linearly with some vari-able, obtained median reaction time mightstill show a discontinuity if response emis-sion were forced into a periodic temporalpattern. Nonetheless, th e major findings ofthese experiments were rather consistentacross subjects.


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VISUAL SEA RCH 223References

Blough, D. S. Photoelectric recording pf pigeon-peckresponses to computer-driven visual displays. Be-havior Research Methods & Instrumentation, 1977,9, 259-262. (a)Blough, D. S. V isual search in the pigeon: Hunt andpeck method. Science, 1977, 196, 1013-1014. (b)Blough, D. S. Reaction times of pigeons on a w ave-length discrimination task. Journal of the Experi-mental Analysis of Behavior, 1978, 30 , 33-37.Blough, P. M. The visual acuity of the pigeon fordistant targets. Journal of the Experimental Analy-sis of Behavior, 1971, 15, 57-67.Chaiken, J. D., Corbin, H. H. , & Volkmann, J. Map-ping a field of short-time visual search. Science,1962, 138, 1327-1328.Chard, R. D ., & Gundlach, R. H. The structure ofthe eye of the homing pigeon. Journal o f Com-parative Psychology, 1938, 25, 249-272.Egeth, H ., Jonides, J., & Wall, S. P arallel processingof multielement displays. Cognitive Psychology,1972, 3, 674-698.Estes, W. K. Interactions of signal and backgroundvariables in visual processing. Perception &Psychophysics, 1972, 12 , 278-286.Galifret, Y. Les diverse aires fonctionelles de lare'tine du pigeon. Zeitschrift fu r Zellforschung,1968, 86, 535-545.Hearst, E. , & Jenkins , H. M. Sign-tracking: Thestimulus-reinforcer relation and directed action.Austin, Tex.: Psychonomic Society, 1974.

Heinemann, E. G. Mult imoda l distributions ofpigeon's reaction time. Bulletin of the Psycho-nomic Society, 1974, 3, 75-77.

Hodos, W ., L eibow itz, R. W ., & Bonbright, J. C., Jr.Near-field visual acuity of pigeons: Effects of headlocation an d st imulus luminance. Journal of theExperimental Analysis of Behavior , 1976, 25, 129-141.Hulse, S., Fow ler, H. , & Honig, W . (Eds.). Cogni-tive processes in animal behavior. Hil l sda le , N .J . :Er lbaum, 1978.Jastrow, J. Studies from the laboratory of experi-mental psychology of the U niversity of Wisconsin.American Journal of Psychology, 1892, 4, 381-428.Jonides, J . , & Glei tman, H . A conceptual categoryeffect in visual search: O as letter or as digit . Per-ception & Psychophysics, 1972, 12, 457-460.Nye, P . W. The monocular ey e movements of thepigeon. Vision Research, 1969, 9, 133-144.Riley, D. A., & Leith, C. R. Multidimensional psy-chophysics an d selective attention in animals .Psychological Bulletin, 1976, 83 , 135-160.Schneider, W ., & Shiffrin, R. M. Controlled andautomatic human information processing: I . De-tection, search, an d attention. Psychological R e-view, 1977, 84 , 1-66.Shiffrin, R. M., & Schneider, W . Controlled an dautomatic human information processing: II.Perceptual learning , autom atic attendin g, and ageneral theory. Psychological Review, 1977, 84 ,127-190.Teichner, W. H ., & Krebs, M. J. V isual search fo rsimple targets. Psychological Bulletin, 1974, 81,15-28.Received July 18, 1978Revision received March 19, 1979

211-223 effects of the number and form of stimuli on visual search in the pigeon

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