1953 - The Use of the Free Operant in the Analysis of Behavior - Ferster

8/12/2019 1953 - The Use of the Free Operant in the Analysis of Behavior - Ferster

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P S Y C H O L O G I C A L B U L L E T I NVol. 50, N o. 4, 1953

THE USE OF THE FREE OPERANT IN THEA N A L Y S I S OF B E H A V I O R1-2C H A R L E S B.FERSTER'

Harvard Universi tyT he increasing nu m be r of experi-ments in the literature using theSkinnerboxm a k e sit m o re im p o rtan tthat the methods and techniquesofthis kind of ex p e rim en tat io nbe p u b -lic. Since th e technical problemsofthe experimentsdo not bear centrallyon the analysis and would proveburdensometo the general reader, itis understandable that publishedac-counts ofthistype of research do notinclude detailed specifications of allthe techniques employed. Yet it isbecoming impossible to repeat ex-periments solely on the basis of thepublished accounts. This paper willdescribe some of the criteria andprinciples\vhich would m ake it possi-ble for the interested researcher toduplicate th e conditions of freeoperant experiments.T he Skinner box is generally con-sideredaspecial tech nique e mployingan apparatus design of the kindoriginally used by Skinner (1). A nypiece of apparatus differing in anyw ayfrom the original design has been

1T h isworkwas carried ou t underContractNSori-07631between Harvard U niversityandthe Office of N a va l Research, U. S. N a v y(Project N R143-943, R e p o r tPP-1). Directedby B . F. Skinner.2 A large number of persons have con-tributed to the techniquessummarized in thispaper so that it is impossible to establishcredit for my particular idea. Cred it for hispart in the design of much of the apparatusmentioned in this paper is due to Mr. RalphGerbrands, 96Ronald Road,A rlington, Mass.3T he autho r h as been collaborating withProfessor Skinner since 1950on a programofresearch (sponsored by the O f f i c e of N a v a lResearch) on the intermittent reinforcemento f operant behavior.

term ed a modified Skinner box. Con-traryto this view,the Skinnerbox isnota specific technique,but ratheramethod of research employing thefree operant. The use of the freeoperant is a me thod of wide generali-ity; it refers to any apparatus thatgenerates a response which takes ashort time to occur and leaves theanimal in the same place ready torespond again. The free operant isused in experiments when th e maindependent variable is the frequencyof occurrence ofbehavior. N early allth e problems of a science of behaviorf it this paradigm whenth e questionsare of the f o r m :w hat is the likelihoodof a piece of behavior occurring onthis particular occasion; how strongis the tendency to behave on thisoccasionrelativetoanothe r occasion?T he free operant has advantages inthis respect, because it removes re-strictions on the frequency withwhicha response can occurand per-mits th e observation of moment-to-moment changes in frequency (4, 5).T he behavior of approaching thefood magazine and eating, which isinsensitive to a large number ofvariables, provides one such restric-tion. Whenthe free operantis placedon an intermittent schedule of rein-forcement, the small number ofrein-forcements relative to the n u m b e rofresponses minimizes the restrictiononfrequency.W ith p roper techniques,animals can be trained so that thebehavior of approaching th e m a g a -zine and eating is under good stimu-lu s control and, for all practical pur-poses,an invariant.26 3


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TH E FREE OPE R A NT IN TH E ANALYSIS OF BEH AVIOR 26 5th e r e pe r tor yof the pigeonan d occurswith little variation in the topogra-ph y other than th e force of the peck.T he pecking response would, ofcourse, be extremely unsuitablewhere variables influencing th e formof the behavior were b eing man ip-ulated,T he occurrence outside of the ex-p e r i m e n t of the be h a vior which ismeasured is a n o t h e r factor whichmu st be con trolled by train ing pro-cedures and apparatus design. Thisis done by selecting a response wh ichisi n f r e q u e n tin the natural repertoryof th e a nim a l , by m a k ing th e m an ip -u lan d u m prominent, or by placingthe behavior under complete stimuluscontrol by proper training procedure.T he pecking response of the pigeondoesnot offer difficulty in this respectsince th e unco nditione d level ofpecking on a wall surface is almostze ro; after the pigeon has been taugh tto peck, the light behind the keyconies to control the behavior to theextentthatif the colorischanged thepeckingstops.T he design of a key for a pigeonoffers special problem s because of theextremely high rate of response thatcan be gene rated. Rates ofresponseas high as 15 per second can occurunde r appropriate schedules of rein-forcement. The advantages thataccrue from the wide range of valuesover whichth erate of peckingvariesare lost if the key does not have asufficiently high na tur a l frequency.A high n a t u r a l frequency isobtainedby m a k i n g th e a r m a tur e of the keyextremely light and the distance th earmature must travel in or de r toclose the actuatingcontact as smallas possible.Severaltypes of keys have been inusein the Harvard laboratory, noneof w h i c h are satisfactory in everyrespect. A keythat isused at present

employs a bake lite f r ame on which ism ounte d, on be a r ing surfaces, apiece of cloude d Plexiglas. Relaycontacts are m o u n t e d on the bakeliteframe on phosphor bronze with th eopposing contact on the Plexiglas.Wh e nthe key isoperated, the flexingof the phospher bronze supplies aslight wiping action w h i c h cleans thecontacts. A spring hold s the Plexi-glas armature against th e bakeliteframe an d the re sult ing contact ar-r a nge m e nt is normally closed, T hem a x i m u m frequency of operation ofthe key can be increased by increas-ing the tension of the spring. Con-versely, however, pecks will have tobe executed with greater force inorder to be recorded. T h e usua lsolution is a c om pr om ise be twe e nm a x i m u m frequency ofoperation andm i n i m u m force necessary to actuateth e switch. W here it is possible topredict th erates ofresponsethat willbe encountered,onepracticeis the useof aheavy spring on the key arma-ture where uniformly high rates ofresponse are expected, and a reducedspring tension where uniformly lownotes are expected. B ecause of themillionsoftimes the key will operate,t he key contacts must be protectedfrom heavy current loads and spark-ing. Thisisdoneby isolatingthe keyfrom th e rest of the programmingcircuitby a spark-suppressed relayofhigh enough impedance. Since th erequired duration of a peck before itcan be recorded depends on theoperate time of the k e ying r e la y, th erelay chosen must be such that am i n i m u m time elapses between th eapplication of a voltage and theoperation of the relay. T h e k e yswitch isnorm ally closed, because th etime between th e application of avoltage to the relay and the closureof its contact may be as much as 40milliseconds longer than th e time


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266 CHARLES B. FERSTERbetween th e discontinuation of thevoltage and the closure of the nor-mally closed contact. Since th e speedof operation of a relay ten s to beinversely related to its im p ed an ce, acompromise is made by choosing arelay whose impedance is low enoughto ensure fast operation yet highenough that excessive current loadsarekept from the keycontacts.A key which is ad eq u ate for m a n yapplications can be constructed byhinging a piece of Plexiglasat th e topand placing a limit switch at thebottom, behind th e Plexiglas. T h em o v em en t of the Plexiglas eithermakes or breaks th e limit switchcontact. T h e characteristics of theke y can be adjusted by changing th ethickness of the spring or the distancebetween the contactson the spring.T h e optimal size of key has notbeen explored, but a key 1 inch 'indiameter h as been found to be satis-factory. T h e pigeon reaches the keythro u g h a 1-inch ap ertu re cut in thewall. T he height of the key from th efloor depends on the size of the bird.For the homing pigeon this distancewould be | inches from the centerof th e key.Recording

T h e most straightforward record-in g arran g em en t that could be usedwould be a p o ly g rap h. This w o u l dgive a complete record from whichany k ind of com puta tion or prese nta-tion could be mad e. T h e amo u n tofwork necessary to transcribe th epolygraph record into a form thatisusable makes this kind of recordingunfeasible, however. This would bet ru e for any ex p erim en t em p lo y in g afree responding situation, but par-ticularly true for experiments withpigeons, where as hig h as 20,000responses an ho u r can be recorded.Polygraph records also result in a loss

in efficiency because th e experi-menter can have little notion of thestate of the experiment until a s u m -m ary is m a d e by laborious trans-cription. An even more seriousdifficulty is that the use of a poly-g rap h p reclu d es m an ip u lat io n s ofvariables in the m id d le of experi-mental periods.It is the practice in the Harvardlaboratory to devise recording in-stru men ts for each experiment whichs u m m a r i z e th e data in that aspectof the dependent variable which willbe used in the analysis. T h e k i n do frecordings that are taken usuallyarises after considerable exploratorywo rk. T h e record most frequentlytaken is cumulative. O t h e r typeshave been used, however, and thefollowing experiment is an example ofuse of a summary record of a n o n -cumulative type.T he latency of a response to a dis-criminative stimulus was beingstudied. A f t e r preliminary explora-tion, th e e x p e r i m e n t w as pro-grammed sothatthelight behind thekey came on once a m i n u t e . T h eresponse to the light was reinforcedand the light w as turned off . T h etime between th e onset of the dis-criminative stimulus (the light be-hind th e key) and the occurrenceofth e reinforced response w asm eas u redand an arbitrary value within thisrange w as selected. T his w as calledth e criterion time. I f the time be-tween the onset of the light and theoccurrence of the response wasgreater thanthis value, the responsewas not reinforced and the criteriont ime w as lengthened by a fixeda m o u n t . If the response occurredwithin th e criterion time, the re-sponse w as reinforced and the cri-terion time w asshortened. T h edatawere analyzed by registering thechanges in cri terion t ime. T h e paper


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TH E FREE OPERANT IN TH E ANALYSIS OF BEHAVIOR 2 6 7w as driven at a constant speed bya clock motor; th e pen, which travelsperpendicular to the direction ofthe paper drive, moved a constantdistance up the paper whenever th ecriterion time was increased andmoved a constant distance down th epaper whenever th e criterion timewas decreased. The resulting recordcould be examined at a glance andshowed concisely th e change in la-tency over the expe rime ntal period.W h e n e v e r an intermittent sched-ule of re in f o rcem en t is used, acumulative record generally provesto be themost convenient and usefulm etho d ofrecording. T h e cumulativerecord represents nonprocesseddata,in th e sense that it is drawn by thebird directly. I f a proper scale ischosen, it is possible torecover all ofthe information that would be athand if a polygraph record weretaken . I n most cases, however, thischoice of scale would vitiate the ad-vantages of the record. T h e c u m u -lativerecord is drawn as follows: Apen is stepped across the paper asmall distan ce for each response. Atthe same time the paper feeds at aconstant speed. T h e slope of the linethat isdrawn isdirectly proportionalto the rate of the response. Thevirtueof the cumulative record is notthat it allows a precise measurementof th e rate at any particular t ime,bu t rather thatit emphasizes changesin rate which can be seen in thecurvaturesof the record. Con tinuousrate changes occurring over a periodas long as several hours can be sum-marized in the rawdata o f a cu m u la-tive record. I n addition, subtlevariations such as the grain of therate show up on local curvatures.T he choice of a scale is d e p e n d e n tu p o n the range ofrates that are ex-pected. These in t u r n are d e p e n d e n tupon the species of anim al used, the

operant chosen, and the sche dule ofreinforcem ent. While the rate of re-sponse is directly proportional to thetangent of the slope over th e wholerange, it is the anglethatthetangentmakes with th e abscissa that is actu-ally observed in inspecting the localvariations in the record. T he rateofchange of the rate of response withrespect to the angle of the tangentto the curve increases very rapidlyas the angle becomes large. A t valueshigher than 80 degrees the ta ngentofth e angle increases so rapidly thatchanges inrate over short periods oftime (the fine grain of the record) areimpossible to measure. T h e scalethatis selected is therefore such that th ehighest rate ex p ected will produce aline whose angle is no more th a n 8 0degrees with the abscissa.A convenient scale for use withpigeons on interval schedulesof rein-f o r cemen t would be a paper feed ofapproximately 11 or 12 inches pe rhour and 1,000 responses for a 6-inchexcursion of the pen. For use withrats,thisscale would be much too re-duced and a better choice would beeither a slower paper feed or fewerresponses per excursion of the paper.Where both fine grain effects andover-all trend are desired, it is oftenthepracticeto use tworecorders,oneof which gives a much reducedrecord.I n order not to contaminate th erecord with eating time, th e pro-gramming equipment is arranged sothat th e paper drivestops d u r i n gth eoperation of the magazine. A m a r k e ris used on the recorder to indicate th eresponse wh ich was reinforc ed.I n discrimination experiments it isthe practice to usetandem recorders,one forSD and one for SA. Both theSD and theS4 records are in tern al lyorderly and show a degree of lawful-ness which would not be immediately


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268 C H A R L E S B. FERST ERapparent if the be h a vior unde r th etw o s t i m u l i had not been recordedseparately. I n e xpe r im e nts on c on-cept formation four recorders havebeen used in tandem. The limitingfactor in the most useful recordingarrangement, in this type of experi-ment, is the expense of the.recorder.Magazine

The magazine is the device bywhich the reinforcement of the freeoperant is instrumented. In thedesign of the magazine it must berecognized thatin conditioninga re-sponse a chain of responses is beingformed, andthatthe critical eventisthe one immediately following th eresponse. W h e n w ewish to reinforcea peck we must follow it by a prom i-nent event which is the discrimina-tive s t i m u l u sfor the first m e m b e rofa chain of responses leading finallyto the ingestionoffood. T heconcernwith the chain ofevents, whichm a in-tains th e reinforcing properties of thestimulus following th e pe c k ,issimplythat it occur with regularity anduniformity under good stimu lus con-trol.T h e types ofm a ga zinethat can beused h a v e tw o k inds of effects, afixeda m o u n t offood or a fixedperiodof access to food. While the first ofthese guarantees that the samea m ount of food will beeaten for eachreinforcement, it makes difficult themaintenance of good stimulus con-trol over behavior in respect to themagazine. T he a m ount of time th eanimal spends at the magazine willvary because there is no clear-cutstimulus correlated with the end ofth e food delivery. I n addition, it isimpossibleto k now wh e th e r the cor-rect amount of time has been sub-tracted from the records. A mo reserious difficulty isencountered w h e nth e bird leaves food in the m a g a z i ne

and finds it later, in the absenceofth e disc rim ina t ive s t im ulus for ap-proaching th e m a ga zine . Th e pa r t ia la bol i sh m e nt of the mag az i n e dis-crimination results in be h a viorwhichwill be in competition with th e peck-ing response.T h e m a ga zine wh ichpresents foodfor a fixed period of time eliminatesall the difficulties that are encoun-tered in the fixed-amount presenta-tion magazine, but suffers a disad-vantage in that the amount of foodingested for each reinforcement isnot necessarily uniform. N e v e rt h e-less, the fixed-time presentationmagazine is undoubtedly superiorfor most purposes, because th e vari-ability introduced by slight varia-tions in the a m o u n t of food perreinforcement is small in respect toth e difficulties e nc ounte r e dfromc om -peting behavior conditioned in thesame termsas the dependent variableof th ee x p e r i m e n t .T h e natural diet for the pigeon isalmost entirelygrain, and gr a in tur nsout to be asuitablereinforcings t i m u -lus, both from th e point of view of thebird 's behavior and thec on str uc t ionof a reliable magazine. T he grainused in the Harvard laboratory con-sists of a m i x t u r e of 50 per centkaffir corn, 40 per cent vetch, and10per cent hemp seed. The grainstend to be of uniform size.The magazinenow at the Harvardlaboratory works as follows:A sole-noid draws a pivoted tray into ahorizontal position where the pigeoncan reach the grain through a smallaperture; a funnel feeds grain asneeded at the rear of the tray; a6-watt lamp, mounted directly overthe aperture through which thepigeoneats, lights wheneverthe trayis raised to the eating position; wh e nth e tray is dro pp ed out of reach thelight goes out and the resulting il-


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TH E FREE OPERANT IN TH E ANALYSIS OF B E H A V IOR 269l um i na t i on is sufficiently dim sothatthe pigeon cannotsee the grain. Thei l luminated magazine serves as anSD for ap p ro ach to the m ag az in e andits termination serves as an Sd inrespect to p eckin g in the m ag az in e.A f t e r o n ly m in u tes of training th ebird m akes no a t t e m p t to reach thegrain whenthetrayis in ther e m o v e dposition and the magazine l ightis o f f .O t h e r types of m ag az in es whichmove the grain in and out of thebird'scompartment, or which coveror uncover the grain, have been usedbut suffer thedisadvantagethat thebird can be h u r t by hav in g its billcaught in the moving part of them a ga z i ne . Wh ile this will occur onlyrarely beforethe bird learns to avoidbeing cau g htby the movingpart,theresulting timidity in respect to them ag az in e will m ake the m ag az in etraining much more difficult and in-t ro d u ce a variable into the experi-

ment whichcan beeasily eliminatedby the design of the i n s t r u m e n t .The Exper imenta l Ch a mbe r

The sizeof the chamber in whichthe bird works is to a large extentarbitrary,so longas the magazine ispositioned in the n eig hb o rho o d of them a n i p u l a n d u m . Limitationsinspace,however, usually requirethatthe ap-paratus be kept as small in size aspossible. A suitable size for a birdwould be approximately 15X 12X 12inches. W h en the b i rd co m p artm en tisthis size, artificialventilationis al-most always required.Sound insulation is aknotty prob-le m whose solution depends upon th etasteof the experimenter and the re-q u irem en tsof the p art icu lar p ro b lem .The physicsof sound absorption in-evitably requ ires insulation by mate-rials of considerable weight an dthickness. This solution is both veryexpensive and space-consuming. For

most applications the degree of soundproofing that is afforded by a well-constructed picnic icebox will pro-vide data w h i c h are unaffect ed byran d o m noises that occur in the ex-perim ental room. Crit ical eventswhich can form th e basis of a dis-crimination, however, must be silent .Since these events are nearly alwaysth e operation or the release of a re-lay, the problem can be solved mosteasily by u s in g hig h im p e d an ce,light-weight relays that are shockm o u n t e d . T he low in ten s i ty of theseclicks, together with th e a m o u n t ofs o u n d insulation applied by a goodquality picnic icebox, will m ake im -possible the f o r m a t i o n of the dis-crimination based on a relay opera-tion. In ad d i t io nto these precautionsit is the practice of the Harvardlaboratory to supply in each ex p eri -mental chambera masking noise con-sisting of a ran d o m s p ectru m of

sound. This type of noise is mosteffective in m as kin g clicks. Fornearly all applications, there is noneed to el im in ate th e recorder noises.The click which the recorder make severy t ime a response is made servesas a conditioned reinforcer for theresponse and help s m a in tain a stableform of theresponse.Control EquipmentBecau s ethe responsethatischosenin the freeoperant situation actuatesa switch every time it occurs, it isnearly always possible to instrumentan e x p e r i m e n t so that it runs auto-matically. Once the programmingeq u ip m en t f o r an ex p erim en t hasbeen devised it requires very littleattendance by a psychologist, andessentially unskilled personnel canrun the ex p erim en t . Ap p aratu s d o esdouble duty, since it can run alln ig ht wi tho u tany atten dan ce. N ig htexperiments are often an efficient


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7 C H A R L E S B. F E R STE Rsolution where i t is decided to m i n i -mize outside sou nd disturbances. T h el imit ing f act o r in the n u m b e r of ex-p er i men t s that can be carr ied outunder condit ions of au t o mat i c p r o -g r a m m i n g is the i n g e n u i t y of thepsychologist , the amount of moneyavailable fo r programming devices,and co mp et en t t echn i calassistants.N i gh t p r o g ra m m i n go fex p er i men t srequires some method of t er mi n at i n gthe experiment in the absence of theexperimenter . T h e problem is easilyhandled in the case of the pigeon be-cause th e pigeon roosts in the d a r k .To t er mi n at e the e x p e r i m e n t , it isnecessary only to d i s co n n ect the re-i nfor ci ng circui t an d t u r n out all ofthe l ightsin the ap p ar at u s . Wh en anocturnal animal is used, however,external events are correlated witht he t er m i n at io n o f t he e x p er i me n tand a d i s cr i mi n at i o n isf o r m e d .There are no simple rules or pro-cedures for the automatic program-m i n g of experiments. T h e program isaccomplished almost exclusively byth e relay an d t iming devices, a n dthere is no subst i tute fo r generalr e l ay kn o w -ho w or t echn i cal assist-ance. Facilityfordesigningrelay cir-cuits for use in p r o g r a m m i n g e x p e r i-mentscomes with a small amountofpractice.4

Care mustbetakenin the designofrelay circuits so that the length ofpulse necessary for the operation ofth e relays in the recording circui t isnot larger than the length of pulsenecessary for the operation of therelays in the reinforcing circuit. I fthe o perate t im e of the reinforcingcircuit isshorter than the recordingcircuit , the force or durat ion of the4 An e x ce l l e n ttext on general principleso frelay circui t design is avai lable in T h eDesign

Swi tching Circui t s , by W. K ei st er , A . E .R i t c hi e , and S. H. W a s h b u r n , D. Van N o s -trand, 1951.

pecks will be dif ferentia ted so thatl ar g e n u mb er s of p ecks will occurwhich are of sufficient force tooperate the m a g a z i n e , but not ca-pable of operat ing th e r eco r d er . O nesolut ion of this problem is to r e q u i r ea larger durat ion and i n t en s i t y ofpecks for the reinforcing circui t thanis r eq u i r ed for the r eco r d i n g c i r cu i t .I n this way the m e a n force or d u r a-tion of pecks that occur is increased,and the l ikel ihood of nonrecordedpecks is correspondingly decreased.

T H E P I G E O N A S A N E X P E R I M E N T A LA N I M A LThe pigeon has advantages overthe rat as an e x p e r i m e n t a l a n i m a lthat j u s t i f y the use of a new animalin spite of a considerable body ofl i terature already deal ing with therat.The pigeon lives as long as I Syears. This makes possible long-term experiments, into w h i c h thematurat ion of the rat would intro-duce radical change s. A variable in-terval schedule, for example, whichtook 30 or 40 days to reach asteadystate, would result in an e x p e r i m e n twhich wouldencompassanimportantfraction of the life span of the rat.Considerableeconomy isachievedbyusing pigeons sequentially in experi-

ments. Forexample,if an experimenton some aspects of s ched u l es of r e i n -forcement resulted finally in stablebehavior which can serve as a baseline, the birdscould bereused in anent i rely different ex p er i men t w hi chrequired abase lineofthatsort.The pigeon has very good visualacuityand color vision. Thisleavesgreat l a t i t u d e and variety in thekinds of stimuli that can be used ind i s c r i m i n a t i o n ex p er i men t s , a nd t h en u mb er of different s t imuli that canbe used within a single experiment . contrast, the rat's very poor


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TH E FREE OPERANT IN TH E ANALYSIS OF BEHAVIOR 271vision offers serious difficulty in thedesign ofe xperiments analyz ing com-plex processes.T h e pigeon comes to the experi-menter with a well-tailored responseextremely suitable for free operanttype ex p er imen tat io n . T h e highratesofpeckingthatcan be generatedin the pigeon result in a dependentvariable which can change over avery wide range, and which is neces-sarily more sensitive to manipula-tion.

T H E P R E P A R A T I O N O F B I R D S F O RA N E X P E R I M E N TT he birds should be banded assoon as they are received from th esupplier and theirwings clipped alongthe second run of feathers so thatt h e y cannot fly. They are fed freelyfor three or four days, after whichtime the ad lib. weightistaken. T h i s

is designated as the normal weight.After this th e birds are not fed againunti l they reach approximately 80 percent oftheir ad lib. weigh t. T h i s willtake about on e week if no food isgiven. A n alternative practice, how-ever, is to feed 5grams of grain everyother day. Wh ile this will make thedeprivation period longer, the disrup-tion of the bird's digestive system willnot be sosevere. T he degreetowhichany particular animal's weight mustbe reduced can be ascertained only byattending to behavior. A given per-centage of reduction in body weightwill not have th e same behavioraleffect on each animal. Therefore, th eweight should be reduced until thea nima l will eat out of the magazinew i t h o u t hesitation. If the bird doesnot eat out of the open magazinewithin 15 minutes after being placedin the apparatus, it should be re-turned to the loft and no t fed foranother day.

After th e bird eats freely and hashad its daily ration on two separatedays (I S grams each day, for thetypical bird), the magazine trainingproper can be started. T he task a th a n d at this point is to establish th emagazine light and sound as the oc-casion on which approach to themagazine will occur with high prob-abil i ty, and to extinguish responsesto the magazine in the absence of themagazine light and sound. T o a c-complish this the magazine is op-erated approximately every 30 sec-onds. T h e bird will approach th emagazine in the absence of the light,and this behavior will soon ex-tinguish. At this stage of its trainingth e bird should be watched carefullyto avoid the development of anysuperstitious behavior (3). If anysuperstitious behavior develops, itcan be eliminated simply by w i t h -holding th e magazine operation untilsome other behavior is in evidence.W h e n th e bird moves rapidly towardth e magazine as soon as it is op-erated, seldom approaches it w h e n itisn ot operated, andeatsc o n t i n u o u s l yt h r o u g h o u t th e operation, th e maga-zine training is complete and thepecking response can be differ-entiated.

T h e pecking response can be differ-entiated by any one ofthree methods.In all three methods the key is con-nected sothat its operation presentsth e mag az in e via a switch ing circuit .I n this way, when the first peck oc-curs th e discriminative stimulus forapproaching th e magazine will beimmediately contingent on the re-sponse.1. The first method depends on theunconditioned level of responding.T h e bird is simp ly left in the ap-paratus with the key connected toth e magazine. T he f irs t key peckthat occurs will almost result in con-


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27 2 C H A R L E S B. FERST ERdi t i oni ngif the m a ga z i ne t ra i ni ng h asbeen effective.2. T h e second method uses a dis-c r i m i na t i on already in the bird'srepertory. A small grain is scotch-taped to the k e y. The bird pecks atth e grain an d i nc i de nt a l l y operatesth e m a ga z i ne . W h e re removal of thegrain disturbs the bi rd , it can be vanished by replacingit w i t h pro-gressively smaller grains.3. T h e t h i rd m e t h od is the differ-entiation of the response. This isdoneby observingthe birdand select-ing some behavior which approxi-mates the peck. O f t e n the firststepis o p e n i n g th e m a g a z i n e w h e n th ebi rd is facing th e key. T h e p r o c e d u r ef r o m this point is one of alternatecondit ioning and extinct ion of vari-ous responses, which progressivelya p p r o x i m a t e k y peck ing. As soon asth e bird is facing the key r e g u l a r l y ,re i nforc e m e nt s are w i t h h e l d u n t i lsome variation occurs in the be h a vi orwhich is in the direct ion of the re-quired response. I f r e i n f o r c e m e n t sare w i t h h e l d too long and the birdextinguishes, the process is begunagain.It is very i m p o r t a n t that theoperat ion of the mag azine be im-mediately contingent on the specifiedbeh avior. If the reaction time of thee x p e r i m e n t e r is too long, th e maga-zine operat ion may strengthen otherb e h a v i o r thanwas i n t e n d e d .Wi t h a sm a l l a m ount of practiceand a h ungry, w e l l m a ga z i ne - t ra i ne dbi rd , it sh oul d take no m o r e thanfivem i n u te s to shapeup a pe c k i ngresponse. I f t h e re is difficulty inshaping up th e pe c k i ng re sponse , th ebird is p ro b ab ly n o t h u n g r y e n o u g h .T h e h u n g e r of the bird cannot bej u d g e d from th e fact that th e birdeats readily out of the open maga-zine, since this b e h a v i o r is relativelyinsensitive to changes in the levelof

deprivation. T he f r e q u e n c y of oc-currence of the kind of response thatis being condit ioned is a m uc h m oresensitive indicant,and the percentageof body w e i gh t re duc t i on that isfinally assigned to the bi rd is bestd e t e r m i n e d by l o o k i n gat the generala m o u n t of activity. If the bird isi na c t ive , c ond i t i oni ngwill b e diff icultand the bird shouldeither ber e d u c e din weight or h a bi t ua t e d to the ap-paratus. A suitable level of depriva-tion is more easi ly determined a f t e ranintermittent sc h e dul e of re inforce-ment isestablished . T h e s u b j e c t sc anbe equated in t e rm s of level of de-privation b y a djust i ng t h e i r bodyweight sothat the rates of responseu n d e r a variable interval scheduleo fre i nforc e m e nt are e qua l .

The ini t ial condit ioning may becarried out ei ther in the cage thatwill be used in the e x p e r i m e n t or ina special ly prepared condit ioningap-paratus w h i c h affords a good viewofth e bi rd . Wh i c h one is use d de pe ndson th e c ond i t i oni ng proc e dure an dth e type of experimental cage. N oparticularadvantageisgainedby theuse of an open cage if thec o n d i t i o n i n gis carried out by the u n c o n d i t i o n e drate or grain ofcorn m e t h o d . I fth e differentiat ion proc e dure i s use d,h ow e ve r , an open cage is necessary,unless the e xpe r i m e nt a l c a ge has aw i ndow w h i c h will allow a clear viewof the bird whenthe cageis at a levelas h i g h as or h i g h e r than th e experi-m e nt e r . T h e bi rds will be se r i ousl ydisturbed i f the experime nter looksdow n on t h e m .I f th e pe ckin g response is c ondi -tioned by d i f f e r e n t i a t i o n , some d i f f i -culty m ay result from th e accidentalco n d i t i o n i n g of unna t ura l pe c k i ngtopographies or supe rst i t i ous be-havior. This is not l ikely to be thecase w h e re the c ondi t i oni ng is ac-complished by the use of a k e r n e lo f


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TH E FREE OPERANT IN TH E ANALYSIS OF BEHAVIOR 7grain or the unconditioned rate ofpecking. Wh ere an unu sual topog-r a p h y does occur, several sessionsofcontinuous reinforcement in the ex-perimental cage will almost alwaysresult in a uniform, natural topog-rap hy .T he length of an experimental ses-sion is limited only by the n u m b e ro f re i n fo rcemen t sthat are given. T hetotal amount of food that is given issuch that the bird's weight will beth e same th e next day. If the birddoes not receive enough food duringth e experiment to bring its weight upto the value determined as the nor-mal w ei g ht , th e difference is fed atthe end of the session. If the birdgains weight, th e n u m b e r of rein-forcements given each session mustbe reduced. About 6 0 reinforc em entswill usually maintain th e weight of abird if a 3-second presentation ofgrain defines a reinforcement.

T HE D E S I G N OF E X P E R I M E N T ST h e r e are no rules for the designof ex p eri men t s on the free operantotherthan those ofaccepted scientificpractice. I nterm ittent reinfo rcem entof th e free operant, how ever, allowsmany experiments to be designedaro u n d base lines generated by aschedule of reinforcem ent. T wo

examples will clarify how the inter-mittent reinforcement of a responsesupplies a base line for judgi ng th ealteration of behavior due to an ex-perimental manipulation.A m o u n t of Reinforcementthe effect

o fvariations in the size of a rein-forcement on the tendency to be-have.T h e pigeonistrained on a variable-interval schedule of reinforcementwhich generates a constant rate ofresponse. A n ari thmetic-intervalschedule of reinforcement produces a

constant rate of response after ap-proximately IS h o u r s of training. I nth e mi d d l eof an experimental periodth e amount of reinforcementiseitherdecreased or increased. I n this wayth e effect of the indep end ent variableon the behavior is uncontaminatedby anypassageoftime or handlingofth e animal. T h e experiment is thencontinued u nt il the beh avior is stableunder the new procedure, and the re-versibility of the process is checkedby re t u rn i n g to the original amountof reinforce me nt. If i t is found thatthe process is not reversible, th eprocedure of al ternating th e a m o u n to f reinforcementis continued in somecounterbalanced order. This allowsan analysis of the dynamic effect ofchanges in the a m o u n toffood in eachreinforcement. If the behavior underdifferent amounts of reinforcement isreversible,th e procedureisrepeated asufficient n u m b e r of times to estab-lish its significance. T he number ofreversals that are needed dependsupon th e size of the effect and thelawfulness of the change in the be-havior from one amount of reinforce-m e n t to another. T h e parameter isexplored over th e complete range in asingle animal; and the experiment isrepeated upon a sufficient n u m b e rofanimals to convinceth e experimenterthat th e effects of the man i p u l at i o nare large inrespect to the variationdue to individual differences. Rela-tively few animals are necessary iftherecords showthatthe dependentvariable changes in the same wayfrom subject to s u b ject , and differsonly in terms of aconstant, such asover-all level of responding. Suffi-cient control is possible for use ofgroups as small as two animals, al-though a group of three is moreefficient statistically. Fu rth er refine-m e n t of the experiment is necessaryif th e subjects differ in the dependent


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74 CHARLES B. FERSTERvariable in both th e constant effectand the nature of the change.Discriminat ion

T h e de ve lopm e nt of a discrimina-tion between two colors will serve asanother example of the use of ani n t e r m i t t e n t s c h e d u l e of reinforce-m e nt as a base line. Afterth e peckingresponse is differentiated, th e bird isgiven several sessions of c o n t i n u o u sr e i n f o r cemen t and the color of thel ight changes af t er each reinforce-me nt. Re inforcem ent then occursequally in the presence of the twocolors on a variable-interval sche duleof r e i n f o r c e m e n t , and it is not pre-dic ta ble wh e th e r th e color will re-main th e same or change after eachreinforcement. Separate recordersoperating in tandem record the be-h a vior unde r the two colors. W henth e rate of response under the twocolors is the same, reinforcements arediscontinued in the presence of one ofth e colors. T he intervals of thepresentation of the color, however,

remain defined by the previous re -inforcement schedule. As the rateofresponse in the presence of the n o n -reinforced color (SA) declines, th er a te unde r th e reinforced color (SD)does not change. I t provides a baseline for j udging th e effects of vari-ables such as level ofde pr iva t ion.Despite th e alternation of the re-inforced and nonreinforced stimuli ,th e decline inrate in SA is extremelyorderly. M oreover, afte r two or threeh o u r s of training th e r a te unde r th enonreinforced color will be less than1/100 th e r a te unde r th e reinforcedcolor. Such an analysis ofdisc r im ina -tion problems permits th e experi-m e n t e r to determine over very shortintervals of time th e tendencies tobehave in the presence of bothstimuli. T he use of an interm ittentschedule of reinforcement as a baseline for the analysis of the develop-m e n t of stimulus control is easilyextended to complex processes, suchas concept fo rm atio n, with similaradvantages.

REFERENCES1. S K I N N E R B . F. On the conditions of elici-tation of certain eating reflexes. Proc.nai Acad.Sci., 1930, 16,433-438.2. S K I N N E R , B. F . The beliavior of organ i sms.

N ew Y ork: D . Ap pleto n-C entu ry, 1938.3. S K I N N E R , B. F. Superstition in thepigeon. J. exp. Psychol 1948,38, 168-172.

4. S K I N N E R B . F. Are t heori es of l e ar n i n gnecessary? Psychol. Rev. 1950, 57, 193-216.5. S K I N N E R ,B. F. Somecontributionsof anexperimental analysis ofbehavior to psy-chologyas awhole. Amer Psychologist,19S3, 8,69-78.

Received June 19, 1952.

1953 - The Use of the Free Operant in the Analysis of Behavior - Ferster

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