VOL. PSYCHOLOGICAL REVIEW...Most learning theorists have at-tempted to reduce the outcomes of these two experimental procedures to a common underlying learning prin-ciple. Such attempts

VOL. 74, No. 3 MAY 1967

PSYCHOLOGICAL REVIEWTWO-PROCESS LEARNING THEORY:

RELATIONSHIPS BETWEEN PAVLOVIAN CONDITIONINGAND INSTRUMENTAL LEARNING 1

ROBERT A. RESCORLA * AND RICHARD L. SOLOMON "

University of Pennsylvania

The history of 2-process learning theory is described, and the logicaland empirical validity of its major postulates is examined. The as-sumption of 2 acquisition processes requires the demonstration of an em-pirical interaction between 2 types of reinforcement contingencies and(a) response classes, (b) reinforcing stimulus classes, or (c) charac-teristics of the learned behavior itself. The mediation postulates of2-process theory which argue that CRs are intimately involved in thecontrol of instrumental responding are emphasized, and 2 majorlines of evidence that stem uniquely from these postulates are ex-amined : (a) the concurrent development and maintenance of instru-mental responses and conditioned reflexes, and (b) the interaction be-tween separately conducted Pavlovian conditioning contingencies andinstrumental training contingencies in the control of instrumentalbehavior. The evidence from concurrent measurement studies pro-vides, at the very best, only weak support for the mediational hypothe-ses of 2-process theory. In contrast, the evidence from interactionstudies shows the strong mediating control of instrumental responsesby Pavlovian conditioning procedures, and demonstrates the sur-prising power of Pavlovian concepts in predicting the outcomes ofmany kinds of interaction experiments.

The procedures which the experi- lovian, or classical, conditioning experi-menter (£) carries out in the Pav- ment are quite different from those he

. . . . . . . carries out in a Thorndikian, or in-1 The research leading to this paper was . . . . ' _supported by Grants MH-04202 from the strumental, training experiment. InUnited States Public Health Service and the Pavlovian conditioning experiment,GB-2428 from the National Science Foun- £ ideally has full control over all ex-da^onl r 11. • j r A • perimental events; he determines theParts of this paper were delivered in a *. 'Harold Schlosberg Memorial Symposium time of occurrence and the duration ofaddress, Eastern Psychological Association, a trial without any regard to the ani-Atlantic City April 1965 This paper is mal's behavior. That is, E arrangesdedicated to the memory of Harold Schlos- . . . , .. . .?,berg relations between stimulus events which

2 National Science Foundation PredoctoralFellow. Now at Yale University. Irwin, and Vincent M. LoLordo for their

3 The authors would like to thank Otello extensive comments on an earlier draft ofL. Desiderate, Henry Gleitman, Francis W. this paper.

151

152 ROBERT A. RESCORLA AND RICHARD L. SOLOMON

he controls. In contrast, in the Thorn-dikian training experiment, E only ar-ranges it such that the animal's be-havior at specified times will yieldpredetermined environmental changes;E arranges relations between the ani-mal's behavior and future stimulusevents.

Because the laws of learning arestated as interrelationships between ex-perimental operations and consequentbehavioral changes, the laws of condi-tioning and those of learning must bedifferent at a descriptive level. Thiswould be so even though the behavioralchanges were identical in the two cases.On the other hand, if some theoreticalsystem could be developed to unify thedifferent empirical laws, to reduce themto the same general underlying prin-ciples, then the laws of behavioral mod-ification would deduce the outcomes ofboth types of experiment. In an im-portant sense, the history of learningtheories is a succession of attempts tospecify the relation between the out-comes of the two types of experiment.

Most learning theorists have at-tempted to reduce the outcomes ofthese two experimental procedures toa common underlying learning prin-ciple. Such attempts led to a periodof vigorous experimenting, in order tosee which of several competing theoriesof learning would survive the data ofboth conditioning and training experi-ments. Pavlov's (1932), Hull's(1943), Tolman's (1932) and Guth-rie's (1935) theories are well enoughknown in the history of learning theoryto excuse us from detailed discussionof them. Suffice to say, these single-process theories challenged each otherduring the period 1930-1950. Then,in recent years, interest in the all-en-compassing, "single, sovereign prin-ciple" theories declined as Es more andmore explained their findings in termsof limited, more specific "miniature

models." Thus, for example, theoriesof the partial reinforcement effect havemultiplied and have been refined to ac-count for a single phenomenon ratherthan all learning phenomena. In thesame way, theories of extinction haveproliferated, until now there are atleast seven distinct accounts of the phe-nomenon. Grand theory testing, in thesense of seeking a crucial experimentaltest of whole theoretical systems, hasclearly subsided.

In contrast, two-process learningtheory has persisted as a systematic in-fluence since 1928, and interest in ithas increased rather than decreased.Lacking the elegance and simplicity ofa postulated, single learning processand the parsimony of a single reinforce-ment principle, as well as the prose-lytizing influence of a vigorous "schoolof thought," two-process learningtheory nevertheless has been a majorheuristic tool in the stimulation of newconditioning and training experiments.

HISTORYHow did two-process learning theory

arise, and what stages of development has itundergone? Two Polish investigators, Ko-norski and Miller, stated in 1928 that thefacts of Pavlovian conditioning and Thorn-dikian learning required the postulation oftwo underlying associative processes for ade-quate explanation; the facts of one could notbe explained by the inferred processes of theother (Miller & Konorski, 1928). They dis-tinguished between responses yielding richsensory feedback and those yielding little orno sensory feedback. They assumed thatPavlovian conditioned reflexes yield poorsensory or proprioceptive feedback; but, incontrast, Thorndikian response learning in-volves extensive and intricate feedback mech-anisms. The associative processes operatingfor responses with poor feedback were postu-lated to be those of an S-S nature: the link-ing of afferent processes set up by the con-ditioned stimulus (CS) and the unconditionedstimulus (US). In contrast, for responsesyielding rich feedback, the feedback itselfwas postulated to be a part of the associativeprocess, and subjects (Ss) learn an S-Rrelationship only insofar as the feedback

TWO-PROCESS LEARNING THEORY 153

from R is distinctive and powerful. Therewas, however, no postulation of a law ofeffect for such feedback-rich responses(Miller & Konorski, 1928).

Thorndike (1932) believed that Pavlovianconditioned reflexes do not reflect the gen-eral laws of ordinary trial-and-error learn-ing. Perhaps, he speculated, Pavlovian con-ditioned responses (CRs) are a special caseof some subclass of learning? Writing inthis vein in the early 1930s, Thorndike had,however, no alternative to his own law ofeffect to propose in order to account forPavlovian phenomena. Relegating such phe-nomena to a limited subclass did not solvethe theoretical problem, and he candidlysaid so.

Skinner (1935) arrived at a two-experi-ment and two-response classification, empha-sizing that the operations of R differbetween the Pavlovian and Thorndikian ex-periment. He proposed that there are twotypes of conditioned reflex, Type S andType R, and two types of response, the oper-ant and respondent. However, Skinner didnot postulate two separate associative proc-esses to explain the two sets of conditioningfacts, nor did he infer two distinctly dif-ferent theoretical reinforcement processes forthem. His distinction between the respond-ent and the operant paralleled, but was notidentical to, Miller and Konorski's (1928)distinction between poor and elaboratedfeedback.

Konorski and Miller felt that Skinner'sdistinction was superficial. In 1937, theyargued that Skinner had not completelyspecified all of the differences between thetwo types of conditioned reflexes, and theypointed out that Skinner's operant condi-tioned reflex ". . . is confined exclusively tostriped muscles, while the classical type hasno restrictions laid on effectors and includesamong them, besides striped muscles, smoothmuscles and glands [Konorski & Miller,1937b, p. 271]." They further argued that:"Being a glandular reaction, salivation can-not by any means be made a conditioned re-action of the new type [p. 271]." Konorskiand Miller were thus speculating that re-spondents cannot be brought under the con-trol of the law of effect.

It is one matter to argue that temporalcontingency between CS and US onsets issufficient for conditioning, or that the re-sponse-reward temporal contingency is suf-ficient for instrumental learning. Here, how-ever, Konorski and Miller were arguing fora strong two-process law: that the con-tingency sufficient for law of effect learning

cannot reinforce a CS-CR relationship ofthe Pavlovian type. Though the conversewas not stated at the time, we might supposethat Konorski and Miller believed it to beso (i.e., instrumental responses cannot beconditioned by a Pavlovian procedure).Their emphasis was on Pavlovian condition-ing, and, indeed, they anticipated Mowrer(1947) in thinking that Pavlovian CRsmight strongly influence instrumental re-sponding; however, they did not have inmind a concept like that of mediation.

The first explicit statement of a completetwo-process learning theory came in 1937, ina paper by Schlosberg entitled, "The Rela-tionship between Success and the Laws ofConditioning." This paper was the basis formost of the more recent elaborations oftwo-process learning theory. In it, Schlos-berg distinguished between (a) the experi-menter operations of Pavlovian conditioningand Thorndikian training, (6) the postulatedassociative processes set up by the two dif-ferent procedures, and (c) the theoreticalreinforcement mechanisms appropriate forthe two processes. He argued that the em-pirical laws of Pavlovian conditioning im-plied that the associations formed are thosebetween stimulus-related or contiguous per-ceptual processes, and that the reinforcementmechanism is brought into play by the initi-ation of a US. He further claimed that theempirical laws of Pavlovian conditioning arethe laws of conditioning of diffuse, prepara-tory responses of an emotional type. In con-trast, he stated that the empirical laws ofThorndikian learning imply that the majorassociative process is that linking stimulusand precise, adaptive, motor response, andthat the reinforcement process is that of"success" or an improvement of the hedonicstate of the S.

Actually, even though Schlosberg wasaware of Konorski and Miller's two-responseidea, Skinner's operant and respondent, andType S and Type R conditioning procedures,he was much more influenced by W. J.Brogden's 1936 APA paper than he wasby the published papers of Konorski andMiller and of Skinner. Brogden, at thattime, reported on the work later to be in-corporated into the article by Brogden, Lip-man, and Culler (1938), the widely citedwork showing that omission of shock couldreinforce running by guinea pigs in a run-ning wheel, in contrast to the poor and un-reliable running obtained with a Pavlovian,inevitable presentation of the shock. Brog-den suggested that the underlying reinforcingmechanism for Pavlovian conditioning might

154 ROBERT A. RESCORI.A AND RICHARD L. SOLOMON

be different from that for Thorndikian learn-ing. He contrasted forepaw conditioning,relatively successful with a Pavlovian pro-cedure, with the conditioning of running,relatively unsuccessful with a Pavlovian pro-cedure, and speculated about possible theo-retical reasons for such a discrepancy.

Schlosberg developed a novel theoreticalexplanation for the discrepancy. He pointedout that some diffuse motor responses arecomposed of reflexes within an emotionalitypattern. These are conditionable only byPavlovian methods. If one tries to makeuse of such motor responses as operants, onewill not be able to train them as such be-cause the law of effect will not "work" withsuch reflexes. Pavlov's laws will. (Konor-ski & Miller, 1937a, 1937b, at about the sametime, made the same claim about visceral,glandular reflexes.)

Schlosberg's (1937) clearly stated two-process theory was listened to, digested, andacknowledged. Yet it did not effectivelyenter into the arena of the theory-testinggiants, those single-process theories strugglingagainst each other during the 1930s and1940s. There was no two-process "school"to match the Hullian, Yale-Iowa group, orthe Tolmanian group at Berkeley, or theGuthrie group at the University of Wash-ington. Instead, due note of the reasonable-ness of the two-process idea appeared hereand there, in sporadic fashion. For example,Hilgard and Marquis (1940) in their influ-ential book, Conditioning and Learning, dis-tinguished between classical and instrumentalconditioning, preferring to use the term"conditioning" for both the Pavlovian andThorndikian experiment. Maier and Schni-erla (1942) espoused a distinction betweentwo acquisition processes, but preferred toclassify the Pavlovian processes as those forperceptual reorganization and the Thorn-dikian processes as those for biologicallyadaptive behavior. Later Tuttle (1946), ina rarely cited paper, argued for the existenceof associative conditioning and law of effectlearning as two completely distinct proc-esses. Birch and Bitterman (1949) notedthe usefulness of the two-process distinctions.

Mowrer (1947) published what wasat the time the longest, most tightlyreasoned, and most persuasive argu-ment for two-process theory. He ad-ded precision to Schlosberg's (1937)specifications, refined the theoreticalrelationships between inferred proc-

esses and experimental operations, anddeveloped the conception of CRs asmotivational mediators of instrumentalresponding. Mowrer argued that thelaws of Pavlovian conditioning are ap-plicable only to visceral responses. TheThorndikian law of effect applies onlyto the training of skeletal motor re-sponses. The specific controlling re-lationship for the establishment of con-ditioned emotional reactions is the tem-poral contiguity between CS onset andUS onset. (US termination conditionsare irrelevant to this process.) Mowrercalled this process the problem-posingprocess. Previously neutral environ-mental events come to have the condi-tioned power to evoke visceral re-sponses. These visceral responses cre-ate emotional and motivational ten-sions which then must be resolved byproblem-solving behavior. The prob-lem-solving is done by skeletal motorresponses alone, as reinforced by drive-reduction (in Mowrer's terms, reduc-tion of visceral tension-states). There-fore, conditioned reflexes are powerfulmediators of instrumental responses.Mowrer argued that, in signalizedavoidance experiments, the CS and USare paired on early trials, and this es-tablished CS anxiety, a type of condi-tioned fear. The anxiety has theproperties earlier assigned to it byMiller (1941); that is, it is both aresponse and an acquired drive. Asa response, it is a pattern of condi-tioned visceral reactions. The driveproperties come from response-pro-duced feedback stimulation arising fromvisceral reactions. Therefore, if CRscan produce drive, they can strongly in-fluence instrumental responses. Themore intense the conditioned anxiety,the more vigorous are avoidance re-sponses, and the shorter are their la-tencies in the presence of fear-produc-ing CSs. The more drive-arousing theCS, the more reinforcing would be a


response that terminated the CS.Avoidance responses are reinforced byanxiety-reduction. Thus did Mowrerprovide both a motivational and a re-inforcement principle for avoidancelearning. Although Mowrer believedthat the mediational principles wouldapply to appetitive CRs and rewardlearning, he did not develop this idea,perhaps because at that time the rele-vant observations were missing.

Because Mowrer's mediational hy-potheses were so important in the de-velopment of and testing of two-proc-ess theory, we shall emphasize the ex-periments relevant to them. Slightmodifications and expansions weremade by other investigators, but themajor ideas remained, and they servedas the basis for a large number of ex-periments appearing in the 1950s, ex-periments which presented grave prob-lems for all of the single-processtheories (see Solomon & Brush, 1956).

Then some notable desertions fromsingle-process learning theory oc-curred. Tolman (1949) argued thatthere might be as many as six kindsof learning, each with its own rein-forcement principle. Spence (1956)acknowledged the possibility that theremight be two reinforcement processes,one for instrumental learning and onefor classical conditioning. However,Spence turned things around. Hetentatively suggested that if he were toadopt a two-process theory, he wouldargue that classical conditioning is ahabit acquired by reinforcement of anappetitional or aversive sort, whereasinstrumental learning is acquired bycontiguity principles without a specificreinforcing event. Spence's importantidea from our particular point of viewwas the postulation of a mediationalrelationship between the Pavlovian,conditioned rg and instrumental re-sponding. This mediational process isthe appetitive counterpart of Mowrer's

description of aversively motivated be-havior.

The manifestations of two-processlearning theory have recently becomedifficult to follow. On the one hand,we have Spence's view of two-processtheory, arguing that Pavlovian condi-tioning is reinforced by the law of ef-fect. On the other hand, heavily in-fluenced by Schlosberg (1937), Solo-mon and Wynne (1954) extendedMowrer's (1947) postulates to coverthe conditioning of skeletal-motor re-flexes by Pavlovian processes, ratherthan confining Pavlovian conditioningto visceral responses. In other re-spects, Solomon and Wynne (1954)and Solomon and Brush (1956) haveheld faithfully to the Schlosberg-Mow-rer concepts, and their experimentshave been guided by these concepts.Yet, in contrast, Mowrer himself hassteadily abandoned his original two-process conception of learning. In themost recent statement of his position,Mowrer (1960) has proposed a learn-ing theory which employs only one un-derlying learning process with two ma-jor types of reinforcing event. Al-though Mowrer continues to call thisa "two-factor" theory, it clearly doesnot involve two learning processes inthe same sense as do previous theories.This latest version of Mowrer's theoryhas not yet been widely used to gen-erate new types of experimentation.In contrast, the number of experimentsinstigated by the Schlosberg-Mowrertype of two-process theory has in-creased steadily. It is this latter typeof two-process theory that we shalldiscuss in detail.

THE SCOPE OF THIS PAPER

In this paper we analyze the twomajor questions posed by various two-process learning theories: (a) Arethere two acquisition processes, a con-ditioning process and a learning proc-


ess, each with its own set of distinctlaws? (b) Does the conditioningprocess serve a mediating function inthe control of instrumentally learnedresponses ?

Our main emphasis will be upon thesecond, mediational, question. In ex-amining the mediation role of condi-tioning processes in the control of in-strumentally learned responses, we willdiscuss two major research strategies:(a) The first is the concomitant mea-surement of Pavlovian CRs and in-strumental responses during the courseof acquisition and extinction of instru-mental behavior. The question of in-terest here is to what degree the twoclasses of behavior are correlated, (b)The interaction of independently es-tablished Pavlovian CSs with instru-mental behavior and the effectivenessof such CSs in controlling that be-havior. In assessing both of theseresearch strategies we concentrate uponthe role of Pavlovian conditioningprocesses in evoking instrumental be-havior. The establishment of rein-forcers of behavior by Pavlovian proc-esses is beyond the scope of this paper.

Although the mediational question isour main concern, it seems appropriateto deal first with the logically priorquestion of whether there are reallytwo kinds of learning. It is obviouslynot possible to deal with this vastlycomplex question in all of its ramifica-tions ; however, in the next section wedo attempt to lay out a logical frame-work within which the question can beanswered.

ARE THERE Two ACQUISITIONPROCESSES ?

A variety of two-process theorieshave been mentioned in the previoussection. In attempting to specify thedomain of the two processes, eachtheory has made a number of logicaland empirical assertions. This sec-

tion attempts a classification of thedistinctions drawn by two-process the-ories and attempts to specify the kindsof evidence that may be taken as sup-porting the proposition that there aretwo acquisition processes. We willnot attempt an exhaustive review ofthe relevant literature; rather we wishto make explicit the logical structureof the evidence that would be relevantto the two-process proposition.

All two-process theories emphasizethe basic operational difference betweenthe Thorndikian and Pavlovian experi-ment. In the former, E's presentationof the reinforcer is dependent upon theorganism's behavior, but in the latterit is independent of that behavior. InPavlovian conditioning the reinforce-ment is made contingent upon the oc-currence of a stimulus; in instrumentaltraining it is made contingent upon theoccurrence of an arbitrarily selectedresponse.

In general, this operational distinc-tion has not been thought sufficient tojustify by itself the assertion of twodifferent learning processes. Theoristshave felt that only if the differencebetween response-contingent (instru-mental) and stimulus-contingent (Pav-lovian) reinforcement has importantimplications for the way in which be-havior is modified would we want toidentify these two operations with dif-ferent underlying processes. There-fore, the assertion of two separatelearning processes has rested upon anassumed interaction between reinforce-ment contingency and other variablesin producing behavior change. In par-ticular, two-process theories havepointed to three sets of such variables(a) response class, (&) reinforcementclass, and (c) characteristics of theproducts of learning. The claim isthat the class of responses affected, theeffective reinforcers, and the resultsof learning, all depend upon whether


response- or stimulus-contingencies areemployed.

Response Distinctions

Theorists have tried to separate thoseresponses subject to modification bystimulus- and response-contingenciesin a variety of different ways. Someof the proposed response distinctionshave been: (a) ANS (visceral orglandular) responses as contrastedwith somatic (skeletal) responses(Mowrer, 1947); ( b ) operant (emit-ted) responses as contrasted with re-spondent (elicited) responses (Skin-ner, 1938); (c~) voluntary responsesversus involuntary responses (Schlos-berg, 1937); (d) so-called "light-weight" responses as contrasted with"heavy-weight" responses (Miller &Konorski, 1928; Osgood, 1953); (<?)diffuse, emotional responses as againstprecise, adaptive responses (Schlos-berg, 1937); and (f) responses highin reflexiveness as contrasted withthose low in reflexiveness (Turner &Solomon, 1962),

We do not wish to review in detailthe evidence for and against the abilityof each of these distinctions to discrimi-nate between responses subject to mod-ification by the two reinforcement con-tingencies. Instead, we will take as anexample the autonomic-skeletal dis-tinction to illustrate the logic and prob-lems of testing the propositions of two-process theory. The autonomic-skeletaldistinction has the advantage of beingthe easiest distinction to make withprecision.

The assertion central to some ver-sions of two-process theory is thatskeletal responses are subject to in-strumental reinforcement contingenciesbut not to Pavlovian reinforcementcontingencies, while autonomic re-sponses are only subject to Pavloviancontingencies. It is worth pointing outthat the autonomic-skeletal distinction

is typical of all response-class distinc-tions in relating response classes andreinforcement contingencies in a one-to-one manner. This particular formof interaction between response classand reinforcement contingency is, how-ever, not a logical requirement of thetwo-process approach. The separationof response classes would be no lessimportant if, for example, severalclasses of responses were subject tomodification by one reinforcement con-tingency while only one class containedresponses affected also by the othercontingency.

Thus the question of theoretical in-terest is whether any autonomic re-sponses are subject to instrumentaltraining procedures and whether anyskeletal responses are subject to modi-fication by Pavlovian conditioning pro-cedures. If both of these possibilitiesoccur, we cannot rely on the interactionof reinforcement contingency with theautonomic-skeletal distinction to justifythe theoretical separation of the effectsof stimulus- and response-contingentreinforcement.

At least three skeletal responses havebeen successfully brought under thecontrol of Pavlovian procedures.Schlosberg (1928) found patellar re-flex conditioning in humans; a numberof investigators have demonstratedconditioned paw flexion in dogs (e.g.,Konorski & Szwejsowska, 1956); andhuman eyelid conditioning has becomea standard procedure for the investiga-tion of acquisition processes. How-ever, in trying to interpret these re-sults we meet a problem typical ofattempts to bring under Pavloviancontrol responses in any class sup-posedly not subject to Pavlovian con-ditioning. We must be able to assureourselves that unwanted response-con-tingencies are not producing the results.There is a common way in which suchcontingencies often enter; occurrence


of the CR may influence the effect ofthe US, thus converting presumedPavlovian to actual instrumental con-tingencies. For example, the inevitableoccurrence of the shock US to a dog'spaw may be less aversive when thepaw is in a flexed position. Schlos-berg (1937) was the first to discussthis kind of possibility in detail. Thetypical way of dealing with this prob-lem is to try to arrange a situation inwhich such an argument seems, at acommon-sense level, implausible. How-ever, an alternative procedure would beto give 51 the choice (following condi-tioning) between (a) presenting him-self with the CS (and thus the CR)followed by the US or (&) presentinghimself with the US alone (or fol-lowed by the CS). Presumably, ifthe role of the CR in altering the ef-fect of the US is important, a clearpreference would be demonstrated. Asimilar design has been suggested forthis purpose by Wagner (1966) whoused it to detect instrumental rein-forcement in cortical conditioning ex-periments. Without this kind of evi-dence we must reserve judgment onsupposed demonstrations of the Pavlo-vian conditioning of skeletal responses.

There is also evidence suggestingthat autonomic responses can come un-der response-contingent control. Al-though Mowrer (1938) and Skinner(1938) reported failure to train au-tonomic responses instrumentally, morerecent investigators have reported suc-cess. Fowler and Kimmel (1962),Kimmel and Kimmel (1963) and Cri-der, Shapiro, and Tursky (1966) haveall reported successful instrumentaltraining of the galvanic skin response(GSR). Lisina (reported by Razran,1961) trained vasodilation as an escaperesponse, and Shearn (1962) obtainedsuggestive evidence that heart ratechanges can be used as avoidance re-sponses. Although Sheffield (1965)

found the salivary response insensitiveto instrumental contingencies, usingfood as the reinforcer, Miller and Car-mona (1967) were able to reinforcesalivary responses in thirsty dogs whenwater was used as the reinforcer.

The interpretative problem thatarises in this sort of experiment con-cerns the need to rule out mediatingoperants. We must be sure that someunnoticed skeletal response is not beinglearned and is not directly producingthe observed autonomic changes. Themost effective argument against thispossibility would be the successful rep-lication of these experiments while 5"is immobilized by curare agents. Thistype of experiment has recently beenreported. Trowill (1967) and Millerand DiCara (1967) produced heart-rate changes in curarized rats usingpositive brain stimulation as the in-strumental reinforcer. Birk, Crider,Shapiro, and Tursky (1966) partiallycurarized a human S and were able toproduce GSR changes using instru-mental avoidance contingencies. Andyet there is the disturbing possibilitythat even the use of curare agents maynot permit us to rule out operant medi-ators. Curare only precludes periph-eral skeletal mediators and allows cen-tral responses to occur. Thus, evenwhile paralyzed, a human can think ofemotional events which will reflexlyproduce peripheral respondent events.It is not at all clear whether such"thoughts," or brain events which areclearly subject to response-contingentreinforcement, should be considered tobe "skeletal" or not. Possibly thedistinction is better made between typesof brain events than between types ofperipheral nervous system association.

These brief comments give an ideaof the logic and problems involved intesting the assertion that the auto-nomic-skeletal distinction is identicalwith the distinction between responses


subject to modification by stimulus-and response-contingencies.

The kinds of experiments and prob-lems generated by other response-classdistinctions are similar. The most fre-quently mentioned of these other dis-tinctions is the operant-respondent dis-tinction. According to Skinner (1938,pp. 20, 21) "behavior that is correlatedwith specific eliciting stimuli may becalled respondent behavior . . ." (elici-ted behavior) and an operant is identi-fied by the fact that "no correlatedstimulus can be detected upon occasionswhen it is observed to occur" (emittedbehavior). A two-process position thenasserts that respondents are subjectonly to stimulus-contingencies andoperants can only be taught by re-sponse-contingent reinforcement. Onecan then attempt to test these notions,as in the case of the autonomic-skeletaldistinction.

The operant-respondent distinctionraises special problems of its own.Although, for many common responses,there is no practical difficulty in identi-fying which are operants and which arerespondents, there are, unfortunately,cases where this is extremely difficult.Many responses seem at times to beoperants and at other times to be re-spondents. This observation suggestedto Turner and Solomon (1962) thatwe examine a continuous dimension,which they called "reflexiveness," onwhich operants and respondents arelocated. This modification of two-process theory claims that the rela-tive effectiveness of response- and stim-ulus-contingent reinforcement wouldvary along this response dimension,each contingency being maximally ef-fective at one end; it has yet to receiveany extensive empirical analysis.

The two remaining, commonly made,response-class distinctions are very dif-ficult to make empirically. Schlosbergsuggested that responses subject to

stimulus-contingencies were "prepara-tory-diffuse" responses while those af-fected by response-contingencies were"precise-adaptive" responses. Theother response-class distinction is thatbetween voluntary and involuntary be-havior. Reliable ways of distinguish-ing between these latter types of be-havior are few. One suspects thatattempts to classify responses intovoluntary and involuntary are not en-tirely independent of reinforcement-contingency distinctions. Thus, themost reasonable objective criterion ofwhether or not a response is voluntarymay be just whether or not it is subjectto modification by response-contingentreinforcement contingencies. If a re-sponse-contingent reinforcement pro-cedure will not modify a response, it isinvoluntary.

Although these different categoriza-tions of response are far from identical,they all seem to be attempting to em-body the idea that behavior subject tomodification through instrumental con-tingencies is somehow "freer," morevaried, and "adaptive," while the re-sponses which are conditionable byPavlovian procedures are more "rigid,"more "specialized," and more auto-matic or "reflexive." In general, manyresults support this correlation be-tween response-class and reinforcementcontingency. However, considerablymore analytic experiments are neededbefore a precise statement about thenature of the response-class distinctioninvolved can be made.

Reinforcement Class

Instead of asking whether the classof responses subject to modificationvaries with type of reinforcement con-tingency, one can ask whether theevents which serve as reinforcers differwhen response- and stimulus-contin-gent delivery of reinforcement are used.Clearly, reinforcers for Pavlovian con-


ditioning experiments are closely re-lated to reinforcers for instrumentaltraining. For example, food servesboth as a US for the conditioning ofsalivation through stimulus-contingentreinforcement and as a reward for thetraining of bar-pressing through re-sponse-contingent reinforcement. Butare there reinforcers which will func-tion only in conjunction with one orthe other contingency?

Two-process theories have generatedseveral theoretical attempts to specifydifferences between reinforcementclasses. Both Schlosberg (1937) andMowrer (1947) have argued that thereinforcement event for instrumentaltraining must have some affectivecharacter but, in contrast, the simplecontingency of CS and US is sufficientfor Pavlovian conditioning. In theinstrumental case, the reinforcer mustbe pleasant or unpleasant, whereas forPavlovian conditioning it is sufficientthat the reinforcer regularly elicit theunconditioned response (UR). Butsuch a characterization is probably notprecise enough to be helpful. Even ourintuitive notions of affect seem strainedby such instrumental reinforcers aslight onset (Kiernan, 1964) or the op-portunity to run in a running wheel(Hundt & Premack, 1963). It isequally easy, in the case of almost allPavlovian reinforcers, to become con-vinced that they produce a modicum ofaffect.

An alternative specification of the dif-ference between instrumental and Pav-lovian reinforcers has been suggestedby Mowrer (1947) : Instrumental rein-forcers are drive-reducers, whereasPavlovian reinforcers do not neces-sarily reduce drives and, indeed, mayeven increase drive level. A typicalexample of the latter is Pavlovian fearconditioning, for which there is nowexcellent evidence (Mowrer & Aiken,1954; Mowrer & Solomon, 1954; Over-

mier, 1966) that the effective US isshock onset, a drive-increasing stimu-lus. Unfortunately, there is no guar-antee that all instrumental reinforcersare drive-reducing; indeed, many ex-amples of reinforcers (brain stimula-tion, light onset, novelty, etc.) strainthis notion. Furthermore, such drive-inducers as shock onset are often in-strumental reinforcers—albeit negativeones (punishers). Thus the notion ofdrive-reduction does not seem helpfulin separating Pavlovian and instru-mental reinforcers.

Perhaps a more fruitful, if morelimited, approach is simply to examineempirically the degree to which thetwo classes of reinforcers overlap."Reinforcer" is used here in both thepositive and negative sense, that is,punishers are instrumental reinforcers,and Pavlovian CRs involving reduc-tion in behavioral output are treated inthe same way as those involving in-crement. Thus, in order to demon-strate that a given stimulus reinforceswhen used with one contingency, anddoes not do so when used with theother contingency, it is not sufficient toshow that it has incremental effects inone case and decremental effects in theother. We here require that a stimu-lus have no effects when used with onecontingency or the other. Of course,it is necessary to employ appropriatecontrol procedures in making this as-sessment. A particular stimulus mayhave nonassociative effects upon a re-sponse which are not dependent uponthe particular contingency with whichit is used. Thus, to show that a givenstimulus is an effective reinforcer instimulus-contingent presentation, it isnecessary to demonstrate, through ap-propriate control procedures, that thechanges which it produces depend uponthe contingency arranged. The prob-lem of control procedures for Pavlo-vian conditioning has been discussed


in detail by Rescorla (1967b). Simi-larly, to demonstrate that a stimulus isa reinforcer when used in a response-contingent fashion requires suitablecontrol procedures.

It seems likely, as has been impliedby the various theoretical attempts toseparate the two kinds of reinforcers,that the class of Pavlovian reinforcersis larger than that of instrumental re-inforcers, and in fact includes as asubclass the set of events which serveas instrumental reinforcers. With thisin mind, we may ask whether there isany stimulus event which will rein-force behavior when made contingentupon prior stimulus presentation butnot when made contingent upon a re-sponse. Unfortunately, this question,though basic to the two-process ap-proach, seems to have received rela-tively little direct experimental atten-tion ; but there are a few hints avail-able. One of the earliest USs to beused in Pavlovian conditioning was atap on the patellar tendon; there is con-siderable evidence that this is an ade-quate stimulus to establish a Pavlovianconditioned knee-jerk (e.g., Schlos-berg, 1928). Yet, if the US is ad-ministered properly, 5"s report being"neutral" toward it. It would be ofconsiderable interest to see whetherthis patellar tap could be used to re-inforce instrumental behavior in a situ-ation comparable to that in which itconditions the knee-jerk. A second,promising source of "pure" Pavlovianreinforcers is the class of interoceptiveUSs described by Bykov (1957).Many of the internal USs used to pro-duce Pavlovian conditioning wouldmost likely go completely unnoticedin an instrumental training situation.However, we do not yet know whetheror not such internal USs can serve asinstrumental reinforcers. This wouldcertainly be an important type of in-vestigation to pursue.

Another example comes closer to ful-filling our experimental requirements.Doty and Giurgea (1961) have re-cently provided considerable evidencethat direct stimulation of the motorcortex will serve as a US for limb-flexion conditioning. Yet, when thesame US is made contingent upon on-going operant behavior, in many casesit produces no change in that behavior.Although one would like similar dem-onstrations with a large number of op-erants, the Doty and Giurgea findingsindicate that this type of brain stimu-lation is indeed a Pavlovian reinforcerwith no instrumental rewarding or pun-ishing properties. Likewise, Malmo(1965) has reported a few cases ofseptal stimulation which serve as USsfor heart-rate conditioning in rats butwhich will not maintain operant bar-pressing.

However, recent evidence presentedby Wagner (1966) indicates the pres-ence of instrumental reinforcement insuch experiments. Paw flexion wasconditioned in dogs, using a motorcenter brain stimulation as US. Whenthe CS was presented, the dogs ap-peared to be positioning themselves insuch a way as to modify the effect ofthe US. When the dogs were latergiven a choice between signaled andunsignaled presentations of the US,they chose the signaled US. This in-dicates that this type of experimentmay not be a pure case of Pavlovianconditioning.

Finally, experiments directed towardexamination of sensory precondition-ing (which fits the Pavlovian, stimuluscontingency paradigm) provide somesupport for the separation of Pavlo-vian and instrumental reinforcers. Un-fortunately, such experiments havefailed to include direct evidence that theneutral "US" used for the sensory pre-conditioning is not also an instrumen-tal reinforcer. Furthermore, the dem-


onstration that sensory preconditioninghas actually occurred is often less thanconvincing.

We can only conclude that the evi-dence on the overlap of Pavlovian andinstrumental reinforcers is scanty. De-spite the fact that most Pavlovian re-inforcers seem also to be instrumentalrewards or punishments, the evidencedoes imply that the overlap is not com-plete. To the degree that Pavlovianand Thorndikian reinforcers are dif-ferent, a two-process theory receivesstrong support. Clearly, this is one ofthe most exciting areas of researchsuggested by a two-process theory oflearning, and considerable work re-mains to be done.

Characteristics of the Learned Behavior

It is often thought that the product of thelearning process differs across stimulus- andresponse-reinforcement contingencies. Sev-eral attempts have been made to specify howthe result of learning is different in thePavlovian and Thorndikian experiments.

iS1-.? versus S-R connections. The firstdistinction is theoretical. Both Schlosberg(1937) and Maier and Schneirla (1942) be-lieved that Pavlovian conditioning proce-dures established S-S connections; at thesame time they suggested that instrumentallearning consists of acquired S-R bonds.Other authors have made similar claims indescribing conditioning as stimulus-substi-tution and instrumental learning as response-substitution (Hilgard & Marquis, 1940).Both of these distinctions arise as directconsequences of the contingencies of rein-forcement which E arranges; however, theyimply that a difference beyond that of ex-perimental manipulation is involved.

Two types of experiment have beenthought to bear on these theoretical distinc-tions. First, a number of investigators haveshown that Pavlovian conditioning is pos-sible even when peripheral responding hasbeen prevented. Salivary conditioning ispossible when salivation is blocked byatropine (Crisler, 1930; Finch, 1938) andPavlovian fear conditioning occurs while 5is paralyzed by curare (Solomon & Turner,1962). To the degree that the S-R bondis conceived to require peripheral skeletalresponding for its establishment, the S-S

alternative is favored for Pavlovian con-ditioning.

The second line of evidence stems fromthe use of direct motor-cortex stimulation asa US for Pavlovian conditioning. Loucks'(1935) failure to obtain conditioning usingsuch a US was taken as evidence that Pav-lovian conditioning involves S-S connec-tions. However, Brogden and Gantt (1937)obtained conditioning with direct stimulationof the cerebellum as the US. And Doty andGiurgea (1961) were able to obtain Pavlo-vian conditioning with electrical stimulationof the motor-cortex as the US. If the S-Sand S-R notions are given physiological in-terpretation as sensory- and motor-cortexconnections, this result suggests that Pav-lovian conditioning may not be S-S innature. We can conclude that the presentevidence does not support a sharp distinc-tion between Pavlovian conditioning andinstrumental training in terms of hypotheti-cal S-S and S-R connections.

Similarity of CR and UR. Greater simi-larity of the CR to the UR is often men-tioned as setting Pavlovian conditioningapart from instrumental learning. However,the CR is by no means identical with theUR, even for Pavlovian conditioning; in-deed, many have suggested that the CR ispreparatory for the US or that it is a frac-tional part of the UR. But there is a grosssimilarity of the CR and UR in Pavlovianconditioning, at least to the extent that theyusually involve the same response system.This is in general not true in instrumentaltraining situations, where, for example, theresponse may be bar-pressing, which bearsno fixed relation to the UR, ingestion of afood pellet.

It is possible that the more valuable dis-tinction here rests in the relation of the CRto the US. Skinner has pointed out that inPavlovian conditioning, once the US isselected, E is no longer free to select theCR at will (except in the trivial sense thathe chooses to ignore parts of the behaviorpattern). In instrumental training, selec-tion of the US does not uniquely determinethe CR which is acquired; E is free to selectarbitrarily the response he will reinforce.Thus, it may be that the apparent CR-URrelationship results from the added con-straint which selection of the US places uponPavlovian conditioning but not upon in-strumental training. The US may uniquelydetermine both the UR and the PavlovianCR, even though the learned and unlearnedresponses are quite different.


Sensitivity to parametric variations. Fi-nally, the Pavlovian CR may differ from theinstrumentally trained response in its sensi-tivity to a variety of parametric variations.In general, as Kimble (1961) points out,there is a striking resemblance in the reac-tion of the two kinds of learning to suchvariables as amount of reinforcement, delayof reinforcement, etc. However, Kimblesuggests one possible difference. Instru-mentally trained responses consistently showgreater resistance to extinction following par-tial reinforcement; this may not be the casefor Pavlovian CRs. The evidence on thispoint is far from clear-cut. However, a sharpdifference in the sensitivity of Pavlovian con-ditioned responses and instrumental behaviorto such a parametric variation would givestrong support to the distinctions of two-process theories. The investigation of suchparameters seems to us to be a fruitful areafor future research.

Conclusion

We have argued that the basic opera-tional distinction between response-and stimulus-contingent reinforcementmay interact with various other vari-ables in such a way as to justify theclaim that two independent processesare acting. In general, the results rele-vant to such interactions are still in-adequate. Our attempt therefore hasbeen not so much to marshal all theevidence in support of such interactionsas to point out the kinds of evidencewhich would be relevant. The ques-tions we have raised here have oftennot received explicit experimental at-tention, although to our minds theseare basic questions in the study of be-havior modification.

EXPERIMENTAL APPROACHES GEN-ERATED BY TWO-PROCESS THEORY

In addition to asserting the existenceof two independent acquisition proc-esses, two-process theories have postu-lated interrelationships between thetwo processes in the control of be-havior. They usually assert that Pav-lovian CRs serve as mediators of in-strumental behavior, functioning as

either instigators or reinforcers. Suchassumptions have given rise to two re-search strategies: (a) concurrent meas-urement of the development and main-tenance of conditioned reflexes andinstrumental responses within instru-mental learning situations; and (b)testing the interaction between sepa-rately conducted Pavlovian condition-ing contingencies and instrumentalcontingencies in the control of instru-mental behavior.

It should be mentioned at the outsetthat the claim that instrumental be-havior is mediated by Pavlovian CRsis by no means a unitary theoreticalidea. As is pointed out below, differ-ent theorists have emphasized differentaspects of the mediational process. Buttheir ideas also differ on the preciserole that CRs play in mediating in-strumental behavior. Some proposethat the observed CR itself, or sensoryfeedback from it, is an event whichelicits and/or reinforces a particularinstrumental act. In this case, the re-search strategy is to seek out and varythose particular CRs which one sus-pects are mediating the instrumentalbehavior. As we will conclude below,this approach does not seem to havebeen a fruitful one, because the at-tempts to find specific mediating CRshave been generally without success.A more viable claim is that operantbehavior is mediated by a complex ofCRs, both autonomic and skeletal; noone of these may be necessary for oper-ant behavior, but each contributes tothat behavior. This position suggestsan extension of the concurrent meas-urement research strategy to the studyof more complex CRs.

Still another position is that the ob-served Pavlovian CRs are not them-selves mediators of instrumental be-havior but rather are merely an indexof a central nervous system state whichdoes mediate that behavior. This posi-

164 ROBERT A. RESCORI.A AND RJCI IARD L. SOLOMON

tion leads to a research strategy re-viewed in the final section of this paper.

It is often difficult to tell which ofthese positions an author intends whenhe describes the mediation of instru-mental behavior by Pavlovian CRs, soit is well to keep these distinctions inmind.

Concurrent Measurement of PavlovianConditioning and Instrumental

LearningAny instrumental training situation has

within it the conditions favoring the de-velopment of Pavlovian conditioned re-sponses. In the discriminated operant para-digm there is a regular sequence of Sa andthe reinforcement; in nondiscriminated op-erant behavior, feedback from various re-sponses leading to reward is also regularlyfollowed by reward. To the degree thatthe stimulus event maintaining the operantbehavior is also a Pavlovian reinforcer, wewould expect the development of PavlovianCRs in addition to the acquisition of in-strumental behavior. Given that food isboth a Pavlovian and an instrumental re-inforcer, we would expect that its use inan instrumental training situation would alsolead to the development of Pavlovian CRssuch as salivation, cardiac changes, licking,swallowing, etc.

However, two-process theories make thestill stronger assertion that these PavlovianCRs are somehow crucial to the maintenanceof instrumental behavior. Two differentmediational roles have been assigned toPavlovian CRs. Some authors have em-phasized their motivational role. This con-ception seems to have originated, at leastfor aversively motivated behavior, with Mil-ler (1948), who postulated that emotionalreactions become associated with previouslyneutral stimuli by the action of drive reduc-tion. The emotional reactions give rise toimmediate sensory feedback, having both cueand drive properties. Thus the "acquireddrive state" is a complex of emotional re-actions and then correlated perceptual events.Such mediators have been given either orboth of two properties, motivational or re-inforcing. The two-process theories ofSpence (19S6) and Mowrer (1947) havelikewise emphasized the motivating functionof conditioned reflexes, arguing that suchCRs as salivation or cardiac change mayreflect the level of motivation or incentive

in instrumental training situations. Condi-tioned responses are assigned the role of in-stigators (or indexes of instigators) of in-strumental behavior. These theories, there-fore, predict a close correspondence betweenthe occurrence or nonoccurrence of Pavlo-vian CRs and the magnitude or probabilityof specific instrumental responses.

On the other hand, Konorski (1948),Soltysik (1963), and Mowrer (I960) haveemphasized the rewarding functions of me-diating respondents. Thus the reduction ofheart-rate or the increase in salivary flowmay be thought to reflect a state which isinstrumentally rewarding. For these theo-ries the important changes in conditionedreflexes reflect instrumental reinforcementand thus occur following instrumental be-havior. Again, a close relation between con-ditioned reflexes and learned instrumentalbehavior is predicted.

The instigating and rewarding functions ofconditioned reflexes in maintaining instru-mental behavior are by no means incompati-ble. For instance, conditioned cardiac ac-celeration may reflect the motivation for anavoidance response while cardiac decelera-tion following the response may reflect areward. This is roughly the picture of avoid-ance behavior which Mowrer (1947) drew.For appetitive behavior, it is not clearwhether salivation should be treated as re-flecting a motivating or a rewarding state;in the former case, we might expect saliva-tion to precede the operant (Spence, 1956),while in the latter it should follow the oper-ant (Konorski, 1948).

The dual role which two-process theoriesassign to conditioning processes leads natu-rally to an examination of the sequence ofCRs and instrumental responses in instru-mental training situations. One researchstrategy for studying these temporal se-quences is to allow the normal instrumentalsequence to be established while takingsimultaneous measures of various CRs andoperants.

The degree to which the various theoriesare bound by these predictions depends uponthe precise role which the theory assigns tomediating CRs. In the preceding few para-graphs we have purposely been vague onthis role, using such phrases as "the CR re-flects a motivational state." It is clear thata theory which claims that a particular CRor complex of CRs is itself mediating theinstrumental behavior predicts a closer cor-respondence between the CR and instru-mental behavior than does a theory whichassigns to the CR the role of indexing a


central mediator. Thus the experiments tobe described below are particularly relevantto the view that the CR itself (or its associ-ated feedback) is mediating the instrumentalbehavior.

Appetitive Behavior

It is probably no accident that thefirst experiment using the concurrentmeasurement technique was performedby Konorski and Miller (1930), whowere the first to propose a two-processtheory. They trained a dog to lift hispaw when a signal sounded, in order toobtain food. They found a close rela-tion between the occurrence of the pawmovement and the magnitude of condi-tioned salivation. But for them the im-portant finding was that the operantconsistently preceded increased salivaryflow. Working in Konorski's labora-tory some years later, Wolf (1963)found similar results using a fixed ratio(FR) schedule of reinforcement.These findings have been substantiatedby similar results of Williams (1965)for FR, and of Shapiro (1961) andKintsch and Witte (1962) for fixedinterval (FI) performance and ex-tinction. In addition, Kintsch andWitte found that the characteristic FIscallop developed prior to a similartemporal discrimination in the sali-vary response.

These studies suggest that, at leastunder some circumstances, conditionedsalivation does not provide the essen-tial motivating state which instigatesoperant behavior. Rather, they sup-port the notion that operant behaviorprecedes (and possibly serves as a CSfor) conditioned salivation. The ob-served temporal sequence of events isthat predicted by Konorski's (1948)notion that salivation indexes a state ofexcitation which serves to reinforceinstrumental behavior.

But results inconsistent with thisconclusion have also been obtained byShapiro (1962), who trained dogs to

obtain food by pressing a panel on adifferential reinforcement of low rates(DRL) schedule. On this schedule,bursts of salivation regularly precededthe operant, even though the occurrenceof the operant itself generally led to afurther increment in salivation. Evi-dently the temporal sequence of CRsand instrumental behavior is not fixed,but depends upon the relations whichthe E arranges between instrumentalresponse and reinforcement. A dra-matic demonstration of this dependenceis provided by Ellison and Konorski(1964). They trained dogs to panel-press on an FR, the completion ofwhich initiated an 8-second waitingperiod at the end of which food wasdelivered. Using this technique, panel-pressing and salivation were kept al-most completely separate temporally,the first occurring only during the FRrequirement and the second only fol-lowing its completion.

It thus appears that although saliva-tion and operant behavior may bear agross relation to each other in typicalinstrumental training situations, thedetails of this relation are not con-stant. Salivation consistently repre-sents neither Spence's rg nor Konor-ski's alimentary excitation; salivationmust neither precede nor consistentlyfollow operant behavior in order forthat behavior to be maintained.

Similar conclusions are in order forother "mediating" CRs. Both Soltysik(1960) and Wenzel (1961) have founda gross temporal correspondence be-tween cardiac acceleration and per-formance on a motor response rein-forced by food. Further, Soltysikfound that cardiac acceleration oc-curred prior to the motor behavior,suggesting that it may be involved ininstigating that behavior. But Wenzelfound, upon administration of reserpineto her cats, that the heart-rate responsewas markedly reduced with no effect

166 ROBERT A. RESCORI.A AND R I C H A R D L. SOLOMON

upon operant behavior. We need fur-ther research, detailing the relation be-tween food-motivated behavior andconditioned cardiac changes; but atthe present time the evidence does notsuggest that cardiac changes are neces-sary mediators for operant behavior.

Another possible CR, licking, hasbeen studied by Miller and DeBold(1965). Using a discriminated bar-press operant reinforced by intraoralliquid, and simultaneously measuringintraoral licking, these investigatorsfound that although licking was moreprobable just prior to a bar-press thanat other times, it was maximal justfollowing an unreinforced bar-press.To the degree that the licking responseis under the control of Pavlovian con-tingencies, this parallels the case ofsalivation; neither the notion that theoperant leads to the respondent, northe notion that the respondent mustoccur prior to the operant, can aloneencompass the data. This empiricalconclusion proves to be an interestingproblem for those two-process theoriesthat postulate the mediation of instru-mental responses by action of periph-eral Pavlovian CRs (and their associ-ated feedback).

Aversive Behavior

Two-process theories of avoidancelearning have typically ascribed moti-vating and rewarding properties toautonomic responses and their afferentfeedback. In a standard, discriminativeavoidance training situation, the pairingof a CS with electric shock leads to thedevelopment of a conditioned "fear"reaction. Increase in sensory feedbackfrom that "fear" reaction is postulatedto instigate the instrumental avoidanceresponse while reduction in the feed-back rewards it. Various CR indexesof this fear-state have been suggested:heart-rate increase, blood-pressure in-crease, pupillary dilation, GSR, defeca-

tion, urination, suppression of appetitivebehavior, etc. If two-process concep-tions of avoidance behavior are ade-quate, and if the various indexes ofemotionality reflect adequately the levelof conditioned fear, a close correspond-ence between the occurrence of instru-mental avoidance behavior and theseindexes is clearly predicted.

Some of the relations which two-process theories require are the follow-ing: (a) Conditioned fear should in-crease in the early stages of avoidancetraining; (b) acquisition of the fearreaction should precede acquisition ofa reliable avoidance response, and ex-tinction of the avoidance responseshould occur concurrently with, or fol-low, extinction of the fear reaction;(c) during avoidance responding, fearshould be greater preceding successfulavoidance responses than on othertrials; (d) fear should decrease follow-ing the avoidance response; and finally(e) physiological manipulation such asadministration of drugs or sympathec-tomy which may directly affect thelevel of conditioned autonomic re-sponses should likewise indirectly affectthe avoidance behavior.

The two most often used indexes ofthe conditioned fear reaction used totest these predictions are heart-rateand suppression of appetitive behavior(conditioned emotional response, orCER, technique) by a CS. The evi-dence on cardiac conditioning is moreextensive, and we will examine it first.

1. Cardiac CRs—-acquisition.Changes in cardiac responding duringacquisition of an avoidance responseprovide some support for the two-proc-ess position. Both Gantt and Dykman(1957) using paw flexion and Black(1959) using a panel-press responsefound general increases in heart-rateduring instrumental avoidance trainingin dogs. Furthermore, both reported


the development of conditioned heart-rate increases during the Sd for theavoidance response. Typically, heart-rate conditioning occurred prior to ac-quisition of the avoidance reaction. Butone of Black's more detailed findingscontradicts at least one form of a two-process position; maximum heart-rateoccurred following the avoidance re-sponse, and it was only some secondslater that the rate declined. This is indisagreement with the two-process re-quirement of rapid reduction in fear asa reinforcement for the avoidance re-sponse. It may be that the influence ofthe skeletal avoidance movement andrespiratory changes upon heart-ratemakes cardiac change suspect as an in-dex of conditioned fear in such situ-ations. The possibility of artifact frommovement is especially great when in-strumental responses are required orpossible.

Performance. The evidence relatingheart-rate to avoidance behavior dur-ing continued performance of the avoid-ance response is conflicting. Soltysik(1960), using a paw-placement re-sponse with dogs, obtained resultswhich fit quite closely with two-processpredictions. In well-trained animals,an increase in heart-rate preceded theavoidance response and a decrease fol-lowed it. Furthermore, heart-rateCRs and avoidance responses werebrought under parallel, discriminativestimulus control. The conditionedheart-rate was maintained through con-tinued, long-term avoidance. In con-trast to Black's findings, Soltysikfound that maximum heart-rate occur-red prior to the avoidance response.The avoidance response was followedby sharp cardiac deceleration. Bersh,Notterman, and Schoenfeld (1956), indisagreement with Soltysik, found thatwith continued avoidance performance,human 5"s showed no heart-rate ac-celeration to the Sd for avoidance.

That is, avoidance behavior was main-tained in the absence of conditionedfear, as indexed by cardiac accelera-tion. Using cats, which show cardiacdeceleration as a CR when shock isthe US, Wenzel (1961) found a grossrelation between magnitudes of de-celeration and the latency of a bar-press avoidance response. However,the introduction of reserpine left unaf-fected the conditioned cardiac decelera-tion although it disrupted the avoidancebehavior. McCleary's (1960) failureto demonstrate interocular transfer ofavoidance responding in fish in spiteof good interocular transfer of Pavlo-vian cardiac conditioning also ques-tions the role of cardiac responses inthe mediation of avoidance behavior.

Extinction. Several relations havebeen found between heart-rate andavoidance responding during extinctionof the avoidance response. Gantt andDykman (1957) found extinction ofthe instrumental response long beforeextinction of the cardiac CR. In con-trast, Soltysik's dogs showed parallelextinction of the cardiac CR and theavoidance response, including trial-by-trial correspondence. To further com-plicate matters, Black (1959) foundmore rapid extinction of the cardiacCR than of the instrumental response,and no relation between the rates ofextinction for the cardiac CR and theavoidance response in individual 5"s.Furthermore, Black (1958) found thatextinction trials under curare facili-tated extinction of the avoidance re-sponse without affecting that of theheart-rate CR.

It is clear that the relation betweencardiac changes and avoidance behavioris not well understood. The sharp dis-agreements in findings suggest thatthe relation, if any really exists, iseasily disturbed by yet unidentifiedvariables. Most likely we have beenna'ive in selecting a single aspect of


cardiovascular change as an index ofconditioned "emotionality." The car-diovascular system is a highly complexone with many self-regulatory mecha-nisms. To expect simple heart-ratechanges, which are only a small por-tion of this system, to mirror adequatelya state such as "fear" is to oversimplifyhopelessly the operation of the cardio-vascular system. When we apply stressto an organism, we affect not only theheart rate but also a number of otheraspects of the circulatory system, suchas blood pressure, peripheral vessel re-sistance, stroke volume, etc. Many ofthese have intricate interrelations suchthat they can compensate for andchange the action of each other withina fraction of a second. It is clear thatwe cannot look at only heart rate inisolation, but must examine the entirecardiovascular system if we hope toestablish a fruitful peripheral index ofa motivational state.

2. Conditioned suppression. If astimulus associated with the onsetof shock is sounded while a hungryrat is pressing a bar to obtain food,it produces a marked decrement in bar-pressing. This is usually interpretedto mean that the stimulus has pro-duced a CER which is incompatiblewith bar-pressing. This CER is notspecified precisely, but is usuallythought to be due to a pattern of Pav-lovian CRs, identical to the conditionedfear reaction postulated by two-processtheories of avoidance learning. Thus,the degree of conditioned suppression,like changes in the heart rate, can beused to assess the amount of fearelicited by the Sd from a signaledavoidance situation.

The suppression measure is not en-tirely unrelated to conditioned heart-rate changes. Stebbins and Smith(1964) found a positive relation be-tween the occurrence of CER suppres-sion and heart-rate acceleration in mon-

keys. But, more recently, deToledoand Black (1966) have found sloweracquisition for cardiac CRs than forCER suppression in a simple Pavlo-vian conditioning situation.

Hoffman and Fleshier (1962) at-tempted concurrent measurement ofavoidance behavior and conditionedsuppression. While rats pedal-pushedfor food, an avoidance Sd was sounded,during which they had to press anearby bar to avoid shock. Suppres-sion of pedal-pushing during the avoid-ance Sd was greater on successfulavoidance trials, in agreement withtwo-process predictions. However,only with further training was condi-tioned suppression less following theavoidance response than it was duringthe Sa. Fear reduction, as indexed byconditioned suppression, did not seemto be the reinforcement for early avoid-ance responses.

Kamin, Brimer, and Black (1963)used a procedure similar to that ofHoffman and Fleshier. However,they separated the avoidance trainingand conditioned suppression situations.After training rats to various criteriaof avoidance acquisition and extinction,Kamin et al. imposed the avoidanceSd upon food-motivated bar-pressing.They found that as extinction of theavoidance response proceeded, condi-tioned suppression was reduced. How-ever, during avoidance acquisition,conditioned suppression produced bythe avoidance Sd first increased andthen decreased as avoidance trainingproceeded. If a conditioned fear reac-tion indexed by conditioned suppres-sion was maintaining the avoidance be-havior, this latter result is difficult tounderstand. It is not entirely consis-tent with that of Hoffman and Fleshierwho found continued conditioned sup-pression with long-term avoidance be-havior. It may be that the suppres-sion found by Hoffman and Fleshierresulted from the incompatibility of the


avoidance response with the appetitivebar-press, rather than from some con-ditioned emotional state.

Like heart-rate changes, conditionedsuppression does not reflect a mediatingfear reaction in a manner completelyconsistent with two-process theories.However, there are two alternative in-terpretations of the CER experimentswhich might make the results com-patible with a two-process descriptionof avoidance learning. First, it maybe that the role of the fear reaction inmaintaining avoidance behavior is dif-ferent from its role in the establishmentand extinction of avoidance behavior.Thus the failure of a CS for a well-learned avoidance response to produceconditioned suppression may indicatethat the CS is not producing fear in theavoidance situation and that the tra-ditional two-process account of avoid-ance learning does not apply to main-tained avoidance behavior. A secondpossibility is that the CER experimentis not an adequate index of the con-ditioned fear reaction. After all, theredoes not exist a closely reasoned ac-count of the fact that the CER pro-cedure produces suppression of the ap-petitively maintained operant. Whyshould we not instead find rate in-creases ?

3. Physiological manipulations. An-other method can be used to examinethe interrelations between PavlovianCRs and instrumental responses. Ifwe suspect that a specific set of CRsis mediating instrumental behavior, wecan simply eliminate those CRs andobserve the effects upon the instru-mental behavior. Solomon and hisco-workers have pursued this line ofresearch for various classes of CRs.

Using sympathectomized dogs,Wynne and Solomon (1955) havedemonstrated that although removalof peripheral autonomic CRs impairsavoidance learning, it does not pre-

vent it; nor does such removal facili-tate extinction of avoidance. Sympa-thectomy after avoidance learning doesnot impair performance in dogs. Pre-sumably, sympathectomy combinedwith vagal blocking as used by Wynneand Solomon eliminates cardiac andblood-pressure changes. Following thesame strategy, Auld (1951) used tetra-ethylammonium (TEA) to blocksympathetic autonomic nervous system(ANS) reactions, and found resultsperfectly paralleling those of Wynneand Solomon. There followed a longseries of experiments, too numerous todescribe here, in which barbiturates,autonomic blocking agents, stimulants,and tranquilizers were used to studythe relationship between autonomicCRs and avoidance behavior. Theydid not importantly affect the conclu-sion that autonomic CRs are not neces-sary mediators of avoidance behavior.

Similarly, the transfer of Pavlovianfear conditioning from the curarized tothe normal state, demonstrated by Sol-omon and Turner (1962) shows thatperipheral skeletal CRs are not re-quired for avoidance behavior. (Theseexperiments are described in detail inthe next section.) Both the sympathec-tomy and the curarization preparationseliminate broad classes of peripheralCRs as necessary mediators of avoid-ance behavior.

In summary, we have not yet identi-fied any peripheral CRs which arenecessary to mediate avoidance behav-ior. From this review of both aver-sively and appetitively motivated be-havior, the simple idea that someperipherally observed CR is essential inthe mediation of operant behaviorseems implausible. In no case that wehave studied does a peripheral CR seemto bear the required strong relation tothe instrumental behavior. However,the two alternative views of the medi-ational process, (a) that a complexof autonomic and skeletal CRs is the

170 ROBERT A. RESCOELA AND RICHARD L. SOLOMON

mediator, and (b) that the peripheralCRs are simply indexes of centralevents, still seem to be reasonable.Both views permit some slippage be-tween instrumental behavior and CRs.That we are not measuring complexenough peripheral behavior is difficultto refute; on the other hand, it is arelatively unattractive position becauseit suggests little that is new by way ofexperimentation except the recordingof a larger number of CR measures.

However, consider a third view, (c),that what concomitance we do observebetween instrumental behavior andperipheral CRs is due to mediation bya common central state. Then the con-current measurement of instrumentalbehavior and Pavlovian CRs is not theoptimal experimental strategy. In-deed, it becomes an irrelevant strategy.

Accepting the third view, then to findthat a particular CR does not controloperant behavior is hardly a refutationof a general two-process approach; in-deed, it would be surprising if weshould be able to select from the com-plex instrumental situation the fewcontrolling CRs. Rather, the essentialpostulate of two-process theory willthen be that manipulation of Pavlovianconditioning procedures should haveimportant effects upon instrumental be-havior. Although we may not be ableto identify the precise Pavlovian CRswhich affect instrumental behavior, wecan demonstrate that Pavlovian condi-tioning procedures exert strong influ-ences over instrumental behavior inthe absence of changes in instrumentalcontingencies. Such studies are re-viewed in the next section.

Manipulation of Instrumental Behaviorby Separately Conducted Pavlovian

Conditioning Procedures

It is one matter to lift Pavlovianconcepts out of Pavlovian theory andexperiments, as Hull and Spence did,

and use them to explain instrumentalbehavior. It is quite another matterto employ the procedures of Pavlovianconditioning in order to influence al-ready established, or to-be-established,instrumental behavior. The latterstrategy is generated by two-processtheory, because it assumes that Pavlo-vian conditioning and instrumentallearning are two distinct processes, eachgoverned by its own appropriate sets ofoperations and laws, and it is typifiedby the experiment of Solomon andTurner (1962). They avoidance-trained dogs in a panel-pressing appa-ratus with a visual Sd. The dogs werethen completely paralyzed by d-tubo-curarine, and were subjected to purelyPavlovian, discriminative condition-ing procedures, with tone CSs and ashock US. When 6"s were later testedin the panel-pressing apparatus, theyretained their avoidance response to thevisual Sd. In addition, they showedreliable panel-pressing responses to thetone paired with shock (CS-)-) duringPavlovian conditioning, but these re-sponses were weak or absent when thetone not paired with shock (CS — )was presented. Thus, the postulatedtwo processes were seen to interact in aparticular way, such that the instru-mental panel-pressing was immediatelyelicited by the introduction of a Pav-lovian CS+, even though S had neverbefore pressed a panel in the presenceof CS + . This experiment can beanalyzed in terms of two propositionsof two-process learning theory: (a)Pavlovian association processes pre-cede the acquisition of emotional reac-tions to previously neutral stimuli; and(b} these emotional reactions havemotivational properties that can influ-ence instrumental responding. It fol-lows that any empirical or theoreticallaw of Pavlovian conditioning has pro-found implications for the control ofinstrumental responding when the two


processes are interactively combined byE's procedures.

What are some of these Pavlovianlaws (see Pavlov, 1927), and howwould they be expected to reveal theirimpact ?

1. The Law of Excitation. A CS con-sistently paired with a US acquires excita-tory properties. Previously neutral stimuli,originally unable to elicit salivary responsesin the dog, come to do so after several tem-poral pairings with meat powder on thetongue (the US for salivation). Irradi-ation of excitation should occur, and sostimuli similar to the CS should elicit theCR.

2. The Law of Internal Inhibition, (a)Differential inhibition. If a CS+ is con-sistently paired with a US on one-half ofthe conditioning trials, and a CS— is con-sistently presented unpaired with the US onthe other half of the conditioning trials, thelast phases of the conditioning show "dif-ferentiation" ; that is, 51 gives a reliable CRto each CS+ presentation but not to CS—.Differential inhibition is postulated to sup-press actively the CR in the presence ofCS—. Salivation is not merely failing tooccur in response to CS—; it is being sup-pressed. That CS— actually has inhibitorypowers can be demonstrated by presentingit along with an effective CS+. When wedo this, a CR that normally would have aspecific magnitude will occur in markedlyreduced magnitude. ( fc ) Conditioned in-hibition. If a compound CS is used as CS—,and one segment of the compound CS isused as the CS+, we meet the conditions forproducing a conditioned inhibitor. For ex-ample, on half of the conditioning trials wepresent CSi, paired with the US, and on theother half of the trials we present CSa andCSt in sequence, unpaired with the US.Then, eventually, good CRs will emerge inthe presence of CSi, and no CRs in the pres-ence of the CSa-CSi sequence. We cantest the properties of CSi and CS2 by pre-senting each one in a test trial togetherwith some effective CS+. CSi with CS+will lead to an enhanced CR, but CSa withCS+ will lead to a diminished CR. There-fore, CSa is a conditioned inhibitor. It hasacquired the property of actively suppressinga CR that would have occurred in greatermagnitude; it is no longer neutral, (c) In-hibition by temporal delay. We carry outthe conditioning of the salivary reflex undera procedure that delays the onset of the US

long after the CS+ has begun. For ex-ample, a tone comes on and remains on for30 seconds before the meat powder is de-livered to the dog's tongue. When thisconditioning technique is used, the excita-tory CR at the end of many conditioningtrials has a long latency, "crowding" theend of the CS-US interval. Pavlov sup-posed that the CR was actively inhibitedduring the early moments of presentation ofCS+ and that this inhibition dissipated intime, allowing the excitatory influence ofCS+ to appear. The CR is thus temporallypaced by an inhibition process. A similarphenomenon occurs if a trace conditioningprocedure is used with a long CS-US in-terval, (d) Extinctive inhibition. If a dogis given SO conditioning trials with CS+always paired with the US, and then theCS+ is presented unpaired with the US forseveral hundred trials, the CR, previouslymeasurable after SO trials, will disappear.This extinction procedure, according to Pav-lov, does not merely reduce the excitatorypower of CS+ but rather builds up its in-hibitory power. Thus, if CS+ is presentedalong with another CS4- which normallyelicits a CR, the CR should be diminished inmagnitude. The extinguished CS+ is thenlabeled an extinctive inhibitor, with power tosuppress CRs. (e) Induction. During thecourse of establishing a differential CR,when CS+ clearly produces a CR of greatermagnitude than does CS—, a few CS—trials will often produce an enhanced CR onthe next CS+ trial. This phenomenon iscalled positive induction. Conversely, a fewCS+ trials are thought to produce a di-minished CR on the next CS— trial. Thisis called negative induction.

(3) The Law of External Inhibition.Novel or distracting stimuli, whether weakor unusually intense, can temporarily dimin-ish the magnitude of a CR. Thus, when aneffective CS+ is presented, the occurrenceof a loud noise will diminish the CR magni-tude. Conversely, novel, distracting, or unu-sually intense stimuli can destroy temporarilythe inhibitory power of a CS—. Any inhibi-tory process is thought to be susceptible todisruption by an external inhibitor. This iscalled disinhibition. Repeated presentationof an external inhibitor diminishes its in-hibitory and disinhibitory power.

We have reviewed in some detail afew of the major laws of Pavlovian con-ditioning. What does two-process the-ory do with such laws? First, it as-


sumes that these laws of Pavlovian con-ditioning of the salivary reflex are prob-ably the laws of emotional conditioningor laws of acquired drive states. Sec-ond, it assumes that conditioned emo-tional states change 6"'s motivation leveland thus can serve either as motivatorsor reinforcers of instrumental re-sponses.

Table 1 provides a convenient sum-mary of the variety of ways in whichPavlovian conditioned emotional statescan interact with instrumental learningto produce changes in instrumental re-sponding.

Note that Table 1 accomplishes threepurposes. First, it allows us to classifyand organize the existing knowledgeabout interactions between the two hy-pothetical processes. Second, it drama-tizes the absence of certain kinds ofknowledge, thus pointing to new ex-periments which need to be done. Andthird, it raises new and interestingtheoretical questions concerning theoutcomes of the experiments in thetable.

The Solomon and Turner (1962)experiment, which we have already de-scribed, can be located in Table 1 in

TABLE 1COMBINATIONS OF SEPARATELY CONDUCTED

PAVLOVIAN PROCEDURES ANDINSTRUMENTAL TRAINING

PROCEDURES

Pavlovianconditioning

Appetitive(alimentary)

Aversive(defensive)

CS +

cs-CS +

cs-

Instrumental training

Appetitive

noSJ

1 T

2 ;

3 1

* T

Sd-SA

5 T

6 i

7

8

Aversive

no SJ

9

10

11 T12 i

S<i-S4

13

14

is T16 ?

Note.— | and J. refer to the effect of the condition-ing-training combination on amount of instrumentalresponse.

the following manner. In this experi-ment, the US for fear conditioning wasaversive (shock) as was the reinforcerfor avoidance behavior. In addition,the Pavlovian conditioning was dis-criminative, and the 5"s were testedwith independent presentations of CS +and CS —. The instrumental trainingwas discriminative, since 5"s learnedto respond by panel-pressing in thepresence of Sa. This allows us to in-sert the results of the Solomon andTurner experiment in Cells 15 and 16.We arbitrarily use a (f) sign in Cell15 to indicate that the Pavlovian CS +produced an enhancement of instru-mental responding. CS—, on the otherhand, had little or no effect, and so wehave inserted a (?) in Cell 16.

The traditional CER experimentfalls into Cell 3. The 6" is trained toperform some instrumental responsereinforced by some appetitive stimulus.Pavlovian conditioning is carried outwith an aversive US. The S is testedwith presentations of CS+ while per-forming the instrumental response.The usual finding is that the responserate is suppressed by CS+, and so Cell3 contains a (|).

Now we can examine the laws ofPavlovian conditioning as they are re-flected in the interactions contained inTable 1. In all cases, the dependentvariable is some measure of instru-mental responding. Yet the independ-ent variables, the influence of whichis being tested, are those contained inthe Pavlovian conditioning experiment.

EXCITATION AND INHIBITION

1. Differential excitation and inhibi-tion. Rescorla and LoLordo (1965)gave dogs discriminative conditioningwith an aversive US (shock). A Pav-lovian law predicts that the stimulusnot paired with shock (CS — ) will be-come a differential inhibitor—that is,it will have the capacity to suppress


actively whatever emotional reflex pat-tern is usually elicited by CS+. Thus,CS — should be able to inhibit "fear ofshock." Prior to discriminative fearconditioning, the dogs had been trainedso that they reliably responded on aSidman avoidance schedule at a rateof seven jumps per minute in a dogshuttlebox. While 5"s were jumping,short presentations of CS+ and CS —were given in some random sequence.

Two-process theory leads to the fol-lowing expectations. If CS+ is aPavlovian excitor, then conditionedfear should be augmented by its pres-ence, and the instrumental respondingrate should increase above the normalrate. In contrast, if CS — is a Pavlo-vian differential inhibitor, then itshould actively suppress conditionedfear and the instrumental respondingrate should decrease below the normalrate.

Rescorla and LoLordo (1965) foundthat the presentation of the CS+ re-sulted in a doubling of the Sidmanjumping rate, but the presentation ofthe CS— resulted in a large reductionin the Sidman jumping rate. It seemsclear that CS— had acquired inhibitoryproperties. The Rescorla and LoLordoexperiment can be inserted in Cells 11and 12 in Table 1. They represent theintersection of aversive, discriminativePavlovian conditioning preceded by un-signalized (Sidman) aversive training,with no Sd.

Another example of differential Pav-lovian conditioning combined with in-strumental training is the experimentby Ray and Stein (1959) who trainedhungry rats to bar-press for milk on aVI-2 schedule. Then, when the ratsattained a steady response rate, an1800 cps tone was presented for 5minutes and it terminated with shockto the feet. A contrasting 200 cpstone was presented at other times, butit terminated without shock. Eventu-

ally, the 1800 cps tone acquired the ca-pacity to suppress bar-pressing com-pletely. In contrast, presentation of the200 cps tone (CS—) often produced in-creases in the bar-pressing rate abovethe normal base-line rate, though thisdifference was not a large one (seeCells 3 and 4, Table 1).

Hoffman and Fleshier (1964) havecarried out a series of experimentssimilar to that of Ray and Stein. Ingeneral, their findings paralleled thoseof Ray and Stein with regard to theCER suppression effect of CS+ pre-sentations. However, unlike Ray andStein they did not find the enhancementof instrumental responding in the pres-ence of CS —. Hammond (1966) re-peated and extended the results of Rayand Stein. He further showed thatenhancement of bar-pressing rate bypresentation of CS— occurred onlywhen the base-line responding rate wasbelow normal.

These interesting findings raise thepossibility that a Pavlovian differentialinhibitor established by aversive condi-tioning can enhance instrumental re-sponding established by an appetitivereinforcer. But what could the find-ing, that an inhibitor of fear enhancesappetitively maintained behavior, mean ?One possibility is that reflex interrela-tions exist between appetitively andaversively based incentive states. Theoccurrence of an elicitor or inhibitor ofa fear state may reflexly depress orenhance positive incentive motivation.On the other hand, it may be that aninhibitor of fear has no effect upon anappetitive motivation state. Instead,there may be a general level of fearproduced by the experimental situa-tion ; since such fear presumably re-duces the response rate, an inhibitor ofthat fear would lead to a rate enhance-ment. The Hammond experimentstrongly supports such an interpreta-tion. These findings are of consider-


able importance to a theory of incen-tive motivation.

So far, we have concerned ourselveswith aversive Pavlovian conditioning,and have traced the effects of CS +and CS—, in their roles as differentialexcitors and inhibitors, on both aversiveand appetitive instrumental respond-ing. There is, in addition, a series ofexperiments in which appetitive, dis-criminative conditioning proceduresare combined with appetitive instru-mental training procedures. These ex-periments come out of the Skinneriantradition. A prototype experiment isthat of Estes (1948), and it was prob-ably the first of its kind to be success-ful in showing that a CS + , previouslypaired with food presentations, couldenhance, during extinction, the rate ofan operant previously reinforced byfood presentations. As is the case inall of the experiments subsequentlyusing the Estes paradigm, CS+ pre-sentations are in reality being pairednot only with food presentations, butalso with magazine approach responses.This is inevitable, because the condi-tioning procedure in these experimentsis done through magazine training.In order to get the food, 5"s must makeinstrumental responses. Even thoughthis procedure is not "pure" Pavlovianconditioning, it resembles it to the ex-tent that the approach response cannotproduce the food US; only the CS +can produce it. Neither can the ap-proach response produce the CS + .(The procedure can be described inSkinnerian terminology as a "discrimi-nated operant.")

Morse and Skinner (1958) trainedpigeons to approach a magazine forfood in the presence of one color(CS + ), but the food never was pre-sented in the presence of a contrastingcolor (CS — ) . The behavior of 5" wasirrelevant for the occurrence of maga-zine operation. Then, in the second

stage of the experiment, 5s learnedto peck at a white key for food. Fi-nally, extinction was instituted, duringwhich there were test periods in whichCS+ and CS— were alternately pre-sented. The pecking rate was higherin the presence of CS+ than it waswhen CS— was present. There was,however, no control for normal extinc-tion in white light alone, so we cannottell whether CS— was inhibitory, orthe CS+ excitatory, or both. Weknow CS+ showed differential excita-tory properties when compared toCS—, and this confirms Estes' (1948)findings, but it leaves in doubt theproper sign to put in Cells 1 and 2of Table 1. It is our guess that Cell 1should contain a (f) sign, and Cell 2a (4) sign, but in the absence of theproper controls we cannot be sure. Arecent experiment by Bower andKaufman (1963) confirms the findingby Morse and Skinner of a differencebetween the effects of CS+ and CS —.

Most of the earlier experimentsshowing the differential excitatory andinhibitory effects of CS+ and CS —on instrumental responding have usedextinction responding as a base line.Recently, however, there have beenseveral experiments exploring the ef-fects of differential Pavlovian condi-tioning procedures on subsequentlearning of discriminative instrumentalresponses. For example, Bower andGrusec (1964) used thirsty rats, con-ditioning them by having tone S,paired with water reinforcements andtone S2 occurring without water rein-forcements. The rats had previouslybeen trained to press a lever to getwater, but this early training was notdiscriminative (no Sd). Then, in athird stage of the experiment, the ratswere given discriminative instrumentaltraining. One group was trained withits CS+ from the conditioning phasenow the Sa for the operant, and its


CS— as SA, while in contrast, anothergroup had its CS+ from the condition-ing phase now the S4 for the operantand its CS— as Sd. Thus, in onegroup the CS/Sd relationship was con-sistent, but in the other group theCS/S" relationship was inconsistent.Bower and Grusec found that the ac-quisition of the Sd — SA discriminationwas enhanced for the consistent groupbut was interfered with in the incon-sistent group. There was, however,no way of ascertaining whether or notCS— had true differential inhibitoryproperties because there was no con-trol group that learned the Sd — SA

discrimination without prior condition-ing with CS+ and CS —. We canspeculate, however, that learning wasinterfered with whenever CS— inhib-ited conditioned appetitive reactions inthe presence of Sd, and learning wasfacilitated whenever CS+ facilitatedexcitatory appetitive reactions in thepresence of Sa.

More recently, Trapold (1966) hasshown that inconsistent differentialconditioning can actually facilitate re-versal learning of a discriminative in-strumental response. Rats were firsttrained, in an operant discrimination,to press a lever for food in the presenceof Sj but to refrain from pressing inthe presence of S2. Then the lever wasremoved, and S2 was paired with foodpresentation while St was paired withabsence of food. Later, when the ratswere presented again with the lever,they were required to reverse the orig-inal operant discrimination and press inthe presence of S2. They learned thisreversal more rapidly than a groupthat had not received pairings of S2

with food presentation. Thus, Pavlo-vian contingencies were powerfulenough to assist the instrumental dis-crimination reversal.

2. Conditioned inhibition. Rescorlaand LoLordo (1965) trained dogs in

our laboratory on a Sidman avoidancecontingency in the shuttlebox, until thedogs performed the avoidance responseat a stable rate. Then they subjectedsome of the dogs to a Pavlovian fearconditioning procedure in which CS +was followed 2-8 seconds later byshock on one-half of the trials, buton the other half of the trials CS+was followed 2-8 seconds later by CS —and no shock. Other dogs, after learn-ing the Sidman avoidance response,were subjected to a Pavlovian fearconditioning procedure in which CS't-was followed by shock on one-half ofthe trials, but on the other half of thetrials CS — was inserted 5 seconds priorto CS+ and no shock followed. CS —in both procedures was shown to haveinhibitory properties. Test presenta-tions of CS — reduced the Sidman avoid-ance response rate significantly. Res-corla and LoLordo infer that condi-tioned fear was inhibited by their CS —.In contrast, fear was aroused by CS + .

3. Inhibition by temporal delay.Rescorla (1967a) trained dogs in ourlaboratory in a Sidman avoidance re-sponse in the dog shuttlebox. Whenthe dogs had acquired a stable jump-ing rate, they were subjected to aPavlovian fear conditioning procedurein which only a long-delay CS+ wasused. A 30-second tone was followedby shock on all conditioning trials.Then later, when the dogs were per-forming their avoidance response in theshuttlebox, the 30-second tone waspresented from time to time. Rescorlafound that the onset of the tone pro-duced a decrease in jumping rate, andthe rate thereafter increased until, atabout 20-second duration, the jumpingrate went above normal, increasingsteadily to the end of the interval.Cessation of the tone produced a de-crease in jumping rate to a below-base-line level, followed by slow recovery tothe base-line rate. Here is a case


where onset of a "danger signal" re-sulted in a temporary decrease ofavoidance responding. This is whatone would expect from Pavlovian ex-periments on long duration CSs. CSonset is never closely paired withshock, and so it serves as a CS—, aninhibitor of conditioned reflexes. Inthe Rescorla experiment, we can inferthat CS+ onset inhibited fear.

Recently, Trapold, Carlson, andMyers (1965) have shown how Pavlo-vian inhibition by temporal delay canoperate in an appetitive situation.They conditioned rats with either fixedor variable temporal delay intervalsbetween food US presentations, in theabsence of any CS (temporal condition-ing). Then they gave the rats FItraining with food reward in a bar-press situation. They found that whenthe temporal interval between US pre-sentations previously used in Pavlovianconditioning was the same as that usedin the subsequent FI training, the de-velopment of a sharp FI "scallop" wasfacilitated. Evidently the delivery ofa US can serve as a stimulus whichtemporarily inhibits an appetitive statethat mediates bar-pressing for food.The temporal pacing of instrumentalbehavior, as in FI contingencies, canbe "sharpened" by Pavlovian condi-tioning treatments of the proper sort.This is evidence quite compatible withthat of Pavlovian experiments.

4. Induction. Not much is knownabout the operation of the Pavlovianinduction phenomenon as it influencesinstrumental responding. Rescorla(1967a) and Rescorla and LoLordo(1965) found that termination of anaversive CS+ during instrumentalavoidance responding reduced the re-sponse rate for a few seconds. This,however, is not the only way of show-ing induction. One might explore theincrease in jumping rate produced by

a CS+ presentation that followed aCS— presentation, as compared withone that followed another CS+ pre-sentation, in order to measure positiveinduction. This has not been done.Neither has negative induction beenstudied in such a way. In order to dothis, one would compare respondingto CS— when it has recently been pre-ceded by a CS+ presentation, as com-pared with being preceded by a CS —presentation.

5. External inhibition and disinhibi-tion. The effects of extraneous stimuliupon CSs imposed on ongoing instru-mental behavior have been little stud-ied. It would be of interest to seewhether a novel stimulus could disruptthe effect which a CS+ has upon in-strumental behavior (external inhibi-tion). Likewise, we would like toknow whether we can remove the in-hibitory effect which a CS— has uponinstrumental behavior by presentationof a novel stimulus (disinhibition).Preliminary experimentation with dogs(Rescorla, 1967a) suggests that dis-inhibition of inhibition of temporal de-lay can be produced with fear condi-tioning; however, no evidence wasfound for external inhibition.

We have covered many of the ex-periments showing how Pavlovian pro-cedures, interacting with Thorndikian(or Skinnerian) procedures, can in-fluence instrumental responding. Indoing so, it became clear that manyof the cells in Table 1 are empty.Some of the empty cells are as interest-ing as the filled ones. For example,take Cell 9. This cell would requirethe testing of the effect of an appetitiveCS + on unsignaled avoidance respond-ing. What would happen ? Would the"irrelevancy" of the fear that mediatesavoidance to the appetitive CRs elicitedby CS+ mean that avoidance ratewould be unaffected by CS+? Or is


there a matrix of interrelationshipsamong emotional-motivational statesthat requires that appetitive CRs inter-fere with or inhibit all aversive statesto some extent? Perhaps, instead, theSpence (1956) view of "generalizedD" would be correct, and any excita-tory CS+ would enhance any responsemediated by any excitatory state in thepresence of Sa. CER experimentation(see Cell 3) would argue against thisexpectation, but perhaps it is too earlyto be certain. Certainly, much workis needed in this area of ignorance.

Another interesting cell is No. 10.Suppose a dog has acquired a stableSidman avoidance response in theshuttlebox. He is then given test pre-sentations with a CS— previously es-tablished in an appetitive Pavlovianconditioning procedure. What mightbe expected to happen? Would CS —be "irrelevant" for fear level, and there-fore leave jumping rate unaffected?Or would CS—, being a signal for thenonoccurrence of an appetitive US, be"disturbing" in some way, thus "add-ing to" fear level and increasing theavoidance response rate? Is there adimension of "pessimism" establishedin the emotional life of laboratory ani-mals, such that a signal that says "foodwon't come," although it may super-ficially seem irrelevant for fear moti-vated avoidance responding, neverthe-less is a "bad" event, just as shock isalso a "bad" event? Amsel (1965) andBrown and Wagner (1964) have re-cently shown the generality of a "per-severance" attribute for laboratory 5"sgiven special types of partial reinforce-ment experiences. Perhaps "pessi-mism" can be similarly established byappropriate Pavlovian and Thorndikiantreatments, such that all CS—'s for ap-petitive differential conditioning, andall CS + 's for aversive differential con-ditioning, can enhance all instrumental

responses reinforced by the avoidanceof aversive USs of any type. It cer-tainly would be valuable to know howthese interactions work. The tech-niques for getting this information havealready been worked out.*

There are other cells in Table 1 thatcommand attention, but the reader cannow generate his own experiments tofill them. We should note, however,that Table 1 does not exhaust the pos-sibilities for analysis of interactions be-tween Pavlovian and Thorndikianprocesses. The Pavlovian condition-ing procedures can either precede, orbe preceded by, the instrumental train-ing procedures (thus focusing attentionon order effects in the interaction ofthe two processes). Or the Pavlovianconditioning procedures can be carriedout either within the situation used forinstrumental training or in another sit-uation as distinctively different as pos-sible from the instrumental trainingsituation. This variation focuses at-tention upon situational stimuli as afactor in the interaction of the twoprocesses. Intrasituational Pavlovianconditioning can, in turn, either be car-ried out "on the base line," that is,

4 After the completion of this manuscriptLoLordo (1966) showed that the summationof fear of two different aversive events isreflected in instrumental responding. Hetrained dogs to avoid shock by pressing apanel, with an unsignalized (Sidman) pro-cedure. He then exposed the dogs to aPavlovian conditioning experience duringwhich the CS+ was paired with a loud blastfrom a horn and CS— was explicitly un-paired with the horn blast. Later, in a testsession, S-second presentations of CS4- andCS— were imposed on Sidman responding.The CS+ elicited an increase in the pressingrate, but the CS— did not produce an in-hibitory effect on the pressing rate. Sucha result indicates that the generalization offear excitation as a mediator is probablyquite great (from noise to shock), but per-haps the generalization of fear inhibition isnot very great.


while the instrumental behavior to beinfluenced is occurring; or, in con-trast, it can be carried out "off the baseline," that is, when instrumental re-sponding is impossible.

Up to this point we have talked onlyof variations in the Pavlovian condi-tioning parameters. Another strategyis to vary the instrumental trainingparameters while holding the Pavlovianconditioning constant. For example,would Pavlovian conditioned stimulibased on a shock US have the same ef-fect on an S during asymptotic, rein-forced, operant performance as it wouldhave during the first moments of ex-tinction of that operant? Such a ques-tion has implications for any theory thatargues for the importance of emotionin the control of instrumental respond-ing, since presumably different emo-tions are present in training and ex-tinction. It seems clear that system-atic variations of both the Pavlovianand instrumental operations in Table1 would be valuable in extending ourunderstanding of the emotional con-trol of instrumental behavior.

The many possible variations in pro-cedure will complicate Table 1 andexpand it to almost unmanageable pro-portions. Nevertheless, such variationsmust be kept in mind, because we al-ready know that they are important.For example, the effects of the orderof Pavlovian conditions and instrumen-tal training are subtle and interesting.Overmier and Leaf (1965), workingin our laboratory, found that the dis-criminative control of avoidance re-sponding by a Pavlovian CS+ andCS— was poorer when the condition-ing preceded avoidance training thanwhen the reverse order was used.However, there is nothing in two-process theory that predicts an ordereffect. This result indicates that two-process theory is rapidly generatingnew data requiring further refinement

and extension of such theory whilenot requiring the abandonment of theapproach.

We can conclude that, following onestrategy suggested by two-processtheory, Pavlovian conditioning pro-cedures can readily be used to controlinstrumental responding. Further-more, it might very well turn out thatinstrumental responding is as sensitive,or perhaps even more sensitive, a meas-ure of the effects of Pavlovian condi-tioning procedures than are the tra-ditionally measured conditioned visceralor motor reflexes themselves. If thisshould turn out to be true, it wouldconstitute a major heuristic, albeitsomewhat ironic, contribution of two-process learning theory.

Finally, we point to the successachieved in controlling instrumental re-sponding by means of a wide variety ofPavlovian procedures, contrasted withthe failure to establish definitive rela-tionships between CRs (as mediators)and concurrent instrumental responses.Such success gives support to the ver-sion of two-process theory postulatingthat the concomitance we do observebetween CRs and instrumental re-sponding is mediated by a common cen-tral state, and the changes in thatstate are subject to the laws of Pavlo-vian conditioning.

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(Received June 16, 1966)

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VOL. PSYCHOLOGICAL REVIEW...Most learning theorists have at-tempted to reduce the outcomes of these two experimental procedures to a common underlying learning prin-ciple. Such attempts