rescorla (1969).pdf

VOL. 72, No. 2 AUGUST 1969

Psycholog ica l Bul le t in

PAVLOVIAN CONDITIONED INHIBITION1

ROBERT A. RESCORLA 2

Yale University

The notion of conditioned inhibition is examined and a definition is suggestedin terms of the learned ability of a stimulus to control a response tendencyopposed to excitation. Two techniques of measuring inhibition are outlined: (a)the summation procedure in which an inhibitor reduces the response that wouldnormally be elicited by another stimulus and (6) the retardation-of-acquisitionprocedure in which an inhibitor is retarded in the acquisition of an excitatoryconditioned response (CR). Examples of the use of these procedures are givenfor a variety of unconditioned stimulus (US) modalities. Several possible opera-tions for generating conditioned inhibitors are reviewed: extinction followingexcitatory conditioning, discriminative conditioning, arrangement of a negativecorrelation between a conditioned stimulus (CS) and a US, use of an extendedCS-US interval, and presentation of a stimulus in conjunction with US termi-nation. A review of the literature on these operations suggests that condi-tioned inhibitors are not generated either by simple extinction procedures orby pairing a stimulus with US termination. By contrast, for both salivary andfear conditioning the other procedures do appear to generate inhibitors. Mostof the procedures generating conditioned inhibitors can be described as arrang-ing a negatively correlated CS and US.

In his work on conditioned salivation,Pavlov (1927) found that he could endow aneutral stimulus with the power either to elicitor to inhibit salivation, depending upon thetemporal relations which he arranged betweenthat stimulus and food. American psycholo-gists have tended to emphasize the acquisitionof the ability to elicit salivation, so-calledexcitatory conditioning, and to ignore whatPavlov himself discussed at length, that somerelations between a neutral stimulus and foodlead to the development of inhibition.

The present paper is concerned with thelogical and empirical procedures involved in

1 Preparation of this manuscript was supported inpart by National Science Foundation grant GB-6493.

2 Requests for reprints should be addressed toRobert A. Rescorla, Department of Psychology, YaleUniversity, 333 Cedar Street, New Haven, Con-necticut 06510.

identifying Pavlovian conditioned inhibition.3

The paper has three objectives: first, to pro-vide at least a working definition of the con-cept of conditioned inhibition; second, toexamine at some length the procedures avail-able for identifying a stimulus as a Pavlovianconditioned inhibitor; and third, to reviewthe scanty evidence on the conditions neces-sary to establish a stimulus as a conditionedinhibitor.

The defining properties of a conditionedinhibitor are not well specified in thepsychological literature. But a few points ofagreement are clear. First, the power of aconditioned inhibitor to produce changes in

3 The review concentrates on Pavlovian condi-tioning. No attempt is made to discuss in detail therole of inhibition in instrumental learning; however,analogous findings from that literature are occasion-ally mentioned.

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© 1969 by the American Psychological Association, Inc.

78 ROBERT A. RESCORLA

behavior is acquired as a result of thepast experience of the organism; that is, itsinhibitory function is conditioned in the mostgeneral sense of the term. Second, the out-come of this past experience is that a stimulusbecomes capable of reducing some behavioralchange which is normally attributed toexcitation.

The criteria denning conditioned excitation(more commonly called a conditioned re-sponse) are somewhat more clear than thosefor conditioned inhibition. Typically, condi-tioned excitation is denned by satisfaction oftwo conditions: an operation relating theconditioned stimulus (CS) and the uncondi-tioned stimulus (US), such as a pairing ofthe two or the arrangement of a positive con-tingency between them, and a change in be-havior resulting from this operation. Themost common example is the increase insalivation resulting from the operation ofpresenting in sequence a neutral CS and afood US. If the conditioning operation isperformed and no change appears in thebehavior of the organism, ostensibly no condi-tioning has occurred. Conversely, if thatoperation is not performed, and still changesin the organism are found, such changes areidentified not with conditioning but with somenonassociative change in the organism. Excita-tory conditioning is thus denned in terms ofboth an operation and a consequent changein the behavior of the organism. Althoughthe laws which a conditioned response (CR)follows, including those specifying the natureof the change which will count as a PavlovianCR, may eventually be included in the defini-tion of conditioned excitation, the presentdefinition is silent on this point. Nothing issaid about the nature of the behavior change.Should one wish to exclude, for instance,changes in which the CRs are unlike theunconditioned response (UR) in form, thiswill only limit somewhat the scope of thepresent discussion.

Parallel attempts to specify conditionedinhibition have not been so sharply laid out.In the first place, the operation involved insetting up a conditioned inhibitor is rarelyspecified. In a subsequent section, severalpossible defining operations are reviewed.

Second, a variety of effects upon behaviorwhich might result from such an operationhave been suggested; however, no single effecthas been generally accepted. Again, in a latersection, the usefulness of some of these pos-sibilities is discussed. For the present, condi-tioned inhibitors are specified only in termsof the general kind of change that they pro-duce in behavior. First, an acceptable condi-tioned excitor is specified in terms of anoperation relating CS and US and the result-ing behavioral change. A stimulus, then, iscalled a conditioned inhibitor if, as a result ofexperience of the organism with some opera-tion relating that stimulus to the US, thestimulus comes to control a tendency oppositeto that of the conditioned excitor. Notice thatthere are two important parts to this defini-tion: (a) The experience must be with thesame US as that forming the basis for theconditioned excitor; thus explicit counter-conditioning with another US does not estab-lish a stimulus as a conditioned inhibitoras the term is used here; (6) the tendencycontrolled by the supposed conditioned inhib-itor must be opposite to that controlled bythe conditioned excitor (cf. Jenkins, 1965).If the excitor produces an increased proba-bility, decreased latency, and increased vigorof a particular response, then a conditionedinhibitor should decrease its probability, in-crease its latency, etc. Furthermore, theconditioned inhibitor should be specific to thebehavior controlled by the excitor; the dis-cussion is concerned only with stimuli whichinhibit specifically the responses generated byexcitatory conditioning with a particular US,not with stimuli that suppress all behavior.The intention here is to demand a specificityparallel to that which is attributed to condi-tioned excitation. When excitatory condition-ing is performed with a given US, the patternof conditioned responding should depend uponthe particular US employed. One would notexpect a general decrement in all the orga-nism's responses from a conditioned inhibitorany more than one would expect a generalincrement in all responses from performingexcitatory conditioning with a particular US.

For the present, the question of whichspecific past experiences might generate condi-

PAVLOV1AN CONDITIONED INHIBITION 79

tioned inhibitors is left open. This is primarilyan empirical question and is dealt with in alater section. But notice that even at thispoint, deciding whether or not a stimulus isa conditioned inhibitor depends upon theoperation specified to obtain a conditionedexcitor as well as upon the outcome of thatoperation. This point is discussed later.

Thus, a conditioned inhibitor has beendefined as a stimulus which through learningcomes to control a tendency which is oppositeto that of a conditioned excitor.4 Any furtherproperties which might accrue to conditionedinhibitors are to be discovered empirically.It may turn out that inhibitory tendencies aremore fragile than excitatory tendencies (asPavlov indeed assumed), or they may bemore robust. Similarly, inhibition may dis-sipate with time (as again Pavlov assumed),or it may not. Conditioned inhibitors maybe positive or negative reinforcers, aversiveor appetitive stimuli. These are possible prop-erties of a conditioned inhibitor but they arenot central to its definition. The definitionhere is made purely in terms of control of atendency opposite to that of an excitor.

THE MEASUREMENT OF INHIBITION

In most conditioning situations, it is con-siderably easier to measure conditioned excita-tion than it is to measure conditioned inhibi-tion. For instance, in salivary conditioning,excitation is measured by the occurrence ofdrops of saliva to a CS after repeated condi-tioning trials. The change that takes place isthat the CS now comes to produce drops ofsaliva, whereas it did not before. If this in-crease in the probability of salivation is theexcitatory reaction, then by the definitiongiven above, a conditioned inhibitor shouldproduce a decrease in the probability of sali-vation. Immediately, there is an asymmetry

4 It may be noted that the term "conditionedinhibition" as used here differs from the usage ofPavlov. The term "conditioned" is used in thegeneral sense of "learned" so that "conditionedinhibition" applies generally to learned inhibition.Pavlov applied the phrase "conditioned inhibition"to inhibition acquired as a result of a particulartraining paradigm. The present use of "conditionedinhibition" corresponds roughly to Pavlov's phrase"internal inhibition."

between the measurement of excitation andinhibition. Prior to conditioning, the CS elicitsno salivation; it is thus difficult to demon-strate the presence of conditioned inhibitionthrough decreases in the amount of salivationelicted by the CS. And surely, the continuedfailure of a CR to occur is not sufficientgrounds for asserting that the CS is a condi-tioned inhibitor.

One source of the problem is relativelyclear: Typically the experimenter selects as"neutral" CSs those stimuli which prior toconditioning do not produce any evidence ofexcitation; that is, stimuli which normallyelicit little or no response. It is then difficultto measure the degree to which a stimulusacquires control over a tendency to decreasethe response. This is not to say that condi-tioned inhibition does not develop to suchstimuli, only that it is difficult to detect itspresence. Difficulty in observing conditionedinhibition should not be mistaken for itsabsence.

To bypass this problem of detection, avariety of special measurement procedureshave been developed. What follows is a briefsummary of such procedures together withmention of some experiments illustratingtheir use.

Summation

Pavlov (1927) suggested what is perhapsthe most direct method of measuring inhibi-tion. Suppose that there is a stimulus S0

which is a known excitor of a response, thenone can tell whether or not Si is a conditionedinhibitor of that response by comparing theresponse elicited by So alone with that elicitedby the combination of Si and So presentedtogether. If the combination produces a re-duced response compared with that to S0

alone, Si is declared an inhibitor. The notionis simply that So elicits a given amount ofexcitation; excitation and inhibition are alge-braically additive; thus if Si reduces theamount of responding that would normallyoccur to So, Si is an inhibitor. A similar logicapplies to the demonstration that Si is a con-ditioned excitor; under those circumstances itshould enhance the responding to S0. How-ever, this test is rarely applied except in the


case that So is the null stimulus which elicitslittle responding.

Notice two important things about the sum-mation technique for demonstrating an inhib-itor. First, it has built into it the assumptionthat excitation and inhibition produce directlyopposite effects; Si is termed an inhibitor ifit reduces the response normally elicited bythe excitor So no matter whether the responseto S0 involves an increment or a decrement inbehavior. This is important since many excita-tory CRs actually reflect themselves as behav-ior decrements. Second, it emphasizes the fact,which is obvious upon reflection, that it isimpossible to demonstrate that a stimulus isan inhibitor of something unless there issomething present to inhibit.

The summation procedure has been usedwith a variety of kinds of stimuli playingthe role of So, the known elicitor of behavior.

1. Reduction of behavior elicited by anexcitatory CS: The most common summationprocedure employed by Pavlov involvedshowing that a suspected inhibitor reducedthe CR elicited by a second, excitatory CS,based on the same US. For instance, in oneexperiment he examined the reduction in sali-vation to an excitatory CS when it was pre-sented in conjunction with a stimulus whichhad repeatedly been presented to the dog buthad never been followed by food (Pavlov,1927, p. 77) . He found this reduction greaterthan that produced when the excitatory CSwas presented in conjunction with a novelstimulus. Pavlov concluded that a CS re-peatedly not followed by food became a condi-tioned inhibitor. Similar demonstrations ofconditioned inhibitors using this techniquewith an excitatory CS occur throughoutKonorski's (1948) book. A more recent ex-ample is found in a salivary conditioningexperiment by Szwejkowska (1957).

This technique has recently been used inthe demonstration that a stimulus consistentlyfollowed by the absence of shock can becomean inhibitor of fear. Bull and Overmier(1968) showed an increase in avoidancelatency in response to a positive conditionedstimulus (CS+) for shock when the CS+was also accompanied by a negative condi-tioned stimulus (CS — ) for shock. Similarly,

in an experiment by Hammond (1967), acombination of a CS— for shock with a CS+for shock showed less condition emotional re-sponse (CER) suppression than the CS +alone. With the GSR, Rodnick (1937) hasused the summation technique to detect in-hibition during the early part of a long-delayCS for shock. Rodnick's experiment is notablebecause it found transfer of conditioned in-hibition between two aversive USs.

Finally, a parallel procedure has evolvedfor demonstrating control of nonresponding inan operant situation. Both Brown and Jenkins(1967) and Cornell and Strub (1965) pre-sented negative stimuli (Ss—) in conjunctionwith discriminative stimuli for operant behav-ior. They interpreted reductions in the operantbehavior produced by these stimuli as evi-dence that they are conditioned inhibitors.

2. Reduction of behavior elicited by a US:Several authors have used an unconditionedstimulus as So in examining the reduction inbehavior produced by an SiSo combination.With salivary conditioning, Sergeev (1961)found that following extinction, a formerlyexcitatory CS now reduced the flow of salivanormally elicited by food. In contrast, duringconditioning, presentation of this CS had en-hanced the UR. Findings of the effects ofconditioned and extinguished stimuli upon theGSR and eyeblink URs which conflict withthose of Sergeev for salivary conditioninghave been reported by Kimble and Ost(1961) and Kimmel (1966). With those re-sponses, conditioned excitatory CSs seemed toinhibit the UR. It is interesting that thisfinding, too, has been interpreted by someas evidence for conditioned inhibition.

With stimulation of the motor cortex asthe US, Tchilingaryan (1963) and Wagner,Thomas, and Norton (1967) used shifts inthreshold value of the US needed to producea UR as a measure of conditioned inhibition.Tchilingaryan found an extinguished CS(which had formerly been paired with motorcortex stimulation eliciting paw flexion)raised the intensity of the US necessary toproduce the UR. A similar report was madeby Wagner et al. for a CS— which had neverbeen paired with motor cortex stimulation.Both of these authors used symmetrical down-

PAVLOV I AN CONDITIONED INHIBITION 81

ward shifts in UR threshold as a measure ofconditioned excitation.

3. Reduction in the general level of condi-tioned excitation: Rescorla and LoLordo(1965) detected conditioned inhibitors of fearby their effect in reducing unsignalled avoid-ance behavior. This behavior may be thoughtof as maintained by a general level of condi-tioned fear for which an explicit stimulus isunidentified, S^. Superimposition of stimuliupon unsignalled avoidance then amounts topresentation of S.rSi combinations. Reduc-tions in behavior produced by such super-impositions have been interpreted as evidencethat stimuli followed by absence of shock orused in a long-delay conditioning procedurewith a shock US become inhibitors of fear(Rescorla, 1967a; Rescorla & LoLordo,1965). Similarly, stimuli increasing the avoid-ance rate have been called conditioned ex-citors. Novel stimuli did not produce suchchanges in avoidance.

4. Stimulus generalization of inhibition:Following the establishment of a potentialconditioned inhibitor, several authors havetested for inhibition by presentation of stimulialong a continuum of similarity to the sup-posed inhibitor. For instance, Krasnagorski(1927) established several points along theskin of a dog as conditioned excitors for sali-vation and then extinguished the response toone point. Stimulation of this set of pointsfollowing extinction showed a differential re-sponse tendency which was minimal at theextinguished point. From these data, Pavlovreasoned that there was a fixed level of excita-tion along the continuum denned by thesepoints; inhibition differed among the pointsand the gradient was a result of summationof a fixed level of excitation with differentlevels of inhibition. Hovland (1937) per-formed a similar experiment with GSR andgave his results a similar interpretation. Inan operant-conditioning situation, some au-thors (e.g., Kalish & Haber, 1963) haveexamined modifications in excitatory stimulusgeneralization gradients produced by non-reinforcement of one point on the continuum.They interpreted such changes as indices ofinhibition.

All of these generalization tests are perhapsbest considered special cases of a summationprocedure. A level of excitation is establishedacross some continuum, either explicitly orthrough stimulus generalization; inhibition isthen produced at one point of the continuumand the resulting changes in the excitatorygradient reflect the summation of excitationand inhibition at each point of the dimension.In tests of generalization of inhibition, com-parison is made between the reduction pro-duced by some supposed inhibitor and thatproduced by a variety of stimuli not treatedso as to produce inhibition but varying insimilarity with that inhibitor. It should benoted that if the magnitude of inhibition isto be measured in this way, considerablyfewer metric assumptions about the summa-tion of excitation and inhibition are necessaryif the amount of the excitation is uniformthroughout the continuum.

Jenkins (1965) recently added an importantrefinement to the generalization procedurewithin an operant setting. He suggested thatto demonstrate that S— is an inhibitor, thecontinuum of stimuli in the generalizationtest must be orthogonal to any dimensionsspanning S— and any Ss+. Otherwise whatappear as inhibitory generalization gradientsmight be due to variations in excitationaround some S+. The assumption seems tobe that if the points on a generalization con-tinuum are "equally" distant from S + , as isassumed to be the case if the dimension isorthogonal to the S+ dimension, they shouldall have some uniform tendency to respond(excitation) which can summate with differ-ent degrees of inhibition which they alsoelicit. This technique recently has been suc-cessfully used by Hearst (1968) and Farthingand Hearst (1968). Failure to meet theJenkins criterion in most of the generalization-of-extinction experiments mentioned previ-ously (e.g., Hovland, 1937; Krasnagorski,1927) makes it impossible to identify thedecrement observed in those experiments asinhibition.

The problem, of course, is to find dimen-sions which are orthogonal to one another;this may not be as easy as it first sounds.Although the dimensions described by the


experimenter may be independent, these maynot be the stimuli to which the animal isresponding. Absence of a line may not havea sensible place on a dimension described byline orientation, but perhaps it does fit on adimension describing the probability of seeinga black area at the top of the visual fieldwhen the head is primarily held erect. Thisdifficulty in specifying the relevant dimensionis particularly troublesome since it is reason-able to expect that discrimination trainingmay itself determine the dimensions to whichthe organism is responding.

Unless the level of excitation along thestimulus dimension can be safely specified,the use of this generalization technique ishazardous. For if either the degree of excita-tion is nonuniform or if it is zero along thisdimension, then variations in responding tostimuli similar to S— are not accurate indicesof inhibition. It is of particular importance todemonstrate independently the presence ofsome level of excitation at each point of thiscontinuum if lack of inhibitory control is tobe inferred from failure to find an inhibitorygradient (cf. Farthing & Hearst, 1968;Terrace, 1966).

The use of any of these summation tech-niques is not without its problems. A fre-quently raised difficulty is that the testpresentation of the S0St combination simplyinvolves presentation of a stimulus complexwhich varies in similarity to the conditionedSo; such decrements as may be observed canthen be attributed to stimulus generalizationdecrement with reference to inhibitory controlof Si. It is clear that such effects of Si canbe assessed by appropriate control proceduresdesigned to identify the treatment of Sinecessary to make it an inhibitor. This gen-eral problem of control procedures is dis-cussed in a subsequent section.

A more subtle problem of presenting Si inconjunction with So arises from the possibilityof differential attention to the two stimuli.The treatment designed to make Si aninhibitor may lead the organism to attend toSi to the detriment of attention to S0. ThenSi might decrement the response to S0 bydrawing attention away from stimuli normallymaintaining the response without actively

controlling a tendency opposite to that con-trolled by So. Presumably if this is the case,Si would disrupt the normal response to S0

largely without regard to the nature of theresponse it controlled or what US generatedthat response. Thus, in some cases it may beimportant to demonstrate that the inhibitorypower of Si is relatively specific to a particu-lar CR elicited by S0.

Retardation of the Development of CRs

A second class of procedures which havebeen used to demonstrate conditioned inhibi-tion focuses on the acquisition of responses re-sulting from excitatory conditioning. The argu-ment here is that if conditioned inhibitionand excitation are subtractive of one another,then setting up inhibition to a stimulus eitherprior to or simultaneously with excitatoryconditioning should retard the development ofan overt CR. Pavlov (1927, p. 302) reportedan experiment by Rickman in which acqui-sition of a salivary CR to a former CS —was greatly retarded.

Konorski (1948) made particular use ofthis criterion for the demonstration of condi-tioned inhibition. In an extended series ofinvestigations, workers in Konorski's labora-tory repeatedly interpreted treatments whichlead to retardation of the development ofsubsequent CRs to a CS as having establishedconditioned inhibition to that CS. Examplesof retarded development of a salivary CRto a former CS— for food can be found inKonorski and Szwejkowska (19S2b), S/wej-kowska (1959), and Szwejkowska and Konor-ski (1959).

Using a CER procedure, Hammond (1968)interpreted retardation of the development ofconditioned suppression to a former CS— forshock as evidence that the CS— had devel-oped into a conditioned inhibitor. Similarly,Carl ton and Vogel (1967) found interferencewith subsequent CER conditioning induced byrepeated CS presentation prior to condition-ing; this might be interpreted as evidence forconditioned inhibition. Precursors to the Carl-ton and Vogel finding can be found in thephenomenon of "latent inhibition" exploredby Lubow and Moore (1959) and Lubow(1965). In both of these studies the authors

PAVLOVIAN CONDITIONED INHIBITION 83

measured inhibition through retardation inthe development of subsequent leg-withdrawalconditioned responses in sheep and goats.

Similar interpretations have been made ofsome instrumental learning experiments. Forinstance, Bower and Grusec (1964) andTrapold and Fairlie (1965) performed dis-criminative Pavlovian conditioning with foodreinforcement in rats. They then trained therats to bar press in the presence of either theformer CS+ or the CS— for food. Althoughtheir interpretations are cautious, they havesuggested that retardation in learning to barpress in the presence of CS— might indicatethat it is a conditioned inhibitor for somePavlovian conditioned incentive-motivationstate.

In all of the above studies, proceduresthought to produce conditioned inhibitionhave been administered prior to trying to setup a CR. Recently, Rescorla (1968b) re-ported that presentation of shock USs in theintertrial interval during excitatory fear con-ditioning can interfere with the developmentof that CR. He suggested that one interpre-tation might be that such shocks lead to theconditioning of inhibition to the CS, thus dis-rupting the appearance of the overt CR. Herethe development of inhibition occurs simul-taneously with the establishment of a CR.

One problem of the retardation of learningas an index of conditioned inhibition is thatsuch learning decrement might result fromother processes. A particularly popular alter-native lies in a selective-attention mechanism.There is now considerable evidence which in-dicates that various conditioning proceduresnot only establish conditioned responses butalso lead animals to attend selectively amongstimulus events. It is reasonable to expectthat an organism which comes to the experi-mental setting attending to stimulus dimen-sions other than that of the CS will be re-tarded in the development of a CR. For in-stance, in the latent-inhibition experiments,the repeated presentation of a CS in the ab-sence of USs might lead the organism tocease attending to the CS without that CSgaining any control over tendencies oppositeto subsequently established CRs. If such werethe case, then the summation technique would

presumably fail to show any inhibitory effectof the CS despite the fact that development ofa CR to the CS is retarded.

A final comment should be made about theretardation and summation techniques. Theyboth determine whether or not a stimulus is aconditioned inhibitor in the sense of having anet response tendency directly opposite toconditioned excitation. It is entirely conceiva-ble that a stimulus have conditioned to itboth excitation and inhibition; it would onlybe deemed a conditioned inhibitor by theseassessment techniques if the conditioned in-hibition is greater. Naturally, similar consid-erations apply to the identification of a con-ditioned excitor. At this point it seems likelythat identification of such "hidden" responsetendencies requires the use of a theory ofinhibition. The assessment techniques dis-cussed below make such additional theoreticalassumptions about inhibition.

Criteria Dependent upon Special Properties ojInhibition

In addition to the above two criteria fordetecting conditioned inhibition, several morespecialized techniques have been proposed.These depend upon phenomena which wereimportant to Pavlov in postulating an inhibi-tory process but which are not central to thenotion of conditioned inhibition as definedhere.

Pavlov noticed that following extinction ofa formerly excitatory CR, a rest period al-lowed the CR to recover much of its strength.He termed this finding "spontaneous recov-ery" and argued from it that the excitatorytendency must have persisted through ex-tinction ; else, how could it reassert itself aftera rest period, without further training? Simi-larly, Pavlov found that if a novel stimuluswas presented in conjunction with an ex-tinguished CS, some of the excitatory CRreappeared, so-called disinhibition. Again, thiswas taken as evidence that the excitatorytendency persisted through extinction andwas simply overcome by a superimposed in-hibitory tendency. By this account, presenta-tion of the novel stimulus disrupted the inhi-bition, leading to an increased response.


The deduction from these findings that theCS has inhibitory properties depends upon theassumption that conditioned inhibition issomehow less stable and more easily disruptedby external stimuli or the passage of timethan is conditioned excitation. This may infact be the case, but there is little empiricalevidence that demands this assumption.Furthermore, this interpretation of spontane-ous recovery and disinhibition depends uponthe assumption that even though conditionedinhibition has developed to a stimulus, theexcitatory tendency has survived whateveroperations lead to the development of inhibi-tion. It is this assumption, rather than thepresence of conditioned inhibition, to whichthe phenomena appear most relevant. It isperhaps more reasonable to account for thesephenomena as special cases of stimulus gen-eralization rather than as demonstrations ofinhibition. In any case, it is clear that condi-tioned inhibition in the sense of controlling atendency opposite to an excitator could de-velop to a stimulus and be detectable by, forinstance, the summation technique in the faceof failure to demonstrate either spontaneousrecovery or disinhibition. These phenomenadepend upon the two further assumptions thatconditioned inhibition is less stable than exci-tation and that conditioned excitation per-sists through conditioned-inhibition treat-ments.

A third phenomenon has often been in-terpreted as evidence for inhibition: induc-tion. Induction was first described by Pavlovas the enhancement of the reaction to a condi-tioned excitor when it is presented just subse-quent to a conditioned inhibitor. The argu-ment is that removal of a supposed inhibitorhas an aftereffect which is opposite to theinhibitory process itself. This in turn sum-mates with the excitor, thereby enhancing itsresponse. An analogous wave of inhibition wassupposed to follow the termination of a con-ditioned excitor. A modern parallel to thisPavlovian observation can be found in be-havioral contrast in operant situations. Herethe introduction of a nonreinforced S— leadsto the enhancement of responding to a rein-forced S+. Several authors have been tempted

to attribute to S— inhibitory properties as aresult of this observation.

The status of that inference is somewhatunclear. Pavlov had in mind a particularmodel of the operation of inhibition in whichthis overshooting aftereffect played an integralrole. But there seems to be no logical neces-sity for such overshooting into excitation tofollow conditioned inhibition. Furthermore,Pavlov himself often described the converseeffect, so-called inhibitory aftereffect, inwhich termination of a conditioned inhibitorcould reduce the response to a subsequentCS+. This, too, he interpreted as evidencefor conditioned inhibition. This overshootingseems not central to the present conception ofconditioned inhibition but rather tied to aparticular theory of inhibition. It remains anempirical question whether induction covarieswith inhibition as measured by the summa-tion and retardation techniques.

In summary, several techniques for themeasurement of conditioned inhibition havebeen reviewed.5 Some of these techniques de-pend upon properties of a conditioned inhibitorother than its ability to produce a tendencyopposite to that of a conditioned excitor. Theinference from these procedures is accord-ingly less secure. The summation and theretardation-of-the-acquisition-of-a-CR tech-niques remain as relatively direct measures

5 The assessment techniques reviewed here allconcern the ability of a CS to evoke responses. Itshould be acknowledged that in some cases excitationand inhibition might be assessed through the abilityof a CS to reinforce responses. Particularly in thecase of Pavlovian fear conditioning, issues havegrown up over the establishment of positive condi-tioned reinforcers through relationships a CS mightbear to an aversive US. In some cases (e.g., Beck,1961; Rescorla, 1969b; Weisman, Denny, & Zerbokio,1967), the conditioning procedures are similar tothose discussed here as setting up conditioned inhib-itors ; however, the results of the conditioning areassessed through the ability of the stimulus to rein-force responses. One way of viewing many of theseexperiments assumes a level of excitation (fear)maintained by other stimuli in the situation, thepresentation of the treated stimulus upon executionof a response inhibits that fear and thus reinforcesthe response. However, the question of positiveconditioned reinforcement in this kind of situationinvolves an extensive literature of its own whichcannot be discussed in detail here.


of conditioned inhibition. They most straight-forwardly measure the ability of a stimulusto control a tendency opposite to excitation.

Finally, some comment should be madeabout the relationship between the conceptsof conditioned inhibition and attention. Aspointed out previously, in many instances,shifts in the attention of the organism amongvarious stimuli will produce effects which aresimilar to those of conditioned inhibition.Conceptually the notions are clearly separa-ble. Conditioned inhibition refers to an effectwhich is specifically opposite to conditionedexcitation; a conditioned inhibitor is specificto the reduction of a particular response. Incontrast, attention, as the term is typicallyused, refers to the effectiveness of a stimulusin producing a response, regardless of whatthat response may be. Given a stimulus Sowhich elicits a response, a second stimulus Siproducing a shift in attention will modify theeffectiveness of S0 independently of the natureof the response that So elicits and the USupon which it is based. However, an Si con-trolling conditioned inhibition should affectthe response to S0 maximally when relatedUSs are used to set up the response to So andthe inhibition to Si. Of course a particulartreatment may endow a stimulus with bothconditioned-inhibition and attention-shiftingproperties.

It is experimentally possible to make arough evaluation of conditioned inhibitionand shifts in attention; if the effect of Si inmodifying So's reaction depends heavily uponthe nature of that reaction, then St is pri-marily a conditioned inhibitor rather than astimulus controlling shifts in attention. Evi-dence on the specificity of conditioned inhi-bition has been reported in salivary condi-tioning by Konorski (1967). Konorski re-ported retardation in salivary conditioning toa former CS— for food but rapid defense con-ditioning to the same CS. The retarded condi-tioning to CS— was dependent upon the na-ture of the response being conditioned, thusindicating that conditioned inhibition ratherthan attention was responsible for the retarda-tion. Similarly the author has unpublisheddata indicating that a conditioned inhibitor

based on shock, although retarded in the sub-sequent acquisition of fear, is not retarded indeveloping a discriminative stimulus (SD)function for food-reinforced bar pressing.

Rescorla and Solomon (1967) reviewedvarious studies of conditioned excitation andinhibition resulting from use of positive andaversive USs. In these studies, which assessPavlovian CRs through their effects on in-strumental behavior, the effect of a Pavlovianconditioned inhibitor was not independent ofthe reinforcer maintaining the instrumentalbehavior. Thus conditioned inhibitors derivedfrom shock disrupt shock-motivated avoid-ance behavior but facilitate or leave unaf-fected food-motivated behavior. Each ofthese effects is specifically opposite to theeffect of a conditioned excitor based on shock.Furthermore, conditioned inhibitors based onfood have quite different effects in these situ-ations. Thus, for some kinds of conditionedinhibitor, there is evidence that the inhibitoris not a general disrupter of behavior but hasrelatively specific effects. Unfortunately,cases in which an investigator of conditionedinhibition reports this sort of evidence arerare.

Even within the two measurement tech-niques for conditioned inhibition discussedhere it may be possible to distinguish betweenconditioned inhibition and attention. It seemsreasonable that a stimulus to which the or-ganism does not attend will be retarded inacquisition of an excitatory CR but will pro-duce little effect in the summation procedure.On the other hand, a stimulus which attractsattention might be expected to produce decre-ments in the summation testing procedure butto lead to facilitated acquisition of an excita-tory CR. Thus if a stimulus affects the atten-tion of the organism it should not behave likea conditioned inhibitor in both the summationand retardation-of-acquisition test procedures.For this reason when attentional accountsseem plausible, it may be valuable to haveinformation from both of these procedures fora stimulus thought to be a conditioned inhibi-tor.


PROCEDURES LEADING TO THE DEVELOPMENTOF CONDITIONED INHIBITION

A variety of different procedures have beenthought to produce conditioned inhibitors. Inthis section, the evidence concerning some ofthose procedures is reviewed briefly.

First, however, another question must beconsidered: How do we identify an inhibitoras conditioned? The previous section reviewedprocedures for asserting that a particularstimulus was an inhibitor but paid little at-tention to the question of how it became aninhibitor. What criteria must be satisfied inorder to assert that a stimulus acquires itsinhibitory power through a conditioningprocess? The view taken here is that condi-tioned inhibition is an associative process, re-sulting from some relation arranged betweenCSs and USs in much the same way thatconditioned excitation is an associative processresulting from such a relation. To be called aconditioned inhibitor, a stimulus must acquireits inhibitory power through some relation itbears to a US in the past experience of theorganism.

If conditioned inhibition is an associativeprocess, then it needs to be evaluated againstthe background of nonassociative changesthat normally take place in conditioning ex-periments. The problem here is parallel tothat in excitatory conditioning. How do weknow that the changes produced when wearrange relations between CS and US dependupon those relations? Perhaps, instead, theobserved changes in the reaction to the CScould be generated, for example, by simplypresenting the US repeatedly. The use of boththe summation and the retardation-of-learningtechniques must include comparisons with theeffects that control CSs produce in these situ-ations. This problem of controls for nonasso-ciative changes has been dealt with for yearsin the case of excitatory conditioning, but lit-tle attention has been paid to the problem ofnonassociative effects in the evaluation of in-hibitory conditioning.

Recently, Rescorla (1967b) reviewed theproblem of specifying nonassociative controlprocedures for excitatory conditioning. Hesuggested a reformulation of excitatory con-ditioning operations and a novel, companion

control procedure for those operations. Ac-cording to this suggestion, the critical opera-tion for excitatory conditioning is a positivecontingency between the CS and US. Such apositive contingency results when the proba-bility of a US is greater following CS onsetthan at other times. On this view, the ap-propriate control procedure for nonassociativeprocesses during conditioning is one involvingpresentation of both CS and US but with nocontingency whatsoever between them, that is,one in which the probability of the US isequal in the presence and absence of the CS.This is best accomplished by randomly dis-tributing CS and US events in time. This con-trol procedure, the "truly random" procedure,might well serve as a control for nonassocia-tive factors in both excitatory and inhibitoryconditioning. Within this scheme, it is reason-able to expect that operations involving anegative contingency between the CS and USwould result in the development of condi-tioned inhibition.

It should be noted that this view of Pav-lovian conditioning differs from that mostfrequently encountered. According to most ac-counts, it is the pairing of CS and US ratherthan the contingency of one upon the otherthat counts in conditioning. On the pairingview, the most appropriate control procedurefor nonassociative effects during excitatoryconditioning would involve presentation of CSand US without pairing them. The most popu-lar procedure for doing this might be calledthe "explicitly unpaired" procedure. In thisprocedure, a subject receives both the CS andthe US, but their occurrence is restricted totimes when the other is not present. Thepairing view of Pavlovian conditioning doesnot specify an operation leading to the devel-opment of inhibition nor a control procedureappropriate to such operations.

In good part, the difference between thetwo views of conditioning centers aroundtheir handling of conditioned inhibition. Onthe contingency view, the "explicitly un-paired" control procedure meets the condi-tions for the development of inhibition; incontrast, from the pairing viewpoint, it issimply a condition in which excitation fails todevelop. This is partly a semantic matter, it


being arbitrary where on a contingency con-tinuum a "neutral" point is placed. However,the contingency viewpoint does seem to gen-erate predictions which do not easily followfrom the pairing view. For this reason, partsof the remainder of this section are presentedwith the contingency view in mind.

Most of the experiments reviewed here havenot been conducted from the contingencyviewpoint. Consequently, it is often the casethat what is considered an appropriate non-associative control from that viewpoint is notavailable. Instead comparison will have to bemade with more traditional control proce-dures. This in itself may not be too serioussince something is known of the properties ofsome of those control procedures. The mostfrequently used control procedures in the fol-lowing studies are comparison with a novelCS or with a CS which has simply been re-peatedly presented to the subject in the ab-sence of all US events. It should be clearthat these procedures are not equivalent to theexperimental procedure in all respects exceptthe relation between a CS and US. For in-stance, neither involves prior experience of theorganism with the US, while the experimentalprocedures discussed do. A similar criticismapplies to the use of generalization gradientsas a control; other stimuli on the dimensionare simply examples of the novel CS control.In the absence of further controls it cannot beasserted that the orderly gradient is due to aconditioning procedure and not simply fa-miliarity with the original CS.

Particular notice should be made of thecontrol procedure involving repeated presenta-tion of the CS in the absence of all USs. Thisprocedure is often called the "latent inhibi-tion" procedure because in some situations ithas been found to retard the development ofa subsequently conditioned excitor. From thepoint of view adopted here this procedure can-not properly be said to produce a conditionedinhibitor; the reason for this is that the stimu-lus does not attain its inhibitory powerthrough any relation with the US. How canthis inhibition result from learning a relationbetween the CS and US when no US has beenpresented? Furthermore, prior to the attemptto produce excitatory conditioning, there is

nothing in the latent-inhibition procedurewhich is specific to the particular US and CRthat will be used. It is difficult to believe thatsuch a stimulus could control a tendencyspecifically opposite to the excitation aboutto be set up, no matter which excitation it is.Rather than speak of such retardation ofthe acquisition of an excitatory CR as due toconditioned inhibition, it may be better toconsider it as due to some general tendencyelicited by the stimulus such as failure toattend to the CS. In this connection it maybe noted that the evidence for the latent in-hibition comes exclusively from studies usingretardation of the development of excitatoryCRs as the measure of inhibition.

Nonreinforcement of a PreviouslyReinforced CS

One of the early phenomena which ledPavlov to postulate an inhibitory process wasthe extinction of a CR with the omission ofthe US following the CS. He conceived of thisas the building up of inhibition to the CSwhich subtracted from a maintained level ofexcitation. Although recently this conceptionof extinction has not been popular, there re-mains the empirical possibility that with ex-tinction a formerly excitatory CS becomes aconditioned inhibitor. Indeed, in the foreignliterature, extinguished CSs are often de-scribed as inhibitory stimuli.

There is only meager evidence bearing onthis question. Although Pavlov claimed thatan extinguished CS was a conditioned inhibi-tor, he in fact never presented evidence whichsatisfies the definition adopted here. He nevermentioned evidence showing that an extin-guished CS reduces the CR elicited by aknown excitor or that an extinguished CS isharder to recondition than a fresh CS. Thereare isolated Russian studies (e.g., Sergeev1961; Tchilingaryan, 1963) claiming that anextinguished CS will reduce the UR in asalivary conditioning situation. Unfortu-nately, insufficient control procedures are re-ported for us to be certain that prior condi-tioning and extinction of the CS is requiredfor a stimulus to have this effect. Furthermore,in most eastern European studies, extinctionof a CS is carried out in a context of con-

ROBERT A. RESCORLA

tinued reinforcement of other CSs. Thus, ineffect, the CS is a discriminative stimulus,CS—; this is perhaps quite a different opera-tion than simple omission of all reinforce-ment.

A few American studies have used a sum-mation technique to evaluate extinguishedfear-eliciting CSs (e.g., LoLordo & Rescorla,1966; Rescorla, 1967a). Typically, these havefound that extinguished CSs do not becomeinhibitors of fear but simply return to thestatus of preconditioned stimuli.

The evidence from studies of retardationof learning is more clear. There seems to belittle question that a formerly conditionedstimulus, no matter how much extinction in-tervenes, will recondition faster than a novelstimulus condition. In simple salivary con-ditioning, for instance, Konorski and Szwej-kowska (1950, 1952b) provided ample evi-dence for this assertion. Pavlov himself (1927,p. 59) reported not retarded but rapid condi-tioning of an extinguished CS.

Once again, care must be taken in interpret-ing negative evidence from such relearningstudies as unequivocally against the inhibi-tory proposition. Animals previously condi-tioned to a CS presumably have learned toattend to that stimulus dimension; it is notunreasonable to expect this to facilitate sub-sequent reacquisition even to the point ofovercoming inhibition. Second, the presence ofthe US within an experiment is an importantcomponent of the functional CS. Simple re-occurrence of the US may lead to reappear-ance of the CR even in the absence of a rein-forcing relation between the CS and US. Inhi-bition may not have been built up to thatpart of the stimulus complex. Thus althoughan extinguished CS might become a condi-tioned inhibitor, a relearning procedure mightfail to detect it. Nevertheless, in the absenceof any strong positive evidence for conditionedinhibition being elicited by an extinguishedCS from summation studies, the strong nega-tive evidence from relearning studies seemsparticularly damaging. We can conclude onlythat there is no reason to believe that anextinguished CS is a conditioned inhibitor.

Discriminative (CS—)

A second procedure which Pavlov suggestedestablished a CS as a conditioned inhibitor isdiscriminative conditioning in which CS+ isconsistently followed by the US and CS— isnot. Under these circumstances, Pavlovclaimed that CS— came to control a tendencyopposite to that of CS + . Using a summationtechnique, Pavlov himself demonstrated thatCS— would reduce the salivation elicited byCS+; he further showed this reduction to begreater than that produced by a novel CS.Since that time a great deal of additionalpositive evidence has accumulated. Below arelisted a few examples illustrating that a CS —in Pavlovian conditioning takes on conditionedinhibitory properties.

The evidence for the conditioned inhibitoryproperties of CS— in salivary conditioningcomes mainly from Konorski's laboratory. Us-ing a summation technique with CS+ forfood as the known response elicitor, Szwej-kowska (1957) and Szwejkowska and Konor-ski (1959) both found that presentation ofCS— depressed the flow of saliva. However,neither of these studies reported evidence thatthe CS— required any special treatment toserve as an inhibitor; presumably it did nothave this effect prior to its contrasted presen-tation with food, but such evidence is notreported. In a study with the retardation-of-conditioning procedure, Konorski and Szwej-kowska (1952b) however did find that a CS-for food could be transformed into a CS+for food only with considerably more trainingthan that required by a novel stimulus. Theycited in support of their results the experi-ments of various other eastern European au-thors.

As mentioned earlier, Bower and Grusec(1964) and Trapold and Fairlie (1965) founda CS— for food retarded in acquisition whenused as an SD for food-reinforced bar pressing.However, comparison was made only betweena CS— and CS+ for food; acquisition for theCS— might not have been retarded whencompared with appropriate controls. Morerecently, Trapold, Lawton, Dick, and Gross(1968) reported that a CS- for food devel-oped more rapidly as an SA for food-rein-

PAVLOV1AN CONDITIONED INHIBITION 89

forced bar pressing than did a control stimu-lus. However, they did not find parallel re-tardation when that CS — was used as an S1'.These results have been taken by the authorsas evidence that this stimulus was an inhibi-tor of a Pavlovian incentive motivator in-volved in instrumental behavior.

One study with paw flexion and motor cor-tex stimulation by Wagner et al. (1967) foundevidence for inhibition from a CS —. A CS —for cortical stimulation raised the thresholdvalue of the cortical US necessary to producea UR. Prior to conditioning, the CS did nothave this effect.

Perhaps the best evidence for inhibition toa discriminated stimulus comes from studiesusing peripheral electric shock as the US.Hammond (1967), using a CER procedure inwhich fear is measured by disruption of on-going appetitively motivated behavior, founda CS— reduced the suppression normallyelicited by a CS+. Furthermore, the effect ofthe CS— was greater than the effect of astimulus treated in a "truly random" controlprocedure. Hoffman (1968) measured inhibi-tory generalization gradients around CS—.When stimuli varying in similarity to CS —were superimposed on CS + , appropriate vari-ations in disruption of CER suppression wereobserved. Using summation with a base-linelevel of excitation in a Sidman avoidanceschedule, Rescorla and LoLordo (1965) foundthat a CS— inhibited fear in the sense of dis-rupting the avoidance response. Within thatsituation, a variety of controls have beenshown to disrupt the avoidance very little:novel CS, CS alone, truly random CS. Gros-sen and Bolles (1968) repeated these resultswith rats, and Bull and Overmier (1968) ob-tained similar findings using summation withan SD for signalled avoidance. Hammond hasfound that a CS— for shock produced ananalogous increase in the ongoing level ofappetitively maintained bar pressing. A con-trol stimulus produced no change in bar press-ing.

Gluck and Rowland (1959) studied theeffects of CS+ and CS- for shock upon EECpattern in sleep-deprived cats. They foundthat CS+ produced signs of activation; CS—,in contrast, induced spindling (a sign of re-

laxation) in some cases when superimposedon CS+. This latter effect was not obtainedprior to using the stimulus as a CS— forshock. Grings and O'Donnell (1956) found aCS— for shock reduced the GSR normallyelicited by a CS+ in humans. A novel CSproduced less decrement in the reaction toCS+.

Evidence for inhibition with a shock USalso comes from several studies on retardationof the subsequent acquisition of fear. For in-stance, Hammond (1968) reported that aformer CS— for shock is harder to establishas a CS+ for shock than either a truly randomcontrol stimulus or a stimulus simply pre-sented in the total absence of shock.

In sum, the evidence that a CS— for foodis a conditioned inhibitor for salivation issparse. Such a stimulus does produce inhibi-tion as measured by both the summation andretardation-of-CR techniques; however, ap-propriate control results are not always avail-able to indicate that this inhibition is learned.On the other hand, the more recent studieswith conditioned inhibition when a shock USis used are better controlled. These studiesgive strong support to the attribution of con-ditioned inhibition to a CS— for shock. Be-yond these two USs, tests for inhibition toCS— have rarely been performed. The mostcommon observation is that some difference inresponse occurs to CS— and CS+, but it isnot common to test for the inhibitory proper-ties of CS— compared with the effects of acontrol stimulus. Thus for many US modali-ties we simply do not have the appropriateevidence to decide whether or not a CS— isa conditioned inhibitor.

Stimuli Negatively Correlated with the US

It is often said that discrimination learningconsists, in part, of a combination of condi-tioning and extinction; for this reason CS— isoften treated in the same fashion as an ex-tinguished CS+. As pointed out above, CS —is much more than that; it is a stimulus con-sistently followed by no US despite the con-tinued occurrence of the US at other times. Itis worth asking whether it is this aspect ofCS— that might give it its inhibitory prop-erties. Consider a situation in which a US can


occur at various times but never occurs duringor closely following a CS. Under these circum-stances the CS and US are negatively corre-lated in time; the CS predicts the absence ofthe US. According to the contingency notionof conditioning such a stimulus (of whichCS— is an example) should take on inhibitoryproperties.

Kosteneckaja (cited in Konorski & Szwej-kowska, 19S2b) found a CS not followed byfood in a situation in which unsignalled foodis otherwise occurring becomes a conditionedinhibitor. Here difficulty in the subsequentestablishment of a CR was the index of inhi-bition.

Virtually all of the recent evidence for theassertion that a conditioned inhibitor resultswhen a stimulus is negatively correlated withthe US comes from situations using a shockUS. One special case of such a stimulus isthat generated by the "explicitly unpaired"procedure in which the US neither precedesnor follows the CS closely in time. Rescorlaand LoLordo (1965) used summation with abase line of fear maintaining Sidman avoid-ance in dogs to assess the inhibitory proper-ties of such a stimulus explicitly unpairedwith shock. They found that a CS kept tem-porally distant from shock during indepen-dently administered Pavlovian conditioningsubsequently produced depression of theavoidance rate. They inferred from this thatit was a conditioned inhibitor of fear. Asmentioned earlier, a variety of control pro-cedures, including the truly random proce-dure, yield stimuli which have little effect inthis kind of summation test procedure.

Rescorla (1966) used a similar procedure toexamine a more general case of negative CS-US contingency. In that experiment a CS pre-dicted the nonoccurrence of shock in a con-text of otherwise randomly occurring shocks.Here shocks could precede that CS, but theCS forecast a time of decreased shock proba-bility. Again the conditioned inhibitoryproperties of that CS were indicated by itsability to disrupt ongoing avoidance behavior.Furthermore, in a recent parametric investi-gation, Rescorla (1969a) found that greaternegative contingencies between a CS andshock made that CS a stronger conditioned

inhibitor. This result occurred in a summa-tion procedure with CER conditioning inrats.

Retardation of the acquisition of fear to aCS has also been used to assess the degree ofinhibition conditioned to a stimulus nega-tively correlated with shock. Rescorla(1968b) performed CER conditioning on ratswith a variety of probabilities of shock dur-ing a CS+. For different groups, shock alsooccurred with various probabilities in theabsence of the CS+. He found that as theprobability of shock in the absence of the CSincreased, the amount of fear elicited by theCS decreased. One way of viewing this findingis that the non-CS shocks are setting up con-ditioned inhibition to the CS and thus retard-ing the development of an overt fear CR. In asecond experiment, Rescorla (1969a) mademore explicit use of the retardation technique.He compared the development of conditionedsuppression to CSs which had a history ofnegative correlation with shock. The CS hadforecast the absence of shock in the contextof otherwise randomly occurring shocks; theprobability of shocks in the non-CS periodvaried among the groups. The notion was thatgreater conditioned inhibition should accrueto a stimulus predicting absence of a highershock probability. Truly random and US-alone procedures were used in control groups.At the values used, a stimulus previously con-trasted with a higher probability of shock washarder to condition as a CS+ for shock. Aswith the summation assessment procedure, theretardation technique indicates not only thatnegative CS-US contingencies yield condi-tioned inhibitors but also that the magnitudeof the conditioned inhibition is greater withgreater negative contingencies.

With CSs negatively correlated with shock,as with CS—, the evidence on inhibition israre largely because the appropriate tests havenot often been made. This, of course, is par-ticularly true of the "explicitly unpaired"procedure because its extensive use as a con-trol procedure shifts interest away from it asa procedure important in its own right. How-ever, at least with a shock US, there is nowaccumulating evidence that under some cir-


cumstances such stimuli become conditionedinhibitors of fear.

Inhibition of Delay

If an extended CS-US interval is used inconditioning, it is often observed that the CRbecomes confined to the later part of the CS.Pavlov (1927) interpreted this finding as in-dicating that the early portion of an extendedCS becomes inhibitory. According to Pavlov'sinterpretation, the early part of the CS elic-its both excitation and inhibition; with con-tinued training the amount of inhibition be-comes greater than that of excitation. He sup-ported this interpretation with a variety ofkinds of evidence, for example, disinhibition,and decrease in salivation to CS onset overtrials. In the present context, the most rele-vant evidence is that the early portion of sucha CS reduced the salivation which wouldnormally have been elicited by a secondexcitatory CS. There is very little additionalevidence on the inhibitory control of the earlypart of such a CS. Barkhundarian (1960)found that the onset of a long-delay CS forfood decreased the response normally pro-duced by a second CS. However, the absenceof relevant control procedures makes thisfinding difficult to interpret.

With shock as a US, only two studies havereported evidence relevant to the notion ofinhibition as used here. Rodnick (1937)found the early portion of a CS+ for shockin GSR conditioning with humans reduced anair-puff-based eyeblink CR normally elicitedby another CS. A control group indicatedthat without prior conditioning this CS didnot affect the eyeblink. The second set ofstudies (Rescorla, 1967a) involved changesinduced in avoidance rate by a CS previouslypaired with shock after an extended CS-USinterval. Such a long-delay CS induced avoid-ance rate depression at its onset; as the CScontinued the rate grew monotonically to amaximum well above the pre-CS rate. Compa-rable results were obtained with long CS-USintervals in a trace-conditioning procedure(Rescorla, 1968a). Rescorla interpreted theseresults as indicating that the onset of the CSwas a conditioned inhibitor of fear. CS—alone and "truly random" control procedures

did not yield CSs whose onset depressed theavoidance rate. Thus there is scattered evi-dence for both salivary and fear conditioningthat the early portion of a long-delay CS is aconditioned inhibitor when the summationtechnique is used.

Very few experiments have assessed the in-hibitory effects of the early part of a long-delay CS using the retardation-of-learningprocedure. Presumably the proper experimentwould find it more difficult to condition a CSwith a short CS-US interval if the CS had ahistory of extensive conditioning with anextended CS-US interval. Konorski andSzwejkowska (19S2b) cited several easternEuropean studies in which this kind of resultwas obtained for salivary conditioning. How-ever, there appears to be no more recent evi-dence on this point.

Stimuli Signalling US Termination

An active line of research in the field ofreinforcement has concerned whether stimulisignalling the termination of a US becomereinforcers. Equally, one may ask whethersuch stimuli become inhibitors of CRs basedon US onset. This is of particular interestsince Konorski (1948) suggested that theparadigm for establishing a conditioned in-hibitor is pairing a CS with a fall in level ofexcitation. Segundo, Galeano, Sommer-Smith,and Roig (1961) reported that stimuli signal-ling the termination of a long duration shockproduce behavioral relaxation and correspond-ing EEC changes in cats. Prior to this condi-tioning such CSs did not have these effects.

Two experiments related to this problemhave been performed by Zbrozyna (19S8a,19S8b). In those experiments, stimuli pre-ceded the withdrawal of food. The basic find-ing was that during such stimuli the dogstopped eating in anticipation of food removal.Possibly this reflects inhibition of the re-sponse normally elicited by the food US.However, no data are presented on inhibi-tion of a Pavlovian salivary UR.

Braud (1968) recently associated stimuliwith the termination of a CS+ for shockrather than with the termination of the USitself. Such stimuli, when subsequently maderesponse contingent, reduced the suppression


normally elicited by fear CSs. Since response-independent CS presentation was not used, itis not clear whether these results should beinterpreted in terms of conditioned inhibitionor conditioned reinforcement.

An interesting confounding arises in mostof the experiments involving stimuli signallingthe termination of a US. When a stimulusprecedes the end of the US it signals not onlyUS termination but also the coming of theintertrial interval. It generally forecasts notonly the end of the present US but also atime free from the onset of the next US. Inthat sense, the CS bears a close resemblanceto the explicitly unpaired procedure. One maythen ask whether the inhibition which maybecome conditioned to such a CS results fromits preceding US termination or from itspreceding a period which is free from the on-set of the next US. Recently, Moskowitz andLoLordo (1968) addressed just this issue.After finding that stimuli bearing a backward-conditioning relation to a shock US becomeconditioned inhibitors of fear, they attemptedto tease these variables apart. They foundthat a stimulus signalling a period free fromshock served as an inhibitor whether or not itwas associated with US termination. But astimulus associated with US termination onlybecame a conditioned inhibitor if it precededa period relatively free from shock. The up-shot is that presenting a stimulus in conjunc-tion with the termination of a US may yielda conditioned inhibitor primarily because ofthis confounding with prediction of a periodfree from shock, that is, because it is nega-tively correlated with shock onset.

SUMMARY AND CONCLUSIONS

The present paper has reviewed the notionof conditioned inhibitor and has suggestedthat it is best viewed as a stimulus whichcomes, through learning, to control a ten-dency directly opposite to that of a condi-tioned excitor. To qualify as a conditionedinhibitor, the action of a stimulus must belargely confined to responses normally elicitedby conditioned excitors based upon the sameUS. Furthermore, conditioned inhibitors areviewed as resulting from associative relationsbetween a CS and US in a way parallel to

conditioned excitation. Conditioned inhibitioncomes about as the result of learning rela-tions among CSs and USs.

Two techniques of measuring conditionedinhibition are suggested: summation, the re-duction in the response normally elicited byan excitatory CS; and retardation of excita-tory conditioning to the supposed inhibitor.Other procedures for measuring conditionedinhibition are discarded because they fail tomeasure the ability of a stimulus to control atendency opposite to conditioned excitation.

A variety of procedures thought to estab-lish stimuli as conditioned inhibitors havebeen reviewed. In both salivary and fear con-ditioning, some relations between CS and USlead to the development of conditioned inhi-bition to the CS as measured by both the re-tardation-of-excitatory-conditioning and thesummation procedures. For other kinds ofconditioning the evidence is too scattered tomake a positive statement. As yet there isnot sufficient experimental analysis to makefinal assertions about the critical conditionsfor producing conditioned inhibition. Extin-guished CSs and CSs preceding US termina-tion do not seem to become conditionedinhibitors; on the other hand, discriminativestimuli, stimuli negatively correlated with theUS, and possibly long-delay CSs do becomeinhibitors.

Most of the experiments which have gen-erated conditioned inhibitors can be describedas using procedures in which the CS and USare negatively correlated in time. That is,most conditioned inhibitors seem to be stim-uli which forecast a decrease in the probabil-ity of occurrence of the US. This generalstatement applies to the case of discriminativestimuli, stimuli explicitly unpaired with theUS, and those positive results with stimulipreceding the termination of the US, as wellas to situations in which a negative relationwas explicitly arranged. This conclusion en-courages the view that Pavlovian conditioningis best described as arising from the contin-gencies arranged between CS and US. Accord-ing to such a view, positive CS-US contingen-cies lead to the development of conditionedexcitation and negative contingencies lead tothe development of conditioned inhibition.


This view receives further encouragementfrom the finding that greater negative cor-relations between CS and US result in greaterconditioned inhibition; this parallels the supe-rior excitatory conditioning found with greaterpositive correlations between CS and US. Ifviewed in this way, conditioned inhibition hasa status equal to that of conditioned excita-tion. Both kinds of learning result fromvariations on a CS-US contingency continuum.Indeed, the definition of conditioned inhibi-tion may be reformulated in terms moreparallel to that of conditioned excitation.Conditioned inhibition could be defined interms of the operation of arranging a nega-tive CS-US contingency and the outcome of aparticular change in behavior.

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(Received August 13, 1968)