The Role of Contextual Stimuli in the Blocking Paradigm

Psychobiology 1988, Vol. 16 (1), 59-66 The role of contextual stimuli in the blocking paradigm

MELANIE S. WEA VER and WILLIAM C. GORDON University of New Mexico, Albuquerque, New Mexico

Two experiments used rats in a conditioned lick-suppression paradigm to investigate the role of contextual stimuli in a traditional blocking paradigm. Experiment 1 demonstrated that the blocking effect could be significantly attenuated by changing contexts between the simple and compound stimulus conditioning phases of the blocking procedure. Experiment 2 demonstrated that the blocking efIect could also be attenuated when both phases occurred in the same context as long as substantial context-alone exposure (i.e., extinction) was given between the two train ing phases. Experiment 2 also showed that the attenuating efIects produced by extinguishing the training context could be alleviated if additional footshocks were given in either the training or a novel context, suggesting that context extinction may result in both a weakening of direct context-unconditioned stimulus (US) associations and adegradation ofthe US memory represen tation. These findings indicate that contextual associations can play an important role in producing the blocking effect. The potential use of contextual manipulations to distinguish between expla nations for blocking deficits shown by animals with hippocampal lesions is discussed.

It is weIl known that prior conditioning to one element 1972). Thus, if aUS is already fully predicted by stimuli of a reinforced compound conditioned stimulus (CS) often in the environment, new stimuli that occur contiguously reduces conditioning to the other element in the compound with the US will fail to become conditioned. According (Kamin, 1968, 1969). This phenomenon, known as "block to this view, the first phase of the blocking paradigm es ing," has had important implications for contemporary tablishes a given CS (CS1) as a reliable signal for US oc theories of conditioning. It suggests, for example, that the currence. In the second phase of training, when CS1 is contiguous occurrence of an effective CS and uncondi placed in compound with a novel CS (CS2) and the com tioned stimulus (US) does not necessarily ensure that the pound is reinforced, no conditioning occurs to CS2 be CS will become conditioned. On the contrary, condition cause the US is already predictable on the basis of the ing to a given CS often depends on the degree to which occurrence of CS 1. other stimuli in the situation already serve as reliable sig A second, related type of explanation is that any CS nals for US occurrence. that is paired with a fully predicted US decreases in More recently, the blocking phenomenon has also be salience and subsequently loses the capacity for entering come important in evaluating the function of certain brain into an association with that US (see Mackintosh, 1975; structures, such as the hippocampus. It has been shown, Moore & Stickney, 1980; Pearce & Hall, 1980). Thus, for example, that both rats and rabbits with hippocampal according to this view, the first phase of the blocking lesions often exhibit substantial attenuations of blocking procedure establishes CS1 as a reliable signal for the USo as compared with sham animals (e.g., Rickert, Bennett, In the second phase, the presence of CS 1 assures that the Lane, & French, 1978; Solomon, 1977). Thus, current novel CS2 will be paired with a fully predicted USo As theories of hippocampal function have had to account for a result, CS2 loses salience and, on subsequent trials, be why blocking deficits sometimes occur in lesioned organ comes less capable of entering into an association with isms (see Schmajuk, 1984). the USo Since the blocking effect has implications for both be It is dear that these two views make different predic havioral and psychobiological theories, it has become in tions conceming the blocking effect in some cases (e.g., creasingly important that we understand the mechanisms Dickinson, Nicholas, & Mackintosh, 1983). However, underlying the effect, as weIl as the conditions that con these hypotheses do share one important assumption. Ac trol its occurrence. To date, most attempts to explain the cording to both of these positions, the degree to which blocking phenomenon have fallen into two general cate blocking is obtained should ultimately depend on how sur gories. One type of explanation is that a US loses its ability prising the US occurrence is in the compound condition to promote conditioning as that US becomes better pre ing phase of the blocking procedure. Both of these views dicted by the stimuli preceding it (Rescorla & Wagner, predict that blocking should occur only when stimuli present in the second phase of training have already be come reliable signals for US occurrence. Requests for reprints should be sent to William C. Gordon, Depart ment of Psychology, University of New Mexico, Albuquerque, NM Given the presumed importance of US predictability in 87131. producing the blocking effect, it is somewhat surprising

59 Copyright 1988 Psychonomic Society, Inc. 60 WEAVER AND GORDON that experimenters have virtually ignored the role ofback EXPERIMENT 1 ground or contextual stimuli in the blocking paradigm. It is weH known, for example, that contextual stimuli are The purpose of the first experiment was to assess capable of entering into associations with a US in a con whether or not a contextual shift between the simple and ditioning situation (e.g., Balaz, eapra, Hart!, & Miller, compound conditioning phases would attenuate blocking. 1981; Bouton & King, 1983; Marlin, 1982). Thus, it is To make this determination, all animals were exposed to reasonable to assume that in the initial conditioning phase aseries of tone-shock pairings and later were presented of a blocking experiment, both es 1 and contextual stimuli with pairings of a compound tone-light es with shock. have the potential for becoming associated with the USo For some animals, both simple and compound es trials This means that in the compound conditioning phase of occurred in the same context. For other animals, these such an experiment, one rnight expect that both es 1 and two training phases occurred in different contexts. Our contextual stimuli would contribute to the predictability assumption was that the former animals would show rela of the USo In effect, contextual stimuli should, in many tively poor conditioning to the light es added during com cases, help to make the US less surprising in the second pound training. In other words, these animals should ex phase of the blocking procedure. hibit evidence of blocking . The question of interest in this Aside from the possibility that contextual stimuli rnight, study was whether or not blocking would be dirninished themelves, become predictors for the US occurrence, in the animals that experienced a context change. there is at least one other reason to expect that such stimuli rnight influence the blocking effect. Several theorists have Method suggested that contextual stimuli can serve as retrieval Subjects. The subjects were 30 male albino rats derived from cues for the memory of a es-us association (e.g., Hirsh, the Sprague-Dawley strain, bred and reared at the University of 1974; Konorski, 1967; Medin, 1975; Nadel & Willner, New Mexico. They were housed in individual cages and maintained under a 12: 12light:dark cycle. At the beginning ofthe experiment, 1980; Spear, 1973). According to this view, when an or all animals were between 90 and 120 days of age, and their body ganism forms a es-us association in the presence of weights ranged from 240 to 300 g. All behavioral testing was con specific contextual stimuli, these stimuli somehow become ducted during the light phase of the light:dark cycle. linked to the memory of that association. Later, if the or Apparatus. Two identicallick chambers, adapted from operant ganism is reexposed to the training context and the es chambers and manufactured by Lafayette Instrument Co. (Model is presented, the contextual stimuli aid in retrieval of the 8000), were employed. Each chamber measured 27x21 x26 cm and was composed of two alurninum and two Plexiglas walls with es-us memory and a conditioned response occurs. eon a Plexiglas lid. The floor of each chamber consisted of 0.6-cm ditioned responding is dirninished if the es occurs out diameter stainless steel rods, spaced 1.7 cm apart. The rods were side the training context, since retrieval ofthe es-us as connected with electrical wire outside the walls of the chamber. sociation is relatively ineffective. An aperture measuring 2.4 cm in diameter was located 12.0 cm From this view, one would also predict that the occur from the floor and centered in one alurninum wall to accommodate rence of blocking should depend on the contextual stimuli a drinking tube. A drinkometer (Lafayette Instruments, Model A121) was placed into a circuit that remained open except when an animal present during compound conditioning. Ifthe same con stood on the chamber floor and contacted the drinking tube. Each textual stimuli are present during both phases of the block such contact resulted in a pulse to a counter that recorded the num ing procedure, the memory of the association established ber of tube contacts. in the simple conditioning phase should be retrieved dur Conditioning trials were presented in a large Plexiglas chamber ing the compound conditioning phase. Under these con measuring 36x36x69 cm. The floor ofthis chamber was identi ditions, blocking should occur, since an organism would cal to that described for the liek chambers. A 1.5-mA scrambled remember that es 1 was already a reliable predictor for grid shock would be delivered to the floor by means of a Grason Stadler shock generator (Model EI064GS). Two CSs were em the USo A change in context between the two phases of ployed. The first was a 2oo-Hz, 9O-dB tone that was delivered training should reduce blocking, because the memory of through a speaker mounted at the top of the chamber. The second simple conditioning should be more difficult to retrieve: CS was a 15-W flashing light, also located at the top of the chamber. Given these kinds of predictions, the present experi Two experimental rooms (A and B) were employed for this study. ments were designed to assess the role of contextual These rooms differed distinctively in terms of odor, size, and light ing. To further differentiate between contexts, the walls of the con stimuli in a traditional blocking paradigm. In the first of ditioning chamber were clear Plexiglas when located in Room A these experiments, we looked at the effect of changing and striped when located in Room B. In Room B, striped poster contexts between the simple and compound conditioning board panels, consisting of altemating I-in. black and white strips, phases. The second experiment was concemed with how were hung on the outside ofthe clear Plexiglas walls. The specific blocking is influenced by extinguishing contextual associ characteristics of the rooms are listed in Table 1. ations between the two phases of the blocking procedure Procedure. Four days prior to any experimental treatment, all when both phases are conducted in the same context. On animals were placed on a 23.75-h water-

Table 1 confinement period in the wooden holding cage, and then plaeing Characteristics of Experimental Treatment Rooms it into a liek chamber for testing. Upon emission of Lick 40, the Room flashing light was presented and remained on until the subject either completed the next 40 licks or suppressed licking for 1,800 sec. A B This preparation, whieh has been employed by Miller and his col Dimensions 16.0 X 11.0 ft 7.5 x 5.0 ft Ieagues (e.g., Balaz, Gutsin, Cacheiro, & Miller, 1982; Schacht Lighting Dirn: 0.17 tL Bright: 1. 0 tL man, Gee, Kasprow, & Miller, 1983), ensures that all subjects are Odor (room deodorizer) None Lemon licking when the nonreinforced testing stimulus is presented. Laten Noise Level None: 45-50 dB Fan: 63-65 dB eies to complete the first and second 40 lieks were recorded, with Gloves Smooth Rough the criticai dependent variable being the latency to complete the Holding Cage Wooden Wire mesh Conditioning Chamber Walls Clear Plexiglas Striped second 40 licks in the presence of the light. Trus same testing proce dure was then employed on Day 12 to assess conditioned suppres sion in the presence of the blocking stimulus (i.e., the tone). Since the purpose of this day was to familiarize the animais with the lick chambers and to initiate drinking from the tube, lick laten Results and Discussion eies were not recorded. On Days 2 and 3, the lick tube was flush Two animals were disearded due to equipment malfune with the wall and lateneies (in seconds) to begin Iicking and laten tions. Both were replaeed with similar animals. On both eies to complete the first 40 and second 40 Iicks were recorded as the liek reeovery day and the 2 test days, all animals were an index of adaptation to the apparatus. Since there were no ap required to hegin lieking within 10 min after heing plaeed preciable differences between subjects in adaptation to the enclosure by Day 3, the animals were assigned randomly to one of three groups in the operant ehambers. All animals in the present study consisting of 10 animals each. met these requirements. Latencies to begin lieking and The simple conditioning phase was conducted on Days 4 and 5 to eomplete the first and second 40 lieks on Day 3 (the in th~ Plexiglas conditioning chamber located in either Room A or last day ofliek training), Day 10 (the liek recovery day) , Room B. The standard blocking group (Same) received four pair and the 2 test days were transformed to log latencies (base ings per day of a lO-sec tone with a 2-sec footshock in Room A. 10) to permit the use of parametric statistics. A rejeetion Pairings occurred at 5, 13, 22, and 28 min during a 30-min ses sion on each day, with tone offset being concurrent with shock on region of .05 was employed for all statistical analyses. set. Anirnals in the context shift blocking group (Different) received An analysis of varianee performed on the Day 3 and exactly the same treatment with the exception that conditioning oc Day 10 lateney data revealed no signifieant differenees curred in Room B instead of Room A. Animals in the third, con between groups to begin lieking or to eomplete the first trol group (Control) remained in their horne cages on these 2 con and second 40 lieks (all ps > .05). Separate analyses fur ditioning days. ther showed that the groups did not differ in terms of the Animals in Groups Same and Different received no explicit treat ment on Days 6 and 7, but instead were taken to Room A, where total number of lieks emitted on these 2 days (all they remained in their horne cages for 30 sec and then retumed to ps > .05). Taken together, these data indieate that liek the animai colony. Animais in Group Control continued to remain rates were relatively equivalent aeross the treatment in their horne cages. These days were inc1uded to ailow for com groups prior to the testing sessions. parisons between trus experiment and the subsequent study, in which On the initial test day (Day 11), the groups did not differ subjects would receive extinction treatments and additional US in terms of their latencies to initiate lieking or in terms presentations between the two training phases of the blocking procedure. of their lateneies to eomplete the first 40 lieks (p > .05 The compound conditioning phase took place on Days 8 and 9 in both eases). The mean log latencies to eomplete Lieks and, for all animals, was conducted in Room A. Conditioning con 40-80 in the presenee of the flashing light are shown in sisted of four pairings of a lO-sec tone-flashing-light compound Figure 1. An analysis of these data revealed a signifieant stimulus followed immediately by the same footshock as was em effeet of treatment [F(2,27) = 8.38, p < .05]. The ployed in the simple training phase. Also, as in the simple condi Newman-Keuls multiple eomparison statistie (Kirk, 1968) tioning phase, pairings occurred at 5, 13, 22, and 28 min during was employed to analyze differenees among the individual a 30-min session on each day with tone-light offset being concur rent with shock onset. group means. On the basis of tbis test, it was found that During both the simple and compound conditioning sessions, Group Same manifested significantly shorter latencies in animais receiving treatments were taken to the appropriate room, the presenee of the flashing light than did either of the one at a time, and placed in a holding cage speeific to the particu other two groups. Groups Control and Different did not lar context (see Table I) for 30 sec prior to being placed in the con differ from eaeh other in terms of this measure. Thus, ditioning chamber. Additionaily, all animais received 15-min ac Group Same exhibited signifieantly less liek suppression cess to water in their horne cages daily. Water was provided after treatment on conditioning days. in the presenee of the light than did the other two groups. On Day 10, ail animals were retumed to Room A, placed in the On the 2nd test day (Day 12), again the groups did not Iick chambers, and given a 30-min lick-recovery session. Laten differ in their latencies to begin lieking or to eomplete eies to begin licking and latencies to complete Licks 0-40 and 40-80 the first 40 lieks (p > .05 in both eases). The mean log were again recorded to assess any generaiization of fear between latencies to eomplete the last 40 lieks in the presenee of the liek chamber and the conditioning apparatus, and to ensure that the tone are shown in Figure 1. This figure suggests that all animals were restored to their baseline Iicking rates as measured on Day 3. all groups exhibited relatively long lateneies in the Testing for conditioned suppression to the blocked stimulus (i.e., presenee of the tone. Furthermore, an analysis of vari the flashing light) was conducted on Day 11. Testing consisted of anee revealed that the groups did not differ in terms of taking each animal individually into Room A, giving it a 3D-sec this measure (p > .05). 62 WEAVER AND GORDON

In SUPPRESSION TO THE BLOCKED STIMULUS (L) 2.20 - o SUPPRESSION TO THE BLOCKING STIMULUS(T) 2.00 -

"'! 1.80 u - w !!l \I) 1.60 :IC - ~ ...J 0.., 1.40 - ~ ~ i :::::::::~ w 1.20 0 - ~ > u z 1.00 w - ~ "'...J 8 .80 - ...J 1)( .60 ~ ~~~~ I CONTROL SAME DIFFERENT

GROUP

Figure 1. Mean test-trial log Iatencies (base 10) to eomplete 40 lieks in the presenee of the flashing light (L) on Day 11 and tone (T) on Day 12 (Experiment 1).

The results of this study reveal that the typical block Finally, the liek-suppression data from the 2nd test day ing effect can be obtained with the present parameters. suggest that the groups exhibited equivalent conditioning This effect is represented by the difference between the to the tone es. These data are only suggestive, since sup Control and Same groups when they were tested with the pression to the tone may have been influenced by the prior flashing light. These groups each received eight total pair test with the light es. Still, these data indicate that by ings of the tone-light compound with shock in Phase 2 the end of compound training, all groups had become con of training. However, animals in the Same groups had ditioned to the tone to an equal degree, whereas condi previously received eight pairings of the tone es with tioning to the light depended on the specific treatment con shock. As a result, the animals in the Same condition show ditions. much less evidence of conditioning to the light es fol lowing the compound conditioning phase. EXPERIMENT 2 Importantly, these results also show that the blocking effect is attenuated significantly by a change in context The results of Experiment 1 indicate that contextual between single stimulus and compound stimulus training. stimuli can play an important role in the production of Animals exposed to a shift in context (Group Different) the blocking effect. The purpose of the second study was failed to exhibit evidence of blocking. These rats sup to determine the degree to which this contextual influence pressed Iicking in the presence of the light to almost the is mediated by direct context-US associations formed dur same degree as the eontrol animals. This finding is con ing the simple training phase of the blocking procedure. sistent with the idea that the contextual stimuli present dur We reasoned that if context-US associations contribute ing compound training help to determine whether or not to the blocking effect, then the extinction of these associ the US will be surprising. Apparently, when novel con ations following the simple training phase should signifi textual stimuli are present in the compound conditioning cantly reduce blocking of the novel es during the com phase, the surprisingness of the US is enhanced even pound conditioning phase. To test this prediction, all rats though a previously conditioned es (the tone) precedes in Experiment 2 received both single stimulus and com each occurrence of the USo As a result, blocking of the pound stimulus training in the same context. However, novel es (the light) fails to occur. some animals were given nonreinforced exposures to the ROLE OF CONTEXT IN BLOCKING 63 context between the single and compound stimulus con fore being returned to the animal colony. Animals in Group Con ditioning phases. The major question addressed in this trol again remained in their horne cages on Day 6. study was whether or not extinction of contextual assoei Day 7 involved giving additional shock exposures to rats in ations would reduce the blocking effect. Groups EXTN(A)/EXP(A) and EXTN(A)/EXP(B). Rats in Group EXTN(A)/EXP(A) received two footshocks in the conditioning Although context-alone exposures following the sim chamber located in Room A. These footshocks were identical to ple conditioning phase should serve to reduce direct con those used during conditioning, and they occurred at 5 and 21 min text-US assoeiations (e.g., Marlin, 1982), the effects of during a 30-min session. These shock presentations occurred in the such an extinction manipulation are not always interpret absence of any nominal CS. Rats in Group EXTN(A)/EXP(B) able on that basis alone. Some investigators have demon received exactly the same treatment, except that the shock presen strated that extinction trials not only reduce associations, tations occurred in Room Brather than in Room A. On this day, animals in Groups Blocking and EXTN(A) were simply given a but may also serve to degrade an organism's memory 30-sec exposure to Room A in their horne cages; rats in Group Con representation ofthe US (e.g., Rescorla & Heth, 1975). trol remained in the animal colony. Thus, decreases in performance following extinction often The compound conditioning phase of this experiment took place result from the combined effects of a reduced association on Days 8 and 9 in Room A. Animals in all groups received four and a degraded US memory. pairings of the compound CS (tone and flashing light) with shock. To determine the mechanism for any extinction effects These pairings were distributed over a 30-min session in the con in the present study, two important control conditions ditioning chamber. Subsequent liek recovery (Day 10) and testing (Days 11 and 12) procedures were identieal to those employed in were used in this experiment. In one of these conditions, Experiment 1. rats were exposed to shock-alone presentations in the previously extinguished context prior to the compound Results and Discussion conditioning phase. We reasoned that this manipulation Two animals were discarded due to equipment malfunc should serve to reestablish any context-US associations tions, 3 were eliminated because of illness, and 2 failed that had been extinguished and should also reinstate an to begin licking within the required 10 min on the lick animal' s memory of the US. In the other of these condi recovery day. All 7 of these animals were replaced with tions, rats were given shock-alone presentations in a novel animals that had similar characteristics. context after extinction of the training context. Such a The lick-Iatency scores for all animals were converted manipulation should reinstate an organism's US represen to log latencies, and the appropriate analyses were per tation, but should not reestablish any associations between formed. As in the first experiment, there were no differ the training context and the USo By comparing the per ences between groups to begin licking or to complete the formance of animals in these conditions, it should be pos first and second 40 licks on the last day of lick training sible to determine the basis for any extinction effects found (Day 3) or liek recovery (Day 10)(all ps > .05); nor did in the present study. the total number of lieks emitted on these 2 days differ between groups (allps> .05). Furthermore, no signifi Method Subjects and Apparatus. Eighty male rats, similar to those em cant differenees between groups were detected in laten ployed in the previous experiment, served as subjects. The condi eies to begin lieking or to eomplete the first 40 licks on tioning apparatus, lick chambers, conditioning contexts, and stimulus either test day (all ps > .05). parameters were the same as those utilized in Experiment 1. Mean log latencies to emit Lieks 40-80 on Day 11 in Procedure. Pretraining and lick-training procedures were car the presence of the flashing light and on Day 12 in the ried out precisely as in Experiment 1. As in the previous study, presence ofthe tone are illustrated in Figure 2. The anal there were no appreciable differences between subjects in adapta ysis of varianee performed on the Day 11 lateney mea tion to the lick chambers by Day 3, and animals were assigned ran domly to one of five groups consisting of 16 animals each. sures revealed a signifieant effeet of treatment [F( 4,75) On Days 4 and 5, animals in four of the treatment conditions = 8.07, p < .05]. As would be expected, the Newman [Groups EXTN(A), EXTN(A)/EXP(A), EXTN(A)/EXP(B), and Keuls statistie indicated that Group Blocking exhibited sig Blocking] underwent the simple conditioning phase of the block nificantly shorter lateneies in the presenee ofthe light than ing procedure. On each of these days, these rats were placed in did Group Control. Of greater importanee is the finding the conditioning chamber in Room A for 30 min. During this time, that the animals that received two eontext-alone exposures they received four tone-shock pairings distributed over the session, as in Experiment 1. Animals in the remaining group (Control) re [i.e., Group EXTN(A)] displayed signifieantly longer mained in their horne cages on these 2 conditioning days. lateneies than did Group Blocking. This finding suggests On Day 6, animals in Groups EXTN(A), EXTN(A)/EXP(A), and that when context-alone exposures follow the simple eon EXTN(A)/EXP(B), received two 30-min exposures to the condi ditioning phase, the typical blocking effeet is attenuated. tioning chamber, stilliocated in Room A. These exposure sessions Regarding the remaining two groups, the results ob simply involved placing each animal in the conditioning chamber tained are somewhat more ambiguous. As evideneed by without any presentations of the tone or shock. The two exposure sessions were separated by approximately 3 h in the case of each the Newman-Keuls statistie, Groups EXTN(A)/EXP(A) anima!. Rats in the standard, Blocking group received no exposure and EXTN(A)/EXP(B) did not differ from one another sessions on this day. Instead, these animals were simply taken to and neither differed from Group Blocking. That is, both Room A. where they remained in their horne cages for 30 sec be- groups displayed blocking regardless of where their US 64 WEAVER AND GORDON

lIilllJ SUPPRESSION TO THE BLOCKED STIMULUS (l) 2.20- o SUPPRESSION TO THE BLOCKING STiMULUSn) 2.00-

1.80 r- ~f 1 . 60~

1.40f-

1 . 20~

1 .00~

8 .80r- ...J 1)( .60~ ~.:.:.:.:

CONTROL BLOCKING EXTN(A) EXTN(A) EXTN(A) EXP(A) EXP(B)

GROUP

Figure 2. Mean test-trial log Iatencies (base 10) to eomplete 40 lieks in the presenee of the f1ashing light (L) on Day 11 and tone (T) on Day 12 (Experiment 2). exposures were given after extinction. This effect was fur ment conditions. This indicates that although the treat ther confrrmed by the finding that both of these groups ment conditions produced differential responding in the manifested significantly shorter latencies than did Group presence of the light, these manipulations had little ef Control. The only comparison that tends to differentiate feet on the degree of conditioning to the tone CS. these groups is that with Group EXTN(A). The group given shock exposures in the training context [EXTN(A)/ GENERAL DISCUSSION EXP(A)] did exhibit significantIy shorter latencies than did Group EXTN(A). However, the group given shock ,The question addressed in these experiments was exposures in the novel context [EXTN(A)/EXP(B)] did whether contextual stimuli could contribute to the occur not differ trom Group EXTN(A) in terms of this measure. rence of the blocking effeet. The results of these studies These findings suggest that shock exposures in the clearly provide an affrrmative answer to this question. In previously extinguished context were somewhat more ef Experiment 1, we found that the blocking effeet could be fective in restoring the blocking effeet than were shock virtually eliminated by changing contexts between the sin exposures in the novel context. However, both types of gle and compound stimulus training phases ofthe block shock exposure tended to alleviate the effeets of extin ing procedure. In Experiment 2, we found that the block guishing the training context. Thus, it appears that in the ing effect could also be attenuated by extinguishing the present case, context extinction reduced the blocking ef single stimulus training context prior to the compound fect both by reducing context - US associations and by conditioning phase. degrading the animals' US memory representation. If ex Although these findings indicate that contextual stimuli tinction had served only to reduce context- US associa can play an important role in the production of blocking, tions, then shock exposures in a novel context should have they do not provide a clear indication of the mechanism done little to restore the blocking effect. underlying this contextual influence. As we have previ Finally, it is notable that all groups exhibited relatively ously noted, contemporary views of contextual effeets pro long latencies in the presence of the tone. Furthermore, vide at least two suggestions as to how contextual stimuli an analysis of variance performed on these latencies re rnight aid in the production of blocking. According to one vealed no significant differences as a function of treat- view, contextual stimuli should become associated with ROLE OF eONTEXT IN BLOeKING 65 the US during the initial stage of the blocking procedure that is already a reliable signal for the USo This means (e.g., Rescorla & Wagner, 1972). Thus, the presence of that the novel es that is added during this stage is viewed these same contextual stimuli in the compound condition as a redundant signal for the US and is progressively tuned ing phase should enhance the predictability of the US and out or ignored. For this reason, little conditioning occurs should, in turn, reduce conditioning to the novel es. Ac to the novel es. According to this view, animals with hip cording to another view (e.g., Hirsch, 1974; Nadel & pocampal lesions are less adept at tuning out redundant Willner, 1980; Spear, 1973), the contextual stimuli stimuli, and thus show greater conditioning to the novel present during the initial simple training phase of the es than do sham subjects. blocking procedure should become effective retrieval cues Within this theoretical framework, contextual stimuli for the es-us association formed during that stage. Thus, can contribute to the blocking effect by becoming reli the presence of these contextual cues during the compound able signals for US occurrence during the initial condi phase of the procedure should enhance blocking by facili tioning phase. However, blocking can also occur in situ tating retrieval of the previously formed association. In ations in which contextual stimuli are relatively ineffective effect, the presence of these stimuli should enhance the as long as the es from the initial conditioning phase be effectiveness ofthe previously conditioned es as a block comes a reliable signal. In effect, this hypothesis predicts ing agent. that animals with hippocampallesions should always ex elearly, the results of Experiment 1 are consistent with hibit deficits in blocking, regardless ofwhether or not con both of these views, since both views would predict a textual stimuli contribute to the blocking effect. reduction in blocking when novel contextual stimuli are Alternatively, Hirsch (1974) accounts for blocking present in the compound conditioning phase. On the other deficits by proposing that lesioned animals are deficient hand, the results of Experiment 2 would seem at first in their ability to use contextual stimuli as memory glance to be more in accord with the idea that context retrieval aids. Hirsch's view suggests that in Stage 2 of US associations are instrumental in the production of the blocking procedure, sham animals utilize the context blocking. This is because this view predicts that extinc to retrieve the memory of the es-US association formed tion of contextual associations formed during the simple in Stage 1. Associations to the novel es are blocked be training phase should attenuate blocking in the compound cause an animal remembers that the original es has al training phase. However, although Experiment 2 did ready become a reliable signal for US occurrence. demonstrate that exposure to the simple training context Animals with hippocampallesions are presumed to ex does reduce blocking, the specific mechanism for this ef hibit blocking deficits because they are unable to use the fect was unclear. These results indicated that the contex context in Stage 2 to help them to remember the eu-us tual exposure may have resulted in adegradation of the association formed in Stage 1. US representation as weIl as a weakening of context-US One implication of this view is that lesioned animals associations. Thus, the degree to which context-US as should show blocking deficits only under conditions in sociations are necessary for the production of blocking which contextual stimuli contribute to the blocking effect. remains unclear. Further work will be necessary to deline If blocking occurs in the absence of contextual control, ate the basis for this contextual control over the blocking then there is little reason to predict that lesioned animals effect. would perform differently than shams. In other words, Although the present studies demonstrate that contex if under some circumstances, blocking does not depend tual stimuli can influence the occurrence ofblocking, these on the presence of Stage 1 contextual stimuli, lesioned findings do not imply that such contextual control will oc animals should show minimal deficits in blocking, since cur under all circumstances. On the contrary, pilot work these deficits are presumed to result from an inability to currently underway in our laboratory suggests that the utilize contextual cues. contextual control of blocking may depend on factors such elearly, such an analysis depends on identifying situa as the number of conditioning trials that occur in the sim tions in which blocking is not influenced by the training ple conditioning phase. This preliminary work indicates context, as weIl as situations in which contextual control that context changes diminish blocking to a much lesser is strong. By understanding the role context plays in this degree when the simple conditioning phase involves paradigm, it may be possible to accomplish more than sim numerous tone-shock pairings that are widely distributed ply increase our understanding of the blocking phenome in time. Should these preliminary findings be confirmed, non. It mayaiso be possible to use contextual manipula it may become possible to use manipulations of context tions to clarify the role of the hippocampus in conditioning to distinguish between explanations for the blocking situations. deficits shown by animals with hippocampallesions. To illustrate this point, consider the explanation of REFERENCES blocking deficits proposed by Moore and Stickney (1980) as opposed to the interpretation offered by Hirsch (1974). BALAZ, M. A., CAPRA, S., HARTL, R., & MILLER, R. R. (1981). Con According to Moore and Stickney, blocking normally oc textual potentiation of acquired behavior after devaluing direct context- curs because animals tend to "tune out" redundant stimuli US associations. Leaming & Motivation, 12, 383-397. BALAZ, M. S., GUTSIN, P., CACHEIRO, H., & MILLER, R. R. (1982). in a conditioning situation. In other words, in Stage 2 of Blocking as a retrieval failure: Reactivation of associations to a blocked the blocking procedure, animals are confronted with a es stimulus. Quarterly Joumal 0/ Experimental Psyclwlogy, 34B, 99-113. 66 WEA VER AND GORDON

BOUTON, M. E., & KING, D. A. (1983). Contextual control ofthe ex NADEL, L., & WILLNER, I. (1980). Context and conditioning: A place tinction of conditioned fear: Tests for the associative value of the con for space. Physiological Psychology, 8, 218-228. text. Journal of Experimental Psychology: Animal Belulvior Processes, PEARCE, I. M., & HALL, G. (1980). A model for Pavlovian learning: 9, 248-265. Variations in the effectiveness of conditioned but not of unconditioned DICKINSON, A., NICHOLAS, D. 1., & MACKINTOSH, N. I. (1983). A re stimuli. Psychological Review, 87, 532-552. examination of one-trial blocking in conditioned suppression. Quar RESCORLA, R. A., & HETH, C. D. (1975). Reinstatement of fear to an terly Journal of Experimental Psychology, 35B, 67-79. extinguished conditioned stimulus. Journal of Experimental Psychol HIRSH, R. (1974). The hippocampus and contextual retrieval of infor ogy: Animal Behavior Processes, 104, 88-96. mation from memory: A theory. Behavioral Biology, 12,421-444. RESCORLA, R. A., & WAGNER, A. R. (1972). A theory of Pavlovian KAMIN, L. I. (1968). "Attention-like" processes in classical condition conditioning: Variations in the effectiveness of reinforcement and non ing. In M. R. Iones (Ed.), Miami Symposium on the Prediction of reinforcement. In A. H. Black & W. F. Prokasy (Eds.), Classical Behavior: Aversive Stimulation (pp. 9-33). Miami: University ofMi conditioning: 11. Current research and theory (pp. 64-99). New York: ami Press. Appleton-Century-Crofts . KAMIN, L. I. (1969). Predictability, surprise, attention, and condition R!CKERT, E. I., BENNETT, T. L., LANE, P .. & FRENCH, I. (1978). Hip ing. In B. A. Campbell & R. M. Church (Eds.), Punishment and aver pocampectomy and the attenuation ofblocking. Belulvioral Biology, sive behavior (pp. 279-296). Appleton-Century-Crofts. 22, 147-160. KIRK, R. E. (1968). Experimental design: Proceduresfor the behavioral SCHACHTMAN, R. R., GEE, J., KASPROW, W. I., & MILLER, R. R. sciences. Belmont, CA: Brooks/Cole. (1983). Reminder-induced recovery from blocking as a function of KONORSKI, I. (1967). Integrative activity ofthe brain. Chicago: Univer the number of compound trials. Leaming & Motivation, 14, 154-164. sity of Chicago Press. SCHMAJUK, N. A. (1984). Physiological theories ofhippocampal func MACKINTOSH, N. I. (1975). A theory of attention: Variations in the as tion. Physiological Psychology, 12, 166-183. sociability of stimuli with reinforcement. Psychological Review, 82, SOLOMON, P. R. (1977). Role ofhippocampus in blocking and condi 276-298. tioned inhibition of the rabbit's nictitating membrane response. Journal MARUN, N. A. (1982). Within-compound associations between the con ofComparative & Physiological Psychology, 91,407-417. text and the conditioned stimulus. Leaming & Motivation, 13, 526-541. SPEAR, N. E. (1973). Retrieval of memory in animals. Psychological MEDIN, D. L. (1975). A theory of context in discrimination learning. Review, 80, 163-194. In G. H. Bower (Ed.), The psychology of leaming and motivation (Vol. 9, pp. 263-314). New York: Academic Press. MOORE, J. W., & STICKNEY, K. J. (1980). Formation of attentional associative networks in real time: Role of the hippocampus and im (Manuscript received Iune 26, 1987; plications for conditioning. Physiological Psychology, 8, 207-217. revision accepted for publication December 7, 1987.)