Journal of Experimental Psychology: Animal Behavior Processes 1978, Vol. 4, No. 3, 237-249
Analysis of Second-Order Odor-Aversion Conditioning in Neonatal Rats: Implications for Kamin's Blocking Effect Martin D. Cheatle and Jerry W. Rudy Princeton University It has been reported that neonatal rats acquire first-order conditioned aversions to olfactory stimulation experienced prior to drug-induced illness. Experiment 1 demonstrates that the neonate will also acquire a second-order conditioned odor aversion and that the second-order aversion is not affected by extinguishing the first-order aversion. The effectiveness of the secondorder conditioning paradigm (i.e., Si~* US; S2—* Sj) and a sequential blocking paradigm (i.e., S t —*US; S 2 ~*Si—>US) also were compared. These comparisons revealed (a) that the occurrence of the US in the second phase of the blocking treatment interfered with second-order conditioning (Experiment 3) and (b) that the temporal proximity of US occurrence to the S2~* Si episode was a critical determiner of the degree to which such interference was observed; that is, the more proximate the S2-Sl and US events, the more US occurrence interfered with second-order conditioning (Experiment 4). Implications of these findings for theories of conditioning are discussed. The paradigms for investigating secondorder conditioning and blocking are similar. In Phase 1 the .subject typically receives a paired presentation of the stimulus, Si, and an unconditioned stimulus (US), and in Phase 2 it experiences both Si and a second stimulus, 82, in close temporal proximity. In a second-order conditioning experiment, the two stimuli are usually presented sequentially, with 82 preceding the occurrence of Si and the US omitted (i.e., S 2 -»Si). In the blocking experiment, Si and 82 generally are presented simultaneously and terminate with the occurrence of the US (i.e., SiS 2 -*US). Pavlov (1927) presented the first evidence on the fate of 82 in the second-order conditioning paradigm. He found that S2 acquired conditioned properties even though This research was supported in part by National Science Foundation Grant GB-38322 to J, W. Rudy. Requests for reprints should be sent to Jerry W. Rudy, Department of Psychology, Princeton University, Princeton, New Jersey 08S40.
the US was not directly paired with it. Rescorla and his colleagues recently have provided a number of demonstrations of this phenomenon. In his laboratory second-order conditioning has been observed with the conditioned emotional response (CER) preparation (Rescorla, 1973; Rizley & Rescorla, 1972) and the appetitive-activity situation (Holland, 1977; Holland & Rescorla, 197Sa, 1975b). Most recently it has also been obtained with Brown and Jenkins's (1968) autoshaping procedure (Rashotte, Griffin, & Sisk, 1977; Rescorla & Furrow, 1977). Kamin (1968) has provided the reference experiment on the fate of S2 in the blocking paradigm. He reported that little or no conditioning occurred to S2, with the specific comparison of interest being that less conditioning occurred than if SiS2-US pairings were not preceded by the Si-US training, The generality of the latter blocking effect is well established (Gillan & Domjan, 1977; Holland, 1977; Kamin, 1968, 1969; Mackintosh, 1975a), and this phe-
Copyright 1978 by the American Psychological Association, Inc. 0097-7403/78/0403-0237$00.7S
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nomenon has provided the impetus for many recent theoretical analyses of first-order conditioning (e.g., Kamin, 1969; Mackintosh, 1975b; Rescorla & Wagner, 1972; Wagner, Rudy, & Whitlow, 1973). Given the formal similarity of the blocking and second-order conditioning arrangements, it is all the more surprising that the animal fails to condition to S2 in the blocking sequence. In the blocking arrangement there are two potential sources of reinforcement available to S2. On the one hand, there is the opportunity for first-order conditioning provided by the US. On the other hand, there is an opportunity for second-order conditioning provided by the occurrence of the previously conditioned Si. Nevertheless, when presented alone, S2 evokes little conditioned responding, and this fact suggests that little first- or second-order conditioning is captured by S2. One potential reason why 82 does not acquire second-order conditioned properties in the typical blocking study is that Si and S2 are presented concurrently, whereas in the typical second-order conditioning experiment, S2 generally precedes the occurrence of Si. There are, however, empirical grounds for questioning the necessity of a sequential, 83 —» Si, presentation for 82 to acquire second-order conditioned properties. For example, Rescorla (1973) has reported second-order conditioning when S2 and Si are presented jointly. An obvious feature distinguishing the second-order conditioning and blocking arrangements is that the US occurs in the latter but not the former paradigm on the occasion of pairing Si and S2. Thus one might argue that the occurrence of the US in a blocking study in some way interferes with the acquisition of second-order conditioned properties by S2 that would otherwise occur if the US were omitted. In this paper we report several studies that analyze the second-order conditioning paradigm and directly compare the results obtained with this procedure (i.e., Si —> US; 82—> Si) with those produced by a sequential blocking paradigm (i.e., $! —» US; S 2 -» Si -* US). Ultimately we will
argue that the results of this work encourage the view that second-order conditioning is adversely influenced when the US is temporally proximate to the occurrence of the 82 —> Si episode. The procedures employed in this research were somewhat unusual. Therefore, we will provide some additional background before describing the experiments. Recently, we have been concerned with investigating the ontogeny of associative-learning processes in altricial animals such as the rat (Rudy & Cheatle, 1977, in press). For this purpose we have developed an odor-aversion conditioning preparation that has proved remarkably sensitive to learning in neonatal rats ranging in age from 2-14 days old. The procedures are quite simple. On the day of conditioning pups are placed into an environment scented with a novel odor. Subsequently they are injected with a drug-US, Lithium Chloride (LiCl), that induces illness. The pups are later given a choice between spending time over shavings scented with the odor previously paired with the drug versus shavings scented with some other odor. Evidence that the pups acquired an aversion to the odor is provided by the fact that they avoid the scent experienced on the conditioning trial, in comparison to the alternative odor. The interested reader may consult the recent paper by Rudy and Cheatle (in press) for a detailed report on this research, indicating that first-order conditioning is easily detected with pups ranging in age from 2-14 days old and that major age-related differences are obtained when the parameters of the task are systematically investigated. Relevant to the present research is our work on second-order, odor-aversion conditioning. We have reported some evidence that pups 7 days of age can acquire a second-order odor aversion (Rudy & Cheatle, in press). To do this, one group of pups first experienced a pairing of the odor, Si, and the US. In Phase 2 they received a new odor, S2, followed immediately by exposure to Si. The pups that received this sequence displayed a greater aversion to 82 than did pups that (a) received the same Phase-2
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pairing but only Si or only the US in Phase 1 or (b) received the same Si-US pairing in Phase 1 but in Phase 2 experienced Si 4 hr after the S2 exposure. The neonatal odor-aversion task was the vehicle for studying second-order conditioning in the present research. In the first experiment further evidence that neonates acquire second-order odor aversions is presented. In the second, some evidence that these procedures are suitable for demonstrating .blocking, as in the Kamin (1968) comparison, are provided. In Experiments 3 and 4 the second-order conditioning and blocking sequence are directly compared. Experiment 1 There were at least two weaknesses in the second-order conditioning studies reported by Rudy and Cheatle (in press). First, although we included comparison treatment groups that were intended to evaluate the necessity of the Phase 1, Si-US pairing, the pups in these control groups experienced only Si or only the US in Phase 1. No comparison group experienced both Si and the US in Phase 1. Since the pups in the experimental group received a pairing involving both Si and the US, the criticism can be offered that the change in the experimental pups' reaction to S2 was a product of some "nonassociative" effect of their having experienced both Sj and the US prior to the test for second-order conditioning, independent of their pairing. Second, we provided no independent evidence that Si had acquired first-order conditioned properties. These shortcomings were met in the present study by employing a control that experienced both Si and the US in Phase 1. They experienced the US and 60 min later received Si. Moreover, following the test for an aversion to S2, all pups were tested for their reaction to Si. In this experiment, evidence was also provided that is relevant to the hypothesis that the second-order odor aversion acquired by the neonate is the result of an S2-Si association. The Sa-Si experience provides the animal the opportunity to acquire any of several associations (Rescorla, 1973), which
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could allow S2 to evoke a conditioned response (CR). These include (a) S2 becoming associated with a representation of Si (the S2-Si association), (b) S2 becoming associated with the CR evoked by Si (the S2-CR association), and (c) S2 becoming associated with a representation of the US activated by Si (the S2-US association). The behavioral reaction to S2 by the first view is seen as reflecting the CR evoked by the activation of Si's representation by S2. Accordingly, the ability of S2 to evoke a conditioned reaction depends critically on the integrity of the conditioned properties of Si. Depreciating the value of Si's conditional properties following second-order conditioning consequently should also weaken the ability of S2 to evoke a CR. According to the remaining alternatives (the S2-CR and Si-US association views), the ability of S2 to evoke a CR does not depend upon the ability of Si to evoke a CR. The above issues were addressed in this experiment by examining four groups of pups. Two groups each received a pairing of a lemon scent, Si, and the US in Phase 1 and a pairing of an orange scent, S2, and Si in Phase 2, Prior to testing their reaction to S2) the pups in one of these conditions, Group P/P/E, received a treatment calculated to extinguish the conditioned aversive properties of Si. They received a series of nonreinforced exposures to the lemon scent. Pups in the other group, Group P/P/MS, in contrast, received a comparable number of nonreinforced exposures to maternity cage shavings. According to the S2-Si association view, Group P/P/E should display a substantially weaker aversion to S2 than Group P/P/MS. Subjects in two control conditions, Group B/P/E and B/P/ MS, were treated, respectively, like pups in Group P/P/E and P/P/MS, except that in Phase 1 they received a "backward" conditioning treatment: They were injected with the US 60 min prior to being exposed to the lemon scent. These groups thus controlled for the necessity of the exposure to both Si and the US in the Phase-1 Sj-US pairing operation. All subjects were tested for their reaction to both S2 and Si.
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Method Subjects. The subjects were 29 male and female Sprague-Dawley derived rats 7 days old at the start of the experiment, where Day 0 is defined as date of birth. These animals were bred in our laboratory and housed with the mother in standard maternity cages containing pine shavings. The shavings were changed approximately twice per week. Four litters contributed to this study. Procedures. On the first day of conditioning (Phase 1) the maternity cages were brought into the experimental area and the mothers were removed. Thereafter, the subjects in Group P/P/E (n — 7) were treated as follows. The pups were taken from the maternity cage and placed directly into a large polyurethane bag (55 X 36 X 24 cm) scented with 2.5 cc of McCormick's lemon extract (Si) mixed with a volume of fresh natural-pine shavings sufficient to fill a 500-ml beaker. After all 7 pups were placed into the shavings the top of the bag was closed except for an opening of 2-3 cm in the top for ventilation. Approximately 5 min later each pup was removed, weighed, and injected (ip) with a 2% body weight, .15-mol solution of LiCl and then returned to the odor environment for an additional 30 min before being returned to its maternity cage. The maternity cages were then returned to the colony room. On the next day (Phase 2) these pups received a second-order conditioning treatment that involved a sequential exposure to a novel orange scent (S2) followed immediately by exposure to the previously conditioned lemon scent (Si). Specifically, they were placed for 5 min into a bag scented with the pine shavings adulterated with 2.5 cc of McCormick's orange extract, then immediately placed into a bag containing lemon-scented shavings. On the third day of the experiment (Phase 3) the pups in Group P/P/E experienced a series of five 30-min exposures to the lemon scent (Si), separated by 90 min. During the 90-min interval the pups and the mother were returned to the maternity cage. Subjects in Group P/P/MS (n = 8) were treated exactly like those in Group P/P/E except that during Phase 3 they received nonreinforced placements into a bag containing maternity cage shavings. On the first day of conditioning the pups in the two control conditions, Group B/P/E (» = 6) and Group B/P/MS (n = 8), were removed from their maternity cages, weighed, injected with LiCl, and then returned to their maternity cages. Sixty min later they were placed into a bag containing lemon-scented shavings for 35 min. Thereafter they were treated, respectively, like Groups P/P/E and P/P/MS. In this study there were no more than two and no fewer than one pup from each litter in each of the four groups. Forty-eight hr following the Phase-2 treatment all pups were tested for an aversion first to the orange scent (82) and then to the lemon scent
(Si). The test apparatus consisted of a 30 X 20 X 10-cm Plexiglas compartment with a wire mesh floor, beneath which were two 30 X 9 X 3-cm containers. During the orange-aversion test one of the containers was filled with a sufficient volume of shavings, adulterated with 2.5 cc of the orange scent, to fill a 500-ml beaker. The other container held an equivalent volume of shavings scented with 2.5 cc of McCormick's garlic juice. During the lemon-aversion test one container was filled with a similar volume of shavings scented with 2.5 cc of lemon extract, the other contained garlic-scented shavings. On a test trial a single pup was placed in the center of the compartment so that its right forepaw and rearpaw were over one container and its left forepaw and rearpaw were over the other container. The pup remained in the test apparatus for 150 sec. The dependent variable of interest, depending upon the test, was the percentage of time the pup spent over the orange- or lemonscented shavings. The pup was judged to be over the scent when its head and both forelegs were across a line dividing the two containers. The judge had no knowledge of the pups pretreatment at the time of the test. After each pup was tested the wire floor was wiped with a damp cloth. After 6 pups were tested the shavings were discarded and replaced with appropriately scented shavings. One hr after the last pup was tested for an aversion to the orange scent the lemon-aversion test was initiated.
Results The results of the tests for both first- and second-order conditioning are presented in Figure 1. It may be seen that the pups in Group P/P/MS displayed evidence of a first-order conditioned aversion: In comparison to Groups B/P/E and B/P/MS, they avoided the lemon scent. In contrast, the subjects in Group P/P/E, if anything, displayed a greater preference for the lemon scent than did the pups in the two control conditions. A different pattern of results was revealed by the second-order conditioning test data. Note that Groups P/P/E and P/P/MS did not differ and that they each spent less time over the orange scent than did the pups in the control groups. Two separate 2 x 2 factorial analyses of variance were performed on these data. The analysis of the second-order conditioning data revealed only a significant main effect of the Phase-1 forward- versus backwardconditioning treatment, F(l, 25) = 12.48,
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P/T/MS
B/p/MS
VHI
P/P/MS
P/P/E
B/P/MS
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B/P/E
EXPERIMENTAL CONDITIONS Figure 1. Mean percentage of time spent over the second-order stimulus, the orange scent (Ss), and the first-order stimulus, the lemon scent (Si), by the groups in Experiment 1.
p < .01. In contrast, the analysis of the firstorder conditioning data revealed an interaction between the Phase-1 acquisition treatment and the Phase-3 extinction treatment, F(l, 25) =4.85, p < .05. A subsequent analysis indicated that the source of this interaction was that Group P/P/E differed from Group P/P/MS, F(l, 25) = 17.98, p < .001, whereas Groups B/P/E and B/P/ MS did not reliably differ, F < 1. Discussion The results of this study join those previously reported by Rudy and Cheatle (in press) in providing evidence that the neonatal rat can acquire a second-order conditioned odor aversion. A principal feature of this experiment was the inclusion of Groups B/P/E and B/P/MS, which were exposed to both Sj and the US in Phase 1 and thus controlled for the necessity of the Phase 1 Si-US pairing. The pups in these two groups experienced both Si and the US in Phase 1, but a delayed backward-pairing, US-Si, procedure was employed. They displayed no evidence of avoiding Si or S2 and
spent significantly more time over S2 than did Group P/P/E or Group P/P/MS. The fact that Group P/P/MS spent less time over the lemon scent (Si) can be taken as evidence that the Si-US pairing experienced by Group P/P/MS resulted in a conditioned first-order odor aversion. The comparisons of Groups P/P/E and P/P/MS on the first and second-order conditioning test were particularly instructive. Whereas only Group P/P/MS avoided Si, both groups equally avoided S2. Nonreinforced exposure to S2 prior to testing was apparently effective in depreciating the firstorder conditioned properties of Si for Group P/P/E. The observation that Groups P/ P/E and P/P/MS did not then differ in their reaction to S2 suggests that the status of the second-order properties acquired by S2 does not depend upon the integrity of the conditioned properties of Si. If this had been the case, then extinguishing the conditioned properties of Si should have also reduced the pups' tendency to avoid S2. This pattern of results thus provides no evidence that the second-order odor aver•sion was a result of the acquisition of an
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Sa-Si association. One could, of course, always maintain that the extinction operation did not completely remove the conditioned properties of Si and that this residual was sufficient to mediate the second-order effects produced by 82. To do so, however, one has to make an additional assumption that the residual conditioned properties of Si were sufficient to mediate a second-order reaction to S2 but were not sufficient to produce evidence of a first-order reaction to Si. This seems implausible on the face of it, although the fact that subjects were tested first on 82 (.presumably generating some extinction) prior to Si precludes its dismissal. Our present judgment is that the S2-Si association is unlikely to be the mediator of second-order odor-aversion conditioning. Rizley and Rescorla (1972) and Holland and Rescorla (1975a, 197Sb) have reached a similar conclusion concerning the mechanisms of second-order conditioning displayed by adult rats in the CER situation and the conditioned appetitive-activity situation. In contrast, Rashotte et al. (1977) have found evidence consistent with the 82Si association view using pigeons in the autoshaping preparation (Brown & Jenkins, 1968). They found, upon extinguishing Si, that the ability of S2 to evoke the key-peck response was greatly reduced. Just why the results yielded by the autoshaping preparation differ from those that have been reported with other conditioning preparations is not at this time understood.
US, would show less responding to 82 than subjects that received only the experience, 82 —» Si -» US, that is, the difference reported by Kamin (1968). The second study serves this purpose. Two groups of pups were examined. One, Group B, received a blocking treatment— a single Phase-1 pairing of the lemon scent (Si) and the LiCl and a single, Phase-2 sequential presentation of orange (S 2 ), lemon (Si), and LiCl. The pups in the control condition, Group C, were treated just like Group B except they did not experience the lemon scent or the LiCl in Phase 1. Method Subjects. The subjects were 18 Sprague-Dawley derived male and female rats bred in our laboratory. They were 7 days old at the start of the experiment. Procedures. Unless noted otherwise, the procedures were identical to those used in Experiment 1. On the first day of training, (Phase 1) the pups in Group B (n — 9) were exposed to the lemon scent for 5 min, injected with LiCl, and returned to the lemon-scented (Si) environment for 30 min prior to being returned to their maternity cages. On the second day (Phase 2) of training they were exposed to the orange scent ( S i ) for 5 min, exposed to the lemon extract (Si) for 5 min, injected with LiCl, and returned to the lemon-scented environment for 30 min. The pups in Group C (n = 9) were treated just like those in Group B except that they experienced neither the lemon scent nor the LiCl in Phase 1. Instead they were placed into a bag containing maternity cage shavings on this day. Each of the two litters of pups in this study contributed no more than five and no fewer than four subjects to each group.
Experiment 2 Results and Discussion The results of Experiment 1, together with those previously reported by Rudy and Cheatle (in press), suggest that the neonatal rat will acquire second-order odor aversions. Prior to comparing directly the results of the second-order conditioning and blocking paradigms, it was deemed appropriate to determine if blocking per se could be obtained with neonatal rats in the odor-aversion task with the specific sequence to be employed. That is, it was appropriate to ask whether or not subjects that received the blocking sequence, Si —» US ; S2 —> Si -»
The results of this study are presented in Figure 2. As may be seen, the groups did not differ in their responding on the lemonscent test. The principal question addressed in this study was, Would the Phase-1 treatment experienced by Group B reduce conditioning that would otherwise have accrued to S2 (the orange scent) ? The answer to this question was affirmative: The animals in Group B spent significantly more time over the orange scent than those in Group C, f(16) =2.01, p< .05.
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This experiment has demonstrated that SO the blocking sequence experienced by Group ORANGE (S2) LEMON (SI) B produced less conditioning to S2 than did UJ the simple compound training experienced 60 by Group C. The blocking sequence in this preparation appears to have effects like those originally observed by Kamin (1968) UJ C5> in the CER situation. This observation is sufficient for present purposes. However, it should be appreciated that we present no UJ O analytic evidence that this blocking effect Cg 20 UJ necessarily depended upon either the Si-US O. pairing in Phase 1 or the presence of Si during the Phase-2 S2-US experience. Thus while it is reasonable to suppose that the B C B C decrement in conditioning to S2 in Group B EXPERIMENTAL CONDITIONS depended upon the associative properties Figure 2. Mean percentage of time spent over the acquired in Phase 1 by Si, which in some orange (Sa) or lemon (Si) scent by the groups in way reduced the effectiveness of the US to Experiment 2. promote first-order conditioning in Phase 2 (e.g., Kamin, 1968; Rescorla & Wagner, those in Group B on the first conditioning day. 1972), the study, in fact, allows only the On the second day, they were placed into the bag more cautious empirical conclusion noted containing orange-scented shavings for 5 min then immediately placed into a bag containing lemonabove. scented shavings for 5 min, removed and inExperiment 3 Together, the results of Experiments 1 and 2 and the work of Rudy and Cheatle (in press) suggest that we have effective preparations for directly comparing the second-order conditioning and blocking paradigms. In this study such a comparison was made. Three treatment groups were examined. Two groups were treated exactly like Group B and Group C of the previous study. A third, Group SOC, received a second-order conditioning treatment. They were treated exactly like Group B, except that on the second day of training they received only a sequential presentation of the orange (S 2 ) and lemon (Si) scents. They were not then injected with LiCl. Method Subjects. The subjects were 24 Sprague-Dawley derived male and female pups 7 days old at the start of the experiment. Procedures. Subjects in Group B (n = 8) and Group C (n = 9) were treated exactly as those in the corresponding groups of Experiment 2. Pups in Group SOC (n = 7) were treated exactly like
jected with the hyperderrnic needle, and then returned to the lemon-scented environment for 30 min before being returned to their maternity cages. Thus the only difference between animals that received the second-order conditioning treatment, Group SOC, and those receiving the blocking treatment, Group B, was that on the second day of training, animals in Group B experienced the illness-inducing US, whereas those in Group SOC received only a sham injection.
Results and Discussion The outcome of this experiment is presented in Figure 3. Although Groups B and SOC spent somewhat less time over the lemon scent (Si) than Group C, there were no statistically reliable differences among the groups when they were tested for their aversion to lemon, F < 1. The major interest of this study concerned the test results for the pups' reaction to the orange scent, S2. It may be seen in Figure 3 that the subjects in Group B spent more time over the orange scent than did the pups in either Group C or Group SOC. An analysis of variance revealed that the groups were significantly different on the orange-aversion test, F(2, 21) =6.27, p
2.95, ps < .01. The results of the second experiment thus were replicated as the Phase-1 Si-US pairing received by Group B, but not by Group C, again blocked conditioning to the orange scent (S 2 ). The most important finding of this study, however, was that Group SOC spent much less time over the orangescented shavings than did Group B. Given that the treatment experienced by Group SOC was sufficient to induce a second-order aversion to the orange scent, one is forced to conclude that the additional experience of the US by Group B in some manner prevented the acquisition of a second-order aversion to the orange scent by the animals in this group. Experiment 4 In discussing the blocking effect obtained in Experiment 2, we made reference to the interpretation of Kamin (1968) and Rescorla and Wagner (1972). The important assumption shared by these theorists is that the ability of the US to promote the associate processing necessary for first-order conditioning depends upon the stimulus context in which the US is experienced. If the
US is experienced in a stimulus context in which it previously has occurred, the capacity of the US to produce conditioning will be reduced. By this account blocking occurs because prior Si-US pairings empower Si with the ability to reduce the reinforcing effects of the US. Consequently, when SiSg-US pairings are experienced, the effectiveness of the US to produce conditioning to S2 is weakened. This account may suffice as an explanation of why S2 failed to acquire first-order conditioned properties in a blocking sequence. It does not, however, speak to why, as seen in Experiment 3, Si fails to reinforce second-order conditioning to S2 in the same sequence. This latter problem requires some additional theoretical machinery. Regardless of precisely what the animal learns in the second-order conditioning sequence (see Experiment 1), it is evident that some attributes of the S2-Si experience become associated to allow S2 to evoke a CR. One way to approach the problem of why this learning does not occur in the similar blocking sequence is to assume that the S2-Si pairing consistently sets up potential associations among the attributes of the S2-Si experience, but that these potential associations require some period of time free of interference in order to become permanently functional (cf. Estes, 1970; Wagner, 1976; Wagner et al., 1973). If some interfering post-S2-Si event occurs, such as another learning experience or some unusual disturbance of the nervous system, then the potential associations will be less likely to become functional. Within this framework, S2 failed to acquire second-order conditioned properties in Group B because the experience of the US disrupted the post-Sjr-Si associative processing necessary for converting the potential associations among the S2-Si attributes into functional ones. According to this account, it is not simply the fact that the animal experiences the US as well as S2-Si that prevents second-order conditioning to S2 in the blocking sequence. The temporal relationship of the two experiences is presumably critical. The more temporally proximate the Ss-Si and the US
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experiences, the less likely will be the ac80 quisition of second-order conditioned propORANGE ( S 2 ) erties by S2. Thus if one were to lengthen the interval separating the S2-Si and the US events in the blocking sequence, then S2 (- 60 should be more likely to acquire second- o u order properties. 40 In this experiment an attempt was made to evaluate this reasoning. To do this, three UJ treatment conditions were examined. Subzo jects in two groups, Groups B and SOC, cr LU a. were treated exactly like the corresponding groups of Experiment 3. Pups in the third condition, Group B-60, however, were B B-60 SOC B B-60 SOC treated like those in Group B, except that EXPERIMENTAL CONDITIONS US was not experienced until 60 min after Figure 4. Mean percentage of time spent over the the S2-Si pairing. If the above account is orange (Si) or lemon (Sa) scent by the groups valid, then the subjects in Group B-60 in Experiment 4. should show evidence of second-order conditioning. both Group SOC, Si —> US training trials and found that little conditioning is acquired by S2. Unfortunately for present purposes, neither Kamin (1968) nor Rescorla (Note 1) included a secondorder conditioning group to evaluate whether less conditioning was observed in the blocking sequence than would be seen with such a group. To be sure, the data that we have reported may have implications for interpretation of blocking experiments. They may also imply the operation of a more general effect of the US than has been previously appreciated. Typically in a learning experiment the US is an integral contributor to the associative learning indexed by the conditioned response. In addition to participating in the specific associative structure underlying the CR, however, a US-like event might have more nonspecific properties. One of these, suggested by our experiments, is that it will disrupt the development of functional associations among the attributes of other events that are experienced. Consequently, it is possible that this disruptive influence of the US might be detected in situations other than the second-order conditioning paradigm. For example, it has been found that animals can associate stimulus events that typically serve as CSs in a conditioning study. Such learning can be inferred from the socalled sensory-preconditioning experiments (e.g., Brogden, 1939; Lavin, 1976; Rizley & Rescorla, 1972). In Phase 1 of a sensorypreconditioning study, the animal receives an 81-82 pairing necessary for establishing association among attributes of Si and S2. In a second phase, S2 is paired with a US. If, when tested, Si evokes a conditioned reaction that would occur to S2, one can infer
that Si and S2 were associated as a consequence of the 81-82 pairings. Granted that such inter-CS associations normally occur, one might then ask if the occurrence of a US-like event in Phase 1 (i.e., an Si-S2-US sequence) might interfere with the development of inter-CS associations. In this context it is useful to note, as did Rizley and Rescorla (1972), the similarity of the sensory preconditioning, Si —» S 2 ; S2 —» US, and second-order conditioning, Si —> US; S2 —> Si, paradigms. Both potentially allow the development of inter-CS associations (noted as the 83-81 association view of second-order conditioning). Recall that Rizley and Rescorla (1972) found no evidence in the CER situation of inter-CS associations developing as a consequence of the Phase-2 82-81 pairings of a second-order conditioning paradigm. It is then instructive to note that using a sensory-preconditioning paradigm, they were able to find evidence of such associations, and this fact prompts the question: Why was no evidence of inter-CS associations obtained in their second-order conditioning study? One might speculate that the failure to observe inter-CS associative learning was because the 82-81 pairing was necessarily accompanied by a US-like event, the CR evoked by Si in the second-order paradigm, but was not in the sensory-preconditioning paradigm. Perhaps the occurrence of the CR interfered with the development of the interCS association. Whether or not this particular interpretation is correct, it is clear that the view that US-like events might, in addition to participating in associative learning, interfere with learning that otherwise would take place, is one that merits serious evaluation. Reference Note 1. Rescorla, R. A. Personal communication, November 1977.
References Atkinson, R. C, & Wickens, T. D. Human memory and the concept of reinforcement. In R. Glaser (Ed.), The nature of reinforcement. New York: Academic Press, 1971.
SECOND-ORDER CONDITIONING AND BLOCKING Brogden, W. J. Sensory preconditioning. Journal of Experimental Psychology, 1939, 25, 323-332. Brown, P. L., & Jenkins, H. M. Autoshaping of the pigeon's key-peck. Journal of the Experimental Analysis of Behavior, 1968,11, 1-8. Corrodi, H., Fuxe, K., HSkfelt, T., & Schou, M. The effect of Kthium on cerebral monamine neurons. Psychopharmacology, 1967,11, 34S-3S3. Estes, W. K. Learning theory and mental development. New York: Academic Press, 1970. Gillan, D. J., & Domjan, M. Taste aversion conditioning with expected versus unexpected drug treatment. Journal of Experimental Psychology: Animal Behavior Processes, 1977, 3, 297-309. Holland, P. C. Conditioned stimulus as a determiner of the form of the Pavlovian conditioned response. Journal of Experimental Psychology: Animal Behavior Processes, 1977, 3, 77-104. Holland, P. C., & Rescorla, R. A. The effect of two ways of devaluing the unconditioned stimulus after first- and second-order appetitive conditioning. Journal of Experimental Psychology: Animal Behavior Processes, 1975, 1, 355-363. (a) Holland, P. C., & Rescorla, R. A. Second-order conditioning with food unconditioned stimulus. Journal of Comparative and Physiological Psychology, 1975, 88, 459-467. (b) Kamin, L. J. "Attention-like" processes in classical conditioning. In M. R. Jones (Ed.), Miami symposium on the prediction of behavior: Aversive stimulation, Miami, Fla.: University of Miami Press, 1968. Kamin, L. J. Predictability, surprise, attention, and conditioning. In B. A. Campbell & R. M. Church (Eds.), Punishment and aversive behavior. New York: Appleton-Century-Crofts, 1969. Landauer, T. K. Reinforcement as consolidation. Psychological Review, 1969, 76, 82-96. Lavin, M. J. The establishment of flavor-flavor associations using a sensory preconditioning training procedure. Learning and Motivation, 1976, 7, 173-183. Mackintosh, N. J. Blocking of conditioned suppression : Role of the first compound trial. Journal of Experimental Psychology: Animal Behavior Processes, 1975, 1, 335-345. (a) Mackintosh, N. J. A theory of attention: Variations in the associability of stimuli with reinforcement. Psychological Review, 1975, 82, 276-298. (b) McGaugh, J. L. Time-dependent processes in memory storage. Science, 1966, 153, 1351-1358.
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Pavlov, I. P. Conditioned reflexes. London: Oxford University Press, 1927. Rashotte, M. E., Griffin, R. W., & Sisk, C. L. Second-order conditioning of the pigeon's key peck. Animal Learning and Behavior, 1977, 5, 25-38. Rescorla, R. A. Second-order conditioning: Implications for theories of learning. In F. J. McGuigan & D. B. Lumsden (Eds.), Contemporary approaches to conditioning and learning. Washington, D.C.: V. H. Winston, 1973. Rescorla, R. A., & Furrow, D. R. Stimulus similarity as a determinant of Pavlovian conditioning. Journal of Experimental Psychology: Animal Behavior Processes, 1977, 3, 203-215. Rescorla, R. A., & Wagner, A. R. A theory of Pavlovian conditioning: Variations in the effectiveness of reinforcement and nonreinforcement. In A. H. Black & W. F. Prokasy (Eds.), Classical conditioning II: Current research and theory. New York: Appleton-Century-Crofts, 1972. Rizley, R. C., & Rescorla, R. A. Associations in second-order conditioning and sensory preconditioning. Journal of Comparative and Physiological Psychology, 1972, 81, 1-11. Rudy, J. W., & Cheatle, M. D. Odor-aversion learning by neonatal rats. Science, 1977, 198, 845846. Rudy, J. W., & Cheatle, M. D. Ontogeny of associative learning: Acquisition of odor-aversions by neonatal rats. In N. E. Spear & B. K. Campbell (Eds.), Ontogeny of learning and memory. Hillsdale, N.J.: Erlbaum, in press. Wagner, A. R. Stimulus validity and stimulus selection in associative learning. In N. J. Mackintosh & W. K. Honig (Eds.), Fundamental issues in associative learning. Halifax, Canada: Dalhousie University Press, 1969. Wagner, A. R. Priming in STM: An information processing mechanism for self-generated or retrieval-generated depression in performance. In T. J. Tighe & R. N. Leaton (Eds.), Habituation: Perspectives from child development, animal behavior, and neurophysiology. Hillsdale, N.J.: Erlbaum, 1976. Wagner, A. R., Rudy, J. W., & Whitlow, J. W. Rehearsal in animal conditioning. Journal of Experimental Psychology, 1973, 97, 407-426. (Monograph) Received September 7, 1977 Revision received January 11, 1978 •
Analysis of Second-Order Odor-Aversion Conditioning in Neonatal Rats: Implications for Kamin's Blocking Effect Martin D. Cheatle and Jerry W. Rudy Princeton University It has been reported that neonatal rats acquire first-order conditioned aversions to olfactory stimulation experienced prior to drug-induced illness. Experiment 1 demonstrates that the neonate will also acquire a second-order conditioned odor aversion and that the second-order aversion is not affected by extinguishing the first-order aversion. The effectiveness of the secondorder conditioning paradigm (i.e., Si~* US; S2—* Sj) and a sequential blocking paradigm (i.e., S t —*US; S 2 ~*Si—>US) also were compared. These comparisons revealed (a) that the occurrence of the US in the second phase of the blocking treatment interfered with second-order conditioning (Experiment 3) and (b) that the temporal proximity of US occurrence to the S2~* Si episode was a critical determiner of the degree to which such interference was observed; that is, the more proximate the S2-Sl and US events, the more US occurrence interfered with second-order conditioning (Experiment 4). Implications of these findings for theories of conditioning are discussed. The paradigms for investigating secondorder conditioning and blocking are similar. In Phase 1 the .subject typically receives a paired presentation of the stimulus, Si, and an unconditioned stimulus (US), and in Phase 2 it experiences both Si and a second stimulus, 82, in close temporal proximity. In a second-order conditioning experiment, the two stimuli are usually presented sequentially, with 82 preceding the occurrence of Si and the US omitted (i.e., S 2 -»Si). In the blocking experiment, Si and 82 generally are presented simultaneously and terminate with the occurrence of the US (i.e., SiS 2 -*US). Pavlov (1927) presented the first evidence on the fate of 82 in the second-order conditioning paradigm. He found that S2 acquired conditioned properties even though This research was supported in part by National Science Foundation Grant GB-38322 to J, W. Rudy. Requests for reprints should be sent to Jerry W. Rudy, Department of Psychology, Princeton University, Princeton, New Jersey 08S40.
the US was not directly paired with it. Rescorla and his colleagues recently have provided a number of demonstrations of this phenomenon. In his laboratory second-order conditioning has been observed with the conditioned emotional response (CER) preparation (Rescorla, 1973; Rizley & Rescorla, 1972) and the appetitive-activity situation (Holland, 1977; Holland & Rescorla, 197Sa, 1975b). Most recently it has also been obtained with Brown and Jenkins's (1968) autoshaping procedure (Rashotte, Griffin, & Sisk, 1977; Rescorla & Furrow, 1977). Kamin (1968) has provided the reference experiment on the fate of S2 in the blocking paradigm. He reported that little or no conditioning occurred to S2, with the specific comparison of interest being that less conditioning occurred than if SiS2-US pairings were not preceded by the Si-US training, The generality of the latter blocking effect is well established (Gillan & Domjan, 1977; Holland, 1977; Kamin, 1968, 1969; Mackintosh, 1975a), and this phe-
Copyright 1978 by the American Psychological Association, Inc. 0097-7403/78/0403-0237$00.7S
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nomenon has provided the impetus for many recent theoretical analyses of first-order conditioning (e.g., Kamin, 1969; Mackintosh, 1975b; Rescorla & Wagner, 1972; Wagner, Rudy, & Whitlow, 1973). Given the formal similarity of the blocking and second-order conditioning arrangements, it is all the more surprising that the animal fails to condition to S2 in the blocking sequence. In the blocking arrangement there are two potential sources of reinforcement available to S2. On the one hand, there is the opportunity for first-order conditioning provided by the US. On the other hand, there is an opportunity for second-order conditioning provided by the occurrence of the previously conditioned Si. Nevertheless, when presented alone, S2 evokes little conditioned responding, and this fact suggests that little first- or second-order conditioning is captured by S2. One potential reason why 82 does not acquire second-order conditioned properties in the typical blocking study is that Si and S2 are presented concurrently, whereas in the typical second-order conditioning experiment, S2 generally precedes the occurrence of Si. There are, however, empirical grounds for questioning the necessity of a sequential, 83 —» Si, presentation for 82 to acquire second-order conditioned properties. For example, Rescorla (1973) has reported second-order conditioning when S2 and Si are presented jointly. An obvious feature distinguishing the second-order conditioning and blocking arrangements is that the US occurs in the latter but not the former paradigm on the occasion of pairing Si and S2. Thus one might argue that the occurrence of the US in a blocking study in some way interferes with the acquisition of second-order conditioned properties by S2 that would otherwise occur if the US were omitted. In this paper we report several studies that analyze the second-order conditioning paradigm and directly compare the results obtained with this procedure (i.e., Si —> US; 82—> Si) with those produced by a sequential blocking paradigm (i.e., $! —» US; S 2 -» Si -* US). Ultimately we will
argue that the results of this work encourage the view that second-order conditioning is adversely influenced when the US is temporally proximate to the occurrence of the 82 —> Si episode. The procedures employed in this research were somewhat unusual. Therefore, we will provide some additional background before describing the experiments. Recently, we have been concerned with investigating the ontogeny of associative-learning processes in altricial animals such as the rat (Rudy & Cheatle, 1977, in press). For this purpose we have developed an odor-aversion conditioning preparation that has proved remarkably sensitive to learning in neonatal rats ranging in age from 2-14 days old. The procedures are quite simple. On the day of conditioning pups are placed into an environment scented with a novel odor. Subsequently they are injected with a drug-US, Lithium Chloride (LiCl), that induces illness. The pups are later given a choice between spending time over shavings scented with the odor previously paired with the drug versus shavings scented with some other odor. Evidence that the pups acquired an aversion to the odor is provided by the fact that they avoid the scent experienced on the conditioning trial, in comparison to the alternative odor. The interested reader may consult the recent paper by Rudy and Cheatle (in press) for a detailed report on this research, indicating that first-order conditioning is easily detected with pups ranging in age from 2-14 days old and that major age-related differences are obtained when the parameters of the task are systematically investigated. Relevant to the present research is our work on second-order, odor-aversion conditioning. We have reported some evidence that pups 7 days of age can acquire a second-order odor aversion (Rudy & Cheatle, in press). To do this, one group of pups first experienced a pairing of the odor, Si, and the US. In Phase 2 they received a new odor, S2, followed immediately by exposure to Si. The pups that received this sequence displayed a greater aversion to 82 than did pups that (a) received the same Phase-2
SECOND-ORDER CONDITIONING AND BLOCKING
pairing but only Si or only the US in Phase 1 or (b) received the same Si-US pairing in Phase 1 but in Phase 2 experienced Si 4 hr after the S2 exposure. The neonatal odor-aversion task was the vehicle for studying second-order conditioning in the present research. In the first experiment further evidence that neonates acquire second-order odor aversions is presented. In the second, some evidence that these procedures are suitable for demonstrating .blocking, as in the Kamin (1968) comparison, are provided. In Experiments 3 and 4 the second-order conditioning and blocking sequence are directly compared. Experiment 1 There were at least two weaknesses in the second-order conditioning studies reported by Rudy and Cheatle (in press). First, although we included comparison treatment groups that were intended to evaluate the necessity of the Phase 1, Si-US pairing, the pups in these control groups experienced only Si or only the US in Phase 1. No comparison group experienced both Si and the US in Phase 1. Since the pups in the experimental group received a pairing involving both Si and the US, the criticism can be offered that the change in the experimental pups' reaction to S2 was a product of some "nonassociative" effect of their having experienced both Sj and the US prior to the test for second-order conditioning, independent of their pairing. Second, we provided no independent evidence that Si had acquired first-order conditioned properties. These shortcomings were met in the present study by employing a control that experienced both Si and the US in Phase 1. They experienced the US and 60 min later received Si. Moreover, following the test for an aversion to S2, all pups were tested for their reaction to Si. In this experiment, evidence was also provided that is relevant to the hypothesis that the second-order odor aversion acquired by the neonate is the result of an S2-Si association. The Sa-Si experience provides the animal the opportunity to acquire any of several associations (Rescorla, 1973), which
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could allow S2 to evoke a conditioned response (CR). These include (a) S2 becoming associated with a representation of Si (the S2-Si association), (b) S2 becoming associated with the CR evoked by Si (the S2-CR association), and (c) S2 becoming associated with a representation of the US activated by Si (the S2-US association). The behavioral reaction to S2 by the first view is seen as reflecting the CR evoked by the activation of Si's representation by S2. Accordingly, the ability of S2 to evoke a conditioned reaction depends critically on the integrity of the conditioned properties of Si. Depreciating the value of Si's conditional properties following second-order conditioning consequently should also weaken the ability of S2 to evoke a CR. According to the remaining alternatives (the S2-CR and Si-US association views), the ability of S2 to evoke a CR does not depend upon the ability of Si to evoke a CR. The above issues were addressed in this experiment by examining four groups of pups. Two groups each received a pairing of a lemon scent, Si, and the US in Phase 1 and a pairing of an orange scent, S2, and Si in Phase 2, Prior to testing their reaction to S2) the pups in one of these conditions, Group P/P/E, received a treatment calculated to extinguish the conditioned aversive properties of Si. They received a series of nonreinforced exposures to the lemon scent. Pups in the other group, Group P/P/MS, in contrast, received a comparable number of nonreinforced exposures to maternity cage shavings. According to the S2-Si association view, Group P/P/E should display a substantially weaker aversion to S2 than Group P/P/MS. Subjects in two control conditions, Group B/P/E and B/P/ MS, were treated, respectively, like pups in Group P/P/E and P/P/MS, except that in Phase 1 they received a "backward" conditioning treatment: They were injected with the US 60 min prior to being exposed to the lemon scent. These groups thus controlled for the necessity of the exposure to both Si and the US in the Phase-1 Sj-US pairing operation. All subjects were tested for their reaction to both S2 and Si.
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Method Subjects. The subjects were 29 male and female Sprague-Dawley derived rats 7 days old at the start of the experiment, where Day 0 is defined as date of birth. These animals were bred in our laboratory and housed with the mother in standard maternity cages containing pine shavings. The shavings were changed approximately twice per week. Four litters contributed to this study. Procedures. On the first day of conditioning (Phase 1) the maternity cages were brought into the experimental area and the mothers were removed. Thereafter, the subjects in Group P/P/E (n — 7) were treated as follows. The pups were taken from the maternity cage and placed directly into a large polyurethane bag (55 X 36 X 24 cm) scented with 2.5 cc of McCormick's lemon extract (Si) mixed with a volume of fresh natural-pine shavings sufficient to fill a 500-ml beaker. After all 7 pups were placed into the shavings the top of the bag was closed except for an opening of 2-3 cm in the top for ventilation. Approximately 5 min later each pup was removed, weighed, and injected (ip) with a 2% body weight, .15-mol solution of LiCl and then returned to the odor environment for an additional 30 min before being returned to its maternity cage. The maternity cages were then returned to the colony room. On the next day (Phase 2) these pups received a second-order conditioning treatment that involved a sequential exposure to a novel orange scent (S2) followed immediately by exposure to the previously conditioned lemon scent (Si). Specifically, they were placed for 5 min into a bag scented with the pine shavings adulterated with 2.5 cc of McCormick's orange extract, then immediately placed into a bag containing lemon-scented shavings. On the third day of the experiment (Phase 3) the pups in Group P/P/E experienced a series of five 30-min exposures to the lemon scent (Si), separated by 90 min. During the 90-min interval the pups and the mother were returned to the maternity cage. Subjects in Group P/P/MS (n = 8) were treated exactly like those in Group P/P/E except that during Phase 3 they received nonreinforced placements into a bag containing maternity cage shavings. On the first day of conditioning the pups in the two control conditions, Group B/P/E (» = 6) and Group B/P/MS (n = 8), were removed from their maternity cages, weighed, injected with LiCl, and then returned to their maternity cages. Sixty min later they were placed into a bag containing lemon-scented shavings for 35 min. Thereafter they were treated, respectively, like Groups P/P/E and P/P/MS. In this study there were no more than two and no fewer than one pup from each litter in each of the four groups. Forty-eight hr following the Phase-2 treatment all pups were tested for an aversion first to the orange scent (82) and then to the lemon scent
(Si). The test apparatus consisted of a 30 X 20 X 10-cm Plexiglas compartment with a wire mesh floor, beneath which were two 30 X 9 X 3-cm containers. During the orange-aversion test one of the containers was filled with a sufficient volume of shavings, adulterated with 2.5 cc of the orange scent, to fill a 500-ml beaker. The other container held an equivalent volume of shavings scented with 2.5 cc of McCormick's garlic juice. During the lemon-aversion test one container was filled with a similar volume of shavings scented with 2.5 cc of lemon extract, the other contained garlic-scented shavings. On a test trial a single pup was placed in the center of the compartment so that its right forepaw and rearpaw were over one container and its left forepaw and rearpaw were over the other container. The pup remained in the test apparatus for 150 sec. The dependent variable of interest, depending upon the test, was the percentage of time the pup spent over the orange- or lemonscented shavings. The pup was judged to be over the scent when its head and both forelegs were across a line dividing the two containers. The judge had no knowledge of the pups pretreatment at the time of the test. After each pup was tested the wire floor was wiped with a damp cloth. After 6 pups were tested the shavings were discarded and replaced with appropriately scented shavings. One hr after the last pup was tested for an aversion to the orange scent the lemon-aversion test was initiated.
Results The results of the tests for both first- and second-order conditioning are presented in Figure 1. It may be seen that the pups in Group P/P/MS displayed evidence of a first-order conditioned aversion: In comparison to Groups B/P/E and B/P/MS, they avoided the lemon scent. In contrast, the subjects in Group P/P/E, if anything, displayed a greater preference for the lemon scent than did the pups in the two control conditions. A different pattern of results was revealed by the second-order conditioning test data. Note that Groups P/P/E and P/P/MS did not differ and that they each spent less time over the orange scent than did the pups in the control groups. Two separate 2 x 2 factorial analyses of variance were performed on these data. The analysis of the second-order conditioning data revealed only a significant main effect of the Phase-1 forward- versus backwardconditioning treatment, F(l, 25) = 12.48,
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P/T/MS
B/p/MS
VHI
P/P/MS
P/P/E
B/P/MS
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B/P/E
EXPERIMENTAL CONDITIONS Figure 1. Mean percentage of time spent over the second-order stimulus, the orange scent (Ss), and the first-order stimulus, the lemon scent (Si), by the groups in Experiment 1.
p < .01. In contrast, the analysis of the firstorder conditioning data revealed an interaction between the Phase-1 acquisition treatment and the Phase-3 extinction treatment, F(l, 25) =4.85, p < .05. A subsequent analysis indicated that the source of this interaction was that Group P/P/E differed from Group P/P/MS, F(l, 25) = 17.98, p < .001, whereas Groups B/P/E and B/P/ MS did not reliably differ, F < 1. Discussion The results of this study join those previously reported by Rudy and Cheatle (in press) in providing evidence that the neonatal rat can acquire a second-order conditioned odor aversion. A principal feature of this experiment was the inclusion of Groups B/P/E and B/P/MS, which were exposed to both Sj and the US in Phase 1 and thus controlled for the necessity of the Phase 1 Si-US pairing. The pups in these two groups experienced both Si and the US in Phase 1, but a delayed backward-pairing, US-Si, procedure was employed. They displayed no evidence of avoiding Si or S2 and
spent significantly more time over S2 than did Group P/P/E or Group P/P/MS. The fact that Group P/P/MS spent less time over the lemon scent (Si) can be taken as evidence that the Si-US pairing experienced by Group P/P/MS resulted in a conditioned first-order odor aversion. The comparisons of Groups P/P/E and P/P/MS on the first and second-order conditioning test were particularly instructive. Whereas only Group P/P/MS avoided Si, both groups equally avoided S2. Nonreinforced exposure to S2 prior to testing was apparently effective in depreciating the firstorder conditioned properties of Si for Group P/P/E. The observation that Groups P/ P/E and P/P/MS did not then differ in their reaction to S2 suggests that the status of the second-order properties acquired by S2 does not depend upon the integrity of the conditioned properties of Si. If this had been the case, then extinguishing the conditioned properties of Si should have also reduced the pups' tendency to avoid S2. This pattern of results thus provides no evidence that the second-order odor aver•sion was a result of the acquisition of an
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Sa-Si association. One could, of course, always maintain that the extinction operation did not completely remove the conditioned properties of Si and that this residual was sufficient to mediate the second-order effects produced by 82. To do so, however, one has to make an additional assumption that the residual conditioned properties of Si were sufficient to mediate a second-order reaction to S2 but were not sufficient to produce evidence of a first-order reaction to Si. This seems implausible on the face of it, although the fact that subjects were tested first on 82 (.presumably generating some extinction) prior to Si precludes its dismissal. Our present judgment is that the S2-Si association is unlikely to be the mediator of second-order odor-aversion conditioning. Rizley and Rescorla (1972) and Holland and Rescorla (1975a, 197Sb) have reached a similar conclusion concerning the mechanisms of second-order conditioning displayed by adult rats in the CER situation and the conditioned appetitive-activity situation. In contrast, Rashotte et al. (1977) have found evidence consistent with the 82Si association view using pigeons in the autoshaping preparation (Brown & Jenkins, 1968). They found, upon extinguishing Si, that the ability of S2 to evoke the key-peck response was greatly reduced. Just why the results yielded by the autoshaping preparation differ from those that have been reported with other conditioning preparations is not at this time understood.
US, would show less responding to 82 than subjects that received only the experience, 82 —» Si -» US, that is, the difference reported by Kamin (1968). The second study serves this purpose. Two groups of pups were examined. One, Group B, received a blocking treatment— a single Phase-1 pairing of the lemon scent (Si) and the LiCl and a single, Phase-2 sequential presentation of orange (S 2 ), lemon (Si), and LiCl. The pups in the control condition, Group C, were treated just like Group B except they did not experience the lemon scent or the LiCl in Phase 1. Method Subjects. The subjects were 18 Sprague-Dawley derived male and female rats bred in our laboratory. They were 7 days old at the start of the experiment. Procedures. Unless noted otherwise, the procedures were identical to those used in Experiment 1. On the first day of training, (Phase 1) the pups in Group B (n — 9) were exposed to the lemon scent for 5 min, injected with LiCl, and returned to the lemon-scented (Si) environment for 30 min prior to being returned to their maternity cages. On the second day (Phase 2) of training they were exposed to the orange scent ( S i ) for 5 min, exposed to the lemon extract (Si) for 5 min, injected with LiCl, and returned to the lemon-scented environment for 30 min. The pups in Group C (n = 9) were treated just like those in Group B except that they experienced neither the lemon scent nor the LiCl in Phase 1. Instead they were placed into a bag containing maternity cage shavings on this day. Each of the two litters of pups in this study contributed no more than five and no fewer than four subjects to each group.
Experiment 2 Results and Discussion The results of Experiment 1, together with those previously reported by Rudy and Cheatle (in press), suggest that the neonatal rat will acquire second-order odor aversions. Prior to comparing directly the results of the second-order conditioning and blocking paradigms, it was deemed appropriate to determine if blocking per se could be obtained with neonatal rats in the odor-aversion task with the specific sequence to be employed. That is, it was appropriate to ask whether or not subjects that received the blocking sequence, Si —» US ; S2 —> Si -»
The results of this study are presented in Figure 2. As may be seen, the groups did not differ in their responding on the lemonscent test. The principal question addressed in this study was, Would the Phase-1 treatment experienced by Group B reduce conditioning that would otherwise have accrued to S2 (the orange scent) ? The answer to this question was affirmative: The animals in Group B spent significantly more time over the orange scent than those in Group C, f(16) =2.01, p< .05.
SECOND-ORDER CONDITIONING AND BLOCKING
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This experiment has demonstrated that SO the blocking sequence experienced by Group ORANGE (S2) LEMON (SI) B produced less conditioning to S2 than did UJ the simple compound training experienced 60 by Group C. The blocking sequence in this preparation appears to have effects like those originally observed by Kamin (1968) UJ C5> in the CER situation. This observation is sufficient for present purposes. However, it should be appreciated that we present no UJ O analytic evidence that this blocking effect Cg 20 UJ necessarily depended upon either the Si-US O. pairing in Phase 1 or the presence of Si during the Phase-2 S2-US experience. Thus while it is reasonable to suppose that the B C B C decrement in conditioning to S2 in Group B EXPERIMENTAL CONDITIONS depended upon the associative properties Figure 2. Mean percentage of time spent over the acquired in Phase 1 by Si, which in some orange (Sa) or lemon (Si) scent by the groups in way reduced the effectiveness of the US to Experiment 2. promote first-order conditioning in Phase 2 (e.g., Kamin, 1968; Rescorla & Wagner, those in Group B on the first conditioning day. 1972), the study, in fact, allows only the On the second day, they were placed into the bag more cautious empirical conclusion noted containing orange-scented shavings for 5 min then immediately placed into a bag containing lemonabove. scented shavings for 5 min, removed and inExperiment 3 Together, the results of Experiments 1 and 2 and the work of Rudy and Cheatle (in press) suggest that we have effective preparations for directly comparing the second-order conditioning and blocking paradigms. In this study such a comparison was made. Three treatment groups were examined. Two groups were treated exactly like Group B and Group C of the previous study. A third, Group SOC, received a second-order conditioning treatment. They were treated exactly like Group B, except that on the second day of training they received only a sequential presentation of the orange (S 2 ) and lemon (Si) scents. They were not then injected with LiCl. Method Subjects. The subjects were 24 Sprague-Dawley derived male and female pups 7 days old at the start of the experiment. Procedures. Subjects in Group B (n = 8) and Group C (n = 9) were treated exactly as those in the corresponding groups of Experiment 2. Pups in Group SOC (n = 7) were treated exactly like
jected with the hyperderrnic needle, and then returned to the lemon-scented environment for 30 min before being returned to their maternity cages. Thus the only difference between animals that received the second-order conditioning treatment, Group SOC, and those receiving the blocking treatment, Group B, was that on the second day of training, animals in Group B experienced the illness-inducing US, whereas those in Group SOC received only a sham injection.
Results and Discussion The outcome of this experiment is presented in Figure 3. Although Groups B and SOC spent somewhat less time over the lemon scent (Si) than Group C, there were no statistically reliable differences among the groups when they were tested for their aversion to lemon, F < 1. The major interest of this study concerned the test results for the pups' reaction to the orange scent, S2. It may be seen in Figure 3 that the subjects in Group B spent more time over the orange scent than did the pups in either Group C or Group SOC. An analysis of variance revealed that the groups were significantly different on the orange-aversion test, F(2, 21) =6.27, p
2.95, ps < .01. The results of the second experiment thus were replicated as the Phase-1 Si-US pairing received by Group B, but not by Group C, again blocked conditioning to the orange scent (S 2 ). The most important finding of this study, however, was that Group SOC spent much less time over the orangescented shavings than did Group B. Given that the treatment experienced by Group SOC was sufficient to induce a second-order aversion to the orange scent, one is forced to conclude that the additional experience of the US by Group B in some manner prevented the acquisition of a second-order aversion to the orange scent by the animals in this group. Experiment 4 In discussing the blocking effect obtained in Experiment 2, we made reference to the interpretation of Kamin (1968) and Rescorla and Wagner (1972). The important assumption shared by these theorists is that the ability of the US to promote the associate processing necessary for first-order conditioning depends upon the stimulus context in which the US is experienced. If the
US is experienced in a stimulus context in which it previously has occurred, the capacity of the US to produce conditioning will be reduced. By this account blocking occurs because prior Si-US pairings empower Si with the ability to reduce the reinforcing effects of the US. Consequently, when SiSg-US pairings are experienced, the effectiveness of the US to produce conditioning to S2 is weakened. This account may suffice as an explanation of why S2 failed to acquire first-order conditioned properties in a blocking sequence. It does not, however, speak to why, as seen in Experiment 3, Si fails to reinforce second-order conditioning to S2 in the same sequence. This latter problem requires some additional theoretical machinery. Regardless of precisely what the animal learns in the second-order conditioning sequence (see Experiment 1), it is evident that some attributes of the S2-Si experience become associated to allow S2 to evoke a CR. One way to approach the problem of why this learning does not occur in the similar blocking sequence is to assume that the S2-Si pairing consistently sets up potential associations among the attributes of the S2-Si experience, but that these potential associations require some period of time free of interference in order to become permanently functional (cf. Estes, 1970; Wagner, 1976; Wagner et al., 1973). If some interfering post-S2-Si event occurs, such as another learning experience or some unusual disturbance of the nervous system, then the potential associations will be less likely to become functional. Within this framework, S2 failed to acquire second-order conditioned properties in Group B because the experience of the US disrupted the post-Sjr-Si associative processing necessary for converting the potential associations among the S2-Si attributes into functional ones. According to this account, it is not simply the fact that the animal experiences the US as well as S2-Si that prevents second-order conditioning to S2 in the blocking sequence. The temporal relationship of the two experiences is presumably critical. The more temporally proximate the Ss-Si and the US
SECOND-ORDER CONDITIONING AND BLOCKING
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experiences, the less likely will be the ac80 quisition of second-order conditioned propORANGE ( S 2 ) erties by S2. Thus if one were to lengthen the interval separating the S2-Si and the US events in the blocking sequence, then S2 (- 60 should be more likely to acquire second- o u order properties. 40 In this experiment an attempt was made to evaluate this reasoning. To do this, three UJ treatment conditions were examined. Subzo jects in two groups, Groups B and SOC, cr LU a. were treated exactly like the corresponding groups of Experiment 3. Pups in the third condition, Group B-60, however, were B B-60 SOC B B-60 SOC treated like those in Group B, except that EXPERIMENTAL CONDITIONS US was not experienced until 60 min after Figure 4. Mean percentage of time spent over the the S2-Si pairing. If the above account is orange (Si) or lemon (Sa) scent by the groups valid, then the subjects in Group B-60 in Experiment 4. should show evidence of second-order conditioning. both Group SOC, Si —> US training trials and found that little conditioning is acquired by S2. Unfortunately for present purposes, neither Kamin (1968) nor Rescorla (Note 1) included a secondorder conditioning group to evaluate whether less conditioning was observed in the blocking sequence than would be seen with such a group. To be sure, the data that we have reported may have implications for interpretation of blocking experiments. They may also imply the operation of a more general effect of the US than has been previously appreciated. Typically in a learning experiment the US is an integral contributor to the associative learning indexed by the conditioned response. In addition to participating in the specific associative structure underlying the CR, however, a US-like event might have more nonspecific properties. One of these, suggested by our experiments, is that it will disrupt the development of functional associations among the attributes of other events that are experienced. Consequently, it is possible that this disruptive influence of the US might be detected in situations other than the second-order conditioning paradigm. For example, it has been found that animals can associate stimulus events that typically serve as CSs in a conditioning study. Such learning can be inferred from the socalled sensory-preconditioning experiments (e.g., Brogden, 1939; Lavin, 1976; Rizley & Rescorla, 1972). In Phase 1 of a sensorypreconditioning study, the animal receives an 81-82 pairing necessary for establishing association among attributes of Si and S2. In a second phase, S2 is paired with a US. If, when tested, Si evokes a conditioned reaction that would occur to S2, one can infer
that Si and S2 were associated as a consequence of the 81-82 pairings. Granted that such inter-CS associations normally occur, one might then ask if the occurrence of a US-like event in Phase 1 (i.e., an Si-S2-US sequence) might interfere with the development of inter-CS associations. In this context it is useful to note, as did Rizley and Rescorla (1972), the similarity of the sensory preconditioning, Si —» S 2 ; S2 —» US, and second-order conditioning, Si —> US; S2 —> Si, paradigms. Both potentially allow the development of inter-CS associations (noted as the 83-81 association view of second-order conditioning). Recall that Rizley and Rescorla (1972) found no evidence in the CER situation of inter-CS associations developing as a consequence of the Phase-2 82-81 pairings of a second-order conditioning paradigm. It is then instructive to note that using a sensory-preconditioning paradigm, they were able to find evidence of such associations, and this fact prompts the question: Why was no evidence of inter-CS associations obtained in their second-order conditioning study? One might speculate that the failure to observe inter-CS associative learning was because the 82-81 pairing was necessarily accompanied by a US-like event, the CR evoked by Si in the second-order paradigm, but was not in the sensory-preconditioning paradigm. Perhaps the occurrence of the CR interfered with the development of the interCS association. Whether or not this particular interpretation is correct, it is clear that the view that US-like events might, in addition to participating in associative learning, interfere with learning that otherwise would take place, is one that merits serious evaluation. Reference Note 1. Rescorla, R. A. Personal communication, November 1977.
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