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Potentiation of a conditioned taste aversion in preweanling and adult rats
BEHAVIORAL AND NEURAL BIOLOGY 4 0 , 44--57
(1984)
Potentiation of a Conditioned Taste Aversion in Preweanling and Adult Rats DAVID KUCHARSKI AND NORMAN E. SPEAR I
State University of New York at Binghamton Preweanling (18 days old) and adult rats were made ill with LiC1 either 2 min or 1 hr after tasting controlled amounts of either one of two single flavors (saccharin or NaCI) or a compound mixture of the two. Conditioning was assessed with a single test 4 days later relative to explicitly unpaired control conditions. Generally, potentiation of the aversions to either flavor occurred for animals conditioned to the compound. The potentiation effect was decreased or eliminated by nonreinforced exposure to the alternative flavor of the compound. These effects tended to be stronger for the younger rats. Specifically, adult animals did not express potentiation of the saccharin aversion whereas preweanlings did, especially at the longer C S - U S interval. Both adults and preweanlings expressed potentiated salt aversions. Nonreinforced exposure to the alternative element eliminated the potentiation effect. Conditions conducive to potentiation are discussed in light of investigators who have not observed this effect in similar studies with compound stimuli.
It has frequently been observed that when two conditioned stimuli (CS) are presented prior to a US, one tends to "overshadow" the conditioning of the other (Pavlov, 1927; Revusky, 1971). However, in some instances of learning it has been observed that the conditioning of one CS element paired with a US is actually greater when presented in compound with another than when presented alone. While this effect, termed potentiation, may seem paradoxical, it has been found in a variety of circumstances (for reviews, see Domjan, 1983; Lett, 1982; Rescorla, 1981; Rescorla & Durlach, 1981), and has become established as one possible consequence of multielement conditioning. According to a multiple association explanation of potentiation proposed by Rescorla (see e.g., Rescorla & Durlach, 1981), three different associations are formed during a pairing of a stimulus AB with a US: A - U S , B - U S , and A-B. Because 1 This research was supported by a grant from the National Institute of Mental Health (l RO1 MH35219) to Norman E. Spear. We are grateful for the secretarial assistance of Teri Tanenhaus and the technical expertise of Norman Richter. Send requests for reprints to Norman E. Spear, Department of Psychology, State University of New York at Binghamton, Binghamton, NY 13901. 44 0163-1047/84 $3.00 Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
POTENTIATION OF A TASTE AVERSION
45
A is associated with B, which itself is aversive due to conditioning, the net aversion to A is inflated beyond its association with the US. The association between the two CSs (A and B) has been referred to as a "within-compound" association. Rescorla (1981) has argued that potentiation occurs when the magnitude of this association is such that it overrides the overshadowing that results from compounding the target CS with another stimulus during conditioning. The present study investigated the possible ontogenetic differences in the overshadowing or potentiation of a conditioned taste aversion. Although not intended to be a decisive experiment determining the factors controlling infantile amnesia (see Campbell, 1983; Campbell & Spear, 1972), this study does address one possible explanation of the effect--that immature animals store too few attributes from a learning episode to support later retrieval of that memory. From the view that "memory" is a representation of several discernable events which constitute the learning episode (Bower, 1967; Spear, 1973; Underwood, 1969) one might predict that if their capacity for information processing is actually less than that of adults, preweanlings should store fewer of these events than adults. In this view, when two tastes are presented simultaneously and followed by lithium chloride (LiC1) poisoning, the preweanling rat should develop less of an aversion to the target taste during the conditioning episode. However, if the nontarget taste is not present during acquisition, the hypothetically lesser informational capacity of the preweanling is less apt to have appreciable impact on the magnitude of the conditioned aversion to the target taste. A second way in which a more limited processing capacity of the preweanlings might work against them in the two-taste situation stems from possible CS-CS associations. Assuming their lesser capacity, preweanlings might acquire the C S - U S associations as readily as adults but be less likely than adults to form the within-compound associations that could, theoretically, mediate the potentiation effect (Rescorla, 1981). Together, these factors lead to the hypothesis that in circumstances where conditioning to a single taste is about equivalent for preweanlings and adults, preweanlings should exhibit overshadowing whereas adults should display potentiated aversions if the target taste were instead compounded with another taste during conditioning. A major variable of interest in the present experiment was composition of the CS during conditioning. One set of animals was presented either a 0.20% saccharin solution or a 1.20% sodium chloride (NaC1) solution as their CS, followed by LiCI as the US, and were tested for the consumption of the respective solution after a 4-day recovery period. Additional animals were given a compound solution made up of 0.20% saccharin and 1.20% NaC1 as their CS, followed by LiC1 as the US. These taste stimuli were used in an experiment reported by Bouton and Whiting
46
KUCHARSKI AND SPEAR
(1982). These animals received both a saccharin preference and NaC1 intake test, with order counterbalanced. This counterbalanced testing procedure permitted assessment of differences in the magnitude of the aversion to the target taste as a function of whether or not animals had previously received a nonreinforced presentation (extinction procedure) of the nontarget taste. Whereas Rescorla has argued that such extinction will attenuate the aversion to the target taste (Rescorla, 1981), Lett has argued that the potentiation effect should not be affected by extinction of the other element of the conditioning compound (Lett, 1982). The LiCI injection was administered either 2 min, 1 hr, or 24 hr after the taste presentation. The 1-hr delay group was included because the potentiation effect is believed to be more robust with a long delay between the CS and US (Lett, 1982), and the 24-hr delay group served as an explicitly unpaired control condition. The present experiment employed several procedures intended to counteract some potential problems of method that sometimes have arisen in taste aversion experiments, especially those including ontogenetic comparisons. First, the hedonic value of the taste stimuli employed in this and other experiments probably change when they are compounded with other taste stimuli. The hedonic value of a taste solution can in some cases determine amount and duration of consumption. The amount and duration of conditioning fluid consumed can, in turn, affect the magnitude of the conditioned taste aversion (Rudy & Cheatle, 1978; Steinert, 1981). In a recent Ph.D. dissertation conducted in our laboratory by Steinert (1981), these variables were controlled by allowing animals access to the conditioning solutions via a syringe with the needle removed, placed inside and perpendicular to the front of the rat's mouth. The animal is allowed to lick and consume the full contents of the syringe. This method allows the experimenter better control of the amount and duration of intake than does the more conventional method of allowing the animal access to fluids through bottles, although especially for adults, it is admittedly less representative of "natural" drinking. Nonetheless, the animal is actively obtaining the fluid unlike procedures employing cannulations. Additionally, whereas preweanlings and adults are able to obtain fluids in this manner with about equal proficiency and latency to consume, preweanling animals are not as efficient as adults at obtaining fluids from bottles until about Postnatal Day 21 (Steinert, 1981). Therefore, the present experiment employed the method of taste presentation used by Steinert (1981). Previous studies have found that preweanlings drinking freely from a bottle consume far less fluid during conditioning than addults (e.g., Steinert, Infurna, & Spear, 1979) albeit in reasonable proportion to body weight. The present study also confounds age with amount of conditioning solution. However, as another precaution this study employed the parameters and
POTENTIATION OF A TASTE AVERSION
47
techniques used in the Steinert dissertation (1981) that served to equate performance across age groups for animals given a taste-LiC1 pairing. Therefore, ontogenetic comparisons of overshadowing/potentiation can be made with some degree of experimental control over the magnitude of the conditioned aversions for animals conditioned to individual elements of a compound.
METHOD
Subjects. The subjects were 192 adult (between 60 and 70 days of age) and preweanling (17 days of age at the beginning of the emperiment) Sprague-Dawley derived rats. Animals were born and maintained in our colony at the State University of New York at Binghamton. The preweanlings were housed with their parents and conspecifics within standard maternity cages. Adult subjects were weaned and sexed on Postnatal Day 21 and housed in groups of six within large wire cages. Animals were allowed access to food and water ad lib unless otherwise specified. All subjects were maintained in the same vivarium with a 16-hr light, 8hr dark illumination cycle with lights going on at 0600 hr. Experimental manipulations were conducted between 1000 and 1600 hr. Design. Animals were randomly assigned to one of four groups. For animals in the Saccharin condition (S), 0.20% saccharin solution was both the conditioning and testing solution. For animals in the Salt condition (N), 1.20% NaC1 was both the conditioning and testing solution. Animals in the Saccharin/Salt condition (S/N-SAC) received a presentation of 0.20% saccharin mixed with 1.20% NaC1 during conditioning, and the saccharin preference test was administered prior to the salt intake test. Finally, animals in the other Saccharin/Salt condition (S/N-SALT) received a presentation of the same compound taste during conditioning, with the salt intake test administered prior to the saccharin preference test. The other factors were age (preweanling or adult) and delay between taste presentation and LiCI injection (2 rain, 1 hr, or 24 hr). The number of animals assigned to each group was eight or nine. The design of the experiment is illustrated in Table 1. Apparatus and procedure. To ensure consumption of the taste stimuli employed throughout the experiment, the animals were deprived of water (and deprived also of their parents, in the case of the pups) prior to familiarization, conditioning, and testing. Steinert (1981; Steinert et al., 1980) found that to equate for motivational state and to minimize differences in percentage bodyweight loss across ages, it was necessary to deprive pups for 16 hr and the adults for 23 hr. For familiarization (when the pups were 17 days of age), each animal was placed into a standard wire-mesh cage for 1 hr. During this hour each animal was removed from the cage and handled by the experimenter for a 5-rain period. At the end of this period, the hand-held syringe was
48
KUCHARSKI AND SPEAR TABLE 1 Illustration of the Experimental Design
Name of group S N S/N-SAC S/N-SALT
Conditioning solution
Test on Day 4
Test on Day 5
Saccharin Salt Saccharin/Salt compound Saccharin/Salt compound
Saccharin Salt Saccharin
Salt
Salt
Saccharin
Note. Other factors were age (preweanling and adult) and delay between taste presentation and LiC1 injection (0, 1, or 24 hr).
placed perpendicular to the front teeth of the animal' s mouth. The animal was allowed to lick and swallow the tap water contained in the syringe, 0.4 ml for the preweanlings and 2 ml for the adults. The experimenter held the infusion at the continuous rate during this period so that the animal consumed the full contents of the syringe within 1 min. Minimal spillage occurred and all animals consumed the fluid. The animals were returned to their home cage at the end of the familiarization hour, allowed access to food, water, and their parents in the case of the preweanlings. For conditioning (on the following day), each animal was placed back into its respective familiarization cage for 10 min. Following this period, each animal was weighed and presented (via syringe) a conditioning solution. Animals of both age groups were required to consume the taste presentation within 1 min, 0.4 ml for preweanlings and 2 ml for the adults. Again, minimal spillage occurred and all animals consumed their fluids. "Experimental" subjects were injected either 2 min or 1 hr later with a 0.3 M LiC1 solution at 1% bodyweight and returned to their home cage 30 min afterward. "Explicitly unpaired" control subjects were injected 24 hr later. For testing (4 days after conditioning), the animals were placed back into their respective cages and given access to fluids from licking tubes. The tubes were attached to bottles graduated for preweanlings to 0.1 ml with a 10-ml capacity and for adults, to 1 ml with a 100-ml capacity (described by Infurna, Steinert, & Spear, 1979). Animals conditioned on saccharin were allowed a choice between a solution of saccharin and water. Animals conditioned with salt were allowed access to a solution of salt; preference test procedure was not possible because even unconditioned rats consume little or none of this concentration of salt if water is the alternative. For animals that received the compound taste during conditioning, the saccharin preference test was administered 4 days after conditioning and a salt intake test was administered on the
POTENTIATION OF A TASTE AVERSION
49
following day, or vice versa. The amount of fluid consumed was recorded after 30 min. RESULTS
Conditioned Saccharin Aversion (Two-Bottle Test) Our definition of an aversion is a significant decrease from the baseline preference established by explicitly unpaired control animals. Therefore, to make age-related comparisons of the conditioned aversion expressed among the different groups, it was necessary to calculate a saccharin preference score (saccharin intake/total intake of saccharin + water) for each subject and then further transform these scores into normalized saccharin preference scores. These scores reflect the difference between saccharin preference scores of experimental subjects and those of their respective controls. Use of this score also promotes communication of the results by providing a single index reflecting strength of conditioning, as well as an index common to both saccharin and salt aversion tests. To calculate these scores, the following formula was used: (individual experimental subject's preference score - X respective control group's preference score/SD of the respective control group's preference score. The "respective control group" refers to those animals given the same taste experience, but with LiCl injections administered 24 hr after presentation of the conditioning solution. It is notable that analyses of simple preference scores yielded the same conclusions as those to be reported for the tansformed scores. These scores are illustrated in Fig 1 and 2. A 3 x 2 x 2 ANOVA [3 conditioning groups (S, S/N-SAC, S/NSALT) × 2 C S - U S delay intervals (2 rain or 1 hr) x 2 ages (preweanling or adult)] performed on the saccharin preference differences scores revealed that animals given a 1-hr delay between presentation and LiC1 injection expressed attenuated aversions relative to animals that experienced only a 2-min delay [F(1, 91) = 22.14, p < .001]. Over all, adult animals expressed greater aversions than did preweanlings IF(l, 91) = 4.93, p < .05]. However, the age x conditioning group (S, S/N-SAC,S/N-SALT) interaction reached significance IF(2, 91) = 5.58, p < .05] which tempers the generality of this effect. This interaction suggests age-related differences in potentiation or overshadowing. No other main effects or interactions reached significance (all p's > .25). These results are illustrated in Fig. 3. It is notable that preweanlings and adults in the S condition expressed equivalent aversions when the US was given just after the CS (2-min delay; p > .25), while preweanlings expressed attenuated aversions in comparison to adults when the C S - U S delay was 1 hr [F(1, 91) = 4.56, p < .05]. Toward understanding the significant age x conditioning group interaction, several planned comparisons were performed to investigate the
50
K U C I t A R S K I AND S P E A R PREWEANLINGS LEAST AVERSION 6C
DELAY I"--] O - HR I-HR
~: 5C 8~n
Lu
[~24-HR (UNPAIRED CONTROL
30
=~ 20 IX
I0
MOST AVERSION SACCHARIN
N:~5 SACCHARIN/SALT SACCHARIN/SALT (NO EXTINCTION) ( AFTER EXTINCTION)
CONDITIONING GROUP
FIG. 1. Saccharin preference scores (saccharin intake/saccharin intake + water intake) for preweanling animals that received saccharin or the compound followed either immediately, 1 hr, or 24 hr later by injections of LiC1. Extinction refers to being given a prior salt intake test.
potentiation effect and extinction of this effect. The interaction between age and the S and S/N-SAC conditioning groups reached significance [F(1, 91) = 7.35, p < .05]. The simple main effects of conditioning group (S vs S/N-SAC) revealed that preweanlings expressed greater saccharin aversions if conditioned with the compound solution rather than saccharin alone [F(1, 91) = 8.81, p < .05]. The effect was in the opposite direction, ADULTS LEAST AVERSJON60
8m 50 z
DELAY
O-.R I{ I-HR
(UNPAIREDI CONTROL}I ~/~
~. 4o
AVERSION SACCHARIN
SACCHAR N/SALT SACCHARIN/SALT (NO EXTINCTION) (AFTER EXTINCTION) CONDITIONING GROUP
FIG. 2. Saccharin preference scores for adults that received saccharin or the compound followed either immediately, 1 hr, or 24 hr later by injections of LiC1. Extinction refers to being given a prior salt intake test.
POTENTIATION OF A TASTE AVERSION MOST AVERSION
o- HR CS-US OELAY
I
51
, - HR CS-US OELAY
TIr-~ PREWEAN,ING I
AOU,T
, I.
Z~
~.-~
LEAST
AVERSION SACCHARIN SAC/SALT SAC/SALT SAC (NO EXT.) (AFTER EXT,.)
SAC/SALT SAC/SALT (NO EXI~.} (AFTER EXI~.)
Fro. 3. Saccharin preference difference scores (saccharin preference for each individual experimental subject -3? respective control group/SD of respective control group) for animals that received presentations of saccharin or the compound followed either immediately or 1 hr later by injections of LiC1. Extinction refers to being given a prior salt intake test.
but not significant (F < 1) for the adults. This verifies that potentiation occurred for the preweanling animals but not for the adults. The interaction between age and the S/N-SAC and S/N-SALT conditioning groups reached significance [F(1, 91) = 9.94, p < .01]. Simple main effects revealed that preweanlings expressed greater saccharin aversions if they did not previously experience a salt intake "extinction" test trial [F(1, 91) = 14.00, p < .01]. Adult animals did not demonstrate such an effect (F < 1). This indicates reduction of the potentiation effect (on S) by extinction of the alternative CS element (N) as described by Rescorla (1981), for preweanling animals but not adults.
Conditioned Salt Aversion (Single-Bottle Test) To make age-related comparisons for aversion to salt during the test situation, it was necessary to obtain normalized salt intake difference scores. Preference test procedure was not possible because even unconditioned rats consume little or no salt if water is available as the alternative, and the great difference between the intake of preweanlings and adults requires use of a relative measure such as a difference score. The present scores reflect the difference between salt intake scores of experimental subjects and those of their respective controls, and were calculated with the following formula: (individual experimental subject's intake score - X respective control group's intake score)/SD of the respective control group's intake score. The nontransformed intake scores are shown in Tables 2a and b for comparison.
52
K U C H A R S K I AND SPEAR T A B L E 2a Salt Intake Scores for Preweanlings ~ C S - U S delay (hours)
Conditioning solution SALT SAC/SALT (salt test first) SAC/SALT (saccharin test first)
0
1
24
2.87 b (.65) 2.27 (.45) 4.05 (.44)
3.20 (.38) 2.62 (.61) 3.45 (.32)
3.80 (.30) 3.81 (.29) 4.54 (.32)
° Mean (milliliter) salt solution consumed during test. b Values are means with standard error o f the mean in parentheses.
A 3 x 2 x 2 ANOVA [3 conditioning groups (N, S/N-SALT, S/NSAC) × 2 C S - U S delay intervals (2 min and 1 hr) x 2 ages (preweanling and adult)] performed on the salt intake difference scores revealed a significant main effect of conditioning group [F(2, 87) = 4.94, p < .05]. No other main effects or interactions attained significance (all p's > .25). These results are illustrated in Fig. 4. Toward an understanding of the significant effect of conditioning group, planned comparisons were performed to assess potentiation of the salt aversion and the extinction of this potentiation by prior testing of saccharin. Animals in the S/N-SALT condition expressed greater salt aversions than animals in the N condition [F(1, 87) = 8.88, p < .05]. The interaction with age did not attain significance (F < 1). This verifies the potentiation effect for both preweanlings and adults. Animals in the S/N-SALT condition expressed greater salt aversions than those in the S/N-SAC condition [F(1, 81) = 22.21, p < .001]. The interaction with age did not attain significance (F < 1). This indicates reduction of potentiation (on N) by T A B L E 2b Salt Intake Scores for Adults" C S - U S delay (hours) Conditioning solution Salt Sac/salt (salt test first) Sac/salt (saccharin test first)
0
1
24
11.88 b (1.88) 9.10 (1.60) 12.75 (1.24)
13.88 (1.40) 10.88 (1.15) 12.75 (1.01)
16.17 (1.50) 17.00 (1.40) 16.42 (1.46)
Mean (milliliter) salt solution consumed during test. Values are means with standard error of the mean in parentheses.
POTENTIATION OF A TASTE AVERSION MOST AVERSION
O-HR CS-USDELAY
oo-:3
[~
I
53
f-HR,,, CS-USDELAY
ADuLTPREWEANLINIJG
o~
IX
-|
LEAST AVERSIONSALT
SAC/SALT SAC/SALT SALT (NO EXT.) (AFTEREXT.) CONDITIONINGGROUP
SAC/SALT SAC/SALT (NO EXT.) (AFTEREXT.)
Fia. 4. Saltintake differencescores (salt intake for each individualexperimentalsubject -J~ respective control group/SD of respective control group) for animals that received presentations of salt or the compoundfollowedeitherimmediatelyor 1 hr later by injections of LiC1. Extinction refers to being given a prior saccharin test. extinction of the alternative element (S), as described by Rescorla (1981), for both preweanlings and adults.
DISCUSSION The results from this experiment do not support the original hypothesis that preweanling rats will be more likely than adults to show overshadowing and less likely to show potentiation of conditioned aversions. Instead, preweanling animals expressed greater potentiation than adults, at least in terms o f the conditioned saccharin aversion, and did not differ in terms of the conditioned salt aversion. This suggests that potentiation is the more basic phenomenon and overshadowing increases with age. This seems consistent with the viewpoint that overshadowing assumes a fair amount of CS processing or information analysis. This interpretation, however, like the within-compound association theory, assumes that animals can discern the elements of the taste compound. Rescorla (1982) has recently argued that potentiation could occur because the animal perceives the compound as a unitary whole and initially mistakes the individual elements for the highly salient compound. Previous studies by Spear and Kucharski (1983a, 1983b) revealed that preweanling and adult animals conditioned on one element did not express generalized aversions to the other element, thus ruling out any "conditioned novelty aversion" hypothesis (Best & Batson, 1977). However, Spear and Kucharski (1984) also reported data indicating that relative to adults, preweanling animals demonstrate greater configuration of a compound
54
KUCHARSKI AND SPEAR
mixture of two tastes. Additionally, when the concentration of the conditioning solution was varied while holding constant the concentration of the test solution, preweanling animals expressed greater aversions if the conditioning solution was somewhat greater than the test solution. Adult animals expressed their greatest aversion when the conditioning and test solutions were equated. Spear and Kucharski (1984) concluded that for preweanling animals, an increase in CS intensity or CS saliency overrides the generalization decrement that is inherent in conditioning and testing the animal with two effectively different stimuli. In analogous fashion, potentiation may occur because the increase in CS saliency that results from compounding two taste stimuli can override the generalization decrement that occurs from training and testing the animal with somewhat different taste solutions. Finally, Rescorla (1982) has argued that repeated experience with the individual elements allows the animal to discriminate between the compound and its elements. This accounts for the finding that when potentiation did occur, and "extinction" test trial with the other element of the compound served to reduce or eliminate the potentiation effect. Some caution may be in order on two accounts. First, the effects of the extinction (test) trial were assessed 5 days after conditioning whereas intake of the target taste without prior extinction was assessed 4 days after conditioning. This procedure was necessary to permit within-subject tests of both flavors without confounding of flavor by interval. This extended retention interval could, conceivably, be responsible for the attenuated aversions associated with the extinction manipulation. This difference of a 4- and 5-day interval has, however, seemed negligible and has yielded no differential aversions in our previous experiments with conditioned taste aversion, which is in any case quite resistant to simple forgetting (Steinert et al., 1981). Moreover, several other experiments conducted in this laboratory (Kucharski & Spear, submitted for publication; also reported in part by Spear & Kucharski, 1983, 1984) revealed similar effects of such an extinction trial while employing quite different taste solutions as the CS. The extinction effect in these experiments occurred only when the CS was a compound solution of two tastes mixed together and not if the two components were presented successively or in relative isolation. Therefore, it seems safe to conclude that this extinction effect is not due simply to an extended retention interval. Second, the enhanced extinction effects on the part of the preweanlings could be due to their greater generalization between saccharin and salt during extinction or to their greater susceptibility to extinction generally. The latter has not been supported by our tests, however, and these same effects have been obtained with other flavors that do not generalize in this way (Spear & Kucharski, 1983). Bouton and Whiting (1982) reported an experiment with adult rats in
POTENTIATION OF A TASTE AVERSION
55
which compounding of saccharin with salt during conditioning resulted in an overshadowing of the conditioned salt aversion. These authors included the results from several blocks of extinction test trials in the data analysis. Again, Rescorla (1982) has reported that presentations of the separate elements of a compound CS results in an attenuation or elimination of the potentiation effect. Thus, the potentiation effect should decrease with greater durations or numbers of tests with a single element and evidence of overshadowing may increase. By including several salt intake test trials in Bouton and Whiting's data analysis, any tendency toward potentiation may have been attenuated. Results from the present experiment indicated that preweanlings and adults express potentiated salt aversion on the first test trial with taste stimuli similar to those employed in the Bouton and Whiting (1982) study. Perhaps potentiation could not be detected in the Bouton and Whiting study on their first test trial because of a floor effect, i.e., their animals conditioned with salt or the saccharin/salt compound consumed virtually no salt on the first test trial. This is not to assert that other of our procedural differences, such as the parameters of CS amount and duration, might not have contributed to the differences in adult behavior seen between the BoutonWhiting and present studies. The results from the present experiment nevertheless confirm our previous evidence, with other flavors, that the potentiation effect (1) can be reciprocal for two flavors, at least for preweanlings, (2) is evident with short or long C S - U S delays but more apparent with the longer C S - U S interval, and (3) can be obtained with minimal exposure to the individual elements (cf. Rescorla, 1981). While the present experiment indicates some reasons why potentiation occurs in some circumstances and not in others, the general phenomenon of "potentiation" deserves further consideration. First, potentiation seems inconsistent with the current theories of learning and memory (e.g., Mackintosh, 1975; Rescorla & Wagner, 1972; Wagner, 1981). As noted above, Rescorla first explained the potentiation phenomenon on the basis of within-compound associations and the general rules of classical conditioning (Rescorla & Durlach, 1981). Other theorists (e.g., Rusiniak, Hankins, Garcia & Brett, 1979; Lett, 1982) regard potentiation as a more specialized adaptation that is limited to the feeding system. According to this position, taste potentiates the association of an accompanying odor cue by endowing it with some of its own properties, e.g., the specialized ability to become assocated with internal malaise over a long C S - U S delay (see Garcia & Koelling, 1966). Weakening the strength of the taste-illness association during the interval between conditioning and testing should therefore not disrupt the potentiation effect (see Lett, 1982). In contrast, the multiple association theory predicts that the strength of the potentiation effect depends on the strength of the taste-illness association (see Recorla, 1981). Results supporting both positions have
56
KUCHARSKI AND SPEAR
been reported (Lett, 1982; Rescorla, 1981). Further complications have arisen in a recent experiment by Mikulka, Pitts, and Philput (1982) that failed to replicate the potentiation results reported by Rusiniak et al. (1979). Clarification of the potentiation effect is needed so theories of compound conditioning can be complete and reformulated if necessary. For this purpose it is notable that the effect is especially robust in the preweanling rat, but notable also that the basis of this potentiation is not necessarily the same as that observed in the adult rat (Spear & Kucharski, 1983a). REFERENCES Bouton, M. E., & Whiting, M. R. (1979). Simultaneous odor-taste and taste-taste compounds in poison-avoidance learning. Learning and Motivation, 13, 472-494. Bower, G. H. (1967). A multi-component theory of memory trace. In K. W. Spence & J. T. Spence (Eds.), The Psychology of Learning and Motivation (Vol. 1). New York: Academic Press. Campbell, B. A. (1984). Reflections on the ontogeny of learning and memory. In R. Kail and N. Spear (Eds.), Comparative Perspectives on the Development of Memory. Hillsdale, NJ: Erlbaum. Campbell, B. A., & Spear, N. E. (1972). Ontogeny of memory. Psychological Review, 79, 215-236. Domjan, M. (1983). Biological constraints on instrumental and classical conditioning: Implications for general process theory. In G. H. Bower (Ed.), The Psychology of Learning and Motivation (Vol. 17). New York: Academic Press. Garcia, J., & Koelling, R. A. (1966). Relation of cue to consequence in avoidance learning. Psychonomic Science, 4, 123-124. Infurna, R. N., Steinert, P. A., & Spear, N. E. (1979). Ontogenetic changes in the modulation of taste aversion learning by home environmental cues. Journal of Comparative and Physiological Psychology, 93, 1097-1098. Kucharski, D., & Spear, N. E. Determinants of pote'.~tiation and overshadowing in preweanling and adult rats. Submitted for publication. Lett, B. T. (1982). Taste potentiation in posion-avoidance learning. In R. Herrnstein (Ed.), Harvard Symposium on Quantitative Analysis of Behavior, (Vol 4). Hillsdale, N.J.: Earlbaum. Mackintosh, N. J. (1975). A theory of attention: Variations in the associability of stimuli with reinforcement. Psychological Review,82, 276-298. Mikulka, P. J., Pitts, E., & Philput, C. (1982). Overshadowing not potentiation in taste aversion conditioning. Bulletin of Psychonomic Science, 20, 101-104. Pavlov, I. P. (1927). Conditioned Reflexes. Oxford: Oxford Univ. Press. Pfaffman, C., Norgren, R., & Grill, H. (1977). Sensory affect and motivation. Annals of the New York Academy of Science, 290, 18-34. Rescorla, R. A. (1981). Simultaneous associations. In P. Harzum & M. D. Zeiler (Eds.), Predictability, Correlation and Continguity. New York: Wiley. Rescorla, R. A. (1982). Simultaneous second-order conditioning produces S-S learning in conditioned suppression. Journal of Experimental Psychology: Animal Behavior Processes, 8, 23-32. Rescorla, R. A., & Durlach, P. J. (1981). Within-event learning and Pavlovian conditioning. In N. E. Spear & R. R. Miller (Eds.), Information Processing in Animals: Memory Mechanisms. Hillsdale, NJ: Erlbaum. Rescorla, R. A., & Wagner, A. R. (1972). A theory of Pavlovian conditioning: Variations in the effectiveness of reinforcement and nonreinforcement. In A. H. Black & W. F.
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Prokazy (Eds.), Classical Conditioning H: Current Research and Theory. New York: Appleton-Century-Crofts. Revusky, S. (1971). The role of interference in association over a delay. In W. K. Honig & P. H. R. James (Eds.), Animal Memory. New York: Academic Press. Rudy, J. W., & Cheatle, M. D. (1979). A role for conditioned stimulus duration in toxophobia conditioning. Journal of Experimental Psychology: Animal Behavior Processes, 4~ 399-411. Rusiniak, K. W., Hankins, W. G., Garcia, J., & Brett, L. P. (1979). Flavor illness aversions: Potentiation of odor by taste in rats. Behavioral and Neurobiology, 25, 1-17. Spear, N. E. (1973). Retrieval of memory in animals. Psychological Review, 80, 163-194. Spear, N. E., & Kucharski, D. (1983). Ontogenetic differences in the processing of multielement stimuli: Potentiation and overshadowing. In H. Roitblat, T. Bever, & H. Terrace (Eds.), Animal Cognition. Hillsdale, N J: Erlbaum. Spear, N. E., & Kucharski, D. (1984). Ontogenetic differences in stimulus selection during conditioning. In R. V. Kail & N. E. Spear (Eds.), Memory Development: Comparative Perspectives. Hillsdale, NJ: Erlbaum. Steinert, P. A. (1981) Stimulus Selection among Preweanling and Adult Rats as a Function of CS Amount and Quality Using a Taste-Aversion Paradigm. Unpublished Ph.D. dissertation, State University of New York at Binghamton. Steinert, P. A. Infurna, R. N., & Spear, N. E. (1980). Long term retention of a conditioned taste aversion in preweanling and adult rats. Animal Learning & Behavior, 8, 375381. Underwood, B. J. (1969). Attributes of memory. Psychological Review, 76, 559-573. Wagner, A. R. (1981). SOP: A model of automatic memory processing in animal behavior. In N. E. Spear & R. R. Miller (Eds.), Information Processing in Animals: Memory Mechanisms. Hillsdale, NJ: Earlbaum.
(1984)
Potentiation of a Conditioned Taste Aversion in Preweanling and Adult Rats DAVID KUCHARSKI AND NORMAN E. SPEAR I
State University of New York at Binghamton Preweanling (18 days old) and adult rats were made ill with LiC1 either 2 min or 1 hr after tasting controlled amounts of either one of two single flavors (saccharin or NaCI) or a compound mixture of the two. Conditioning was assessed with a single test 4 days later relative to explicitly unpaired control conditions. Generally, potentiation of the aversions to either flavor occurred for animals conditioned to the compound. The potentiation effect was decreased or eliminated by nonreinforced exposure to the alternative flavor of the compound. These effects tended to be stronger for the younger rats. Specifically, adult animals did not express potentiation of the saccharin aversion whereas preweanlings did, especially at the longer C S - U S interval. Both adults and preweanlings expressed potentiated salt aversions. Nonreinforced exposure to the alternative element eliminated the potentiation effect. Conditions conducive to potentiation are discussed in light of investigators who have not observed this effect in similar studies with compound stimuli.
It has frequently been observed that when two conditioned stimuli (CS) are presented prior to a US, one tends to "overshadow" the conditioning of the other (Pavlov, 1927; Revusky, 1971). However, in some instances of learning it has been observed that the conditioning of one CS element paired with a US is actually greater when presented in compound with another than when presented alone. While this effect, termed potentiation, may seem paradoxical, it has been found in a variety of circumstances (for reviews, see Domjan, 1983; Lett, 1982; Rescorla, 1981; Rescorla & Durlach, 1981), and has become established as one possible consequence of multielement conditioning. According to a multiple association explanation of potentiation proposed by Rescorla (see e.g., Rescorla & Durlach, 1981), three different associations are formed during a pairing of a stimulus AB with a US: A - U S , B - U S , and A-B. Because 1 This research was supported by a grant from the National Institute of Mental Health (l RO1 MH35219) to Norman E. Spear. We are grateful for the secretarial assistance of Teri Tanenhaus and the technical expertise of Norman Richter. Send requests for reprints to Norman E. Spear, Department of Psychology, State University of New York at Binghamton, Binghamton, NY 13901. 44 0163-1047/84 $3.00 Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
POTENTIATION OF A TASTE AVERSION
45
A is associated with B, which itself is aversive due to conditioning, the net aversion to A is inflated beyond its association with the US. The association between the two CSs (A and B) has been referred to as a "within-compound" association. Rescorla (1981) has argued that potentiation occurs when the magnitude of this association is such that it overrides the overshadowing that results from compounding the target CS with another stimulus during conditioning. The present study investigated the possible ontogenetic differences in the overshadowing or potentiation of a conditioned taste aversion. Although not intended to be a decisive experiment determining the factors controlling infantile amnesia (see Campbell, 1983; Campbell & Spear, 1972), this study does address one possible explanation of the effect--that immature animals store too few attributes from a learning episode to support later retrieval of that memory. From the view that "memory" is a representation of several discernable events which constitute the learning episode (Bower, 1967; Spear, 1973; Underwood, 1969) one might predict that if their capacity for information processing is actually less than that of adults, preweanlings should store fewer of these events than adults. In this view, when two tastes are presented simultaneously and followed by lithium chloride (LiC1) poisoning, the preweanling rat should develop less of an aversion to the target taste during the conditioning episode. However, if the nontarget taste is not present during acquisition, the hypothetically lesser informational capacity of the preweanling is less apt to have appreciable impact on the magnitude of the conditioned aversion to the target taste. A second way in which a more limited processing capacity of the preweanlings might work against them in the two-taste situation stems from possible CS-CS associations. Assuming their lesser capacity, preweanlings might acquire the C S - U S associations as readily as adults but be less likely than adults to form the within-compound associations that could, theoretically, mediate the potentiation effect (Rescorla, 1981). Together, these factors lead to the hypothesis that in circumstances where conditioning to a single taste is about equivalent for preweanlings and adults, preweanlings should exhibit overshadowing whereas adults should display potentiated aversions if the target taste were instead compounded with another taste during conditioning. A major variable of interest in the present experiment was composition of the CS during conditioning. One set of animals was presented either a 0.20% saccharin solution or a 1.20% sodium chloride (NaC1) solution as their CS, followed by LiCI as the US, and were tested for the consumption of the respective solution after a 4-day recovery period. Additional animals were given a compound solution made up of 0.20% saccharin and 1.20% NaC1 as their CS, followed by LiC1 as the US. These taste stimuli were used in an experiment reported by Bouton and Whiting
46
KUCHARSKI AND SPEAR
(1982). These animals received both a saccharin preference and NaC1 intake test, with order counterbalanced. This counterbalanced testing procedure permitted assessment of differences in the magnitude of the aversion to the target taste as a function of whether or not animals had previously received a nonreinforced presentation (extinction procedure) of the nontarget taste. Whereas Rescorla has argued that such extinction will attenuate the aversion to the target taste (Rescorla, 1981), Lett has argued that the potentiation effect should not be affected by extinction of the other element of the conditioning compound (Lett, 1982). The LiCI injection was administered either 2 min, 1 hr, or 24 hr after the taste presentation. The 1-hr delay group was included because the potentiation effect is believed to be more robust with a long delay between the CS and US (Lett, 1982), and the 24-hr delay group served as an explicitly unpaired control condition. The present experiment employed several procedures intended to counteract some potential problems of method that sometimes have arisen in taste aversion experiments, especially those including ontogenetic comparisons. First, the hedonic value of the taste stimuli employed in this and other experiments probably change when they are compounded with other taste stimuli. The hedonic value of a taste solution can in some cases determine amount and duration of consumption. The amount and duration of conditioning fluid consumed can, in turn, affect the magnitude of the conditioned taste aversion (Rudy & Cheatle, 1978; Steinert, 1981). In a recent Ph.D. dissertation conducted in our laboratory by Steinert (1981), these variables were controlled by allowing animals access to the conditioning solutions via a syringe with the needle removed, placed inside and perpendicular to the front of the rat's mouth. The animal is allowed to lick and consume the full contents of the syringe. This method allows the experimenter better control of the amount and duration of intake than does the more conventional method of allowing the animal access to fluids through bottles, although especially for adults, it is admittedly less representative of "natural" drinking. Nonetheless, the animal is actively obtaining the fluid unlike procedures employing cannulations. Additionally, whereas preweanlings and adults are able to obtain fluids in this manner with about equal proficiency and latency to consume, preweanling animals are not as efficient as adults at obtaining fluids from bottles until about Postnatal Day 21 (Steinert, 1981). Therefore, the present experiment employed the method of taste presentation used by Steinert (1981). Previous studies have found that preweanlings drinking freely from a bottle consume far less fluid during conditioning than addults (e.g., Steinert, Infurna, & Spear, 1979) albeit in reasonable proportion to body weight. The present study also confounds age with amount of conditioning solution. However, as another precaution this study employed the parameters and
POTENTIATION OF A TASTE AVERSION
47
techniques used in the Steinert dissertation (1981) that served to equate performance across age groups for animals given a taste-LiC1 pairing. Therefore, ontogenetic comparisons of overshadowing/potentiation can be made with some degree of experimental control over the magnitude of the conditioned aversions for animals conditioned to individual elements of a compound.
METHOD
Subjects. The subjects were 192 adult (between 60 and 70 days of age) and preweanling (17 days of age at the beginning of the emperiment) Sprague-Dawley derived rats. Animals were born and maintained in our colony at the State University of New York at Binghamton. The preweanlings were housed with their parents and conspecifics within standard maternity cages. Adult subjects were weaned and sexed on Postnatal Day 21 and housed in groups of six within large wire cages. Animals were allowed access to food and water ad lib unless otherwise specified. All subjects were maintained in the same vivarium with a 16-hr light, 8hr dark illumination cycle with lights going on at 0600 hr. Experimental manipulations were conducted between 1000 and 1600 hr. Design. Animals were randomly assigned to one of four groups. For animals in the Saccharin condition (S), 0.20% saccharin solution was both the conditioning and testing solution. For animals in the Salt condition (N), 1.20% NaC1 was both the conditioning and testing solution. Animals in the Saccharin/Salt condition (S/N-SAC) received a presentation of 0.20% saccharin mixed with 1.20% NaC1 during conditioning, and the saccharin preference test was administered prior to the salt intake test. Finally, animals in the other Saccharin/Salt condition (S/N-SALT) received a presentation of the same compound taste during conditioning, with the salt intake test administered prior to the saccharin preference test. The other factors were age (preweanling or adult) and delay between taste presentation and LiCI injection (2 rain, 1 hr, or 24 hr). The number of animals assigned to each group was eight or nine. The design of the experiment is illustrated in Table 1. Apparatus and procedure. To ensure consumption of the taste stimuli employed throughout the experiment, the animals were deprived of water (and deprived also of their parents, in the case of the pups) prior to familiarization, conditioning, and testing. Steinert (1981; Steinert et al., 1980) found that to equate for motivational state and to minimize differences in percentage bodyweight loss across ages, it was necessary to deprive pups for 16 hr and the adults for 23 hr. For familiarization (when the pups were 17 days of age), each animal was placed into a standard wire-mesh cage for 1 hr. During this hour each animal was removed from the cage and handled by the experimenter for a 5-rain period. At the end of this period, the hand-held syringe was
48
KUCHARSKI AND SPEAR TABLE 1 Illustration of the Experimental Design
Name of group S N S/N-SAC S/N-SALT
Conditioning solution
Test on Day 4
Test on Day 5
Saccharin Salt Saccharin/Salt compound Saccharin/Salt compound
Saccharin Salt Saccharin
Salt
Salt
Saccharin
Note. Other factors were age (preweanling and adult) and delay between taste presentation and LiC1 injection (0, 1, or 24 hr).
placed perpendicular to the front teeth of the animal' s mouth. The animal was allowed to lick and swallow the tap water contained in the syringe, 0.4 ml for the preweanlings and 2 ml for the adults. The experimenter held the infusion at the continuous rate during this period so that the animal consumed the full contents of the syringe within 1 min. Minimal spillage occurred and all animals consumed the fluid. The animals were returned to their home cage at the end of the familiarization hour, allowed access to food, water, and their parents in the case of the preweanlings. For conditioning (on the following day), each animal was placed back into its respective familiarization cage for 10 min. Following this period, each animal was weighed and presented (via syringe) a conditioning solution. Animals of both age groups were required to consume the taste presentation within 1 min, 0.4 ml for preweanlings and 2 ml for the adults. Again, minimal spillage occurred and all animals consumed their fluids. "Experimental" subjects were injected either 2 min or 1 hr later with a 0.3 M LiC1 solution at 1% bodyweight and returned to their home cage 30 min afterward. "Explicitly unpaired" control subjects were injected 24 hr later. For testing (4 days after conditioning), the animals were placed back into their respective cages and given access to fluids from licking tubes. The tubes were attached to bottles graduated for preweanlings to 0.1 ml with a 10-ml capacity and for adults, to 1 ml with a 100-ml capacity (described by Infurna, Steinert, & Spear, 1979). Animals conditioned on saccharin were allowed a choice between a solution of saccharin and water. Animals conditioned with salt were allowed access to a solution of salt; preference test procedure was not possible because even unconditioned rats consume little or none of this concentration of salt if water is the alternative. For animals that received the compound taste during conditioning, the saccharin preference test was administered 4 days after conditioning and a salt intake test was administered on the
POTENTIATION OF A TASTE AVERSION
49
following day, or vice versa. The amount of fluid consumed was recorded after 30 min. RESULTS
Conditioned Saccharin Aversion (Two-Bottle Test) Our definition of an aversion is a significant decrease from the baseline preference established by explicitly unpaired control animals. Therefore, to make age-related comparisons of the conditioned aversion expressed among the different groups, it was necessary to calculate a saccharin preference score (saccharin intake/total intake of saccharin + water) for each subject and then further transform these scores into normalized saccharin preference scores. These scores reflect the difference between saccharin preference scores of experimental subjects and those of their respective controls. Use of this score also promotes communication of the results by providing a single index reflecting strength of conditioning, as well as an index common to both saccharin and salt aversion tests. To calculate these scores, the following formula was used: (individual experimental subject's preference score - X respective control group's preference score/SD of the respective control group's preference score. The "respective control group" refers to those animals given the same taste experience, but with LiCl injections administered 24 hr after presentation of the conditioning solution. It is notable that analyses of simple preference scores yielded the same conclusions as those to be reported for the tansformed scores. These scores are illustrated in Fig 1 and 2. A 3 x 2 x 2 ANOVA [3 conditioning groups (S, S/N-SAC, S/NSALT) × 2 C S - U S delay intervals (2 rain or 1 hr) x 2 ages (preweanling or adult)] performed on the saccharin preference differences scores revealed that animals given a 1-hr delay between presentation and LiC1 injection expressed attenuated aversions relative to animals that experienced only a 2-min delay [F(1, 91) = 22.14, p < .001]. Over all, adult animals expressed greater aversions than did preweanlings IF(l, 91) = 4.93, p < .05]. However, the age x conditioning group (S, S/N-SAC,S/N-SALT) interaction reached significance IF(2, 91) = 5.58, p < .05] which tempers the generality of this effect. This interaction suggests age-related differences in potentiation or overshadowing. No other main effects or interactions reached significance (all p's > .25). These results are illustrated in Fig. 3. It is notable that preweanlings and adults in the S condition expressed equivalent aversions when the US was given just after the CS (2-min delay; p > .25), while preweanlings expressed attenuated aversions in comparison to adults when the C S - U S delay was 1 hr [F(1, 91) = 4.56, p < .05]. Toward understanding the significant age x conditioning group interaction, several planned comparisons were performed to investigate the
50
K U C I t A R S K I AND S P E A R PREWEANLINGS LEAST AVERSION 6C
DELAY I"--] O - HR I-HR
~: 5C 8~n
Lu
[~24-HR (UNPAIRED CONTROL
30
=~ 20 IX
I0
MOST AVERSION SACCHARIN
N:~5 SACCHARIN/SALT SACCHARIN/SALT (NO EXTINCTION) ( AFTER EXTINCTION)
CONDITIONING GROUP
FIG. 1. Saccharin preference scores (saccharin intake/saccharin intake + water intake) for preweanling animals that received saccharin or the compound followed either immediately, 1 hr, or 24 hr later by injections of LiC1. Extinction refers to being given a prior salt intake test.
potentiation effect and extinction of this effect. The interaction between age and the S and S/N-SAC conditioning groups reached significance [F(1, 91) = 7.35, p < .05]. The simple main effects of conditioning group (S vs S/N-SAC) revealed that preweanlings expressed greater saccharin aversions if conditioned with the compound solution rather than saccharin alone [F(1, 91) = 8.81, p < .05]. The effect was in the opposite direction, ADULTS LEAST AVERSJON60
8m 50 z
DELAY
O-.R I{ I-HR
(UNPAIREDI CONTROL}I ~/~
~. 4o
AVERSION SACCHARIN
SACCHAR N/SALT SACCHARIN/SALT (NO EXTINCTION) (AFTER EXTINCTION) CONDITIONING GROUP
FIG. 2. Saccharin preference scores for adults that received saccharin or the compound followed either immediately, 1 hr, or 24 hr later by injections of LiC1. Extinction refers to being given a prior salt intake test.
POTENTIATION OF A TASTE AVERSION MOST AVERSION
o- HR CS-US OELAY
I
51
, - HR CS-US OELAY
TIr-~ PREWEAN,ING I
AOU,T
, I.
Z~
~.-~
LEAST
AVERSION SACCHARIN SAC/SALT SAC/SALT SAC (NO EXT.) (AFTER EXT,.)
SAC/SALT SAC/SALT (NO EXI~.} (AFTER EXI~.)
Fro. 3. Saccharin preference difference scores (saccharin preference for each individual experimental subject -3? respective control group/SD of respective control group) for animals that received presentations of saccharin or the compound followed either immediately or 1 hr later by injections of LiC1. Extinction refers to being given a prior salt intake test.
but not significant (F < 1) for the adults. This verifies that potentiation occurred for the preweanling animals but not for the adults. The interaction between age and the S/N-SAC and S/N-SALT conditioning groups reached significance [F(1, 91) = 9.94, p < .01]. Simple main effects revealed that preweanlings expressed greater saccharin aversions if they did not previously experience a salt intake "extinction" test trial [F(1, 91) = 14.00, p < .01]. Adult animals did not demonstrate such an effect (F < 1). This indicates reduction of the potentiation effect (on S) by extinction of the alternative CS element (N) as described by Rescorla (1981), for preweanling animals but not adults.
Conditioned Salt Aversion (Single-Bottle Test) To make age-related comparisons for aversion to salt during the test situation, it was necessary to obtain normalized salt intake difference scores. Preference test procedure was not possible because even unconditioned rats consume little or no salt if water is available as the alternative, and the great difference between the intake of preweanlings and adults requires use of a relative measure such as a difference score. The present scores reflect the difference between salt intake scores of experimental subjects and those of their respective controls, and were calculated with the following formula: (individual experimental subject's intake score - X respective control group's intake score)/SD of the respective control group's intake score. The nontransformed intake scores are shown in Tables 2a and b for comparison.
52
K U C H A R S K I AND SPEAR T A B L E 2a Salt Intake Scores for Preweanlings ~ C S - U S delay (hours)
Conditioning solution SALT SAC/SALT (salt test first) SAC/SALT (saccharin test first)
0
1
24
2.87 b (.65) 2.27 (.45) 4.05 (.44)
3.20 (.38) 2.62 (.61) 3.45 (.32)
3.80 (.30) 3.81 (.29) 4.54 (.32)
° Mean (milliliter) salt solution consumed during test. b Values are means with standard error o f the mean in parentheses.
A 3 x 2 x 2 ANOVA [3 conditioning groups (N, S/N-SALT, S/NSAC) × 2 C S - U S delay intervals (2 min and 1 hr) x 2 ages (preweanling and adult)] performed on the salt intake difference scores revealed a significant main effect of conditioning group [F(2, 87) = 4.94, p < .05]. No other main effects or interactions attained significance (all p's > .25). These results are illustrated in Fig. 4. Toward an understanding of the significant effect of conditioning group, planned comparisons were performed to assess potentiation of the salt aversion and the extinction of this potentiation by prior testing of saccharin. Animals in the S/N-SALT condition expressed greater salt aversions than animals in the N condition [F(1, 87) = 8.88, p < .05]. The interaction with age did not attain significance (F < 1). This verifies the potentiation effect for both preweanlings and adults. Animals in the S/N-SALT condition expressed greater salt aversions than those in the S/N-SAC condition [F(1, 81) = 22.21, p < .001]. The interaction with age did not attain significance (F < 1). This indicates reduction of potentiation (on N) by T A B L E 2b Salt Intake Scores for Adults" C S - U S delay (hours) Conditioning solution Salt Sac/salt (salt test first) Sac/salt (saccharin test first)
0
1
24
11.88 b (1.88) 9.10 (1.60) 12.75 (1.24)
13.88 (1.40) 10.88 (1.15) 12.75 (1.01)
16.17 (1.50) 17.00 (1.40) 16.42 (1.46)
Mean (milliliter) salt solution consumed during test. Values are means with standard error of the mean in parentheses.
POTENTIATION OF A TASTE AVERSION MOST AVERSION
O-HR CS-USDELAY
oo-:3
[~
I
53
f-HR,,, CS-USDELAY
ADuLTPREWEANLINIJG
o~
IX
-|
LEAST AVERSIONSALT
SAC/SALT SAC/SALT SALT (NO EXT.) (AFTEREXT.) CONDITIONINGGROUP
SAC/SALT SAC/SALT (NO EXT.) (AFTEREXT.)
Fia. 4. Saltintake differencescores (salt intake for each individualexperimentalsubject -J~ respective control group/SD of respective control group) for animals that received presentations of salt or the compoundfollowedeitherimmediatelyor 1 hr later by injections of LiC1. Extinction refers to being given a prior saccharin test. extinction of the alternative element (S), as described by Rescorla (1981), for both preweanlings and adults.
DISCUSSION The results from this experiment do not support the original hypothesis that preweanling rats will be more likely than adults to show overshadowing and less likely to show potentiation of conditioned aversions. Instead, preweanling animals expressed greater potentiation than adults, at least in terms o f the conditioned saccharin aversion, and did not differ in terms of the conditioned salt aversion. This suggests that potentiation is the more basic phenomenon and overshadowing increases with age. This seems consistent with the viewpoint that overshadowing assumes a fair amount of CS processing or information analysis. This interpretation, however, like the within-compound association theory, assumes that animals can discern the elements of the taste compound. Rescorla (1982) has recently argued that potentiation could occur because the animal perceives the compound as a unitary whole and initially mistakes the individual elements for the highly salient compound. Previous studies by Spear and Kucharski (1983a, 1983b) revealed that preweanling and adult animals conditioned on one element did not express generalized aversions to the other element, thus ruling out any "conditioned novelty aversion" hypothesis (Best & Batson, 1977). However, Spear and Kucharski (1984) also reported data indicating that relative to adults, preweanling animals demonstrate greater configuration of a compound
54
KUCHARSKI AND SPEAR
mixture of two tastes. Additionally, when the concentration of the conditioning solution was varied while holding constant the concentration of the test solution, preweanling animals expressed greater aversions if the conditioning solution was somewhat greater than the test solution. Adult animals expressed their greatest aversion when the conditioning and test solutions were equated. Spear and Kucharski (1984) concluded that for preweanling animals, an increase in CS intensity or CS saliency overrides the generalization decrement that is inherent in conditioning and testing the animal with two effectively different stimuli. In analogous fashion, potentiation may occur because the increase in CS saliency that results from compounding two taste stimuli can override the generalization decrement that occurs from training and testing the animal with somewhat different taste solutions. Finally, Rescorla (1982) has argued that repeated experience with the individual elements allows the animal to discriminate between the compound and its elements. This accounts for the finding that when potentiation did occur, and "extinction" test trial with the other element of the compound served to reduce or eliminate the potentiation effect. Some caution may be in order on two accounts. First, the effects of the extinction (test) trial were assessed 5 days after conditioning whereas intake of the target taste without prior extinction was assessed 4 days after conditioning. This procedure was necessary to permit within-subject tests of both flavors without confounding of flavor by interval. This extended retention interval could, conceivably, be responsible for the attenuated aversions associated with the extinction manipulation. This difference of a 4- and 5-day interval has, however, seemed negligible and has yielded no differential aversions in our previous experiments with conditioned taste aversion, which is in any case quite resistant to simple forgetting (Steinert et al., 1981). Moreover, several other experiments conducted in this laboratory (Kucharski & Spear, submitted for publication; also reported in part by Spear & Kucharski, 1983, 1984) revealed similar effects of such an extinction trial while employing quite different taste solutions as the CS. The extinction effect in these experiments occurred only when the CS was a compound solution of two tastes mixed together and not if the two components were presented successively or in relative isolation. Therefore, it seems safe to conclude that this extinction effect is not due simply to an extended retention interval. Second, the enhanced extinction effects on the part of the preweanlings could be due to their greater generalization between saccharin and salt during extinction or to their greater susceptibility to extinction generally. The latter has not been supported by our tests, however, and these same effects have been obtained with other flavors that do not generalize in this way (Spear & Kucharski, 1983). Bouton and Whiting (1982) reported an experiment with adult rats in
POTENTIATION OF A TASTE AVERSION
55
which compounding of saccharin with salt during conditioning resulted in an overshadowing of the conditioned salt aversion. These authors included the results from several blocks of extinction test trials in the data analysis. Again, Rescorla (1982) has reported that presentations of the separate elements of a compound CS results in an attenuation or elimination of the potentiation effect. Thus, the potentiation effect should decrease with greater durations or numbers of tests with a single element and evidence of overshadowing may increase. By including several salt intake test trials in Bouton and Whiting's data analysis, any tendency toward potentiation may have been attenuated. Results from the present experiment indicated that preweanlings and adults express potentiated salt aversion on the first test trial with taste stimuli similar to those employed in the Bouton and Whiting (1982) study. Perhaps potentiation could not be detected in the Bouton and Whiting study on their first test trial because of a floor effect, i.e., their animals conditioned with salt or the saccharin/salt compound consumed virtually no salt on the first test trial. This is not to assert that other of our procedural differences, such as the parameters of CS amount and duration, might not have contributed to the differences in adult behavior seen between the BoutonWhiting and present studies. The results from the present experiment nevertheless confirm our previous evidence, with other flavors, that the potentiation effect (1) can be reciprocal for two flavors, at least for preweanlings, (2) is evident with short or long C S - U S delays but more apparent with the longer C S - U S interval, and (3) can be obtained with minimal exposure to the individual elements (cf. Rescorla, 1981). While the present experiment indicates some reasons why potentiation occurs in some circumstances and not in others, the general phenomenon of "potentiation" deserves further consideration. First, potentiation seems inconsistent with the current theories of learning and memory (e.g., Mackintosh, 1975; Rescorla & Wagner, 1972; Wagner, 1981). As noted above, Rescorla first explained the potentiation phenomenon on the basis of within-compound associations and the general rules of classical conditioning (Rescorla & Durlach, 1981). Other theorists (e.g., Rusiniak, Hankins, Garcia & Brett, 1979; Lett, 1982) regard potentiation as a more specialized adaptation that is limited to the feeding system. According to this position, taste potentiates the association of an accompanying odor cue by endowing it with some of its own properties, e.g., the specialized ability to become assocated with internal malaise over a long C S - U S delay (see Garcia & Koelling, 1966). Weakening the strength of the taste-illness association during the interval between conditioning and testing should therefore not disrupt the potentiation effect (see Lett, 1982). In contrast, the multiple association theory predicts that the strength of the potentiation effect depends on the strength of the taste-illness association (see Recorla, 1981). Results supporting both positions have
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been reported (Lett, 1982; Rescorla, 1981). Further complications have arisen in a recent experiment by Mikulka, Pitts, and Philput (1982) that failed to replicate the potentiation results reported by Rusiniak et al. (1979). Clarification of the potentiation effect is needed so theories of compound conditioning can be complete and reformulated if necessary. For this purpose it is notable that the effect is especially robust in the preweanling rat, but notable also that the basis of this potentiation is not necessarily the same as that observed in the adult rat (Spear & Kucharski, 1983a). REFERENCES Bouton, M. E., & Whiting, M. R. (1979). Simultaneous odor-taste and taste-taste compounds in poison-avoidance learning. Learning and Motivation, 13, 472-494. Bower, G. H. (1967). A multi-component theory of memory trace. In K. W. Spence & J. T. Spence (Eds.), The Psychology of Learning and Motivation (Vol. 1). New York: Academic Press. Campbell, B. A. (1984). Reflections on the ontogeny of learning and memory. In R. Kail and N. Spear (Eds.), Comparative Perspectives on the Development of Memory. Hillsdale, NJ: Erlbaum. Campbell, B. A., & Spear, N. E. (1972). Ontogeny of memory. Psychological Review, 79, 215-236. Domjan, M. (1983). Biological constraints on instrumental and classical conditioning: Implications for general process theory. In G. H. Bower (Ed.), The Psychology of Learning and Motivation (Vol. 17). New York: Academic Press. Garcia, J., & Koelling, R. A. (1966). Relation of cue to consequence in avoidance learning. Psychonomic Science, 4, 123-124. Infurna, R. N., Steinert, P. A., & Spear, N. E. (1979). Ontogenetic changes in the modulation of taste aversion learning by home environmental cues. Journal of Comparative and Physiological Psychology, 93, 1097-1098. Kucharski, D., & Spear, N. E. Determinants of pote'.~tiation and overshadowing in preweanling and adult rats. Submitted for publication. Lett, B. T. (1982). Taste potentiation in posion-avoidance learning. In R. Herrnstein (Ed.), Harvard Symposium on Quantitative Analysis of Behavior, (Vol 4). Hillsdale, N.J.: Earlbaum. Mackintosh, N. J. (1975). A theory of attention: Variations in the associability of stimuli with reinforcement. Psychological Review,82, 276-298. Mikulka, P. J., Pitts, E., & Philput, C. (1982). Overshadowing not potentiation in taste aversion conditioning. Bulletin of Psychonomic Science, 20, 101-104. Pavlov, I. P. (1927). Conditioned Reflexes. Oxford: Oxford Univ. Press. Pfaffman, C., Norgren, R., & Grill, H. (1977). Sensory affect and motivation. Annals of the New York Academy of Science, 290, 18-34. Rescorla, R. A. (1981). Simultaneous associations. In P. Harzum & M. D. Zeiler (Eds.), Predictability, Correlation and Continguity. New York: Wiley. Rescorla, R. A. (1982). Simultaneous second-order conditioning produces S-S learning in conditioned suppression. Journal of Experimental Psychology: Animal Behavior Processes, 8, 23-32. Rescorla, R. A., & Durlach, P. J. (1981). Within-event learning and Pavlovian conditioning. In N. E. Spear & R. R. Miller (Eds.), Information Processing in Animals: Memory Mechanisms. Hillsdale, NJ: Erlbaum. Rescorla, R. A., & Wagner, A. R. (1972). A theory of Pavlovian conditioning: Variations in the effectiveness of reinforcement and nonreinforcement. In A. H. Black & W. F.
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