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Proximal, but not distal, pre-exposure reduces serial overshadowing in one-trial taste aversion learning
Behavioural Processes 118 (2015) 111–114
Contents lists available at ScienceDirect
Behavioural Processes journal homepage: www.elsevier.com/locate/behavproc
Short report
Proximal, but not distal, pre-exposure reduces serial overshadowing in one-trial taste aversion learning Dorothy W.S. Kwok, Robert A. Boakes ∗ University of Sydney, Sydney, Australia
a r t i c l e
i n f o
Article history: Received 25 February 2015 Received in revised form 4 June 2015 Accepted 5 June 2015 Available online 9 June 2015 Keywords: Conditioned taste aversion Long delay learning Serial overshadowing Retroactive interference Latent inhibition Pre-exposure
a b s t r a c t This experiment tested whether pre-exposing a taste would reduce its ability to overshadow conditioning to a target taste and whether this effect would depend on the delay between pre-exposure and conditioning. Two groups of rats were pre-exposed to an interfering taste (HCl) either a week before conditioning (Group Distal) or the day preceding conditioning (Group Proximal). In the single conditioning trial, rats were given the target taste (sucrose) and 65 min later were injected with lithium. The groups differed as to what they were given to drink 50 min after sucrose: The Distal, Proximal and Novel groups were given HCl, while the Control group was given water. Pre-exposure to HCl reduced overshadowing of the sucrose aversion by HCl in Group Proximal but not in Group Distal. Possible explanations for the latter result include extinction of the context-HCl association and loss of context control over an HCl-no outcome association. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Latent inhibition (LI) occurs when a conditioned stimulus (CS) is presented initially without consequence and when later paired with an unconditioned stimulus (US), its subsequent ability to evoke a conditioned response (CR) is impaired compared to that of a novel CS (Lubow & Moore, 1959). This effect has been demonstrated in many kinds of conditioning paradigms (Lubow, 2009). A related consequence of pre-exposing a stimulus is to reduce its effectiveness in overshadowing conditioning of a target stimulus. While almost all experiments examining this type of pre-exposure effect have employed a simultaneous compound (Holmes & Harris, 2010), the experiment reported here used a serial overshadowing procedure. Thus, using a conditioned taste aversion procedure, the potentially overshadowing taste was introduced during the delay between presentation of the target taste and lithium chloride (LiCl) injection. Such a procedure was first employed by Revusky (1971) using saccharin as the target taste followed by a vinegar solution as the overshadowing stimulus. The presence of vinegar was found to reduce conditioning of an aversion to saccharin, an example of serial overshadowing that has been replicated in several subse-
∗ Corresponding author at: School of Psychology (A18), University of Sydney, NSW 2006, Australia. Fax: +612 9351 2603. E-mail address: [email protected] (R.A. Boakes). http://dx.doi.org/10.1016/j.beproc.2015.06.006 0376-6357/© 2015 Elsevier B.V. All rights reserved.
quent studies (Bond, 1983; Cannon et al., 1985; Kaye et al., 1988; Kwok et al., 2012). Revusky (1971) also included a brief report of an experiment in which he pre-exposed his rats to vinegar prior to using this solution as the potentially overshadowing stimulus. Pre-exposed vinegar was less effective in overshadowing the saccharin aversion than when the vinegar solution was novel. To our knowledge, the present experiment is the first since then to examine the effect of pre-exposing a potentially overshadowing stimulus in long delay taste aversion or, for that matter, in any other form of serial overshadowing. A theoretically important property of LI is its sensitivity to the length of the interval between pre-exposure of a stimulus and its use as a CS in Pavlovian conditioning. Thus, standard LI is typically reduced as the interval between pre-exposure and conditioning is increased (e.g. Aguado et al., 2001, 1994). Delay-induced attenuation of LI has not, however, always been reported (e.g. Alvarez & Lopez, 1995; De la Casa & Lubow, 2002). The more usual finding that a long delay between pre-exposure and conditioning attenuates the effects of pre-exposure has been important for accounts of LI proposing that stimulus pre-exposure causes failure in a postconditioning test to fully retrieve CS-US associations (e.g. Aguado et al., 1994; Bouton, 1991, 1993; Kraemer & Spear, 1992; Westbrook et al., 2000). The above accounts were developed to explain temporal effects in standard LI. Whether the same effect of passage of time is found with an overshadowing test does not appear to have been tested.
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As noted above, not all studies have reported loss of LI over a delay interval and so it was considered important to test whether the effect can be found with a serial overshadowing procedure. Consequently, the aim of the present experiment was to examine how a delay between pre-exposure and conditioning would affect the extent to which the pre-exposed stimulus (a hydrochloric acid solution; HCl) overshadowed the novel target stimulus (sucrose). Three interference groups received HCl in the interval between sucrose and a lithium injection, where the crucial manipulation was the nature of the pre-exposure to HCl: either nine days before conditioning (Distal), immediately before conditioning (Proximal), or no pre-exposure at all (Novel). A fourth group (Control) was presented with water instead of the interfering taste. The control rats were split into two subgroups that received proximal or distal HCl pre-exposure to ensure that HCl pre-exposure itself was not a factor for differences in consumption at test. It was predicted that greater overshadowing would be obtained when there was a short gap (Proximal condition) between pre-exposure and conditioning than with a long gap (Distal condition). 1. Method
Fig. 1. Mean intakes (plus SEMs) during the first and second, and tenth and eleventh sessions during pre-exposure. HCl intakes of both the Proximal and Distal groups can be compared with their corresponding water intakes. Group Novel received only water throughout pre-exposure while HCl intakes are shown for the Control Group.
received water. On Days 18–19 rats were given effectively unlimited access to sucrose and on Days 20–21 were given unlimited access to HCl.
1.1. Subjects 2. Results Thirty-two male naive Albino Wistar rats, aged 70 days with a mean weight of 342 g (range 298–374 g) were purchased from ARC Perth. They were group-housed, 4 per cage, and had unrestricted access to chow throughout. In an initial 4-day period access to water was progressively restricted from 4 h to 30 min. From then on rats were given 30-min water in their home cages following each daily session. 1.2. Apparatus A separate laboratory contained sixteen transparent acrylic cages, 33 × 21 × 19 cm high, serving as drinking chambers. Flooring was commercial cat litter. Plastic drinking bottles of 100-ml capacity with a stainless steel ball-bearing spout could be attached though the roof. Fluid intakes were measured by weighing bottles to the nearest 0.1 g before and after each session. The target taste was an 8% sucrose solution and the interfering taste was a .005 M HCl, both mixed with tap water. 1.3. Procedure Experimental sessions were conducted daily and always took place in the drinking chambers. During pre-training on Days 1–3 water access was progressively reduced from 20 to 10 min. From then on all sessions lasted 10 min, unless otherwise specified below. Groups (n = 8) were matched for water intake on Day 3. In the eleven pre-exposure sessions unlimited access to a solution was given. On Days 4 and 5 rats in Group Distal and half the Control rats were given HCl, while all other rats were given water. This was followed by 7 days of water consumption (Days 6–12) for all rats. On Days 13 and 14 rats in Group Proximal and the remaining Control rats were given HCl, while all other rats were given water. Two days of such pre-exposure were given in case HCl consumption in the first session was particularly low due to neophobia. No pre-exposure was given to Group Novel. These rats received daily water sessions for the entire pre-exposure phase. In the single conditioning session (Day 15) all rats were given 5-min access to 8 ml of sucrose followed 65 min later by an intraperitoneal injection of 0.15 M LiCl at 10 ml/kg body weight. The groups differed as to the 7 ml of fluid for 5 min they were given 50 min after sucrose: Groups Distal, Proximal and Novel were given HCl, while Group Control was given water. On Days 16–17 all rats
The four groups did not differ in water intake on the final pretraining session (Day 3), F < 1, when the mean (and SEM) intakes were: Distal, 14.5 (0.4) ml; Proximal, 13.0 (1.0) ml; Novel, 13.8 (0.5) ml; and Control, 14.4 (1.4) ml. These intakes were compared with water intakes on Day 17 (immediately prior to the first sucrose test) as a check on whether there might be differences in context conditioning across the groups. Mean (and SEM) intakes on Day 17 were: Distal, 16.2 (0.7) ml; Proximal, 15.3 (0.7) ml; Novel, 16.5 (1.0) ml; and Control, 15.7 (1.1) ml. A mixed 4 × 2 ANOVA, with Group and Session as factors, detected a main effect of Session, such that rats increased their drinking from Day 3 to Day 17, F(1,28) = 13.19, p = .001, but no main effect of Group or their interaction was found, Fs < 1. Intakes on Days 4, 5, 13 and 14 are shown in Fig. 1. Proximal and Distal rats that received HCl pre-exposure on Days 4–5 and Days 13–14 respectively, drank similar amounts of HCl, t(14) = 1.55, p > .05. Sucrose intakes in the single conditioning session and two test sessions are shown in the left panel of Fig. 2. Intakes of sucrose in the conditioning session were similar across the four groups; a one-way ANOVA of sucrose consumption failed to reveal any group differences, F(3,28) = 2.33, p > 0.05. Test intakes of sucrose were examined in a repeated-measures ANOVA with Group and Session as variables. This revealed both a main effect of Session, F(1,28) = 154.99, p < .001, a main effect of Group, F(3,28) = 14.18, p < .001, but no significant interaction between Group and Session, F(3,28) = 3.25, p > .05. Planned orthogonal contrasts on mean intakes over the two tests sessions were undertaken. These revealed, first, that the pooled results of the three interference groups (Proximal, Distal and Novel) given HCl drank more sucrose than the Control group, F(1,28) = 32.29, p < .001, demonstrating serial overshadowing; second, that the HCl-preexposed groups (Distal and Proximal) drank more than Group Novel, F(1,28) = 4.59, p = .041; and, finally, that Distal rats consumed more sucrose than Proximal rats, F(1,28) = 5.64, p = .025. Test intakes of HCl are shown in the right-hand panel of Fig. 2. A repeated measures ANOVA with Group and Session as variables revealed both a main effect of Session, F(1,28) = 16.61, p < .001, a main effect of Group, F(3,28) = 11.63, p < .001, but no significant interaction between Group and Session, F(3,28) = 2.25, p > .05. Again, mean intakes over the two HCl sessions were examined.
D.W.S. Kwok, R.A. Boakes / Behavioural Processes 118 (2015) 111–114
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Fig. 2. Left-hand panel: Sucrose intakes in the conditioning and two test sessions. Right-hand panel: Hydrochloric acid intakes over two test sessions. Means and SEMs are shown.
Planned orthogonal contrasts revealed, first, that groups that had received HCl in the conditioning session (Proximal, Distal and Novel) drank less HCl on test than the Control group, F(1,28) = 25.87, p < .001; second, Group Novel drank less than the groups given preexposure to HCl (Proximal and Distal), F(1,28) = 6.27, p < .018; but, finally, no difference was found between Groups Distal and Novel, F(1,28) = 2.75, p > .05. 3. Discussion The most important findings from this experiment were as follows. First, proximal exposure to HCl produced a strong LI effect, in that HCl was less effective in overshadowing acquisition of the sucrose aversion in rats that had recently been pre-exposed to HCl than in rats for which HCl was novel in the conditioning session; this confirms an effect reported by Revusky (1971). The new finding from the present experiment was that introducing an extended delay between pre-exposure and conditioning weakened this LI effect: for Distal rats, an aversion to HCl was acquired and sucrose was overshadowed, as if these rats had never been pre-exposed to HCl. Despite its limitations as a general account of LI (e.g. Holmes & Harris, 2010), Wagner’s (1981) SOP model provides a plausible account of the present results. This explains LI as a reduction in the associability of a stimulus produced by the development of an association between the context and the stimulus; casually speaking, the occurrence of the stimulus in the conditioning session is less of a surprise because it is already expected in the experimental context. In the present Distal group an association between the context of the drinking chambers and HCl acquired over two sessions of exposure to HCl would have been extinguished during the seven subsequent sessions when these rats were given water in the same context and replaced by a context-water association. It seems likely that, if these Distal rats had instead been given water in their home cages over these seven days, their access to HCl in the conditioning session would have shown some LI effect, although probably still not as strongly as that shown by the Proximal group. The present results are not anticipated by attentional theories (Mackintosh, 1975; Pearce & Hall, 1980). Unlike Wagner’s (1981) SOP theory, these attentional accounts of overshadowing are not only unable to account for one-trial overshadowing but also do not include any mechanism for passage of time or context association. Thus, according to attentional theories the associative strength and salience of HCl in the present experiment should have been the same in the Distal as in the Proximal group. The theories considered so far are ones proposing that LI results from decreased associability of the pre-exposed stimulus. A different approach is provided by theories of LI based on the assumption
that pre-exposure to stimulus X produces a X-no outcome association that is subject to context retrieval. Such theories explain loss of LI over a retention interval as resulting from a change in the effective context with time, even in the absence of a physical context change (e.g. Hall & Rodriguez, 2010). Such theories suggest that for the Distal rats in the present study the experience of HCl in the conditioning session occurs in a different context from the original experience and thus there is little or no retrieval of the HCl-no outcome association. The assumption that pre-exposure to X produces an X-no outcome association is also central to performance-deficit accounts of LI (e.g. Aguado et al., 1994; Bouton, 1991). Thus, when testing in a standard LI procedure the change in temporal context is supposed to result in poorer retrieval of the pre-exposure memory than of the conditioning memory, with the result that interference is reduced and conditioning performance is restored (Rosas & Bouton, 1997). When applied to overshadowing, as in the present study, such theories need to appeal to a comparator-type process (e.g. Denniston et al., 2001) whereby performance at test depends—in the present case - on the strength of the sucrose aversion relative to that of the HCl aversion. One test for such a process is whether release from overshadowing can be obtained; with the present procedures this involves assessing whether extinguishing the HCl aversion results in a stronger sucrose aversion in a subsequent test. In an unpublished experiment that used procedures almost identical to those of the present experiment we failed to find any such evidence for release from overshadowing and therefore doubt whether the present LI effect results from a performance deficit at test.
References Aguado, L., Symonds, M., Hall, G., 1994. Interval between preexposure and test determines the magnitude of latent inhibition: implications for an interference account. Anim. Learn. Behav. 22 (2), 188–194. Aguado, L., de Brugadade, I., Hall, G., 2001. Tests for inhibition after extinction of a conditioned stimulus in the flavour aversion procedure. Quart. J. Exp. Psychol. 54B, 201–217. Alvarez, R., Lopez, M., 1995. Effects of elements or compound preexposure on conditioned taste aversion as a function of retention interval. Anim. Learn. Behav. 23, 391–399. Bond, N.W., 1983. Reciprocal overshadowing in flavour-aversion learning. Quart. J. Exp. Psychol. 35, 265–274. Bouton, M.E., 1991. Context and retrieval in extinction and in other examples of interference in simple associative learning. In: Dachowski, L., Flaherty, C.E. (Eds.), Current Topics in Animal Learning: Brain, Emotion, and Cognition. Erlbaum, Hillsdale, NJ, pp. 25–53. Bouton, M.E., 1993. Context, time, and memory retrieval in the interference paradigms of Pavlovian learning. Psychol. Bull. 114 (1), 80–99. Cannon, D.S., Best, M.R., Batson, J.D., Brown, E.R., Rubenstein, J.A., Carrell, L.E., 1985. Interfering with taste aversion learning in rats: the role of associative interference. Appetite 6 (1), 1–19. De la Casa, L.G., Lubow, R.E., 2002. An empirical analysis of the super-latent inhibition effect. Anim. Learn. Behav. 30 (2), 112–120.
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Denniston, J.C., Savastano, H.C., Miller, R.R., 2001. The extended comparator hypothesis: Learning by contiguity. In: Mowrer, R.R., Klein, S.B. (Eds.), Handbook of Contemporary Learning Theory. Erlbaum, Mahwah, NJ, pp. 65–117. Hall, G., Rodriguez, G., 2010. Associative and nonassociative processes in latent inhibition: An Elaboration of the Pearce-Hall model. In: Lubow, R.E., Weiner, I. (Eds.), Latent Inhibition: Cognition, Neuroscience, and Applications to Schizophrenia. Cambridge University Press, Cambridge, pp. 114–136. Holmes, N., Harris, J., 2010. Latent inhibition. In: Mitchell, C.J., Le Pelley, M.E. (Eds.), Attention and Associative Learning From Brain to Behaviour. Oxford University Press, Oxford, pp. 99–130. Kaye, H., Gambini, B., Mackintosh, N.J., 1988. A dissociation between one-trial overshadowing and the effect of a distractor on habituation. Quart. J. Exp. Psychol. 40B, 31–47. Kraemer, P.J., Spear, N.E., 1992. The effect of nonreinforced stimulus exposure on the strength of a conditioned taste aversion as a function of retention interval: do latent inhibition and extinction involve a shared process? Anim. Learn. Behav. 20, 1–7. Kwok, D.W.S., Livesey, E.J., Boakes, R.A., 2012. Serial overshadowing of taste aversion learning by stimuli preceding the target taste. Learn. Behav. 40 (4), 427–438.
Lubow, R.E., 2009. Conditioned taste aversion and latent inhibition: A review. In: Reilly, S., Schachtman, T.R. (Eds.), Conditioned Taste Aversion: Behavioral and Neural Processes. Oxford University Press, New York, pp. 37–57. Lubow, R.E., Moore, A.U., 1959. Latent inhibition: the effect of non reinforced preexposure to the conditional stimulus. J. Comp. Physiol. Psych. 52, 415–419. Mackintosh, N.J., 1975. A theory of attention: variations in the associability of stimuli with reinforcement. Psychol. Rev. 82, 276–298. Pearce, J.M., Hall, G., 1980. A model for Pavlovian conditioning: variations in the effectiveness of conditioned but not unconditioned stimuli. Psychol. Rev. 87, 332–352. Revusky, S., 1971. The role if interference in association over a delay. In: Honig, W.K., James, P.H.R. (Eds.), Animal Memory. Academic Press, New York, pp. 155–213. Rosas, J.M., Bouton, M.E., 1997. Additivity of the effects of retention interval and context change on latent inhibition: toward resolution of the context forgetting paradox. J. Exp. Psychol.: Anim. Behav. Processes 23, 283–294. Wagner, A.R., 1981. SOP: a model of automatic memory processing in animal behavior. In: Spear, N.E., Miller, R.R. (Eds.), Informal Processing in Animals: Memory Mechanisms. Erlbaum Associates, Hillsdale, NH, pp. 5–47. Westbrook, R.F., Jones, M.L., Bailey, G.K., Harris, J.A., 2000. Contextual control over conditioned responding in a latent inhibition paradigm. J. Exp. Psychol.: Anim. Behav. Processess 26 (2), 157.
Contents lists available at ScienceDirect
Behavioural Processes journal homepage: www.elsevier.com/locate/behavproc
Short report
Proximal, but not distal, pre-exposure reduces serial overshadowing in one-trial taste aversion learning Dorothy W.S. Kwok, Robert A. Boakes ∗ University of Sydney, Sydney, Australia
a r t i c l e
i n f o
Article history: Received 25 February 2015 Received in revised form 4 June 2015 Accepted 5 June 2015 Available online 9 June 2015 Keywords: Conditioned taste aversion Long delay learning Serial overshadowing Retroactive interference Latent inhibition Pre-exposure
a b s t r a c t This experiment tested whether pre-exposing a taste would reduce its ability to overshadow conditioning to a target taste and whether this effect would depend on the delay between pre-exposure and conditioning. Two groups of rats were pre-exposed to an interfering taste (HCl) either a week before conditioning (Group Distal) or the day preceding conditioning (Group Proximal). In the single conditioning trial, rats were given the target taste (sucrose) and 65 min later were injected with lithium. The groups differed as to what they were given to drink 50 min after sucrose: The Distal, Proximal and Novel groups were given HCl, while the Control group was given water. Pre-exposure to HCl reduced overshadowing of the sucrose aversion by HCl in Group Proximal but not in Group Distal. Possible explanations for the latter result include extinction of the context-HCl association and loss of context control over an HCl-no outcome association. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Latent inhibition (LI) occurs when a conditioned stimulus (CS) is presented initially without consequence and when later paired with an unconditioned stimulus (US), its subsequent ability to evoke a conditioned response (CR) is impaired compared to that of a novel CS (Lubow & Moore, 1959). This effect has been demonstrated in many kinds of conditioning paradigms (Lubow, 2009). A related consequence of pre-exposing a stimulus is to reduce its effectiveness in overshadowing conditioning of a target stimulus. While almost all experiments examining this type of pre-exposure effect have employed a simultaneous compound (Holmes & Harris, 2010), the experiment reported here used a serial overshadowing procedure. Thus, using a conditioned taste aversion procedure, the potentially overshadowing taste was introduced during the delay between presentation of the target taste and lithium chloride (LiCl) injection. Such a procedure was first employed by Revusky (1971) using saccharin as the target taste followed by a vinegar solution as the overshadowing stimulus. The presence of vinegar was found to reduce conditioning of an aversion to saccharin, an example of serial overshadowing that has been replicated in several subse-
∗ Corresponding author at: School of Psychology (A18), University of Sydney, NSW 2006, Australia. Fax: +612 9351 2603. E-mail address: [email protected] (R.A. Boakes). http://dx.doi.org/10.1016/j.beproc.2015.06.006 0376-6357/© 2015 Elsevier B.V. All rights reserved.
quent studies (Bond, 1983; Cannon et al., 1985; Kaye et al., 1988; Kwok et al., 2012). Revusky (1971) also included a brief report of an experiment in which he pre-exposed his rats to vinegar prior to using this solution as the potentially overshadowing stimulus. Pre-exposed vinegar was less effective in overshadowing the saccharin aversion than when the vinegar solution was novel. To our knowledge, the present experiment is the first since then to examine the effect of pre-exposing a potentially overshadowing stimulus in long delay taste aversion or, for that matter, in any other form of serial overshadowing. A theoretically important property of LI is its sensitivity to the length of the interval between pre-exposure of a stimulus and its use as a CS in Pavlovian conditioning. Thus, standard LI is typically reduced as the interval between pre-exposure and conditioning is increased (e.g. Aguado et al., 2001, 1994). Delay-induced attenuation of LI has not, however, always been reported (e.g. Alvarez & Lopez, 1995; De la Casa & Lubow, 2002). The more usual finding that a long delay between pre-exposure and conditioning attenuates the effects of pre-exposure has been important for accounts of LI proposing that stimulus pre-exposure causes failure in a postconditioning test to fully retrieve CS-US associations (e.g. Aguado et al., 1994; Bouton, 1991, 1993; Kraemer & Spear, 1992; Westbrook et al., 2000). The above accounts were developed to explain temporal effects in standard LI. Whether the same effect of passage of time is found with an overshadowing test does not appear to have been tested.
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As noted above, not all studies have reported loss of LI over a delay interval and so it was considered important to test whether the effect can be found with a serial overshadowing procedure. Consequently, the aim of the present experiment was to examine how a delay between pre-exposure and conditioning would affect the extent to which the pre-exposed stimulus (a hydrochloric acid solution; HCl) overshadowed the novel target stimulus (sucrose). Three interference groups received HCl in the interval between sucrose and a lithium injection, where the crucial manipulation was the nature of the pre-exposure to HCl: either nine days before conditioning (Distal), immediately before conditioning (Proximal), or no pre-exposure at all (Novel). A fourth group (Control) was presented with water instead of the interfering taste. The control rats were split into two subgroups that received proximal or distal HCl pre-exposure to ensure that HCl pre-exposure itself was not a factor for differences in consumption at test. It was predicted that greater overshadowing would be obtained when there was a short gap (Proximal condition) between pre-exposure and conditioning than with a long gap (Distal condition). 1. Method
Fig. 1. Mean intakes (plus SEMs) during the first and second, and tenth and eleventh sessions during pre-exposure. HCl intakes of both the Proximal and Distal groups can be compared with their corresponding water intakes. Group Novel received only water throughout pre-exposure while HCl intakes are shown for the Control Group.
received water. On Days 18–19 rats were given effectively unlimited access to sucrose and on Days 20–21 were given unlimited access to HCl.
1.1. Subjects 2. Results Thirty-two male naive Albino Wistar rats, aged 70 days with a mean weight of 342 g (range 298–374 g) were purchased from ARC Perth. They were group-housed, 4 per cage, and had unrestricted access to chow throughout. In an initial 4-day period access to water was progressively restricted from 4 h to 30 min. From then on rats were given 30-min water in their home cages following each daily session. 1.2. Apparatus A separate laboratory contained sixteen transparent acrylic cages, 33 × 21 × 19 cm high, serving as drinking chambers. Flooring was commercial cat litter. Plastic drinking bottles of 100-ml capacity with a stainless steel ball-bearing spout could be attached though the roof. Fluid intakes were measured by weighing bottles to the nearest 0.1 g before and after each session. The target taste was an 8% sucrose solution and the interfering taste was a .005 M HCl, both mixed with tap water. 1.3. Procedure Experimental sessions were conducted daily and always took place in the drinking chambers. During pre-training on Days 1–3 water access was progressively reduced from 20 to 10 min. From then on all sessions lasted 10 min, unless otherwise specified below. Groups (n = 8) were matched for water intake on Day 3. In the eleven pre-exposure sessions unlimited access to a solution was given. On Days 4 and 5 rats in Group Distal and half the Control rats were given HCl, while all other rats were given water. This was followed by 7 days of water consumption (Days 6–12) for all rats. On Days 13 and 14 rats in Group Proximal and the remaining Control rats were given HCl, while all other rats were given water. Two days of such pre-exposure were given in case HCl consumption in the first session was particularly low due to neophobia. No pre-exposure was given to Group Novel. These rats received daily water sessions for the entire pre-exposure phase. In the single conditioning session (Day 15) all rats were given 5-min access to 8 ml of sucrose followed 65 min later by an intraperitoneal injection of 0.15 M LiCl at 10 ml/kg body weight. The groups differed as to the 7 ml of fluid for 5 min they were given 50 min after sucrose: Groups Distal, Proximal and Novel were given HCl, while Group Control was given water. On Days 16–17 all rats
The four groups did not differ in water intake on the final pretraining session (Day 3), F < 1, when the mean (and SEM) intakes were: Distal, 14.5 (0.4) ml; Proximal, 13.0 (1.0) ml; Novel, 13.8 (0.5) ml; and Control, 14.4 (1.4) ml. These intakes were compared with water intakes on Day 17 (immediately prior to the first sucrose test) as a check on whether there might be differences in context conditioning across the groups. Mean (and SEM) intakes on Day 17 were: Distal, 16.2 (0.7) ml; Proximal, 15.3 (0.7) ml; Novel, 16.5 (1.0) ml; and Control, 15.7 (1.1) ml. A mixed 4 × 2 ANOVA, with Group and Session as factors, detected a main effect of Session, such that rats increased their drinking from Day 3 to Day 17, F(1,28) = 13.19, p = .001, but no main effect of Group or their interaction was found, Fs < 1. Intakes on Days 4, 5, 13 and 14 are shown in Fig. 1. Proximal and Distal rats that received HCl pre-exposure on Days 4–5 and Days 13–14 respectively, drank similar amounts of HCl, t(14) = 1.55, p > .05. Sucrose intakes in the single conditioning session and two test sessions are shown in the left panel of Fig. 2. Intakes of sucrose in the conditioning session were similar across the four groups; a one-way ANOVA of sucrose consumption failed to reveal any group differences, F(3,28) = 2.33, p > 0.05. Test intakes of sucrose were examined in a repeated-measures ANOVA with Group and Session as variables. This revealed both a main effect of Session, F(1,28) = 154.99, p < .001, a main effect of Group, F(3,28) = 14.18, p < .001, but no significant interaction between Group and Session, F(3,28) = 3.25, p > .05. Planned orthogonal contrasts on mean intakes over the two tests sessions were undertaken. These revealed, first, that the pooled results of the three interference groups (Proximal, Distal and Novel) given HCl drank more sucrose than the Control group, F(1,28) = 32.29, p < .001, demonstrating serial overshadowing; second, that the HCl-preexposed groups (Distal and Proximal) drank more than Group Novel, F(1,28) = 4.59, p = .041; and, finally, that Distal rats consumed more sucrose than Proximal rats, F(1,28) = 5.64, p = .025. Test intakes of HCl are shown in the right-hand panel of Fig. 2. A repeated measures ANOVA with Group and Session as variables revealed both a main effect of Session, F(1,28) = 16.61, p < .001, a main effect of Group, F(3,28) = 11.63, p < .001, but no significant interaction between Group and Session, F(3,28) = 2.25, p > .05. Again, mean intakes over the two HCl sessions were examined.
D.W.S. Kwok, R.A. Boakes / Behavioural Processes 118 (2015) 111–114
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Fig. 2. Left-hand panel: Sucrose intakes in the conditioning and two test sessions. Right-hand panel: Hydrochloric acid intakes over two test sessions. Means and SEMs are shown.
Planned orthogonal contrasts revealed, first, that groups that had received HCl in the conditioning session (Proximal, Distal and Novel) drank less HCl on test than the Control group, F(1,28) = 25.87, p < .001; second, Group Novel drank less than the groups given preexposure to HCl (Proximal and Distal), F(1,28) = 6.27, p < .018; but, finally, no difference was found between Groups Distal and Novel, F(1,28) = 2.75, p > .05. 3. Discussion The most important findings from this experiment were as follows. First, proximal exposure to HCl produced a strong LI effect, in that HCl was less effective in overshadowing acquisition of the sucrose aversion in rats that had recently been pre-exposed to HCl than in rats for which HCl was novel in the conditioning session; this confirms an effect reported by Revusky (1971). The new finding from the present experiment was that introducing an extended delay between pre-exposure and conditioning weakened this LI effect: for Distal rats, an aversion to HCl was acquired and sucrose was overshadowed, as if these rats had never been pre-exposed to HCl. Despite its limitations as a general account of LI (e.g. Holmes & Harris, 2010), Wagner’s (1981) SOP model provides a plausible account of the present results. This explains LI as a reduction in the associability of a stimulus produced by the development of an association between the context and the stimulus; casually speaking, the occurrence of the stimulus in the conditioning session is less of a surprise because it is already expected in the experimental context. In the present Distal group an association between the context of the drinking chambers and HCl acquired over two sessions of exposure to HCl would have been extinguished during the seven subsequent sessions when these rats were given water in the same context and replaced by a context-water association. It seems likely that, if these Distal rats had instead been given water in their home cages over these seven days, their access to HCl in the conditioning session would have shown some LI effect, although probably still not as strongly as that shown by the Proximal group. The present results are not anticipated by attentional theories (Mackintosh, 1975; Pearce & Hall, 1980). Unlike Wagner’s (1981) SOP theory, these attentional accounts of overshadowing are not only unable to account for one-trial overshadowing but also do not include any mechanism for passage of time or context association. Thus, according to attentional theories the associative strength and salience of HCl in the present experiment should have been the same in the Distal as in the Proximal group. The theories considered so far are ones proposing that LI results from decreased associability of the pre-exposed stimulus. A different approach is provided by theories of LI based on the assumption
that pre-exposure to stimulus X produces a X-no outcome association that is subject to context retrieval. Such theories explain loss of LI over a retention interval as resulting from a change in the effective context with time, even in the absence of a physical context change (e.g. Hall & Rodriguez, 2010). Such theories suggest that for the Distal rats in the present study the experience of HCl in the conditioning session occurs in a different context from the original experience and thus there is little or no retrieval of the HCl-no outcome association. The assumption that pre-exposure to X produces an X-no outcome association is also central to performance-deficit accounts of LI (e.g. Aguado et al., 1994; Bouton, 1991). Thus, when testing in a standard LI procedure the change in temporal context is supposed to result in poorer retrieval of the pre-exposure memory than of the conditioning memory, with the result that interference is reduced and conditioning performance is restored (Rosas & Bouton, 1997). When applied to overshadowing, as in the present study, such theories need to appeal to a comparator-type process (e.g. Denniston et al., 2001) whereby performance at test depends—in the present case - on the strength of the sucrose aversion relative to that of the HCl aversion. One test for such a process is whether release from overshadowing can be obtained; with the present procedures this involves assessing whether extinguishing the HCl aversion results in a stronger sucrose aversion in a subsequent test. In an unpublished experiment that used procedures almost identical to those of the present experiment we failed to find any such evidence for release from overshadowing and therefore doubt whether the present LI effect results from a performance deficit at test.
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