Rat pups prefer their siblings to their mothers: Possible implications for the development of kin recognition

Previous experiments have indicated that for rat pups some learning of their kin's characteristics is required for kin recognition. By differentially scenting the mother and litter with artificial odours, the possible role of each in the development of this ability can be assessed. 12- and 20-day-old rat pups were given a preference tests between the odour associated with their mother and an odour associated with their littermates. It was hypothesized that, because of the importance of the mother to her pups, individuals would preferentially acquire information from her and thus exhibit a preference for that odour. The results, however, did not support this: Both 12- and 20-day-old rat pups were found to prefer their littermate's odour to that of their mother. A further experiment revealed this did not reflect an aversion to their mother's odour. This preference for sibling littermates probably reflects the greater exposure of littermates to one another. The possible advantages of learning about one's kin from littermates are discussed.     

Undernutrition during the brain growth period of the rat significantly delays the development of processes mediating Pavlovian trace conditioning

Pavlovian trace conditioning procedures allow one to assess the ability of animals to associate events that are temporally separated because the conditioned stimulus terminates prior to the occurrence of the unconditioned stimulus. We report that undernutrition during Postnatal Days 2-18 significantly delays the development of trace conditioning to a visual stimulus. Previously undernourished 20-day-old rats conditioned when no temporal interval separated the termination of the CS and US onset (0-s trace interval). However, it was not until the undernourished pups were 30 days old that they conditioned when the trace interval was 10 or 30 s. In contrast, control pups only 20 days old were able to condition when a 10-s trace interval separated the CS and US events, and 25-day-old control pups conditioned when the interval was either 10 or 30 s. These results suggest that undernutrition delays the development of processes that enable the rat to sustain a representation of a visual CS during the trace interval.     

Early learning as ontogenetic adaptation for ingestion by rats

Ontogenetic adaptation is an “ecological” concept in which mammalian maturation is seen as a coordinated sequence of specializations (stages) that enables the infant to survive within a sequence of distinct niches created by the parent(s) and the rest of the environment. Natural selection is presumed to operate at each point in development. The development of ingestive behavior in the rat pup as a series of ontogenetic adaptations enabling the infant while in the nursing niche to obtain and utilize mother's milk and progress gradually into the feeding niche in which a variety of solid foods must either be obtained or avoided is discussed. Data to show that learning is an integral part of these ontogenetic adaptations for ingestion are reviewed and interpreted. Preweanling rats that are capable of learning and remembering toxiphobic food aversions do not display an aversion to a flavor conditioned stimulus (CS) paired with illness if the CS is presented during suckling. In contrast, 20-day-old pups (weanlings) trained identically do show conditioned taste aversions under these conditions. A perspective of ontogenetic adaptations to this phenomenon of “blockade of toxiphobia by suckling” and its dissolution at weaning is applied. The generality of the blockade phenomenon and its mechanisms are explored, and it is shown that feeding experience regulates the stage-appropriate learning strategies used by the pups.     

Ontogeny of second-order odor-aversion conditioning in neonatal rats

In Experimental 1, rat pups 1, 3, 5, or 7 days old received a Phase 1 S1-US (Stimulus 1-unconditioned stimulus) pairing calculated to establish a first-order conditioned aversion to S1 (lemon scent). On the next day each pup received a Phase 2 S2-S1 pairing designed to establish a second-order aversion to S2 (orange scent). Pups at all ages displayed a first-order aversion to S1, but only pups given Phase 2 training when 6 or 8 days old displayed a second-order aversion to S2. Experiment 2 revealed that pups that received the S1-US pairing when 3 days old acquired an aversion to S2 if they were 6 days old when they received the S2-S1 pairing but not if they received the pairing when 4 days old. In Experiment 3, two Phase 2 treatments were compared. One was identical to that employed in Experiments 1 and 2: Pups 4 days old received a sequential exposure to S2 and S1. Another set of pups received a modified treatment: They were exposed first to S2 and then jointly to both S2 and S1. Only pups in the latter set displayed an aversion to S2. It is suggested that these results reflect developmental differences in the memory processes mediating the rat's experience with olfactory stimulation.     

The importance of contextual elements in taste-aversion learning

The effect of exteroceptive contextual cues, presented during the conditioned stimulus (CS) and unconditioned stimulus (US) intervals, upon the later development of conditioned saccharin aversions was studied in two experiments. It was found that the presence of the contextual cues during the CS + US and CS intervals resulted in a greater degree of saccharin aversion; the presence of these cues during the US interval was relatively less important. These findings are discussed in terms of conditioning models and retrieval models derived from animal learning and human information-processing data.     

Trace decay and the priming of short-term memory in long-delay taste aversion learning: Disruptive effects of novel exteroceptive stimulation

A variation of Kalat and Rozin's two-presentation paradigm was used to test the hypothesis that the first, as opposed to the second, presentation of a flavor conditioned stimulus (CS) constitutes the functional CS in two-presentation experiments involving moderate interflavor intervals (IFIs), and results in flavor aversions that are a function of the primary, as opposed to the secondary, conditioned stimulus-unconditioned stimulus (CS-US) interval. Contrary to the hypothesis, it was shown in Experiment 1 that holding the primary CS-US interval constant at 4 hr for each of three groups, while decreasing the secondary CS-US interval (i.e., the interval between the second flavor presentation and the illness) from 3.75 hr to 2.5 hr to .5 hr, resulted in the flavor aversion increasing as the secondary CS-US interval decreased. However, the aversion acquired by the group with a 0.5 hr secondary CS-US interval was also found to be significantly weaker than that acquired by a single-presentation 0.5 hr control group. In Experiment 2 it was demonstrated that animals exposed to novel exteroceptive stimulation (NES) immediately prior to a second flavor presentation that preceded the US by 0.5 hr acquired an aversion as strong as that acquired by a 0.5-hr control group. In Experiment 3 it was demonstrated that, in the absence of a second flavor presentation, animals exposed to novel exteroceptive stimulation 0.5 hr prior to the US acquired a weaker flavor aversion than did animals not exposed to novel exteroceptive stimulation during the 4-hr flavor CS-illness US interval. The contrasting effects of novel exteroceptive stimulation observed in Experiments 2 and 3 were replicated in Experiment 4. The results suggest, consistent with the trace-decay hypothesis and Wagner's (1976) general model of stimulus processing, that exposure to novel exteroceptive stimulation disrupts continued processing of the short-term memory (STM) trace of the initial presentation of a flavor CS, and hence minimizes stimulus preexposure effects attributable to the priming of STM.     

Conditioned taste aversion is enhanced when the unconditioned stimulus is presented in a different context

Using a conditioned taste aversion procedure with rats as the subjects, two experiments examined the effect of presenting a conditioned stimulus (CS saccharin solution) in one context followed by an unconditioned stimulus (US LiCl) in a different context. Experiment 1 showed that animals which received the above-mentioned procedure (Group D) showed a more marked conditioned aversion to the CS than animals which were given both the CS and the US in the same context (Group S). Experiment 2 found that in both Group D and Group S, aversion to the CS increased when the subjects were exposed to the conditioned context after the conditioning. These findings supported the argument that the strength of the CS-US association acquired during conditioning is compared with that of the context-US to determine the magnitude of aversion revealed to the CS.     

What you see is what will change: Evaluative conditioning effects depend on a focus on valence

This study investigated whether evaluative conditioning (EC) effects depend on an evaluative focus during the learning phase. An EC effect is a valence change of an originally neutral stimulus (conditioned stimulus or CS) that is due to the former pairing with a positive or negative stimulus (unconditioned stimulus or US). In three experiments, the task focus during the conditioning phase was manipulated. Participants judged CS–US pairings either with respect to their valence or with respect to another stimulus dimension. EC effects on explicit and implicit measures were found when valence was task relevant but not when the non-valent stimulus dimension was task relevant. Two accounts for the valence focus effect are proposed: (1) An additional direct learning of the relation of CS and evaluative responses in the valence focus condition, or (2) a stronger activation of US valence in the valence focus condition compared to the non-valent focus condition.     

Rho-associated kinase in the gustatory cortex is involved in conditioned taste aversion memory formation but not in memory retrieval or relearning

Rho-associated kinase (ROCK) is intimately involved in cortical neuronal morphogenesis. The present study explores the roles of ROCK in conditioned taste aversion (CTA) memory formation in gustatory cortex (GC) in adult rat. Microinjection of the ROCK inhibitor Y-27632 into the GC 30 min before CTA training or 10 min after the conditioned stimulus (CS) impaired long-term CTA memory (LTM) formation. ROCK inhibitor had no effect on taste aversion when injected before the first LTM test day and did not alter taste aversion on subsequent test days. Microinjection of ROCK inhibitor into GC 30 min before preexposure to the taste CS had no effect on latent inhibition of CTA learning suggesting that ROCK is involved in CS-US association rather than taste learning per se. Cumulatively, these results show that ROCK is needed for normal CTA memory formation but not retrieval, relearning or incidental taste learning.     

The resistance of renewal to instructions that devalue the role of contextual cues in a conditioned suppression task with humans

The renewal of extinguished conditioned behaviour appears to reflect context-dependent learning. The present research used a conditioned suppression task with humans to examine whether instructions concerning the context could influence renewal. Pairings of a conditional stimulus (CS) and unconditional stimulus (US) were made in one context, followed by extinction trials of CS alone in a second context, and test trials of CS alone upon return to the original learning context. Four experiments tested whether the renewal of conditioned suppression observed during test would be attenuated if participants were instructed that the context changes were irrelevant to the predictive relationship between the CS and US. Using a differential conditioning design, no attenuation was found when the instructions were given prior to conditioning (Experiment 1) or immediately prior to the test trials (Experiment 2). The latter result was replicated with a single-cue conditioning design and further controls for exposure to the extinction contexts (Experiment 3). The collection of on-line ratings about the relationship between the contextual changes and the predictive nature of the CS indicated that participants did attend to and believe the instructions (Experiment 4). The results point to the resistance of renewal to explicit instructions that attempt to devalue the role of the contextual cues.     

A failure to find socially mediated taste aversion learning in Norway rats (R. norvegicus)

Observer rats interacted with conspecific demonstrators immediately after demonstrators ate a novel diet and were made ill by LiCl injection. Following their interaction with demonstrators, observers were tested for aversion to their ill demonstrator's diet. Previous research has shown that (a) an observer can extract information from a demonstrator sufficient to permit identification of the demonstrator's diet (Galef & Wigmore, 1983) and (b) a rat ill from LiCl toxicosis is an adequate unconditioned stimulus in a taste aversion learning paradigm (Lavin, Freise, & Coombes, 1980). Further, two of the present experiments demonstrated that cues emitted by a rat, reflecting the particular diet it has eaten, are an adequate conditional stimulus in a toxicosis-induced aversion learning situation. Observer avoidance of a diet previously ingested by an ill demonstrator was, however, not demonstrated. The implications of the failure to find socially mediated aversion learning are discussed.     

Learned Irrelevance Is Not the Sum of Exposure to CS and US

In three experiments hungry rats received appetitive conditioning trials with a light that signalled the delivery of sucrose solution. In Experiment 1, prior exposure to uncorrelated presentations of the conditioned stimulus (CS) and unconditioned stimulus (US) retarded conditioning significantly more than did prior exposure to the CS alone. In Experiments 2 and 3, groups exposed to uncorrelated presentations of the CS and US within the same session conditioned significantly more slowly than groups given separate sessions of exposure to the CS followed by sessions of exposure to the US (or vice versa). Some part of the learned irrelevance effect depends on exposure to a zero correlation between the CS and US, perhaps because this promotes learning that the CS predicts no change in the probability of the US.     

Mismatch between what is expected and what actually occurs triggers memory reconsolidation or extinction

In previous experiments on contextual memory, we proposed that the unreinforced re-exposure to the learning context (conditioned stimulus, CS) acts as a switch guiding the memory course toward reconsolidation or extinction, depending on reminder duration. This proposal implies that the system computes the total exposure time to the context, from CS onset to CS offset, and therefore, that the reminder presentation must be terminated for the switching mechanism to become operative. Here we investigated to what extent this requirement is necessary, and we explored the relation between diverse phases in the reconsolidation and extinction processes. We used the contextual memory model of the crab Chasmagnathus which involves an association between the learning context (CS) and a visual danger stimulus (unconditioned stimulus, US). Administration of cycloheximide was used to test the lability state of memory at different time points. The results show that two factors, no-reinforcement during the reminder (i.e., CS re-exposure) and CS offset are the necessary conditions for both processes to occur. Regardless of the reminder duration, memory retrieved by unreinforced CS re-exposure emerges intact and consolidated when tested before CS offset, suggesting that neither reconsolidation nor extinction is concomitant with CS re-exposure. Either process could only be triggered once the definitive mismatch between CS and US is confirmed by CS termination without the expected reinforcement.     

What you see is what will change: evaluative conditioning effects depend on a focus on valence

This study investigated whether evaluative conditioning (EC) effects depend on an evaluative focus during the learning phase. An EC effect is a valence change of an originally neutral stimulus (conditioned stimulus or CS) that is due to the former pairing with a positive or negative stimulus (unconditioned stimulus or US). In three experiments, the task focus during the conditioning phase was manipulated. Participants judged CS-US pairings either with respect to their valence or with respect to another stimulus dimension. EC effects on explicit and implicit measures were found when valence was task relevant but not when the non-valent stimulus dimension was task relevant. Two accounts for the valence focus effect are proposed: (1) An additional direct learning of the relation of CS and evaluative responses in the valence focus condition, or (2) a stronger activation of US valence in the valence focus condition compared to the non-valent focus condition.     

The effects of lesions in the central nucleus of the amygdala on appetitive classical conditioning in rats

Abstract: Lesions in the central nucleus of the amygdala (cAMY) have been known to interfere with the acquisition of fear classical conditioning when footshock is used as an unconditioned stimulus (US). The present study examined whether or not a similar interference would occur with an appetitive US. Five rats with lesions in the cAMY (the cAMY group), and eight unoperated control rats were trained in an appetitive classical conditioning paradigm, which did not include elements of operant learning, using a visual conditioned stimulus (CS) (5 W of light for 10 s duration) paired with a food pellet US (45 mg, cheese flavor). The behavioral index of appetitive conditioning was an increase in rearing approach behavior to the CS after CS and US pairings. During CS and US pairings, the movement of the rat was limited so that this approach behavior could not occur. As a result, all control rats showed an increase in rearing, but the cAMY group did not. These results suggest that the cAMY is critical for appetitive as well as fear classical conditioning.     

Disrupting the conditioned stimulus preexposure effect in flavor-aversion learning: effects of interoceptive distractor manipulations

Rats exposed to a flavor prior to a conditioning trial involving that stimulus learn a significantly diminished flavor aversion relative to nonpreexposed control animals. A series of four experiments investigated the ability of the conditioned stimulus (CS) preexposure effect to be disrupted by the introduction of a distractor flavor stimulus between the preexposure and conditioning episodes. Experiment 1 demonstrated that the preexposure effect could be reduced by a distractor presented immediately following the preexposure. In Experiment 2, it was discovered that a novel distractor was more effective than a familiar distractor, even though both stimuli were sensorily equivalent. Experiment 3 further analyzed the distractor effect and demonstrated that the magnitude of disruption was more pronounced with immediate than with delayed (3 hr) distractor manipulations. Finally, Experiment 4 assessed the effects of the distractor in the absence of CS preexposure. The relation of the results from these experiments to general information theory is discussed.     

Event representation in Pavlovian conditioning: image and action

In a typical Pavlovian conditioning experiment, a relatively insignificant event, the conditioned stimulus (CS), is paired with a biologically more meaningful event, the unconditioned stimulus (US). As a consequence of those pairings, the CS is thought to acquire response characteristics of the US. In this article I describe experiments with rats that suggest that under some circumstances, the CS acquires control of perceptual processing of the US, in the absence of that US itself. I present three kinds of evidence for this surrogate processing, which I liken to imagery or hallucination: (1) CSs come to control specific, sensory-evaluative responses normally evoked only by the USs; (2) CSs can substitute for USs in the establishment of new learning about those USs themselves; (3) CSs can substitute for USs in the modulation of conditioning to other events, either overshadowing (interfering) or potentiating learning, in the same manner as the USs themselves. Finally, I compare these data with evidence for conditioned sensation and imagery in humans, and suggest that imagery may be a very basic process, evolutionarily derived from perceptual and conditioning processes adapted to deal with remote or absent objects.     

Learning with arbitrary versus ecological conditioned stimuli: Evidence from sexual conditioning

Laboratory investigations of Pavlovian conditioning typically involve the association of an arbitrary conditioned stimulus (CS) with an unconditioned stimulus (US) that has no inherent relation to the CS. However, arbitrary CSs are unlikely to become conditioned outside the laboratory, because they do not occur often enough with the US to result in an association. Learning under natural circumstances is likely only if the CS has a preexisting relation to the US. Recent studies of sexual conditioning have shown that in contrast to an arbitrary CS, an ecologically relevant CS is resistant to blocking, extinction, and increases in the CS-US interval and results in sensitized responding and stronger second-order conditioning. Although the mechanisms of these effects are not fully understood, these findings have shown that signature learning phenomena are significantly altered when the kinds of stimuli that are likely to become conditioned under natural circumstances are used. The implications of these findings for an ecological approach to the study of learning are discussed.     

Learned irrelevance and retrospective correlation learning

In 1973 Mackintosh reported an interference effect that he called learned irrelevance in which exposure to uncorrelated (CS/US) presentation of the unconditional stimulus (US) and the conditioned stimulus (CS) interfered with future Pavlovian conditioning. It has been argued that there is no specific interference effect in learned irrelevance; rather the interference is the sum of independent CS and US exposure effects (CS + US). We review previous research on this question and report two new experiments. We conclude that learned irrelevance is a consequence of a contingency learning and a specific learned irrelevance mechanism. Moreover even the “independent exposure controls”, used in previous experiments to support the CS and US exposure account, provide support for the correlation learning process.     

The interval between the CS and the UCS as a determiner of generalization performance

Rats were trained in a conditioned taste aversion paradigm in order to determine whether a trace interval between the conditioned stimulus (CS) and the unconditioned stimulus (UCS) would result in the forgetting of stimulus attributes. Accordingly, subjects were conditioned with milk (CS), given either an immediate or a delayed injection of lithium chloride (UCS), and tested 48 h later with either milk or chocolate milk, a generalized flavor. The rats conditioned immediately following the presentation of the CS avoided milk more than chocolate milk, indicating discrimination between the two flavors. Those conditioned after a trace interval avoided both flavors equally, suggesting a loss of stimulus attributes of the original CS. Delay rats, however, still exhibited substantial learning when compared with controls not experiencing the UCS. These results allege a role for the forgetting of stimulus attributes during a trace interval in addition to following a complete learning episode.