Potentiation and overshadowing in preweanling and adult rats

Sex Differences in Cue Competition Effects With a Conditioned Taste Aversion Preparation

This study aimed to test whether male and female rats might show differences in cue competition effects in a conditioned taste aversion (CTA) model. Experiment 1 tested for sex differences in overshadowing. After conditioning of a flavored compound AB or only one simple flavor A (being A and B a solution of sugar 10% and salt 1%, counterbalanced), consumption of the A solution at test was larger in the former than in the latter case only in males. Thus, the usual effect of overshadowing was observed in males but not in females. Experiment 2 examined sex differences in blocking with the same stimuli used in Experiment 1. After conditioning of AB, the consumption of B was larger for the animals that previously received a single conditioning trial with A than for those that received unpaired presentations of A and the illness. As observed in Experiment 1, the typical blocking effect appeared only in males but not in females. The present findings thus support the hypothesis that sex dimorphism might be expressed in classical conditioning, or at least, in cue competition effects such as overshadowing and blocking with a taste aversion model.

Effortlessly strengthening infant memory: associative potentiation of new learning

We recently discovered that young infants learn a new association better in the presence of a prior association than when they learn it alone - a phenomenon we call associative potentiation. When 6-month-olds observed actions modeled on a puppet (remembered for 1 day) in the presence of a toy train they had learned to activate (remembered for 2 weeks), they remembered the demonstration for 2 weeks too. Currently, we examined the generality of associative potentiation. We found that when 6-month-olds learned the train task (remembered for 5 days) in the presence of two previously associated puppets (remembered for 4 weeks); they remembered the train task for 4 weeks too - more than five times longer. We conclude that associative potentiation is a general phenomenon: Cues for any stronger prior association will correspondingly increase the memory strength of any weaker new association that young infants learn in their presence, eliminating the need for repeated practice. We view associative potentiation as an adaptive mechanism that counteracts the rapid forgetting of younger infants by instantly increasing the strength of their new learning to a level characteristic of older infants. Neuromaturational models of infant memory cannot account for associative potentiation, but an ecological model does.

CS-US interval determines the transition from overshadowing to potentiation with flavor compounds

The present series of five flavor aversion experiments with rat subjects examined compound conditioning at varying CS-US intervals. Using a taste-taste design, Experiments 1A and 1B demonstrated overshadowing at a 0-min CS-US interval and potentiation at a 120-min CS-US interval, and these effects occurred with both tastes of the compound. Experiment 2 showed that the aversion to a single element is reduced when the CS-US interval is increased to 120 min, but the aversion for a compound taste is not. Experiments 3A and 3B explored odor + taste compound conditioning; the results demonstrated odor potentiation across the trace interval and a transition from taste overshadowing to taste potentiation. Collectively, the data show that the change from overshadowing to potentiation was not due to changes in the aversions produced by compound conditioning but, instead, was due to a more rapid loss of conditionability across a trace interval prior to the US in single-element conditioning. These experiments suggest that following compound conditioning, the aversion to each element represents generalization decrement from the configured compound, but the designation of overshadowing or potentiation actually depends on the status of conditioning in the single-element control.

Redundant amodal properties facilitate operant learning in 3-month-old infants

The current study examined the role redundant amodal properties play in an operant learning task in 3-month-old human infants. Prior studies have suggested that the presence of redundant amodal information facilitates detection and discrimination of amodal properties and potentially functions to influence general learning processes such as associative conditioning. The current study examined how human infants use redundant amodal information (visual and haptic) about the shape of an object to influence learning of an operant response. Infants learned to kick to move a mobile of cylinders while either holding a cylinder, a rectangular cube, or no object. Kick rate served as the dependent measure. The results showed that infants given matching redundant amodal properties (e.g., viewed cylinders while holding a cylinder) showed facilitated operant learning whereas infants given mismatching redundant amodal properties showed inhibited operant learning. These results support and extend the Intersensory Redundancy Hypothesis by demonstrating that amodal redundancy influences complex learning processes such as operant conditioning.

Why a neuromaturational model of memory fails: exuberant learning in early infancy

The characteristics of memory in infants and adults seem vastly different. The neuromaturational model attributes these differences to an ontogenetic change in the basic memory process, namely, to the hierarchical maturation of two distinct memory systems. The early-maturing (implicit) system is functional during the first third of infancy and supports the gradual learning of perceptual and motor skills; the late-maturing (explicit) system supports representations of contextually specific events, relationships, and associations. An alternative model holds that the basic memory process does not change, but what infants and adults select to encode for learning does. This ontogenetic change in selective attention has been mistaken for an ontogenetic shift in the basic memory process. Over the last 25 years, evidence from transfer studies with developing rats and human infants has revealed that the first third of infancy is actually a period of exuberant learning that ends, not coincidentally, at the same age that the late-maturing memory system presumably emerges. This article reviews data from recent studies of sensory preconditioning, potentiation, associative chains, and transitive inference with human infants that support this conclusion-data for which the neuromaturational model cannot account. Fast mapping is a general learning mechanism that accounts for this evidence.

Multiple memory systems are unnecessary to account for infant memory development: an ecological model

How the memory of adults evolves from the memory abilities of infants is a central problem in cognitive development. The popular solution holds that the multiple memory systems of adults mature at different rates during infancy. The early-maturing system (implicit or nondeclarative memory) functions automatically from birth, whereas the late-maturing system (explicit or declarative memory) functions intentionally, with awareness, from late in the first year. Data are presented from research on deferred imitation, sensory preconditioning, potentiation, and context for which this solution cannot account and present an alternative model that eschews the need for multiple memory systems. The ecological model of infant memory development (N. E. Spear, 1984) holds that members of all species are perfectly adapted to their niche at each point in ontogeny and exhibit effective, evolutionarily selected solutions to whatever challenges each new niche poses. Because adults and infants occupy different niches, what they perceive, learn, and remember about the same event differs, but their raw capacity to learn and remember does not.

Within-compound associations are not sufficient to produce taste-mediated odor potentiation

Rats were used in two flavor-aversion experiments to determine if within-compound associations could be detected with a taste+odor compound that would not support taste-mediated odor potentation. In Experiment 1, following taste+odor compound conditioning, postconditioning taste extinction significantly weakened the odor aversion. In Experiment 2, following taste+odor compound conditioning, postconditioning taste inflation significantly strengthened the odor aversion. There was no evidence that taste potentiated the odor aversion in either Experiment 1 or 2. Thus, the results demonstrate that the presence of within-compound associations is not sufficient to produce taste-mediated odor potentiation. We offer a mediated conditioning explanation to account for the results of these two experiments.

Limitations on representation-mediated potentiation of flavour or odour aversions

Odour aversion learning is often potentiated in the presence of flavour stimuli. Establishment of an aversion to an odour is greater when an odour + flavour compound is paired with illness than when the odour alone is paired with illness. Holland (1983) showed that under some circumstances auditory or olfactory stimuli previously paired with flavours may also potentiate odour aversion learning. The present experiments examined limitations on this representation-mediated potentiation of aversion learning. The results indicated that conditioned stimuli (CSs) that activate representations of potentiating cues are themselves immune to potentiation by other CS-activated representations, but remain susceptible to potentiation by their real stimulus associates.

Taste + odor interactions in compound aversion conditioning

In three experiments with rats, taste + odor interactions in compound aversion conditioning were investigated. In Experiment 1, two odors (0.02% almond and 0.02% orange) were compared on single-element odor aversions, taste (denatonium) potentiated odor aversions, and potentiated odor aversions following taste extinction. Although no odor differences were seen following single-element conditioning, both types of potentiated orange odor aversions were stronger than their almond odor counterparts. These data show that odors of similar conditionability are differentially potentiated by the same taste. To determine whether these differences were due to unique perceptual representations, the effects of elemental extinction or compound extinction on aversions to the compound were investigated in Experiments 2 and 3. In Experiment 2, orange odor extinction weakened responding to the compound significantly more than taste extinction did. In contrast, almond odor extinction and taste extinction produced similar decrements in responding to the compound in Experiment 3. These results suggest that the perceptual representation of these specific taste + odor compounds are different, and they are discussed in regard to configural and within-compound association accounts of potentiation.

Sequential learning of pheromonal cues modulates memory consolidation in trainer-specific associative courtship conditioning

BACKGROUND

Associative memory formation requires that animals choose predictors for experiences they need to remember. When an artificial odor is paired with an aversive experience, that odor becomes the predictor. In more natural settings, however, animals can have multiple salient experiences that need to be remembered and prioritized. The mechanisms by which animals deal with multiple experiences are incompletely understood.

RESULTS

Here we show that Drosophila males can be trained to discriminate between different types of female pheromones; they suppress courtship specifically to the type of female that was associated with unsuccessful courtship. Such "trainer-specific" learning is mediated by hydrocarbon olfactory cues and modifies the male's processing of those cues. Animals that are unable to use olfactory cues can still learn by using other sensory modalities, but memory in this case is not specific to the trainer female's maturation state. Concurrent and serial presentation of different pheromones demonstrates that the ability to consolidate memory of pheromonal cues can be modified by the temporal order in which they appear.

CONCLUSION

Suppression of memory by new learning demonstrates that the dynamics of memory consolidation are subject to plasticity in Drosophila. This type of metaplasticity is essential for navigation of experience-rich natural environments.

Prenatal experience and postnatal perceptual preferences: evidence for attentional-bias in bobwhite quail embryos (Colinus virginianus)

Previous studies have indicated that concurrent multimodal sensory stimulation can interfere with prenatal perceptual learning. This study further examined this issue by exposing 3 groups of bobwhite quail embryos (Colinus virginianus) to (a) no supplemental stimulation, (b) a bobwhite maternal call, or (c) a maternal call paired with a pulsating light in the period prior to hatching. Experiments differed in terms of the types of stimuli presented during postnatal preference tests. Embryos receiving no supplemental stimulation showed no preference between stimulus events in all testing conditions. Embryos receiving exposure to the unimodal maternal call preferred the familiar call over an unfamiliar call regardless of the presence or absence of pulsating light during testing. Embryos exposed to the call-light compound preferred the familiar call only when it was paired with the light during testing. These results suggest that concurrent multimodal stimulation does not interfere with prenatal perceptual learning by overwhelming the young organism's limited attentional capacities. Rather, multimodal stimulation biases what information is attended to during exposure and subsequent testing.

Behavioral detection of low concentrations of ethanol in milk in the preweanling rat

Previous animal models testing infantile reactivity to ethanol (EtOH) in maternal milk used EtOH doses that vastly exceeded levels actually encountered in a mildly or moderately intoxicated dam. The present study assessed whether 12- and 16-day-old rats are capable of detecting EtOH in milk at levels actually recorded in an intoxicated dam. Experiment 1 determined representative levels of EtOH in maternal milk as a function of maternal intragastric administration of EtOH (0.5-3.0 g/kg). Experiment 2A assessed generalization of conditioned taste aversions accrued with a high level of EtOH (6%) in either water or milk vehicles towards lower, more representative EtOH levels obtained from Experiment 1. With body weight gain as the dependent measure, conditioned aversions to milk were evident with the milk vehicle, but there was no detection of EtOH at any level at either age. Detection of the high level of EtOH (6%) in milk, however, was observed by 16 day olds within an habituation paradigm (Exp. 2b) via cardiac and behavioral (locomotion, mouthing) indexes. In Experiment 3 application of Experiment 2's more sensitive, behavioral index to assess generalization of the conditioned taste aversions revealed detection of a lower, more representative concentration of EtOH (175 mg%) in milk in 16-day-old rats. Overall the results show that the unweaned rat is capable of detecting very low concentrations of EtOH in milk and can modify their behavior accordingly. The expression of this capability is not, however, homogeneous across different response indexes. In conjunction with prior research it is clear that the infant rat's perception of EtOH in milk, including the very low levels of EtOH found in maternal milk during mild or moderate intoxication, is a relevant experience for generating new responses towards EtOH.

Aversions to alcohol's orosensory cues in infant rats: generalization to compounds of alcohol with sucrose or sodium chloride

Prior research has demonstrated that rat pups perceive alcohol's orosensory consequences during an acute state of intoxication with the drug and are able to associate these orosensory stimuli with aversive reinforcement. The present two experiments tested whether the resulting conditioned aversion to ethanol orosensory consequences generalized to two basic tastants (sucrose or sodium chloride) and if ethanol's orosensory consequences were detected when this agent was configured with these tastants. Conditioned aversions to alcohol were expressed only in rejection of an intraoral infusion of an ethanol solution alone or ethanol in compound with sucrose (experiment 1). A sucrose aversion was recorded in pups that had been subjected to infusions of a sucrose-ethanol compound paired with aversive reinforcement. An aversion to sodium chloride was not induced, however, by analogous procedures (experiment 2). The results indicate that, as in adults, ethanol aversions do not generalize directly to sucrose alone or sodium chloride alone. The infant is, however, capable of detecting the drug in compound with sucrose, and an acquired aversion to ethanol can be transferred to sucrose through ingestion of a sucrose-alcohol compound.

Olfactory and gustatory stimuli in food-aversion learning of rats

Two studies of food-aversion learning in the rat examined some crucial conditions for the potentiation of an odor component by a novel taste. Wistar rats were exposed twice to an odor-taste compound and then were treated either immediately or the next day with lithium chloride, a noxious substance. Immediately poisoned rats acquired an aversion to the odor component when it was paired with a familiar taste. Only in one experimental condition did some weak conditioning occur: when the new odor was presented together with a new taste. Thus, the classical phenomenon of overshadowing of the less salient odor component by the more salient taste was observed in all of our studies. This was true for situations in which the odor was presented near the drinking spout as well as for situations in which the odor was presented at some distance in front of the drinking source (Experiment 1). In addition, the weaker an odor component was, the more easily it was overshadowed by the taste (Experiment 2). All results fit well into classical rules of Pavlovian conditioning and fail to support the assumption that special mechanisms exist in food-related odor conditioning in rats.

Weanling and senescent rats process simultaneously presented odor and taste differently than young adults

Weanling, young-adult, and senescent Wistar albino rats had a novel odor/taste stimulus or a single taste stimulus either paired or explicitly unpaired with the unconditioned stimulus, lithium chloride. Animals were then given a saccharin vs water preference test. Standard preference scores indicated that the odor competed with taste for association with the US in young-adult rats but potentiated the conditioned aversion to taste in weanling and senescent rats. Results were interpreted in terms of age-related attentional differences which were hypothesized to account for infantile amnesia and for the memory dysfunction typically observed in senescent animals.

Alcohol drinking patterns of young adult rats as a function of infantile aversive experiences with alcohol odor

Recent studies have demonstrated that in the rat, alcohol intake patterns can be regulated by prior experiences with the odor of this drug. The efficacy of such regulation appeared to be limited to early postnatal stages of development. The present study supports the possibility, however, that early ethanol odor experiences are retained over considerable amounts of time and may play an effective role in the control of adult patterns of alcohol ingestion. Specifically, it was observed that rats exposed to pairings of ethanol odor and lithium chloride toxicosis (Etoh-LiCl group) during Postnatal Days 5, 10, 15, and 20 displayed as young adults (52-58 days) significant decreases in voluntary alcohol intake scores. This decrease was determined relative to control rats that had been safely exposed to ethanol odor (Etoh group) or given only lithium toxicosis but no ethanol odor (LiCl group) during Postnatal Days 5, 10, 15, and 20. Relative to these controls (Etoh and LiCl groups) the experimental subjects (Etoh-LiCl group) also exhibited significant decreases in their adult preference for ethanol odor, as assessed through an olfactory locational test. The present results indicate several ways in which conditioned aversion to ethanol intake may arise and imply that the transfer between olfactory and gustatory aversions takes place at the time of memory storage rather than at some later stage of memory processing. In a more general sense, the present results add to others in our series to support the notion that consideration of the effects of early experience with alcohol may aid in the clarification and control of voluntary alcohol intake patterns.

Weanlings' transfer of conditioned ethanol aversion from olfaction to ingestion depends on the unconditioned stimulus

Weanling (21-day-old) rats were exposed to an alcohol odor paired with either an interoceptive (apomorphine-induced illness) or exteroceptive (footshock-induced distress) reinforcer. Twenty-four hours later, ethanol preferences were measured in a locational olfactory test (ethanol vs lemon odor) or an ingestion test (5.6% v/v ethanol vs 0.25% v/v citric acid solution). Weanling rats expressed substantial olfactory aversions, independent of the reinforcer employed in conditioning. During the drinking test, however, only rats that had experienced the ethanol odor paired with internal malaise showed a significant reduction in the intake of the ethanol solution when compared to unpaired controls. Furthermore, rats that had experienced the ethanol odor paired with external distress drank significantly more of the ethanol solution than their controls. These results provide further evidence that olfactory experiences with ethanol can lead to changes in ethanol ingestion, and indicate that the nature of the unconditioned stimulus is critical in establishing the ingestive effect.