The development of spatial location coding: Place learning and dead reckoning in the second and third years

There are two possible ways to code spatial location: viewer-referenced and externally referenced systems. The mature form of each system makes use of metric information (in forms of coding termed dead reckoning and place learning, respectively). Each system also exists in a simpler form not using metric information (termed response learning and cue learning, respectively). We report an experiment designed to examine the development of the two mature forms of spatial coding. Children aged 16 to 36 months were asked to search for objects hidden in a long rectangular sandbox, after they moved to the opposite side of the box, either with visual landmarks available or in a curtained environment. When moving in a curtained environment, and hence needing to rely primarily on dead reckoning, children across this age range performed at levels above chance but reliably less accurate than when they did not move. The size of the decrement due to movement did not decrease with age. Thus, the dead reckoning system may become available before 16 months but undergo no further improvement through 3 years. On the other hand, comparisons between children who moved in the curtained environment and children who moved with visual landmarks available showed that only children older than 21 months did better when external landmarks could be seen. This finding suggests a relatively late emergence of place learning.     

Facilitation of acquisition and retention in preweanling but not postweanling rats by the presence of familiar home-nest material

A series of four experiments examined the effect of the presence of stimuli from the home nest on the acquisition and retention of aversively motivated behaviors in preweanling and adult rats. In Experiment 1, training in the presence of home-nest shavings facilitated acquisition of a T-maze discrimination to escape footshock for 16-day-old rats but not for adults. Experiment 2 demonstrated that the extent to which preweanlings were familiar with the home shavings determines the degree to which these stimuli facilitate spatial discrimination learning. When clean shavings were made more familiar than soiled home-nest stimuli (by changing the shavings every day) clean shavings enhanced discrimination performance, whereas no enhancement of learning by home shavings was observed. Experiment 3 extended the generality of the enhancement effect to a conditioned location aversion and examined the extent to which this facilitative effect was due to the tendency for home-nest shavings to elicit approach responses. Expression of the conditioned aversion was enhanced in subjects conditioned in the presence of home shavings, regardless of whether the home shavings were presented with the CS+, the CS-, or both. Experiment 4 determined that the enhanced expression of learning in the context of home-nest stimuli observed for preweanlings did not occur among subjects trained shortly after weaning. Collectively, these data suggest that whereas the enhancement of learning and retention by familiar home-nest stimuli enjoys generality across a number of conditioning situations, the effect may be limited to a relatively brief period during ontogeny.     

Influence of a beacon on spatial learning based on the shape of the test environment.

In 5 experiments rats were required to escape from a triangular shaped pool by swimming to a submerged platform. The principal group of interest in each experiment received training with a beacon attached to the platform. The purpose of the experiments was to assess if the beacon overshadowed (Experiments 1-4) or blocked (Experiment 5) learning about the position of the platform with reference to the shape of the pool. The platform was located in the center of the pool for the first 2 experiments and in a corner for the remaining experiments. Although there was an overshadowing effect in Experiment 1, the remaining experiments failed to reveal any disruptive influence of the beacon on learning based on the shape of the pool. Moreover, in Experiments 3-5 there was an indication that the beacon facilitated such learning. The results suggest that spatial learning based on the shape of a test environment may not take place in the same way as that based on more discrete landmarks.     

The effects of pre-exposure on escape from a Morris pool

In two experiments rats were pre-exposed to the landmarks surrounding a Morris pool while they swam to a platform with a beacon attached to it. They were then required to escape from the pool by finding the platform, without the beacon, in a new position. When the platform remained in the same place for each pre-exposure session, but was moved from session to session, then subsequent escape from the pool was more rapid than when the landmarks were not visible during pre-exposure (Experiment 1). But when the platform was moved from trial to trial during pre-exposure, then subsequent escape from the pool was disrupted (Experiments 1 and 2). It is proposed that pre-exposure to the landmark alters the attention that is paid to them, which then influences how readily the landmarks can be used to identify the new position of the platform.     

Spatial memory of domestic dogs (Canis familiaris) for hidden objects in a detour task

Spatial memory of domestic dogs (Canis familiaris) for hidden objects was investigated via a visible displacement problem of object permanence with a detour paradigm. Experiment 1 showed that dogs were able to spontaneously locate a disappearing object in a detour situation. In Experiments 2 and 3, dead reckoning and allocentric spatial information were put in conflict. Results revealed that dogs simultaneously encoded both sources of information when they had to bypass an obstacle to locate a hidden object. Experiment 3 also revealed that, over the course of testing, dogs gradually learned to rely predominantly on allocentric cues when the detour involved several reorientations. The current study reveals that spatial memory of dogs for hidden objects in a detour task was guided by flexibility in processing spatial information. Dogs could simultaneously encode dead reckoning and allocentric information to locate a disappearing object and used them hierarchically according to the complexity of the detour they encountered in the environment.     

Relationship between vomiting and taste aversion learning in the ferret: studies with ionizing radiation, lithium chloride, and amphetamine.

The relationship between emesis and taste aversion learning was studied in ferrets (Mustela putorius furo) following exposure to ionizing radiation (50-200 cGy) or injection of lithium chloride (1.5-3.0 mEq/kg, ip). When 10% sucrose or 0.1% saccharin was used as the conditioned stimulus, neither unconditioned stimulus produced a taste aversion, even when vomiting was produced by the stimulus (Experiments 1 and 2). When a canned cat food was used as the conditioned stimulus, lithium chloride, but not ionizing radiation, produced a taste aversion (Experiment 3). Lithium chloride was effective in producing a conditioned taste aversion when administration of the toxin was delayed by up to 90 min following the ingestion of the canned cat food, indicating that the ferrets are capable of showing long-delay learning (Experiment 4). Experiment 5 examined the capacity of amphetamine, which is a qualitatively different stimulus than lithium chloride or ionizing radiation, to produce taste aversion learning in rats and cats as well as in ferrets. Injection of amphetamine (3 mg/kg, ip) produced a taste aversion in rats and cats but not in ferrets which required a higher dose (> 5 mg/kg). The results of these experiments are interpreted as indicating that, at least for the ferret, there is no necessary relationship between toxin-induced illness and the acquisition of a CTA and that gastrointestinal distress is not a sufficient condition for CTA learning.     

Intertrial interval as a contextual stimulus: further analysis of a novel asymmetry in temporal discrimination learning

Four experiments investigated discrimination learning when the duration of the intertrial interval (ITI) signaled whether or not the next conditional stimulus (CS) would be paired with food pellets. Rats received presentations of a 10-s CS separated half the time by long ITIs and half the time by short ITIs. When the long ITI signaled that the CS would be reinforced and the short interval signaled that it would not be (Long+/Short-), rats learned the discrimination readily. However, when the short ITI signaled that the CS would be reinforced and the long interval signaled that it would not (Short+/Long-), discrimination learning was much slower. Experiment 1 compared Long+/Short- and Short+/Long- discrimination learning with 16-min/4-min or 4-min/1-min ITI combinations. Experiment 2 found no evidence that Short+/Long- learning is inferior because the temporal cue corresponding to the short interval is ambiguous. Experiment 3 found no evidence that Short+/Long- learning is poor because the end of a long ITI signals a substantial reduction in delay to the next reinforcer. Long+/Short- learning may be faster than Short+/Long-because elapsing time involves exposure to a sequence of hypothetical stimulus elements (e.g., A then B), and feature-positive discriminations (AB+/A-) are learned quicker than feature-negative discriminations (A+/AB-). Consistent with this view, Experiment 4 found a robust feature-positive effect when sequentially presented CSs played the role of elements A and B.     

Golden hamsters on the eight-arm maze in light and darkness: The role of dead reckoning

This paper examines whether golden hamsters can rely on dead reckoning (getting positional information from updated signals generated during locomotion) on an eight-arm maze. Two groups of hamsters were tested: Group L under ordinary room light, Group D in darkness. To enhance the role of dead reckoning, each subject could climb from its own home cage onto the central platform of the maze. In a first experimental phase (15 trials), the L subjects learned to master the maze through developing a locomotor rule (arm chaining) after three to four trials. The D subjects developed arm chaining less readily and fluctuated more in their performance than did the L subjects. In a second experimental phase (15 trials), four arms were blocked at the beginning of each trial. In both experimental groups the performance decreased, yet remained well above chance level. Success and arm chaining were positively correlated in Phase 1 and negatively correlated with success in Phase 2. We assume that in Phase 2 the L subjects switched to the predominant use of visual cues, and the D subjects to dead reckoning.     

Failure to replicate visual discrimination learning with a 1-min delay of reward

Two experiments were carried out in an attempt to replicate Lett's (1974) demonstration of visual discrimination learning with a 1-min delay of reward. In Experiment 1, controls for olfactory cues were introduced by testing animals in different orders from day to day and by reversing the positions of the discrimination chambers within days; also, secondary reinforcement effects were precluded by the use of opaque rather than transparent doors to the discrimination chambers. No evidence of learning was found either in a group of rats returned to the home cage during the delay or in a group left in the apparatus during the delay. It was hypothesized that the failure to find improvement in Experiment 1 might have resulted from either the controls for odor cues or from a lack of memory retrieval cues provided by the apparatus. In Experiment 2, both of these hypotheses were tested and neither was supported. The Lett group in Experiment 2 was run according to the exact procedures used by Lett and still no learning was demonstrated. It was concluded that further demonstrations of visual discrimination learning with a 1-min delay of reward will be necessary for the acceptance of the validity of this phenomenon.     

Emotional mood and memory in young children

Two experiments examined affect-dependent memory with preschool/kindergarten and third-grade children. A two-list intentional learning procedure was used to assess the effects of the congruent versus incongruent relationship between affect (happy vs sad) during initial list learning and affect (happy vs sad) during a delayed recall test. When induction of emotional mood was preceded by relaxation exercises in Experiment 1, no evidence of affect dependence was observed. When the relaxation procedure was omitted in Experiment 2, the affect-dependent pattern was obtained in both free recall and cued recall for both age groups. The results of Experiment 2 show that affect-dependent memory is a reasonably robust phenomenon in children and that hypnosis is not necessary for its appearance. However, the phenomenon is apparently absent under conditions of relaxation, a result consistent with two-factor theories of emotion.     

Differential dependence of Pavlovian incentive motivation and instrumental incentive learning processes on dopamine signaling

Here we attempted to clarify the role of dopamine signaling in reward seeking. In Experiment 1, we assessed the effects of the dopamine D(1)/D(2) receptor antagonist flupenthixol (0.5 mg/kg i.p.) on Pavlovian incentive motivation and found that flupenthixol blocked the ability of a conditioned stimulus to enhance both goal approach and instrumental performance (Pavlovian-to-instrumental transfer). In Experiment 2 we assessed the effects of flupenthixol on reward palatability during post-training noncontingent re-exposure to the sucrose reward in either a control 3-h or novel 23-h food-deprived state. Flupenthixol, although effective in blocking the Pavlovian goal approach, was without effect on palatability or the increase in reward palatability induced by the upshift in motivational state. This noncontingent re-exposure provided an opportunity for instrumental incentive learning, the process by which rats encode the value of a reward for use in updating reward-seeking actions. Flupenthixol administered prior to the instrumental incentive learning opportunity did not affect the increase in subsequent off-drug reward-seeking actions induced by that experience. These data suggest that although dopamine signaling is necessary for Pavlovian incentive motivation, it is not necessary for changes in reward experience, or for the instrumental incentive learning process that translates this experience into the incentive value used to drive reward-seeking actions, and provide further evidence that Pavlovian and instrumental incentive learning processes are dissociable.     

Opioid regulation of spinal cord plasticity: evidence the kappa-2 opioid receptor agonist GR89696 inhibits learning within the rat spinal cord

Spinal cord neurons can support a simple form of instrumental learning. In this paradigm, rats completely transected at the second thoracic vertebra learn to minimize shock exposure by maintaining a hindlimb in a flexed position. Prior exposure to uncontrollable shock (shock independent of leg position) disrupts this learning. This learning deficit lasts for at least 24h and depends on the NMDA receptor. Intrathecal application of an opioid antagonist blocks the expression, but not the induction, of the learning deficit. A comparison of selective opioid antagonists implicated the kappa-opioid receptor. The present experiments further explore how opioids affect spinal instrumental learning using selective opioid agonists. Male Sprague-Dawley rats were given an intrathecal injection (30 nmol) of a kappa-1 (U69593), a kappa-2 (GR89696), a mu (DAMGO), or a delta opioid receptor agonist (DPDPE) 10 min prior to instrumental testing. Only the kappa-2 opioid receptor agonist GR89696 inhibited acquisition (Experiment 1). GR89696 inhibited learning in a dose-dependent fashion (Experiment 2), but had no effect on instrumental performance in previously trained subjects (Experiment 3). Pretreatment with an opioid antagonist (naltrexone) blocked the GR89696-induced learning deficit (Experiment 4). Administration of GR89696 did not produce a lasting impairment (Experiment 5) and a moderate dose of GR89696 (6 nmol) reduced the adverse consequences of uncontrollable nociceptive stimulation (Experiment 6). The results suggest that a kappa-2 opioid agonist inhibits neural modifications within the spinal cord.     

The Role of Simple Semantics in the Process of Artificial Grammar Learning

This study investigated the effect of semantic information on artificial grammar learning (AGL). Recursive grammars of different complexity levels (regular language, mirror language, copy language) were investigated in a series of AGL experiments. In the with-semantics condition, participants acquired semantic information prior to the AGL experiment; in the without-semantics control condition, participants did not receive semantic information. It was hypothesized that semantics would generally facilitate grammar acquisition and that the learning benefit in the with-semantics conditions would increase with increasing grammar complexity. Experiment 1 showed learning effects for all grammars but no performance difference between conditions. Experiment 2 replicated the absence of a semantic benefit for all grammars even though semantic information was more prominent during grammar acquisition as compared to Experiment 1. Thus, we did not find evidence for the idea that semantics facilitates grammar acquisition, which seems to support the view of an independent syntactic processing component.     

Signalling and incentive processes in instrumental reinforcer devaluation

We have previously reported that conditioning an aversion to the reinforcer using an isotonic lithium chloride (LiCl) solution following instrumental training reduces performance in a subsequent extinction test only if animals are re-exposed to the reinforcer prior to the test. Rescorla (1992), in contrast, reported an immediate devaluation effect using a hypertonic LiCl solution that did not depend upon re-exposure. In two experiments we examined the effect of using a hypertonic LiCl solution to condition the aversion to the reinforcer on subsequent instrumental performance in extinction, with and without re-exposure. In Experiment 1 thirsty rats were trained to press a lever for a sucrose solution before being injected with 0.6 M LiCl either immediately or after a delay. Half of the immediate and delay groups were then re-exposed to the sucrose in the absence of the lever, with the remainder being exposed to water. Contrary to the previously reported effects of isotonic LiCl, a hypertonic solution induced a reinforcer devaluation effect in all the immediately poisoned animals, which did not depend upon re-exposure to the reinforcer. In Experiment 2 the possibility that this devaluation effect was induced by the discomfort associated with the hypertonicity of the solution was assessed by replicating Experiment 1 but, in addition, using two immediately poisoned groups given the LiCl injection under anaesthesia. In the absence of anaesthesia, the devaluation effect observed without re-exposure to the reinforcer in Experiment 1 was replicated. When the injection was given under anaesthesia, however, a reinforcer devaluation effect was observed only in animals that were re-exposed to the reinforcer prior to the extinction test. These results were interpreted as evidence that a reinforcer devaluation effect induced by pairing the reinforcer with illness depends upon a process of incentive learning, whereas a devaluation effect mediated by learning a signalling relationship between the reinforcer and somatic discomfort does not.     

Beacon training in a water maze can facilitate and compete with subsequent room cue learning in rats

In Stage 1 of 4 experiments in which rats completed a water-maze blocking procedure, experimental groups were trained to use a predictive beacon (hanging above, connected to, or displaced from the platform) to find a submerged escape platform in the presence of predictive or irrelevant background cues and in the presence or absence of irrelevant landmarks. In Stage 2, a fixed beacon, landmarks, and background cues all predicted the platform location. A Room Test (landmarks and background cues only) showed that Stage 1 training with a fixed hanging beacon or the moving displaced beacon facilitated Stage 2 learning of predictive room cues for experimental relative to control subjects. In contrast, Stage 1 training with a moving pole beacon interfered with Stage 2 learning about predictive room cues relative to controls, whereas training with a fixed pole or moving hanging beacon had no effect. We conclude that multiple spatial learning processes influence locating an escape platform in the water maze.     

Two tests of the stuck-in-time hypothesis

The authors report 2 experiments that test the stuck-in-time hypothesis, which argues that animals cannot time-date events and thus do not remember when events occurred and do not anticipate future events. In Experiment 1, rats in the experimental condition could earn a large reward by reentering the 1st arm that they visited on a radial maze. They did not learn to reenter this arm early and did no better than did a control group that was not given a large reward for reentering the first arm. In Experiment 2, rats in the experimental group could earn a large reward by delaying entry into a distinctive arm. These rats did not learn to delay entry into the distinctive arm, and they performed no better than did the control-group rats, which did not receive a large reward for delayed entry. These experiments provide further evidence in support of the stuck-in-time hypothesis.     

Preference for novel flavors in adult Norway rats (Rattus norvegicus)

The authors fed rats 1 of 2 distinctively flavored, roughly equipalatable diets for 3 days then offered them an ad libitum choice between the 2 diets. For 3 days, subjects exhibited a reduced relative intake of whichever diet they had previously eaten (Experiment 1). Such reduction in relative intake was as effective as a toxicosis-induced conditioned aversion in determining subjects' food choices (Experiment 2). The strength of exposure-induced reduction in relative intake did not depend on similarity of the 2 diets offered for choice either to each other or to subjects' maintenance diet (Experiment 3) but did require continuous exposure to a diet (Experiment 4). These experiments provide the first evidence of a robust, exposure-induced decrease infood preference in rats lasting for days rather than minutes.     

Resistance to satiation: Reinforcing effects of food and eating under satiation

It has been shown previously that rats which have learned a response when hungry will continue to make that response when tested satiated, a phenomenon labeled resistance to satiation. Here we showed that rats which were previously trained hungry will learn a new response for the opportunity to consume pellets in a new situation when tested satiated. In four experiments various groups received each of the components of the training given when rats learn an instrumental response when hungry. Rats were placed in the goalbox of a straight alley and given food pellets when hungry or were hungry only in their home cages prior to running a straight alley in the satiated test in Experiment 1. In Experiments 2, 3, and 4 learning of a differential conditioning problem for pellets in S+ (nonreward in S?) was measured in the satiated test. Groups given pellets in their home cages when hungry with or without alley exposure learned to run more rapidly in S+ than in S? in the satiated test phase. The tendency to eat pellets in the apparatus and the reinforcing effect of eating the pellets was larger for rats which ate the pellets when hungry in their home cage than for rats which ate the pellets when satiated in their home cage. Being hungry in the home cage with no pellets was not sufficient to produce eating or running for pellets in the satiated test, indicating that any inherent reinforcing effect of the pellets is not sufficient to produce eating or running, and that incomplete satiation cannot account for the learning. These data indicate that a reinforcing effect of eating pellets under satiation is an important determiner of resistance to satiation.     

Aggression as a reinforcer: operant behavior in the mouse-killing rat

Two experiments examined mouse killing as a reinforcer of key pressing by rats that killed mice. In Experiment I, mouse-killing rats performed the key-pressing response when each press was reinforced with presentation of a mouse. Offered a choice between a key that yielded presentation of mice and one that did not, the rats preferred the key that yielded mice. When the contingency was reversed, the rats preferred the other key and continued to kill mice. In Experiment II, mouse-killing rats that did not kill rat pups performed a key-pressing response reinforced with presentation of mice on a variable-interval schedule. In tests for responding reinforced on that schedule with presentation of normal mice, anesthetized mice, dead mice, or rat pups, these rats that killed mice but not rat pups exhibited a decline in response rate when rat pups were the reinforcer. Altering the condition of the mice did not significantly affect performance.     

Social influence in pigeons (Columba livia): the role of differential reinforcement

Socially-influenced learning was studied in observer pigeons that observed a demonstrator in an adjacent chamber performing a target response comprising standing on a box and pecking a key 10 times. In Experiment 1 there was no evidence for social learning in the absence of reinforcement of the observer's behavior. When the target response was already established in the observer's repertoire, but was not differentially reinforced in relation to the demonstrator's behavior, rates of extinction were not influenced by the demonstrator's behavior (Experiment 2). Reinforcement of the observer's target response in the presence of the modeled target response, and not in its absence, resulted in control of the observer's responding by the behavior of the demonstrator (Experiments 3 and 4). This control was extended in Experiment 5 to deferred responses that occurred following a delay since the demonstrator's target responses. The acquisition of social influence depended on differential reinforcement of the observer's target response, with the demonstrator's target behavior serving as the explicit discriminative stimulus.