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.     

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.     

Development of place navigation in rats from weaning to puberty

Young hooded rats were trained to escape onto a hidden platform after swimming in a pool of opaque water. Subjects 21, 28, 35, 42, and 64 days of age on the first training day were given 28 trials on 5 consecutive days. Half of the rats were required to localize the platform in relation to external room cues only ("place only" condition) and the other half were helped by the presence of a visible cue on the platform ("cue + place" condition). A deficiency in place navigation was observed in the 21- and 28-day groups; they showed slow escape and took circuitous routes more often than older rats. This deficiency was related to a poor spatial bias toward the training position when the subjects were allowed to swim for 30 s in the absence of the platform, at the end of the 28-trial training period (probe trial). The 35-day group showed adult-like learning ability in both training conditions, but failed to show searching behavior during the probe trial after having been trained in the presence of the proximal cue. Only rats older than 40 days showed typical adult behavior such as swimming directly toward the platform from any starting position and localized searching around the absent platform's position during the probe trial, no matter what the training conditions were. These results suggest that central nervous system structures responsible for place learning in the rat are functional from around 32 days of age, but fail to trigger searching behavior following cued training before the sixth week.     

Evidence of blocking with geometric cues in a virtual watermaze

Three computer based experiments, testing human participants in a non-immersive virtual watermaze task, used a blocking design to assess whether two sets of geometric cues would compete in a manner described by associative models of learning. In stage 1, participants were required to discriminate between visually distinct platforms. In stage 2, additional spatial information was provided by the shape or the color of the walls of the pool. In a test trial, the platforms were removed and the spatial knowledge acquired regarding the position of the platform was assessed. Experimental groups were compared against control groups which did not receive stage 1 training. The unique color of the correct platform, in Experiments 1 and 3, disrupted learning about the colored walls but not the geometry of the pool. In Experiment 2, the correct platform was identifiable from its position within the three platform array. Learning the relative position of the correct platform within the array disrupted learning about its position relative to the geometry of the pool, but not to the colored walls. The results suggest that learning the position of a goal in relation to the geometry of the environment can be blocked but only by an alternative geometric cue.     

Latent spatial learning in an environment with a distinctive shape

Four experiments were conducted with rats in order to determine whether being placed on a platform in one corner of a rectangular swimming pool results in latent spatial learning. Rats in Experiments 1-3 received four trials a day of being placed on the platform. During a subsequent test trial, in which they were released into the pool without the platform, the rats exhibited a preference for swimming in the correct corners of the pool (those with the same geometric properties as the corner containing the platform during training), than the two remaining, incorrect corners. This effect was seen when the interval between the final placement trial and the test trial was as much as 24 hr (Experiment 2) and after varying numbers of sessions of placement training (Experiment 3). Experiment 4 revealed that when the test took place in a kite-shaped arena, after placement training in a rectangle, a stronger preference was shown for the corner that was geometrically equivalent to the correct rather than the incorrect corners in the rectangle. The placement treatment is said to result in latent spatial learning based on the development of S-S associations.     

How do room and apparatus cues control navigation in the Morris water task? Evidence for distinct contributions to a movement vector

The present study compared the relative influence of location and direction on navigation in the Morris water task. Rats were trained with a fixed hidden or cued platform, and probe trials were conducted with the pool repositioned such that the absolute spatial location of the platform was centered in the opposite quadrant of the pool. Rather than swimming to the platform location, rats swam in the direction that was reinforced during training, resulting in navigation to the relative location of the platform in the pool and search at the appropriate distance from the pool wall. Pool relocation tests revealed disruptions in cued navigation if the cued platform remained at the absolute location, whereas no disruption was observed if the platform remained at the relative location (same direction). The results indicate that direction holds greater influence than does location and further demonstrate that this observation is not altered by the amount of training or time on the platform. The authors propose that navigation in the water task involves a movement vector in which the distal cues and apparatus provide direction and distance information, respectively.     

Pre-training to find a hidden platform in the Morris water maze can compensate for a deficit to find a cued platform in a rat model of Parkinson's disease

The bilateral intranigral infusion of 1 micromol 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in adult male Wistar rats caused a specific and partial loss of substantia nigra pars compacta (SNc) dopamine neurons, a partial depletion of striatal dopamine, and a deficit to learn the intra-maze cued version of the Morris water maze. Pre-training the SNc rats in the spatial version of the water maze or simply maintaining the animals on the water maze platform reversed this deficit. This improvement was even observed when the order of the extra-maze cues presented to the rats during pre-training of the spatial version was changed during training of the intra-maze cued version. However, this deficit was not reversed either by maintaining the animals on the platform if the spatial cues were surrounded and covered with a curtain or by swimming sessions in the maze without the escape platform and the curtain. These findings suggest that none of the following elements alone, learned during the spatial task pre-training, could help SNc rats learn the intra-maze cued task: improvement of swimming skills or knowledge of the existence of the escape platform; distance between the platform and the border of the pool; location of a particular extra-maze cue; relations among extra-maze cues. However, the simultaneous presence of the escape platform and extra-maze cues (irrespective of their relational configuration) during the pre-training sessions proved to be necessary for this improving effect to occur.     

Spatial memory in the rat on a hierarchical maze

Rats were trained and tested on a hierarchical radial maze, which consisted of eight primary alleys radiating from a central platform and three secondary alleys which branched off the end of each primary alley. In four experiments, rats in groups 1, 2, and 3 were tested on maze configurations consisting of one, two, or three secondary alleys, respectively, at the end of each primary alley. In Experiment 1, each group was trained to collect food pellets in the secondary alleys. By the end of training, rats in each group collected all pellets efficiently, with little repetition of entrances into either primary or secondary alleys. In Experiments 2 and 3, tests were carried out which required retention of entrances into secondary alleys, as well as primary alleys. Two trials were run in succession, with selected secondary alleys blocked on trial 1 but open and baited with food on trial 2. Animals in groups 2 and 3 showed very accurate retention of blocked secondary alleys, regardless of variation in pattern and number of alleys blocked. A fourth experiment controlled for the possible use of food cues in Experiments 2 and 3 by rebaiting all secondary alleys between trials 1 and 2. Subjects continued to choose previously blocked alleys on trial 2, thus demonstrating that choice of blocked alleys was based on memory and not on a tendency to approach visual or olfactory food cues. Several findings of these experiments suggest that memories for primary and secondary alley choices are encoded and stored within separate memory systems. Possible coding mechanisms for these systems are discussed with reference to a process of cognitive mapping.     

Latent learning in a swimming pool place task by rats: Evidence for the use of associative and not cognitive mapping processes

Using a latent learning paradigm, the experiments examine two hypotheses of how rats solve place navigation tasks. According to associative theory, experience with all relevant cues is required for accurate navigation. According to cognitive mapping theory, animals can generate novel trajectories from knowledge of spatial relations of objects in the environment. Rats that had been placed on a platform, which was submerged in a pool of opaque water and was moved each day, were tested later for their ability to find the platform using only surrounding room cues. One 30-sec exposure to a platform location was effective in improving performance. Improvement was greatest when tests were given within minutes of placement, but facilitation was obtained for as long as 4 hr. Improved performance was obtained as soon as rats acquired the procedural aspects of the task but did not increase with subsequent practice. Improved performance was also obtained when pre-trained rats were tested in a novel environment. Despite the advantage conferred by exposure to the target platform, test swims were not accurate, placement-induced improvement was not as great as that following a single swimming trial, and placement combined with a swim resulted in best performance. The results suggest that rats use associative learning processes rather than cognitive mapping to solve place problems in a swimming pool.     

Single landmark learning in rats: Sex differences in a navigation task

In Experiments 1 and 2, rats were trained in a Morris pool to find a hidden platform located some distance away from a single landmark. Males learned to swim to the platform faster than females, but on test trials without the platform, males, unlike females, spent less time in the platform quadrant of the pool in the second half of each test trial than in the first. They also showed greater persistence in searching in the platform quadrant over a series of extinction trials. In Experiments 3a and 3b, the problem was made easier by locating the platform closer to the solitary landmark. Now males and females learned to swim to the platform equally rapidly, and both stopped searching in the platform quadrant in the second half of each test trial. Experiment 4 ruled out the possibility that males´ shorter latencies to find the platform in Experiment 2 were due to their swimming faster than females.     

Selective attention in human associative learning and recognition memory

Four experiments examined the role of selective attention in a new causal judgment task that allowed measurement of both causal strength and cue recognition. In Experiments 1 and 2, blocking was observed; pretraining with 1 cue (A) resulted in reduced learning about a 2nd cue (B) when those 2 cues were trained in compound (AB+). Participants also demonstrated decreased recognition performance for the causally redundant Cue B, suggesting that less attention had been paid to it in training. This is consistent with the idea that attention is preferentially allocated toward the more predictive Cue A, and away from the less predictive Cue B (e.g., N. J. Mackintosh, 1975). Contrary to this hypothesis, in Experiments 3 and 4, participants demonstrated poorer recognition for the most predictive cues, relative to control cues. A new model, which is based on N. J. Mackintosh's (1975) model, is proposed to account for the observed relationship between the extent to which each cue is attended to, learned about, and later recognized     

Re-examination of the role of within-compound associations in the retrospective revaluation of causal judgements

We investigated blocking and retrospective revaluation of causal judgements using a scenario in which food cues acted as potential causes of an allergic reaction as the outcome. In the blocking contingency,the treatment cues were either paired or unpaired with the outcome prior to a second stage in which sequential compounds of treatment and target cues were paired with the outcome. The order of this compound and treatment training was reversed in retrospective revaluation contingencies. When the interstimulus interval between the treatment and target cues was unfilled on compound trials (Experiments 1 and 3), both blocking and retrospective revaluation were observed in that the target cue trained in compound with the paired treatment cue attracted lower causal ratings than the target cue trained in compound with the unpaired treatment cue. By contrast, performing a mental arithmetic task using numerals presented during the interstimulus interval had no effect on the magnitude of blocking but rendered retrospective revaluation unreliable (Experiments 2 and 3). These results provide further support for accounts of revaluation based upon within-compound associations.     

Effect of absolute spatial proximity between a landmark and a goal

In two experiments rats were trained in a Morris pool to find a hidden platform in the presence of a single landmark. Circular black curtains surrounded the pool, with the single landmark inside this enclosure, so that no other room cues could provide additional information about the location of the platform. This landmark was hung from a false ceiling and rotated from trial to trial, and the position of the platform also changed on each trial, thus preserving a constant relation between the platform and the landmark. In Experiment 1, the position of the landmark was exactly above the hidden platform in Group Above and was relatively close to the hidden platform in Group Near. At the end of acquisition, test trials without the platform revealed a difference between the groups. Although a preference for searching in the correct quadrant of the pool, where the platform should have been, was found in both groups, this preference was significantly higher for Group Above. In Experiment 2, new rats in Group Near were compared to rats for which the position of the landmark was relatively far from the hidden platform in Group Far. Test trials revealed a preference for searching in the correct quadrant of the pool in both groups, but this preference was significantly higher for Group Near. The implication of these results is that the control acquired by a single landmark is different depending on its relative distance from a hidden platform: Closer landmarks acquire better control than distant ones. These results show a clear parallelism in comparison with the effect of absolute temporal proximity of the CS to the US in classical conditioning.     

Spatial separation of target and competitor cues enhances blocking of human causality judgements

Three experiments were carried out. Each required subjects to make judgements about the causal status of cues following a two-stage blocking procedure. In Stage 1 a competitor cue was consistently paired with an outcome, and in Stage 2 the competitor continued to be paired with the outcome but was accompanied by a target cue. It was predicted that causal judgements for the target would be reduced by the presence of the competitor. In Experiments 1 and 2 the blocking procedure was implemented as a computer simulation of a card game during which subjects had to learn which cards produced the best payouts. The cues that subjects used to make their judgement were colours and symbols that appeared on the backs of the cards. When the target and competitor cues appeared on the same card blocking effects did not emerge, but when they appeared as part of different cards blocking effects were found. Thus, spatial separation of target and competitor cues appeared to facilitate blocking. Experiment 3 replicated the blocking result using spatially separated target and competitor cues.     

An automated swim alley for small animals: II. Training and procedures

Experiments were conducted to establish training procedures and testing conditions that produce uniform and reliable swimming performance of rats in an automated swim alley. Data are given on the effect of water temperature and of platform rest interval on swimming speeds. In this system, rats will swim intermittently for as long as 24 h for a distance of more than 7 km.     

A room with a view and a polarizing cue: individual differences in the stimulus control of place navigation and passive latent learning in the water maze

We investigated individual differences in the stimulus control of navigational behavior in the water maze by comparing measures of place learning in one environment to measures of latent learning (via passive placement on the goal platform) in a novel environment. In the first experiment, 12 rats were trained to find a slightly submerged hidden platform at a fixed location in room A for 10 days (4 trials/day). Fast and slow place learners were identified by their mean escape latency and cumulative distance to the goal during acquisition. The same animals were then given a 2-min passive placement on the submerged platform in room B. Latent learning was assessed by the animal's escape latency on a single swim trial immediately following the placement in room B. The results showed that the good latent learners in room B were not necessarily the fast place learners in room A. This weak correlation may be related to the fact that some rats swam near the area in room B that corresponded to the former goal location in room A relative to a common polarizing cue (i.e., the door/entrance to both rooms). When the view of the door was blocked in a second experiment a significant positive correlation between place acquisition and the latent learning test was obtained, although escape performance following passive placement was not improved. These findings suggest that while place navigation and latent learning via passive placement may involve some common cognitive-spatial function, other associative (S-S and/or S-R) processes that occur during place navigation/active movement may be required for animals to exhibit truly accurate navigational behavior characteristic of asymptotic escape performance in the water maze. Additional implications for neurobiological studies using a procedural pretraining design are discussed.     

Changes in attention to relevant and irrelevant stimuli during spatial learning

Rats were trained in 2 experiments to find a submerged platform that was situated in 1 of 2 of the 4 corners of a rectangular pool with a curved long wall. Different landmarks occupied 2 of the corners on every trial, and the platform was always situated near a landmark. For the place group in each experiment, the location of the platform was indicated by the shape of the pool and stimuli outside the pool (place cues), but not the landmarks within the pool. For the landmark groups, the landmarks, not the place cues, indicated where the platform could be found. During Stage 2, 2 of the place cues were relevant, and 2 of the landmarks were irrelevant, for a new discrimination. The place cues better controlled searching for the platform in the place group than in the landmark group when the place cues had initially been relevant by signaling the presence (Experiment 1) or the absence (Experiment 2) of the platform. The results show that animals pay more attention to relevant than irrelevant cues.     

A single unstable visual cue impairs spatial learning in a water maze

The role of a prominent visual cue (a light) in spatial learning was examined. Rats were trained to locate a hidden platform in a 4-arm radial water maze. For some rats the light predicted platform location. For other rats the light moved from trial to trial and did not predict platform location. While the predictive light did not facilitate performance above that seen in rats trained without a light, the moving light significantly impaired performance. In a second experiment, the temporal parameters of the moving light were manipulated. For one group the light was visible for the entire trial. In a second group the light was visible only before the rats reached the platform and in a third group the light was visible only after the rats reached the platform. The moving light impaired performance only when it was presented for the entire trial. When rats were subsequently retrained with a new platform location and a moving light in the familiar room, the light did not impair performance in any of the groups. We suggest that the prominent visual cue served as a directional cue, that competed with other directional cues, in the unfamiliar environment and interfered with performance.     

Effect of swimming session duration and repetition on metabolic markers in rats

The aim of this study was to investigate the profile of metabolites in male rats subjected to 50-60 min of swimming on three protocols: group A, a single 50 min swimming session; group B, one session a day for three days (5 min on day 1, 15 min on day 2 and 30 min on day 3); and group C, one session a day for 5 days, with increasing duration from 5 min on day 1, 15, 30, 45 and 60 min on consecutive days. The interval between sessions was 24 h. Measurements were made after the last swimming session. Controls did not swim. The glycogen content of liver and gastrocnemius and soleus muscle was depleted in the three groups that swam, but blood glucose concentration was significantly increased only in group B. Serum lactate concentrations were greater than the controls in groups A and B. There were significant increases in serum free fatty acid concentrations in all groups that swam. The increases in plasma free fatty acids may have resulted from lipolysis stimulated by endogenous catecholamines in groups A and C, since basal lipolysis measured in vitro was unchanged by swimming. The large increase in basal lipolysis in group B may have contributed to the rise in plasma free fatty acids. Adipocytes from rats in groups A and B were supersensitive to epinephrine, whereas those from group C were not. We conclude that the metabolic alterations were less pronounced after the last of five swimming sessions over 5 days than after a single session, even though session duration and the contribution of the physical component were similar. Glucose mobilization, but probably not utilization, was similar in the three groups that swam. The mechanisms of lipid mobilization from adipose tissue differed, depending on the stress paradigm. The metabolic changes in groups A and B indicated that three daily swimming sessions were insufficient to cause adaptation. The results contrast with previous findings for foot-shock stress, which leads to sensitization rather than adaptation in response to repeated stimuli.