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.     

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.     

Generalization decrement and not overshadowing by associative competition among pairs of landmarks in a navigation task

When they are trained in a Morris water maze to find a hidden platform, whose location is defined by a number of equally spaced visual landmarks round the circumference of the pool, rats are equally able to find the platform when tested with any two of the landmarks (Prados, & Trobalon, 1998; Rodrigo, Chamizo, McLaren, & Mackintosh, 1997). This suggests that none of the landmarks was completely overshadowed by any of the others. In Experiment 1 one pair of groups was trained with four equally salient visual landmarks spaced at equal intervals around the edge of the pool, while a second pair was trained with two landmarks only, either relatively close to or far from the hidden platform. After extensive training, both male and female rats showed a reciprocal overshadowing effect: on a test with two landmarks only (either close to or far from the platform), rats trained with four landmarks spent less time in the platform quadrant than those trained with only two. Experiment 2 showed that animals trained with two landmarks and then tested with four also performed worse on test than those trained and tested with two landmarks only. This suggests that generalization decrement, rather than associative competition, provides a sufficient explanation for the overshadowing observed in Experiment 1. Experiment 3 provided a within-experiment replication of the results of Experiments 1 and 2. Finally, Experiment 4 showed that rats trained with a configuration of two landmarks learn their identity.     

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.     

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.     

The influence of landmark salience in a navigation task: an additive effect between its components

In 2 experiments, rats were trained in a Morris pool to find a hidden platform in the presence of 1 landmark. After acquisition, the rats were tested without the platform. Experiment 1 tested whether the size of a landmark and its relative distance from the platform are additive effects. On test, the rats' best performance was with a near and big landmark; intermediate performance was with either a near and small landmark or a far and big one; and the worst performance was with a far and small landmark. The results of Experiment 2 suggested that the different distances from the goal of the 2 landmarks might not be sufficient to explain the previous results.     

Strategies in landmark use by children, adults, and marmoset monkeys

Common marmosets (Callithrix jacchus jacchus), human children, and human adults learned to find a goal that was located in the center of a square array of four identical landmarks. The location of the landmark array and corresponding goal varied across trials, so the task could not be solved without using the landmark array. In Experiment 1, a matrix of discrete goal locations was presented and the landmarks surrounded and were adjacent to the correct location during training. After training, an expansion test was given in which the distance between landmarks was increased. Marmosets, children (ages 5–9), and adults all readily learned to use the landmarks to search accurately during training. On the expansion test, adults uniformly searched in the center of the array. Monkeys and children concentrated their searching near the landmarks rather than in the center. The monkeys, but not the children, searched more often on the directionally appropriate side of the landmarks than on other sides of the landmarks. In Experiment 2, children (ages 3–5) were trained with a continuous search space and with the goal farther from the landmarks so that a beaconing strategy rule could not be used. Several of the children failed to acquire the training task. Of those who learned to find the goal, three searched in the middle on expansion tests but most searched nearer to the landmarks. The “middle rule” strategy that is uniformly used by adult humans does not appear to be a preferred strategy for children or non-human primates.     

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.     

Effects of Unilateral Entorhinal Cortex Lesion on Retention of Water Maze Performance

In a previous study, adult male Sprague-Dawley rats with unilateral, electrolytic entorhinal cortex lesions showed significant deficits in acquisition of a water maze task that measured working memory. The 10 days of testing used two trials per day with an intertrial interval of 1 h, and the rats with entorhinal damage were impaired in total distance to the platform in both trials. In the present retention study, rats who learned the same task prior to injury and were then retested for 5 days after lesion showed only a first day deficit in total distance to platform in the second trial. Analysis of swim patterns indicated that rats with unilateral entorhinal lesions used an altered strategy in retention testing to find the platform in the second trial of each day and incorporated the use of headings appropriate for Trial 1 only. This altered or compensatory strategy was not the optimum choice for problem solution. Although the rats then were able to switch headings and find the platform without significant impairment in total distance to platform on days 2–5 of testing, the use of an initial incorrect strategy indicated subtle residual deficits in cue integration and use of working memory.     

Integration and representation in rats' serial pattern learning in the T-maze

Rats were exposed to three-trial series consisting of reinforced (R) trials and one nonreinforced (N) trial in a fixed order, RRN and RNR (Experiments 1 and 2) or NRR and RRN (Experiment 3), on extended visually distinct runways in a T-maze. When initially presented with the same sequence on each series in a session (separate presentations) with the same runway on all trials within a series (Experiments 1 and 3), all the rats developed slower running speeds on N than on R trials. When a runway was sometimes changed between the first and next two trials during separate presentations training (Experiment 2) or both sequences were later intermixed within each session in each experiment, only rats exposed to each sequence on a specific runway maintained these serial running patterns. Rats displayed serial running patterns on a test RNN sequence similar to that on the RNR sequence (Experiment 2), as would be predicted by an intertrial association model of serial pattern learning (Capaldi & Molina, 1979), but responded on test RRR and NRN sequences (Experiment 3) as would be predicted by an ordinal-trial-tag/intratrial association model (Burns, Wiley, & Payne, 1986). Results from test series of free-choice trials in Experiments 1 and 2 failed to support a prediction of the intratrial association model that these rats would integrate RRN and RNR sequences. Rather than always selecting a baited runway on both the second and the third free-choice trials, the rats only selected a baited runway on the third trial on the basis of their choice on the second trial, as would be predicted by the intertrial association model. Only after experiencing all possible outcome sequences during forced-choice training in Experiment 3 did these rats predominantly select a baited runway on every free-choice trial.     

Failure of a landmark to restrict spatial learning based on the shape of the environment

Rats were required to find a submerged platform in the corner of a swimming pool with a distinctive shape. A landmark near the platform did not interfere with the control acquired by the pool's shape over searching for the platform. This outcome was observed with an overshadowing and a blocking design. A comparison of the ease with which the landmark and the pool's shape gained control over searching for the platform indicates that the failure of overshadowing and blocking was not a consequence of the landmark being less salient than the shape of the pool. The results are not readily explained by theories of associative learning, but they are consistent with the claim that learning about the shape of the environment takes place in a dedicated module, which excludes information about the significance of individual landmarks.     

Altered grooming responses to stress in rats exposed prenatally to ethanol

The effects of prenatal alcohol exposure on grooming, locomotion, and rearing in response to stress were examined in adult rats whose mothers consumed a liquid diet containing 35% ethanol-derived calories (EDC). Offspring of both pair-fed 0% EDC mothers and ad libitum chow-fed mothers were included as controls. In Experiment 1, females groomed more than males following placement into a novel test chamber, but no differences due to prenatal treatment were observed. Ethanol-exposed animals groomed more than controls following the stress of a forced 1-min swim (Experiment 2), but when rats tested in Experiment 1 were observed again after forced swim stress (Experiment 3), no differences due to prenatal treatment or sex were observed. Experiment 4 examined the effects of pretreatment with 1 mg/kg naloxone on novelty-induced grooming and as in Experiment 1 prenatal treatment did not affect grooming responses. Females again groomed significantly more than males and naloxone reduced grooming equally for all groups. The results suggest that novelty-induced grooming is a sex-influenced behavior, with females grooming more than males, and that animals exposed prenatally to alcohol and tested as adults may have altered responses to certain stressors (i.e., forced swim) under specific conditions. The altered grooming response of alcohol-exposed rats to swim stress can be eliminated by preexposing them to novelty stress.     

The development of landmark and beacon use in young children: evidence from a touchscreen search task

Children ages 2, 3 and 4 years participated in a novel hide-and-seek search task presented on a touchscreen monitor. On beacon trials, the target hiding place could be located using a beacon cue, but on landmark trials, searching required the use of a nearby landmark cue. In Experiment 1, 2-year-olds performed less accurately than older children on landmark trials but performed equivalently on beacon trials. In Experiment 2, the number of items on the screen was reduced and 2-year-olds' performance improved. Use of the landmark transformation technique in Experiment 3 revealed that older children formed a more precise landmark-target spatial relationship than 2-year-olds. Experiment 4 showed that the transformation itself was not responsible for the youngest participants' decreased accuracy in Experiment 3. Overall, beacons were utilized effectively by all participants, but the use of landmark cues is refined between the ages of 2 and 4.     

Allocentric spatial learning by hippocampectomised rats: A further test of the “spatial mapping” and “working memory” theories of hippocampal function

This paper reports a series of three experiments that tested the “spatial-mapping” and “working-memory” theories of hippocampal function. The experimental designs incorporate separate reference- and working-memory procedures of a water-escape task, using both spatial and non-spatial learning. In Experiment 1 (Reference memory), rats with hippocampal (HC) or cortical (CC) lesions and unoperated (UNOP) rats learned to swim to a rigid visible escape platform while avoiding contact with a floating one. In the nonspatial task, the platforms each occupied any of 8 possible positions in the pool over successive trials but differed in appearance. In the spatial task, the platforms were of identical appearance but the safe one always occupied a single fixed location. The HC rats showed a highly specific spatial learning impairment but did learn to perform consistently above chance towards the end of training. In Experiment 2 (working memory), new groups of rats were trained on similar spatial and nonspatial tasks, but the platform designated correct-in terms of its visual appearance or its spatial location-was randomly changed each day. No animal learned the nonspatial task despite extensive training. Performance on the spatial version unexpectedly revealed an impairment in the CC as well as the HC group relative to the UNOP rats. However, the HCs again performed at above chance levels and demonstrated rapid (I-trial) spatial learning towards the end training. Experiment 3 used a place navigation matching-to-sample task examine spatial working memory further. Each day, an underwater platform was hidden at any of 4 possible locations, and the rats were given 2 trials to search for it. Both UNOP and CC rats located the platform faster on Trial 2 than on Trial 1, even when the inter-trial interval was long as 30min. HC rats were no faster on Trial 2 than on Trial 1. We conclude that hippocampal lesions (1) severely but partially impair spatial but not visual reference memory and (2) give rise to different patterns impairment in different working-mermory tasks. The results are a chal lenge to both the spatial-mapping and working-memory theories.     

Overcoming unlearned response biases: delayed escape following errors facilitates acquisition of win-stay and win-shift working memory water-escape tasks in rats

Previous research has demonstrated that perseveration in escape situations is an unlearned response bias of rats and mice which is difficult to overcome. In Experiment I, Sprague-Dawley rats were trained to a criterion of nine correct choices in 10 consecutive trials in a win-stay working memory water-escape task wherein an escape platform was hidden in one of two compartments. Subjects were given a forced choice on an Information Trial followed by a free choice on a Test Trial 5 min later. Subjects who were given a 30-s forced swim in the incorrect section following errors on the Test Trial showed faster acquisition and less use of response perseveration than control subjects. In Experiment II, the delayed escape procedure was used to compare acquisition of win-stay and win-shift strategies. Contrary to previous research both groups learned the tasks, although the win-stay group showed better performance. It is concluded that unlearned response biases of perseverating and returning to previous escape sites can be overcome if experimental conditions are appropriately arranged.     

Dissociation of Hippocampal and Striatal Contributions to Spatial Navigation in the Water Maze

Two experiments were conducted to compare the effects of fornix/fimbria and caudate-putamen lesions in Long–Evans hooded rats (Rattus norvegicus) trained on two water maze tasks that differed in the type of spatial localization required for optimum solution. In Experiment 1, the lesioned rats and surgical controls were trained on the standard place task in the water maze (Morris, 1981) and given two postacquisition tests (a platform removal probe and platform relocation test). In Experiment 2, rats with similar lesions and control rats were trained on a modified cue navigation task. Fornix/fimbria lesions impaired a late stage of place task acquisition but did not impair acquisition of the cue task. Caudate-putamen lesions resulted in a severe place acquisition impairment and a transient cue acquisition impairment, both of which were characterized by an initial tendency to swim near the wall of the pool. Post-hoc analyses of the direction and angles of departure from the start points suggested that rats with fornix/fimbria lesions used non-allocentric spatial strategies to solve the place task. These rats also demonstrated a significantly weakened spatial bias for the former training quadrant on the platform removal probe and reduced flexibility in navigating to a novel platform location on the platform relocation test. In contrast, rats with caudate-putamen lesions showed a significant spatial bias for the former training quadrant but failed to cross the exact location within the quadrant where the platform was formerly positioned. The results suggest that the hippocampus mediates the allocentric spatial component of the water maze place task while the dorsomedial striatum may play an important role in the acquisition of the procedural aspects of both place and cue versions of the task.     

Penetrating the geometric module: catalyzing children's use of landmarks

We used a reference memory paradigm to examine whether 4- and 5-year-old children could be trained to use landmark features to relocate targets after disorientation. In Experiment 1, half of the children were pretrained in a small equilateral triangle-shaped room. Each of the three walls was a different color, and the target was always in the middle of the yellow wall. These children and a control group were tested in a small rectangular room with three white walls and one yellow wall; the target was placed in one of the corners. Children with pretraining responded more frequently to the correct corner than to the diagonally congruent corner on their first set of four trials in the rectangular room, whereas the children in the control group used geometric cues exclusively. Three additional groups of children (Experiment 2) showed that the use of landmark features--both salient and subtle--can be learned in as few as four practice trials in a small rectangular room. The data support the view that both geometry and landmark features are adaptively combined in the same representation.     

A computer simulation model of rats’ place navigation in the Morris water maze

Rats rapidly learn to swim from a variety of starting locations, including novel ones, to a small invisible platform submerged in a pool of cool opaque liquid. A computer simulation based on a simple perceptual memory-matching model successfully mimics this ability. The model assumes that the rat, when it successfully reaches the platform, notes the distance to prominent extramaze landmarks and stores this perceptual information in memory. When placed in the pool on subsequent trials, the simulated rat attempts to match perceived distance between itself and the landmarks to the remembered distances from the platform to the landmark. The model accounts for many of the known facts about rats’ behavior in the swimming pool and makes some interesting predictions that could be easily tested experimentally. The model has the advantage, relative to other cognitive map models, of specifying how spatial information is represented in memory and how this information guides behavior.     

Representation of serial order in pigeons (Columba livia)

In Experiment 1, 2 groups of pigeons were trained to respond to either a 4-item (A→B→C→D) or 5-item (A→B→C→D→E) list. After learning their respective list, half of the subjects were trained on a positive pair with reinforcement provided when pairs were responded to in the order true to that of the original sequence (4-item: B→C; 5-item: B→D). The remaining subjects were trained on a negative pair with reinforcement provided for responding to the pairs in the order opposite to that learned in the original sequence (4-item: C→B; 5-item: D→B). Subjects in the positive pair condition learned their respective pair faster than did subjects in the negative pair condition. In Experiment 2, after reaching criterion on a 4-item list, subjects received 16 BC probe trials spread across 4 sessions of training. Subjects performed significantly above chance on the probe trials. The performance of our subjects in Experiments 1 and 2 demonstrates that, similar to monkeys, pigeons form a representation of the lists that they learn.     

The relative influence of place and direction in the Morris water task

Previous work from our laboratory has demonstrated that rats display a preference for directional responding over true place navigation in the Morris water task. The present study evaluated the range of situations in which this preference is observed and attempted to identify methods that favor navigation to the precise location of the escape platform in the room. A preference for directional responding over place navigation was observed in a wide range of procedures that included providing extensive training (Experiment 1), providing only platform placement experience in the absence of active swim training (Experiment 2), training navigation to multiple platform locations in a moving platform variant of the task (Experiment 3), and explicitly training navigation to a precise location in the room, versus navigation in a particular direction, regardless of the pool's position in the room (Experiments 4-5). A modest preference for navigation to the precise spatial location of the platform was observed when the pool wall was virtually eliminated as a source of control by filling it to the top with water (Experiment 6).