Absence of overshadowing between a landmark and geometric cues in a distinctively shaped environment: A test of Miller and Shettleworth (2007).

Rats in the first 2 experiments, which were designed to test predictions from a model of spatial learning by N. Y. Miller and S. J. Shettleworth (2007), had to escape from a triangular pool by swimming to a submerged platform in a geometrically unique corner. A spherical landmark was suspended above the platform for an overshadowing group. A control group was trained with the same arrangement and with a second, identical landmark suspended in another corner. The platform could thus be found by reference to the landmark or the geometric cues in the overshadowing group, whereas the control group had to rely on geometric cues. There was no indication of overshadowing between the geometric cues and the landmark in the overshadowing group. The final 2 experiments revealed that the absence of overshadowing was not a consequence of the landmark being an ineffective cue for overshadowing. The results indicate either that the landmark and geometric cues were not in competition for the control they acquired over behavior or that an additional process compensated for any such competition that might have occurred in the overshadowing group. This additional process could involve between-cues associations or the provision of a stable spatial framework. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Spatial learning based on the shape of the environment is influenced by properties of the objects forming the shape.

In 3 experiments rats had to find a submerged platform that was located in a corner of a kite-shaped pool. The color of the walls creating this corner provided an additional cue for finding the platform in the shape + color condition but not the shape-only condition. During tests in a pool with walls of a uniform color but no platform, more time was spent in the corner where the platform was originally located after training in the shape + color than in the shape-only condition. The results challenge theories that assume either that learning about the shape of the environment takes place in a dedicated module or that cues compete for the control they acquire over behavior. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Control of spatial behavior by an unstable landmark.

In 5 experiments rats were required to escape from a pool of water by finding a submerged platform that varied in position from session to session. The platform was in a fixed direction; and at a fixed distance from a landmark that was located in the pool. Experiments 1–3 revealed that the landmark was used as a reference point for information about the direction and the distance of the platform. In Experiments 4 and 5, the landmark and platform remained in the same place for one group but moved as one from session to session for another. Both groups were then placed in the pool for periods without the landmark and platform. Testing with the landmark then revealed that it controlled more accurate searching by the group trained with the unstable than with the static landmark. The results do not support the proposal that animals are more likely to use a static than a moving landmark as a reference point for finding a goal. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Between-cue associations influence searching for a hidden goal in an environment with a distinctive shape.

In Experiment 1 rats had to escape from a kite-shaped pool by swimming to a submerged escape platform in a right-angled corner. The two walls creating this corner were white and the two walls creating the opposite, incorrect, right-angled corner were black. The rats were then trained in a square pool with two white walls forming one corner and two black walls forming the opposite corner. The platform was in the white corner for a consistent group and the black corner for an inconsistent group. A test in an entirely white kite revealed a stronger preference for the correct than the incorrect corner in the consistent but not the inconsistent group. This outcome is attributed to the formation of associations between geometric cues, provided by the shape of the pool, and the color of the walls. The results were replicated in a second experiment in which the walls of the test pool were the same color as the incorrect corner during initial training. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Potentiation, overshadowing, and blocking of spatial learning based on the shape of the environment.

Rats were trained in Experiment 1 to find a submerged platform in 1 corner of either a rectangular or a kite-shaped pool. When the walls creating this corner were a different color than the opposite walls, then learning about the shape of the pool was potentiated in the kite but not in the rectangle. Experiments 2-4 revealed that learning about the rectangle can be overshadowed and blocked when information about the wall color indicates the location of the platform. The results mimic findings that have been obtained with Pavlovian conditioning, and they challenge the claim that learning about the shape of the environment takes places in a dedicated geometric module. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Absence of an Interaction Between Navigational Strategies Based on Local and Distal Landmarks.

In 3 experiments, rats were required to escape from a Morris pool by swimming to a submerged platform that was located at the apex of a notional, equilateral triangle with 2 different landmarks occupying the corners at the base. Training for 1 group was always conducted in view of the landmarks surrounding the pool and with the triangular array in a fixed orientation. Subjects could therefore identify the direction of the platform from a single landmark within the pool by reference to cues outside the pool or to the other landmark within the pool. Both strategies were used, and the results from additional groups revealed that the first of these strategies did not affect the acquisition of the second one. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioned inhibition and superconditioning in an environment with a distinctive shape.

In Experiments 1 and 2a rats received an A+/AX? discrimination in a rectangular pool with two submerged platforms in diagonally opposite corners—the correct corners—for A+ trials. For AX? trials, rats were placed in the pool without the platforms but with identical landmarks, X, in the correct corners. Landmark X subsequently passed both a summation and retardation test for inhibition in Experiment 1. Upon completion of the discrimination in Experiment 2a, the platforms were placed near identical landmarks in the correct corners of the rectangle. The landmarks were those used for discrimination training for a superconditioning group (AX+ trials), but for a control group they were novel (AY+ trials). During a final test in the pool without the landmarks and the platforms, the superconditioning group spent more time than the control group searching in the correct corners. This finding, which was replicated in a kite-shaped pool in Experiment 2b, demonstrates successful superconditioning by landmark X of the cues created by the shapes of the pools. The results pose a problem for the theory of Miller and Shettleworth (2007). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Further evidence that rats rely on local rather than global spatial information to locate a hidden goal: Reply to Cheng and Gallistel (2005).

Naive male Hooded Lister rats (Rattus norvegicus) were required to find a submerged platform in a right-angled corner between a long and a short wall of a pool in the shape of an irregular pentagon. Tests in a rectangular pool revealed a preference for the corners that corresponded with the correct corner in the pentagon. These findings indicate that rats identified the correct corner in the pentagon by local cues. They contradict the suggestion that rats navigate by moving in a particular direction relative to the principal axis of the shape of their environment. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Spatial learning based on boundaries in rats is hippocampus-dependent and prone to overshadowing.

In three experiments, rats were required to find a submerged platform by referring to the boundaries of a circular swimming pool. In the first experiment, rats with lesions of the hippocampus were impaired at finding the hidden platform, lending support for the proposal that learning to find a goal that is a certain direction and distance from a boundary is dependent upon the hippocampus. Experiments 2 and 3 offered preliminary tests to see if such boundary learning occurred incidentally, irrespective of the presence of a reliable landmark. In contrast to this proposal, a landmark hanging above the platform successfully restricted learning about the location of the platform with respect to the boundary of the arena. The discussion explores the capacity of the hippocampus to encode boundary information, as well as interprets the behavioral results on the basis of an associative learning framework. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Lesions of the perirhinal cortex do not impair integration of visual and geometric information in rats.

Rats with lesions of the perirhinal cortex and a control group were required to find a platform in 1 corner of a white rectangle and in the reflection of this corner in a black rectangle. Test trials revealed that these groups were able to integrate information regarding the shape of the pool and the color of its walls (black or white) to identify the correct location of the platform. A clear effect of the perirhinal cortex lesions was, however, revealed using an object recognition task that involved the spontaneous exploration of novel objects. The results challenge the view that the perirhinal cortex enables rats to solve discriminations involving feature ambiguity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Transfer of Spatial Behavior Between Different Environments: Implications for Theories of Spatial Learning and for the Role of the Hippocampus in Spatial Learning.

In 3 experiments, rats were required to find a submerged platform located in 1 corner of an arena that had 2 long and 2 short sides; they were then trained to find the platform in a new arena that also had 2 long and 2 short sides but a different overall shape. The platform in the new arena was easier to find if it was in a corner that was geometrically equivalent, rather than the mirror image, of the corner where it had previously been located. The final experiment revealed that hippocampal lesions impaired rats' ability to find the platform in these arenas. The results suggest that rats did not use the overall shape of the arena to locate the platform but relied on more local cues and that the hippocampus plays a role in navigation based on these cues. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Rotation of water in the Morris water maze interferes with path integration mechanisms of place navigation

The idea that place navigation in the Morris water maze is implemented by path integration between locations determined by landmark sighting was investigated in a 200-cm-diameter pool in which circular (7.2 degrees/s) motion of water could be induced by tangentially arranged water jets. The rats were trained at 8 trials per day to navigate to an erectable platform which was raised after the rat had spent a criterion time in the target annulus (30 cm in diameter) in the midpoint of the NW quadrant. Asymptotic escape latency of 7 s was reached after 9 days in moving water (n = 8) and after 6 days in stationary water (n = 8). The group overtrained for 13 days in stable water performed well even after it was transferred to moving water. Changing the sense of rotation of water from counterclockwise to clockwise did not affect the asymptotic performance. The above findings show that overtrained rats rely on landmark sighting rather than on path integration. The influence of water movement reappeared when place navigation to a new target (SW) was examined in alternating 2-s periods of light (L) and darkness (D). On the first day, the latencies were 15.2 +/- 1.2 and 22.8 +/- 1.9 s in stable and moving water, respectively, but dropped to 10 s on the following day. The tracks generated in the L period were more tortuous than those generated in the D period and this difference was more pronounced in moving than in stable water. It is concluded that path integration mechanisms supporting navigation during intervals of darkness are impaired in moving water but that this impairment disappears in overtrained animals.     

Spatial learning and memory deficits induced by dopamine administration with decreased glutathione

Administration of buthionine sulfoximine (BSO) selectively inhibits glutathione (GSH) biosynthesis and induces a GSH deficiency. Decreased GSH levels in the brain may result in less oxidative stress (OS) protection, because GSH contributes substantially to intracellular antioxidant defense. Under these conditions, administration of the pro-oxidant, dopamine (DA), which rapidly oxidizes to form reactive oxygen species, may increase OS. To test the cognitive behavioral consequences of decreased GSH, BSO (3.2 mg in 30 microliters, intracerebroventricularly) was administered to male Fischer 344 rats every other day for 4 days. In addition, DA (15 microliters of 500 microM) was administered every day [either 1 h after BSO (BSO + DA group) or 1 h before BSO (DA + BSO group), when given on the same day as BSO] and spatial learning and memory assessed (Morris water maze, six trials/day). BSO + DA rats, but not DA + BSO rats, demonstrated cognitive impairment compared to a vehicle group, as evidenced by increased latencies to find the hidden platform, particularly on the first trial each day. Also, the BSO + DA group utilized non-spatial strategies during the probe trials (swim with no platform): i.e., less time spent in the platform quadrant, fewer crossings and longer latencies to the previous platform location, and more time spent in the platform quadrant, fewer crossings and longer latencies to the previous platform location, and more time spent around the edge of the pool rather than in the platform zone. Therefore, the cognitive behavioral consequences of decreasing GSH brain levels with BSO in conjunction with DA administration depends on the order of administration. These findings are similar to those seen previously on rod and plank walking performance, as well as to those seen in aged rats, suggesting that the oxidation of DA coupled with a reduced capacity to respond to oxidative stress may be responsible for the induction of age-related cognitive deficits.     

Behavioral effects of intracerebroventricular microinfusion of agmatine in adult rats.

The present study investigated the behavioral effects of intracerebroventricular microinfusion of agmatine. Rats with low dose (10 μg), but not high dose (100 μg), of agmatine spent significantly less time in the enclosed arm and more time in the open arm in the elevated plus maze. In the water maze task, the high dose group displayed a transient impairment in searching for a hidden platform, whereas the low dose group had reduced latency in the first probe test. In the object recognition task, all groups could detect the novel object, but the low dose group spent significantly more time exploring displaced objects. Furthermore, the low dose group made significantly fewer errors in the working, but not the reference, memory version of the radial arm maze task. These results suggest that the behavioral effects of agmatine are task- and dose-dependent, and agmatine may be an anxiolytic and memory modulator. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temperament in the Rat: Sex Differences and Hormonal Influences on Harm Avoidance and Novelty Seeking.

This study showed large and consistent individual differences in 64 rats (32 males) in the hole board and canopy test, which are considered to measure exploration and anxiety, respectively. Nonestrous females were more active than males and nose poked more in the hole board. In the canopy test, nonestrous females, compared with males, showed greater intraindividual variability in time spent outside the protective canopy. Estrous females spent significantly more time outside the canopy. Gonadectomy reduced nose poking in males and hole board locomotion in both sexes. Principal-components analysis disclosed 2 temperamental dimensions reflecting harm avoidance and novelty seeking. More males had high levels of psychometric harm avoidance, and fewer males than females had a low-harm-avoidance/high-novelty-seeking, sanguine profile. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effects of combined lesions of the subicular complex and the entorhinal cortex on two forms of spatial navigation in the water maze.

The role of the subicular complex and entorhinal cortex (SUB–EC) in spatial learning was examined in 2 water maze experiments. In Experiment 1, rats had to locate a hidden platform that was always a fixed distance and direction from an intramaze landmark. Each day, the landmark and platform were moved to a new location. Both control and SUB–EC-lesioned rats learned to locate the platform equally readily during training. However, the control group was impaired in locating the platform when the visual extramaze cues were concealed, whereas the lesioned group was unaffected by this manipulation. In Experiment 2, the lesioned rats were impaired in finding a hidden platform that was in a fixed place in the water maze and showed no evidence of having learned its location in a probe test. These results suggest that damage to the SUB–EC impairs the integration of geometric information but spares a more general navigational-directional strategy. (PsycINFO Database Record (c) 2010 APA, all rights reserved)