Vestibular stimulation disrupts acquisition of place navigation in the Morris water tank task

The significance of vestibular input for place navigation in the Morris water maze was examined in 11 hooded rats. A 5-min rotation (2 Hz) immediately preceding retrieval of an overtrained place navigation task prolonged escape latencies by about 10 s corresponding to circular swimming induced by postrotatory aftereffects, but did not otherwise interfere with target location. A 1-min rotation immediately following the acquisition trial in the working memory version of the water maze task did not deteriorate performance in the retrieval trial performed 5 min later. Two components of the acquisition trial, i.e., active search of the hidden target (up to 1 min) and latent learning during the 30-s stay on the platform, contribute almost equally to the formation of the working memory trace. A 1-min rotation immediately preceding the isolated platform learning component impaired subsequent retrieval but was ineffective when applied 3 min earlier. Latent learning was completely disrupted when the platform with the animal was rotated during the entire 30-s stay on the target platform or when the 30-s stay on the stationary platform was followed 0 or 3 min later by a 30-s rotation on the platform. The interfering effect of the latter procedure was suppressed by covering the platform with the animal by an opaque cylinder. It is concluded that vestibular cues are particularly important for the orientation of the animal in the gravitation field and for the estimation of the angles between vectors plotted from the animal toward external landmarks. Agreement between vestibular and visual signals is a prerequisite of efficient navigation.     

Generalization gradients in a navigation task with rats

In two experiments in a Morris pool, rats were trained to find a hidden platform which was located in a specific position in relation to two objects, B and F, which were presented together, one in front of the other. One object, B, was just above the platform (a beacon for the platform, the critical object) while the second object, F, was above the edge of the pool (the frame of reference). Then the rats received test trials, without the platform, in which B was presented in different positions in relation to F (i.e., in relation to its original position). In the two experiments the test results showed a generalization gradient as a function of the relative distance of the two objects: more time searching in the B segment, where the platform should have been, when B was in the original position (i.e., in front of F), which decreased symmetrically with distance of B from F. The present experiments show for the first time generalization gradients with rats across spatial locations when working with a navigation task.     

Generalization in place learning and geometry knowledge in rats

Rats were trained to search for a food reward hidden under sawdust in the center of a square-shaped enclosure designed to force orientation on the basis of the overall geometry of the environment. They were then tested in a number of enclosures differing in shape and in size (rectangular-, double-side square-, and equilateral triangle-shaped enclosures). Results showed that rats transferred their place-finding ability to the novel enclosures. Our results add evidence to the hypothesis that the evolutionary roots of spatial cognition entail a primitive encoding of geometric relationships, as already shown using other tasks in rats.     

The contribution of nonvisual information to simple place navigation and distance estimation: an examination of path integration.

In Experiment 1, participants walked without vision to a target location they had either previously viewed, were led to and from blindfolded, or both viewed and were led to and from blindfolded. A course to the target could be set and held without vision only if prior vision of its location was available. The locomotor group reproduced the heading and distance to the target less accurately than the other groups, which did not differ significantly. However, when nonvisual information accompanied vision of the target location, it served to subtly influence performance. Participants in Experiment 2 estimated the distance of a target they either viewed or were led to blindfolded. When vision was available, men overestimated target distance and women underestimated it. When target distance was learned nonvisually, no sex differences in distance estimations emerged. Our findings suggest that deriving navigation information from nonvisual locomotion is difficult and may be dependent on prior visual information.     

Binge eating proneness emerges during puberty in female rats: a longitudinal study

Puberty is a critical risk period for binge eating and eating disorders characterized by binge eating. Previous research focused almost entirely on psychosocial risk factors during puberty to the relative exclusion of biological influences. The current study addressed this gap by examining the emergence of binge eating during puberty in a rat model. We predicted that there would be minimal differences in binge eating proneness during pre-early puberty, but significant differences would emerge during puberty. Two independent samples of female Sprague-Dawley rats (n = 30 and n = 36) were followed longitudinally across pre-early puberty, mid-late puberty, and adulthood. Binge eating proneness was defined using the binge eating resistant (BER)/binge eating prone (BEP) model of binge eating that identifies BER and BEP rats in adulthood. Across two samples of rats, binge eating proneness emerged during puberty. Mixed linear models showed little difference in palatable food intake between BER and BEP rats during pre-early puberty, but significant group differences emerged during mid-late puberty and adulthood. Group differences could not be accounted for by changes in nonpalatable food intake or body weight. Similar to patterns in humans, individual differences in binge eating emerge during puberty in female rats. These findings provide strong confirming evidence for the importance of biological risk factors in developmental trajectories of binge eating risk across adolescence.     

Comparative analysis of movement characteristics during dead-reckoning-based navigation in humans and rats

Human and rat movement organization was investigated as they searched for randomly located rewards without access to visual information. Under dark conditions, rats foraged for randomly located food pellets (Experiment 1). Blindfolded humans were instructed to search for an ostensible hidden coin using a metal detector (Experiment 2). After locating the food pellet, rats carried it back to the refuge, and after a designated searching time, humans were instructed to return to the start location. Although both species exhibited a high degree of similarity in searching path movement organization and ability to return to the start location, disruption of human searching path organization was associated with impairments in returning to the start location. These results support the vestibular "gain" account of movement organization during dead-reckoning-based navigation.     

Delayed backward conditioning of place preference induced by wheel running in rats

Backward conditioning of place preference has been obtained in hungry rats when wheel running (the rewarding US) is followed immediately by exposure to a distinctive chamber (the CS). We tested whether such backward conditioning would occur with a 10-min delay. In each of four paired exposures, a period of wheel running (2 or 22 h, in separate experiments) was followed by 10 min in the home cage and then 30 min in the CS chamber. Control rats were put in the CS chamber without wheel running. In other similar experiments, a 0-, 10-, or 30-min delay separated wheel running from exposure to the CS chamber. Reliable conditioned place preference (CPP) occurred when the delay was 0 or 10 but not 30 min. The strength of CPP decreased as the delay increased. These findings imply that the reward process initiated by wheel running remains active for some time after running stops. They support the view that the suppression of feeding produced by wheel running in hungry rats (activity anorexia) is mediated by this reward process.     

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.     

Sex differences in object location memory and spatial navigation in Long-Evans rats

In both humans and rodents, males typically excel on a number of tasks requiring spatial ability. However, human females exhibit advantages in memory for the spatial location of objects. This study investigated whether rats would exhibit similar sex differences on a task of object location memory (OLM) and on the watermaze (WM). We predicted that females should outperform males on the OLM task and that males should outperform females on the WM. To control for possible effects of housing environment, rats were housed in either complex environments or in standard shoebox housing. Eighty Long-Evans rats (40 males and 40 females) were housed in either complex (Complex rats) or standard shoebox housing (Control rats). Results indicated that males had superior performance on the WM, whereas females outperformed males on the OLM task, regardless of housing environment. As these sex differences cannot be easily attributed to differences in cognitive style related to linguistic processing of environmental features or to selection pressures related to the hunting gathering evolutionary prehistory of humans, these data suggest that sex differences in spatial ability may be related to traits selected for by polygynous mating strategies.     

Hippocampal lesion syndrome: Switching to and from a place hypothesis

Spatial discrimination learning (Experiment I) and strategy shifts (Experiment II) were examined in experimentally lesioned rats. Sixty adult male rats were in one of five lesion groups (N = 12/group): operated controls, fornical, entorhinal, fornicoentorhinal and dorsal-ventral hippocampal. All animals reached criterion on the discrimination learning test. In the strategy shift experiment, however, animals with hippocampal lesions had more difficulty than animals without hippocampal lesions. The findings suggest that the hippocampus may not be involved in learning strategies, but in switching a strategy.     

Effect of neonatal dentate gyrus lesion on allothetic and idiothetic navigation in rats

Goal-directed navigation is believed to be the combined product of idiothetic and allothetic orientation. Although both navigation systems require the hippocampal formation, it is probable that different circuits implement them. Examination of Long-Evans rats with dentate gyrus lesions induced by neonatal X-ray irradiation may show the dissociation of these two components of navigation. Two recently developed place avoidance tasks on a rotating circular arena were used to test this hypothesis. In the first test, the position of the punished area is stable in the room frame but is permanently changing on the surface of the arena. This task requires the rat to use allothetic orientation and to disregard idiothetic orientation. In the second test, the prohibited area is fixed in the coordinate system of the arena and the experiment is conducted in complete darkness, forcing the rat to rely exclusively on idiothesis supported by substratal cues. The results suggest that the dentate gyrus lesion interferes less with idiothetic orientation than with allothetic orientation. In addition, an attempt was made to control the number of developing granule cells by exact timing of a single high dose of perinatal irradiation, and to measure the ensuing behavioral deficits. Rats irradiated at 6, 18, or 24 h after birth were tested as adults in the Morris water maze. Irradiated animals showed significant, but highly variable, learning deficit, but histological examination indicated that the granule cell loss did not correlate with the degree of behavioral impairment.     

Dorsal striatal dopamine depletion impairs both allocentric and egocentric navigation in rats

Successful navigation requires interactions among multiple but overlapping neural pathways mediating distinct capabilities, including egocentric (self-oriented, route-based) and allocentric (spatial, map-based) learning. Route-based navigation has been shown to be impaired following acute exposure to the dopaminergic (DA) drugs (+)-methamphetamine and (+)-amphetamine, but not the serotoninergic (5-HT) drugs (±)-3,4-methylenedioxymethamphetamine or (±)-fenfluramine. The dopaminergic-rich neostriatum is involved in both allocentric and egocentric navigation. This experiment tested whether dorsal striatal DA loss using bilateral 6-hydroxydopamine (6-OHDA) injections impaired one or both types of navigation. Two weeks following 6-OHDA injections, rats began testing in the Cincinnati water maze (CWM) followed by the Morris water maze (MWM) for route-based and spatial navigation, respectively. 6-OHDA treatment significantly increased latency and errors in the CWM and path length, latency, and cumulative distance in the MWM with no difference on cued MWM trials. Neostriatal DA levels were reduced by 80% at 2 and 7 weeks post-treatment. In addition, 6-OHDA increased DA turnover and decreased norepinephrine (NE) levels. 6-OHDA injections did not alter monoamine levels in the prefrontal cortex. The data support that neostriatal DA modulates both types of navigation.     

Increased reinforcement when timeout from avoidance includes access to a safe place

Three experiments investigated the reinforcing value of access to a safe place during timeout from an avoidance schedule. Rats were trained on conjoint schedules in which responding both postponed shock on a free-operant avoidance schedule and produced periods of timeout on fixed-ratio schedules. In some conditions, a shelf was inserted into the operant chamber during timeout, enabling subjects to get off the grid floor. The combination of timeout and shelf maintained substantially higher response rates than the baseline avoidance schedule with ratio requirements as high as 90 (Experiment I). Adding the shelf to timeouts in one component of multiple fixed-ratio schedules of timeout resulted in higher response rates in the component where the shelf was included (Experiment II). When timeouts with and without the shelf were arranged on concurrent schedules, the shelf-timeout combination was preferred, even when of shorter duration than timeout alone (Experiment III). In all three experiments, subjects climbed on the shelf, although all shocks were cancelled during timeout periods. The results could not be accounted for solely in terms of the reinforcing properties of changes in shock rates, but required an interpretation that ascribed conditioned reinforcing value to stimuli associated with such changes.     

Recognition of food by individual, food-naive, weaning rats (Rattus norvegicus)

To control and observe first feeding experiences, pups were reared without access to food. At weaning age, individual pups received simultaneous access to a palatable, noncaloric diet and a similar diet containing starch. Each diet contained a flavor cue. Within 20 min of sampling both diets pups preferentially ingested the caloric alternative. Further tests indicated that pups learned a preference for flavor cues paired specifically with ingestion of starch. In caloric discrimination tests in which the diets were not distinguished by artificial flavors, pups did not show the rapid preference for the caloric diet seen in earlier experiments. We suggest that recognition and preference for the caloric diet is based on its rapid postingestive effects rather than readily preferred sensory features of the starch. Our findings demonstrate the abilities of individual, food-naive pups to rapidly recognize food and indicate a role for associative learning in the onset of independent feeding.