Rat spatial memory: Resistance to retroactive interference at long retention intervals

An attempt was made to disrupt memory for spatial information by interpolating a task of high similarity to the to-be-remembered task during a long retention interval. Rats were trained in an 8-arm maze in which choosing each arm without repetition was the optional strategy. A 4-h delay was imposed between the 4th and 5th choices. At various times during the retention interval, the rats ran a second identical maze located in another room. No evidence of retroactive interference was observed. In the second experiment, the rat was required to remember the interpolated spatial task during the retention test. This was accomplished by allowing the rat to make four choices in the first maze and then, after a variable period of time, four choices in the second maze. Four hours after exposure to each maze, retention was tested. Choice accuracy on the retention tests was high and equivalent on both mazes. Requiring the rat to remember which arms it had visited in a second maze did not impair memory for the first maze. These results demonstrate that rats can segregate spatial memories established in different contexts with considerable proficiency.     

The sensory basis of spatial memory in the rat

Rats were given three stages of training on an eight-arm, elevated radial maze with food reward at the end of each arm. In Stage 1, rats were allowed to choose freely among the arms from the beginning of a trial. In Stage 2, three initial forced choices were followed by a series of free choices. In Stage 3, the central platform of the maze was rotated with the rat on it between the initial forced choices and the free choices. Following testing on these three stages, the animals were divided into four groups and deprived of selected senses. One group was made blind, a second anosmic, a third blind and anosmic, and a fourth was left normal. The same three stages of testing that had been conducted preoperatively then were run again post-operatively. Throughout these tests, the possible use of auditory cues was tested by presenting white noise on alternate trials. Finally, two further tests were carried out, the multiple rotations test and the removal-replacement test. The results indicated that visual cues, but not olfactory or auditory cues, played a critical role in the rat’s ability to avoid previously entered alleys. There was evidence also that rats used internal cues from kinesthetic and/or vestibular receptors when visual cues were absent.     

Rats remember not wisely but too well

Rats were trained in a three-alternative spatial delayed matching-to-sample task in a starburst maze. Samples consisted of rewarded forced choices of one arm, and retention was indicated by rats’ returning to that arm after a 90-sec delay. If a rat made an error on its first choice, it was returned to the start compartment and allowed a second choice. Unlike in previous experiments with this task, all three arms were available during the animals’ second choices. The rats tended to perseverate in their second choices by returning to the arm that they had erroneously visited on their first choice. In Experiment 1, the accuracy of second choices following first-choice errors was below chance during the first block of sessions, when a 90-sec delay intervened between the first choice and the second choice, and at chance during the second block of sessions, when a short (5–6 see) delay intervened between first and second choices. In Experiment 2, long-delay and short-delay sessions were randomly presented to naive subjects. Similar results were obtained. In both experiments, the tendency to repeat the erroneous first choice was greater when long delays separated the two choices than when short delays were used. The results suggest that rats make their first-choice errors because they erroneously encode or remember the location of the sample and that they base their second choices on the same erroneous-memory. The increase in perseveration at long delays implies some kind of rehearsal-like mechanism that slows forgetting of the memory controlling the first choice.     

Evidence for a shift in the choice criterion of rats in a 12-arm radial maze

Rats were trained in a standard 12-arm radial maze task. Following training, each trial consisted of a sequence of 2, 4, 6, 8, or 10 choices, followed by a 15-min delay, which then was followed by a choice between a single arm and a response manipulandum mounted in the center of the maze. An arm visit was reinforced if the arm had not been visited prior to the delay, whereas a manipulandum response was reinforced if the arm had been visited. It was found that rats are relatively more likely to reject arms by responding to the manipulandum following a delay occurring late in the choice sequence. This indicates that the choice criterion used by rats in the radial maze becomes more strict as the choice sequence progresses. Such a process provides an alternative explanation for some of the data recently reported by Cook, Brown, and Riley (1985).     

Foraging on the radial-arm maze: Effects of altering the reward at a target location

Thirsty rats were trained to collect small water rewards from the end of each arm of an eight-arm radial maze. During these training trials and subsequent testing trials, the subjects were allowed to choose a maximum of eight arms. “Preference” for a target maze location was studied by noting when, in the sequence of eight choices, the target was selected. During testing, when one maze location was consistently devoid of water, rats decreased their preference for this arm over trials (Experiment 1). Similarly, rats that learned a saccharin-lithium association demonstrated lower preferences for a maze location that consistently held the conditioned saccharin solution. This was true for animals that received saccharin-lithium conditioning on the maze (Experiment 3A) and for animals conditioned to saccharin in a separate context (Experiment 3B). An increase in preference for a target maze location consistently containing a sweet chocolate milk solution was observed in animals that were water- and food-deprived (Experiment 2). These studies demonstrate that animals will modify their responses toward (preferences for) maze locations that predictably contain an altered reward.     

Maze-arm length affects a choice criterion in the radial-arm maze

Male rats were tested in a 12-arm radial maze with 6 arms that were standard in length and 6 arms that were half the standard length. As previously reported by Brown (1990), revisits to short arms were more likely than revisits to long arms. Two explanations of this effect of mazearm length on choice accuracy were experimentally contrasted. The first attributes the effect to diminished discriminability of visited and unvisited arms when the arms are short. The second attributes the effect to a relatively lax choice criterion being applied to short arms. An analysis of the microstructure of choices, applying the logic of signal detection theory, provided evidence for the latter explanation.     

Response cost and time-place discrimination by rats in maze tasks

Time-place discrimination has been shown reliably in several avian and insect species, but only occasionally in rats and fish. In the present experiments, we explored the effects of response cost on time-place discrimination by rats. In the first experiment, we increased the cost of making a choice and the cost of recovering from a wrong choice in two types of maze, a radial arm and a vertical maze. In the radial arm maze, we found only general place preference, whereas in the vertical maze, we obtained evidence of time-place discrimination. In the second experiment, we found that the proportion of rats showing time-place discrimination increased with the height and, therefore, the response cost of the vertical maze. These results suggest that rats do not automatically store and/or retrieve the time and place of reward events but that response cost is an important trigger for time-place discrimination.     

Spatial memory in rats: Resistance to retroactive interference

Treatments that interfere with animals’ short-term retention (e.g., in delayed matching-to-sample) were studied using a spatial memory task. Rats performed in an eight-arm radial maze in which choosing each arm without repetition was the optimal behavior. Performances were interrupted between fourth and fifth choices for a delay of 15 sec to 2 min. A variety of events occurring during the delay interval did not disrupt memories for prior choices (as assessed by the accuracy of postdelay choices). The ineffective treatments included variations in visual and auditory environments, removal from the maze, food consumed during the delay, a distinctive odor added to the maze, or combinations of these manipulations. Additionally, performance on another spatial task (a four-arm maze) during the delay between Choices 4 and 5 did not interfere with performance in the eight-arm maze. These findings suggest that rats’ memories for spatial locations are immune to retroactive interference, at least within the range of conditions reported, and that the rat can successfully segregate memories for spatial locations established in different contexts.     

Social working memory: Memory for another rat's spatial choices can increase or decrease choice tendencies

In two experiments using a radial-arm maze, pairs of rats made choices among eight maze locations, each containing a large quantity of one of two food types. The choices made by 1 rat affected the choices made by the other rat. Under most conditions, visits by 1 rat increased the tendency of the other rat to subsequently choose that maze location. However, the effect depended on the quality of the food available in a particular location. When it was possible for the rats to observe each other on the maze arms and a rat had experienced that a location contained the less preferred food type, a previous visit to that location by the foraging partner decreased the tendency to visit that location. These effects are attributed to working memory for the spatial choices of another rat, and they indicate that memory produced by a rat’s own visit to a maze location is integrated with memory for the behavior of another rat to determine spatial choice     

The influence of spatial irregularity upon radial-maze performance in the rat

In two experiments, we examined how the introduction of vertical or horizontal irregularities in the perfectly regular shape of a radial maze affected rats’ performances. The introduction of various tilts in each arm of an eight-arm radial maze had a slightly positive effect on accuracy. However, when intra- and extraraaze cues were dissociated by rotating the maze before maze completion, the rata relied preferentially on extramaze cues associated with the horizontal direction of the arms but not with the tilts. On the other hand, the rats showed poor performances when trained on a horizontally distorted maze (uneven angles between the arms instead of repeated 45° angles). The high number of errors was related to the neglect of particular arms, the disorganization of the patrolling sequences, and the tendency to chain the visit of five arms that formed a regular ahape. Other animals, trained in the same maze, displayed similar biases even after a pretraining phase with constrained choices. Results from the horizontally distorted maze confirm and extend data from the spontaneous alternation literature that choice behavior is influenced by rules of movement that favor large angle transitions and regular subdivisions of space. They also stress the relation between performance in the radial maze and spontaneous exploratory and foraging behaviors.     

Retroactive interference in rat radial maze performance: The role of point of delay interpolation and the similarity and amount of interpolated material

The conditions necessary for producing retroactive interference (RI) were examined in a 12-arm radial maze. Rats were first given either three or nine forced choices in a to-be-remembered maze. During a 2-h delay, they received one or two trials in a second 12-arm maze, located either in a different room or the same room as the to-be-remembered maze. During the postdelay memory test, RI from the interference trials was produced only when nine choices had been made in the to-be-remembered maze and two interference trials had been conducted during the delay interval. RI was not found when only three forced choices had to be retained or after a single interference trial. The similarity between the interpolated and to-be-remembered mazes had no effect on choice accuracy. It was concluded that two conditions are required for the production of RI in the radial maze. First, a “large amount” of information should be resident in working memory. Second, a substantial number of interpolated trials or choices must be made during the delay.     

The effects of maze-arm length on performance in the radial-arm maze

Rats trained in a 16-arm radial maze with arms half the standard length demonstrated extremely low, but above chance, choice accuracy (Experiment 1). Rats trained in a 12-arm maze with short arms demonstrated a substantially higher degree of adjacent-arm responding than did rats trained in the same maze with long maze arms and, when response stereotypy was disrupted by a forced-choice procedure, the short-arm group chose less accurately than the long-arm group (Experiment 2). In a 16-arm maze with 8 short arms and 8 long arms, there was a strong preference for short arms and no evidence for a difference in the ability to discriminate previously visited arms from unvisited arms as a function of arm length, as measured by a two-alternative forced-choice procedure (Experiment 3). These results are interpreted as indicating that arm length affects a choice criterion, with a relatively lax criterion being applied to shorter arms.     

Two sources of proactive interference in spatial working memory: Multiple effects of repeated trials on radial maze performance by rats

Rats were trained in 8- and 12-arm radial mazes. Each trial began with a study phase (forced choices of 4 arms). The trial ended after a 2-h delay in a test phase consisting of free choices among 8 arms; choices of the 4 arms not yet visited were correct (rewarded). Proactive interference (PI) was induced by an interference phase that occurred on some days 2 or 3 h prior to the study phase. In the PI-repetition condition, the interference phase consisted of forced choices of the 8 arms that were later presented in the study and test phases; in the PI-nonrepetition condition, the interference consisted of forced choices of the 4 arms that were correct during the test phase. Test-phase performance was most accurate in the No PI (single-trial) condition and least accurate in the PI-repetition condition. A second experiment showed that repetitions per se were not responsible for the PI; when the interference phase consisted only of choices of the same 4 arms later presented in the study phase, no PI was observed. These findings suggest two sources of PI. One source, measured by the difference between No PI and PI-nonrepetition conditions, appears to be a difficulty in discriminating the temporal order of visits to arms in the interference and study phases. The other source of PI, measured by the difference between the nonrepetition and repetition conditions, remains to be identified; some possibilities are discussed.     

Retroactive inhibition in rat spatial memory

Two experiments were carried out in which rats first were given four forced choices on an eight-arm radial maze, then were given interpolated maze experiences, and finally were given a free choice retention test on the first maze. In Experiment 1, interpolated experiences consisted of forced choices made on one, two, or three other mazes, each placed in a different room. Retroactive inhibition (RI) was not found with one and two interpolated mazes but was found with three interpolated mazes. In Experiments 2a and 2b, an attempt was made to produce RI within a single context by using two mazes placed side by side or on top of one another and by using interpolated forced choices that were different, random, or the same with respect to forced choices onMaze 1. These conditions failed to yield any evidence of RI. In Experiment 2c, forced choices were followed by interpolated direct placements on the same maze on different, random, or the same maze arms, and retention tests revealed RI under these conditions. It was concluded that rats encode memories of specific places visited in space and that RI will arise only if (1) memory is greatly overloaded with interpolated information or (2) an interpolated visit is made to exactly that position in space to which an animal must travel in order to achieve a correct choice on the retention test.     

Motivation and incentives in education: Evidence from a summer reading experiment

Policymakers and economists have expressed support for the use of incentives in educational settings. In this paper, rather than asking whether incentives work, we focus on a different question: For whom and under what conditions do incentives work? This question is particularly important because incentives’ promise relies on the idea that they might take the place of some cognitive failing or set of preferences that otherwise would have led students to make choices with large long-term benefits. In this paper, we explore whether that is the case. In the context of a summer reading program called Project READS, we test whether responsiveness to incentives is positively or negatively related to the student's baseline level of motivation to read. As a part of the program, elementary school students are mailed books weekly during the summer. We implemented this book-mailing program as a randomized experiment with three treatment arms. Students in the first treatment arm were mailed books as a part of the standard Project READS program. Students in the second treatment arm were mailed books as a part of Project READS, and were also offered an incentive to read the books they were mailed. Students in the third experimental group served as a control and were given books after posttesting occurred in the fall. We find that, if anything, more motivated readers are more responsive to incentives to read, suggesting that to the extent that incentives are effective, they may not effectively target the students whose behavior they are intended to change.     

Rats acquire win-stay more readily than win-shift in a water escape situation

Independent groups of Sprague-Dawley rats were found to acquire a win-stay, but not a win-shift, escape resporse in a circular water maze in each of three experiments that varied with respect to swim time and distance prior to escape following an incorrect first choice. The subjects were given pairs of trials: an information trial and a test trial, separated by 10 min. On the information trial the camouflaged escape platform was randomly placed in one of two positions. On the test trial the platform was placed in the same position for the win-stay task and in the opposite position for the win-shift task. Animals that did not acquire either the stay or the shift response perseverated in their responses, consistently going to the same escape location first on both information and test trials. In the fourth experiment, in which win-shift, win-stay, and perseveration all led to escape, all rats perseverated in their responses. It was concluded that response perseveration and win-stay are more natural responses than win-shift for rats in a water escape situation. This finding contrasts with the spontaneous alternation and readily acquired win-shift behavior previously demonstrated in rodents in exploratory and appetitive situations.     

Social effects on spatial choice in the radial arm maze

Social memory was investigated in the context of a spatial working memory task. Pairs of rats were tested in an eight-arm radial maze. Under most conditions, there was a tendency to choose maze locations that had been visited earlier by the other rat. The possibility that this tendency is produced by common preferences for particular maze locations was ruled out. An opposite tendency to avoid visits to locations that had been visited earlier during the trial by another rat was found only when the maze location contained two pellets (rather than an undepletable supply), the rats’ ability to see each other in the maze was restricted to the central arena, and the maze location had been previously visited by the focal rat. The amount of food available in maze locations did not otherwise modulate social influences on spatial choice. The results indicate that memory for a rat’s own previous choices is combined with memory for the choices made by another rat.     

Central-place foraging by rats on the radial maze: The effects of patch size,food distribution,and travel time

Rats foraged on a four-arm radial maze with one, two, three, and four food items (0.65.g pieces of cheese) placed on different arms (patches) of the maze. In two experiments, the hypothesis was tested that rats should carry food to the center of the maze more often when a patch contains one food item than when it contains multiple food items. Support for this prediction was found when the tendency to carry initial items encountered in patches was compared among the different sized patches. However, a further observation failed to support the hypothesis: Food carrying declined from first to last item encountered in multiple-item patches with clustered food items. Experiment 1 revealed that food carrying was reduced when travel time was increased by barriers placed at arm entrances. Both Experiments 1 and 2 indicated that the tendency for rats to carry food to the center of the radial maze increased as the distance of food encountered on an arm increased from the center. In both experiments, some rats dealt with the problem of multiple items by resorting to multiple-item loading, and some rats carried food items from the end of an arm to a point on the arm nearer the center for consumption.     

Spatial pattern learning in the radial arm maze

Rats experienced a spatial pattern of baited and unbaited arms in an eight-arm radial maze. The spatial pattern remained constant over trials, but the spatial locations that were baited varied unpredictably. Although there was no evidence of control by the spatial pattern during free choice training trials, the rats’ ability to locate baited arms in forced choice test trials was superior to that of animals in a control condition for which maze arms were not baited in a consistent spatial pattern. This is consistent with the results of experiments showing that spatial choices by rats in a pole box maze are controlled by abstract spatial patterns.     

In the dark: Spatial choice when access to spatial cues is restricted

The cognitive mechanisms involved in spatial choice when access to visual cues is restricted were examined in three experiments using male rats. A specially constructed radial-arm maze was used, in which extramaze visual cues could not be perceived from the central arena, but could be perceived from the maze arms. Choices were very accurate when the maze was not rotated during each trial, but inaccurate when the maze was rotated. This suggests that intramaze cues were involved in the control of choices. However, the data clearly showed that choices were not simply controlled by intramaze cues. Rather, control by intramaze cues interacted in a more complex manner with representations of the spatial locations of goals. Such spatial representations were involved in the control of choice despite the absence of visual spatial cues at the time choices were made.