Intramaze cues and “odor trails” fail to direct choice behavior on an elevated maze

The relative importance of intramaze cues and extramaze cues in directing choice behavior on a radial arm maze was examined using a discrimination procedure which selectively rewarded rats for following only one set of cues. Rats in the intramaze group obtained food from a food cup on the end of each arm. Rats in the extramaze group obtained food from a food cup on a small platform just beyond the end of each arm. All rats were first shaped to perform correctly with the maze in a constant position. Then the maze was rotated to a new position after every choice. For rats in the intramaze group, the food moved with the arms, making intramaze cues relevant. For rats in the extramaze group, the food remained on the platforms (in the same position in the room), making extramaze cues relevant. Rats in the extramaze group performed almost perfectly during maze rotation, demonstrating that intramaze cues were not necessary to support accurate choice behavior. Rats in the intramaze group never performed better than chance, demonstrating that intramaze cues (from the rats, the reinforcement, and the apparatus) were not adequate to control choice behavior. The results of the present experiment are compared to those of other experiments describing the influence of “odor trails” or other olfactory stimuli on choice behavior in mazes.     

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).     

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.     

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.     

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.     

Performance ofBetta splendens in a radial arm maze

Siamese fighting fish (Betta splendens) were tested in aquatic versions of radial arm mazes. In the first experiment, the fish were trained to find tubifex worms in an eight-arm maze in which the optimal strategy was to choose each arm once without repetition. After initial training, the fish entered approximately 6.63 different arms in eight choices, showing a strong tendency to choose sequences of adjacent arms, moving about the maze in a Stereotypic direction. This algorithmic response pattern was not, however, sufficient to predict the high performance level of the fish. In the second experiment, a delay of .5 or 5 min was interposed between the fourth and fifth choices. Similar Stereotypic patterns continued in Experiment 2, but choice accuracy following the longer delay declined to a level not significantly above chance. In the third experiment, different fish were tested in a three-arm maze, reinforced either for returning from the second arm to the arm in which they had most recently been fed (win-stay) or for visiting a third arm (win-shift). The fish were significantly faster at acquiring the win-shift contingency than the win-stay contingency. These results demonstrate that solution of spatial tasks depends on the interaction of appropriate behavioral strategies and cognitive capacities that may have little generality across species.     

Rats' performance on an interval time-place task: Increasing sequence complexity

Rats were trained on an interval time-place learning (TPL) task in which the location of food availability depended on the time since the start of the session. Each of four levers (numbered 1, 2, 3, 4) provided food on an intermittent schedule for two nonconsecutive 3-min periods. The order in which the levers provided food was 1, 2, 4, 3, 2, 3, 1, 4. This order was consistent across sessions. Previous research conducted in our lab has shown that when only four “places” are used, rather than the eight in the present study, rats use a timing strategy to track the location of food. Pizzo and Crystal (2004) recently trained rats on an interval TPL in which each of eight arms of a radial arm maze provided food. They found evidence suggesting that rats used both spatial and temporal information. In the present study, in which a revisiting strategy was used (i.e., each lever provided food on more than one occasion), the rats tracked both the spatial and the temporal availability of food for the first half of the session. Interestingly, in the second half of the sessions, the rats appeared to be timing the availability of food even though they did not know where it would occur. That is, the rats knew the temporal, but not the spatial, contingencies for the second half of the session. It appears that the requirement of revisiting a previously reinforced lever resulted in rats' no longer being able to solve the spatial aspect of the task.     

The effects of response cost and species-typical behaviors on a daily time–place learning task

Two theories that have been hypothesized to mediate acquisition in daily time–place learning (TPL) tasks were investigated in a free operant daily TPL task: the response cost hypothesis and the species-typical behavior hypothesis. One lever at the end of one of the choice arms of a T-maze provided food in the morning, and 6 h later, a lever in the other choice arm provided food. Four groups were used to assess the effect of two possible sources of response cost: physical effort of the task and costs associated with foraging ecology. One group was used to assess the effect of explicitly allowing for species-typical behaviors. If only first arm choice data were considered, there was little evidence of learning. However, both first press and percentage of presses on the correct lever prior to the first reinforcement revealed evidence of TPL in most rats tested. Unexpectedly, the high response cost groups for both of the proposed sources did not perform better than the low response cost groups. The groups that allowed animals to display species-typical behaviors performed the worst. Skip session probe trials confirmed that the majority of the rats that acquired the task were using a circadian timing strategy. The results from the present study suggest that learning in free operant daily TPL tasks might not be dependent on response cost.     

Stimulus control and function of arm and wall travel by rats on a radial arm floor maze

Hoffman, Timberlake, Leffel, and Gont (1999) concluded that the tactic of effective trail following (in the form of arm and wall travel), rather than distance minimizing, central-place search, or random search, best characterized the locomotion of rats on a radial arm maze placed flat on the floor of an arena (a floor RAM). The present experiments analyzed further the stimulus control and function of arm and wall travel. Experiment 1 showed that arm travel was controlled more by the edge of a maze arm than by its surface. Experiment 2 showed that rats with whiskers clipped on one side traveled along arms less and along walls more than did intact rats. Experiment 3 showed that maze arms increased search effectiveness and decreased suppression of locomotion by bright light and a novel environment. The results support the hypothesis that arm and wall travel are based on mechanisms of trail following, which, in natural settings, contribute to food finding and regulation of social relations and fear.     

Rats in a levered T-maze task show evidence of time–place discriminations in two different measures

It is difficult for rats to learn to go to an arm of a T-maze to receive food that is dependent on the time of day, unless the amount of food in each daily session is different. In the same task, rats show evidence of time–place discriminations if they are required to press levers in the arms of the T-maze, but learning is only evident when the first lever press is considered, and not the first arm visited. These data suggest that rats struggle to use time as a discriminative stimulus unless the rewards/events differ in some dimension, or unless the goal locations can be visited prior to making a response. If both of these conditions are met in the same task, it might be possible to compare time–place learning in two different measures that essentially indicate performance before and after entering the arms of the T-maze. In the present study, we investigated time–place learning in rats with a levered T-maze task in which the amounts of food varied depending on the time of day. The first arm choices and first lever presses both indicated that the rats had acquired time–place discriminations, and both of these measures became significantly different from chance during the same block. However, there were subtle differences between the two measures, which suggest that time–place discrimination is aided by visiting the goal locations.     

Effects of varying trial distribution,intra- and extramaze cues,and amount of reward on proactive interference in the radial maze

Rats are typically less accurate in their arm selections in the radial maze over successive trials in a session (Roberts & Dale, 1981). In the present study, rats’ choice accuracy declined when such trials were separated by 2-min (massed) but not by 2-h (spaced) intertriai intervals. Changing intramaze visual/tactile arm stimuli (Experiments 1 and 3) or extramaze landmark stimuli (Experiment 4) between trials weakened the massed-trials effect, but changing the number of food pellets per arm, either alone or in conjunction with changes in intramaze cues (Experiments 2 and 3), did not. The rats also tended to avoid the spatial locations of their last four choices on a previous trial during their first four choices on a current trial, and more so with massed than with spaced trials. These findings indicate that intertriai proactive interference (PI) occurred only with massed trials and was weakened by changing intra- and extramaze cues between such trials.     

Foraging for covered and uncovered food on a radial maze

Rats were trained to forage for food on a four-arm radial maze. Each arm of the maze was defined as a patch and contained four feeding stations. Each patch contained a total of 20 45-mg food pellets, with the first feeding station in each patch baited with 1 pellet and the remaining stations baited with 1, 5, or 13 pellets. In Experiment 1, one group of rats was tested with feeders open and food readily accessible, and another group was tested with metal covers on the feeders, which necessitated extra time to gain access to food. With open feeders, the rats visited each feeder in a patch in the order in which they encountered the feeders, from the center of the maze to the end of the arm. The rats in the group with the covered feeders often visited the feeders containing 5 or 13 pellets first and the feeders containing 1 pellet last. In Experiment 2, it was found that the rats switched readily between these two foraging strategies when tested with covered and open feeders on alternate sessions. The extra time and effort required to uncover food appeared to produce selective foraging in rats.     

Testing optimal foraging theory on the radial maze: The role of learning in patch sampling

A four-arm radial maze containing 10 feeders in each arm (patch) was used to study patch sampling in rats. In each of three experiments, rats foraged for 30 sessions. On each session, two randomly chosen patches were baited with food and the remaining two patches were empty. In Experiment 1, the number of baited feeders in baited patches (6) was varied from 1–10 over five groups of subjects. Mean visits to empty patches was an inverse function of 6, as predicted by an optimal foraging model. In Experiments 2 and 3, rats’ ability to discriminate between baited and empty patches was examined when food in baited patches was placed in fixed locations, either in clumps (Experiment 2) or distributed throughout the patch (Experiment 3). Rats in fixed-food-location conditions reliably visited fewer feeders in empty patches than did rats in randomly changing control groups. Examination of within-patch foraging patterns indicated that rats in fixed-food-location groups selectively sampled potentially baited locations and abandoned the patch if food was not found. It is suggested that processes of patch discrimination were responsible for these effects.     

Rats acquire a low-response-cost daily time-place task with differential amounts of food

Gallistel (1990) theorized that when animals encounter a biologically significant event, they automatically form a tripartite code consisting of the time, place, and nature of the event. Recent research examining such time-place learning (TPL) has shown that rats are reluctant to perform TPL tasks and appear to do so only under high-response-cost situations (Thorpe, Bates, & Wilkie, 2003; Widman, Gordon, & Timberlake, 2000). In the present study, we trained rats on a low-response-cost daily TPL task, in which the amount of food varied with the spatiotemporal contingencies. It was found that rats readily learned this task. We hypothesize that, rather than automatically encoding a tripartite code when faced with a biologically important event, rats instead automatically encode bipartite codes consisting of time-event and event-place information.     

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.     

Spatial configuration and list learning of proximally cued arms by rats in the enclosed four-arm radial maze

In an enclosed four-arm radial maze, after sampling three experimenter-selected baited arms (thestudy segment) and following rotation of the maze, rats had to find the fourth baited arm among all four unblocked arms (test segment). The rats learned this task with two sets of arm cues, objects at arms’ entrances and full arm inserts, each maintained in a fixed configuration. When we changed the configuration of one set of arms to itsmirror image and that of the other set to a moremixed variation by switching opposite and adjacent cued arms, the rats’ accuracy was similarly disrupted (Experiment 1). In Experiment 2, the same rats rapidly recovered their high search accuracy on four new configurations recombined from pairs of adjacent arms and pairs of opposite cued arms from the previous final two configurations. Their test segment search accuracy, however, was again disrupted when these configurations were varied either only over trials’ study segments or only over trials’ test segments. In Experiment 3, however, these rats attained accuracy as high on two sets of cued arms with constantly changing configurations as on two sets with constant configurations. Thus, the rats were able to separately represent four different spatially stable configurations, and then they could learn to represent two of these configurations as lists of spatially irrelevant items. We discuss these findings in terms of association theory and parallel map theory (Jacobs & Schenk, 2003).     

Spatial localization of a goal: Beacon homing and landmark piloting by rats on a radial maze

We performed six experiments in order to examine the ability of rats to use moving beacons and landmarks as cues to the location of reward on an eight-arm radial maze. In Experiments 1–4, the cues and goals were moved before each trial, and groups in which a single beacon was placed on the rewarded arm, a single landmark indicated that reward was on the arm immediately to the left of a landmark, or two landmarks were placed on each side of the reward arm were compared. The rats rapidly learned to track the reward in the beacon condition, failed to find the reward sooner than chance expectation with a single landmark, and did only slightly better than chance with two landmarks. In Experiments 5 and 6, the rats were trained in five trials per day, with the landmark and goal locations constant over daily rewarded trials, and in two extinction trials that were inserted among the rewarded trials. The rats found the goal arm at substantially better than chance expectancy with both one and two landmarks. Our results, in agreement with data from recent swimming pool experiments (A. D. L. Roberts & Pearce, 1998), show that rats will use the relationship between moving landmarks and a goal in order to find reward.     

Maze patrolling by rats with and without food reward

The effects of food reward on rats’ behavior in radial and Dashiell tunnel mazes were examined in two experiments. In the first, with animals at ad-lib body weights, food reward reduced speed of movement at the food locations, but did not affect the patterns of movement in either maze. Exploratory efficiency in the Dashiell maze was unaffected by food reward, and spontaneous patrolling of the radial maze by the nonrewarded animals was comparable to the behavior, reported by others, of rats running for food reward on elevated eight-arm mazes. In the second experiment, with subjects maintained at 80% of ad-lib body weights, there was some evidence for “winstay” learning: food-rewarded rats in the Dashiell maze were relatively more active near the food locations than were the nonrewarded animals, and more rewarded than nonrewarded rats revisited all food locations in the radial maze. Nonetheless, exploratory efficiency in the Dashiell maze was unaffected by food reward, as was patrolling efficiency in the radial maze, which was again comparable to that of rats on elevated mazes. The similarity in behavior of rewarded and nonrewarded animals in these mazes implies that the major determinant of their behavior, whether or not food reward is provided, is a spontaneous tendency to avoid places recently visited.     

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.     

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.