Christina M. Thorpe

Rats have trouble associating all three parts of the time–place–event memory code

The ability of animals to associate an event with predictable time and place information confers a major biological advantage. The current research uses a variety of procedures and paradigms (e.g. place preference, radial arm maze, Morris water maze, T-maze, go no-go) to show that rats, unlike pigeons [e.g. Anim Learn Behav 22 (1994) 143] do not readily make an event-time-place association. They do make associations between event-time and event-place information, however. These findings are in disagreement with Gallistels (The Organization of Learning, MIT Press, Cambridge, MA ) theory that claims that animals automatically store a memory code that has these three pieces of information. The present research is in line with the work of others who also find that rats do not readily make daily time-place associations [Behav Processes 23 (1997) 232; Behav Processes 52 (2000) 11; Behav Processes 49 (2000) 21; Anim Learn Behav 28 (2000) 298]. An interesting finding that did emerge from the present research was that at least some rats can use a circadian timer to solve a time-of-day discrimination if the task is a go no-go discrimination.

Individual Differences in Conceptual and Procedural Fraction Understanding: The Role of Abilities and School Experience.

Recent research on childrens conceptual and procedural knowledge has suggested that there are individual differences in the ways that children combine these two types of knowledge across a number of mathematical topics. Cluster analyses have demonstrated that some children have more conceptual knowledge, some children have more procedural knowledge, and some children have an equal level of both. The current study investigated whether similar individual differences exist in childrens understanding of fractions and searches for explanations for these differences. Grade 6 students (n=119) and Grade 8 students (n=114) were given measures of conceptual and procedural knowledge of fractions as well as measures of general fraction knowledge, general conceptual ability, and general procedural ability. Grade 6 children demonstrated a four-cluster solution reflecting those who do poorly on procedural and conceptual fraction knowledge, those who do well on both, those whose strength is procedural knowledge, and those whose strength is conceptual knowledge. Grade 8 children demonstrated a two-cluster solution reflecting those whose strength is procedural knowledge and those whose strength is conceptual knowledge. Cluster in either grade, however, did not vary in distribution across schools and was not related to general conceptual ability or general procedural ability. Overall, these results provide a more detailed picture of individual differences in conceptual and procedural knowledge in mathematical cognition.

Unequal interval time-place learning.

In time-place learning tasks food availability depends upon both spatial and temporal variables. For example, food might be first available at location one, then location two, then location three, and finally location four. To date, the duration of food availability at each of the locations have been identical (e.g. for 4 min). The major purpose of the present experiment was to determine if rats can successfully learn a time-place task in which four locations provided food for different durations. Lever 1 intermittently produced food for 6 min, then Lever 2 produced food for 4 min. Lever 3 and 4 provided food for 2 and 8 min, respectively. Rats were able to learn this unequal interval time-place task. However, their behavior on this unequal interval time-place task was not in agreement with Scalar Expectancy Theory/Webers Law.

Spatial problem solving and hippocampal place cell firing in rats: control by an internal sense of direction carried across environments.

Rats learned to find the baited corner of a box surrounded by a curtain, regardless of whether they had a fixed or random point of entry (POE) through the curtain. On probe trials, rats used an internal direction sense carried from outside the curtain to solve the problem, and only used the visual cue inside the curtain if disoriented and denied access to a view of the room en route. Similar disorientation procedures were required to obtain cue control of hippocampal place fields. The results suggest that: (1) POE effects previously found in the water maze may be task-specific; (2) an undisrupted internal sense of direction carried from one environment to another may provide the preferred solution to spatial problems in the second environment, even when this second environment is a familiar one with stable visual cues; and (3) choice behaviour is sometimes, but not always, representative of the hippocampal representation of space.

The effects of bilateral lesions to the dorsal tegmental nucleus on spatial learning in rats.

The head-direction (HD) signal is believed to originate in the dorsal tegmental nucleus (DTN) and lesions to this structure have been shown to disrupt HD cell firing in other areas along the HD cell circuit. To investigate the role of the DTN in spatial navigation, rats with bilateral, electrolytic (Experiment 1), or neurotoxic (Experiment 2) lesions to the DTN were compared with sham controls on two tasks that differed in difficulty and could be solved using directional heading. Rats were first trained on a direction problem in a water T maze where they learned to travel either east or west from two locations in the experimental room. DTN-lesioned rats were impaired relative to sham controls, both early in training, on the first block of eight trials, and on the total trials taken to reach criterion. In the food-foraging task, rats were trained to leave a home cage at the periphery of a circular table, find food in the center of the table and return to the home cage. Again, DTN-lesioned rats were impaired relative to sham rats, making more errors on the return component of the foraging trip. These data extend previous cell-recording studies and behavioral tests in which rats with electrolytic DTN lesions were used, and they demonstrate the importance of the direction system to spatial learning.

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.

Interval time-place learning by rats: Varying reinforcement contingencies

Two experiments with rats were conducted to study interval time-place learning when the spatiotemporal contingencies of food availability were more similar to those likely to be encountered in natural environments, than those employed in prior research. In Experiment 1, food was always available on three levers on a variable ratio (VR) 35 schedule. A VR8 schedule was in effect on Lever 1 for 5 min, then on Lever 2 for 5 min, and so forth. While rats learned to restrict the majority of their responding to the lever that provided the highest density of reinforcement, they seemed to rely on a win-stay/lose-shift strategy rather than a timing strategy. In Experiment 2, the four levers provided food on variable ratios of 15, 8, 15, and 30, each for 3 min. As expected the rats learned these contingencies. A novel finding was that the rats had a spike in response rate immediately following a change from a higher to lower reinforcement density. It is concluded that rats exposed to spatiotemporal contingencies behave so as to maximize the rate of obtained reinforcement.

A Two-Platform Task Reveals a Deficit in the Ability of Rats to Return to the Start Location in the Water Maze

The ability of rats to return to the start location was examined with a 4-arm radial water maze. The task required rats to find 2 hidden platforms in sequence. Rats were released from 1 of 3 arms and there was a platform located in the fourth arm. Once a rat found this platform, a 2nd platform was raised in another location, which was either the start location, for 1 group, or another fixed location, for a control group. Across 3 experiments, all rats learned the location of the 1st fixed platform in 80 to 120 trials. However, rats had difficulty finding a 2nd platform if it was at the start location. Control groups revealed that rats could learn 2 platform locations and that the difficulty in learning to return to the start location did not seem to be attributable to its aversive nature. In separate groups, exposure to the start location was increased by starting the rats from an initially stable platform. Rats still did not readily learn to return to the start location. The authors suggest that start location, when varied, cannot readily be used to define the location of a hidden platform.

Hippocampal spatial mapping and the acquisition of competing responses

Response reversal learning is facilitated in many species, including humans, when competing responses occur in separate contexts. This suggests hippocampal maps may facilitate the acquisition of competing responses and is consistent with the hypothesis that contextual encoding permits rapid acquisition of new behaviors in similar environments. To test this hypothesis, the pattern of Arc expression was examined after rats completed a series of left/right response reversals in a T‐maze. This reversal training occurred in the same room, two different rooms, or within a single room but with the maze enclosed in wall‐length curtains of different configurations (i.e., black/white square or circle). Across CA1 and CA3, successive T‐maze exposures in the same room recruited the same cells to repeatedly transcribe Arc, while a unique population of cells transcribed Arc in response to each of two different rooms as well as to the two unique curtain configurations in the same room. The interference from original learning that was evident on the first reversal in animals without a context switch was absent in groups that experienced changes in room or curtain configuration. However, only the use of unique rooms, and not changes in the curtained enclosure, facilitated learning across response reversals relative to the groups exposed to only one room. Thus, separate hippocampal maps appear to provide protection from the original learning interference but do not support improved reversals over trials. The present data suggest changes in heading direction input, rather than remapping, are the source of facilitation of reversal learning.

The Effects of Lesions to the Postsubiculum or the Anterior Dorsal Nucleus of the Thalamus on Spatial Learning in Rats

To investigate the role of the head direction (HD) cell circuit in spatial navigation, rats with bilateral, neurotoxic lesions to the postsubiculum (PoS; Experiment 1) or the anterior dorsal nucleus of the thalamus (ADN; Experiment 2) were compared to sham controls on 2 tasks that could be solved using directional heading. Rats were first trained on a direction problem in a water T maze where they learned to travel either east or west from 2 locations in the experimental room. ADN lesioned rats were impaired relative to sham controls on the first block of 8 trials, but not on the total trials taken to reach criterion. This transient deficit was not observed in rats with lesions to the PoS. In the food-foraging task, rats were trained to leave a home cage at the periphery of a circular table, find food in the center of the table, and return to the home cage. Both PoS and ADN lesioned rats showed impairments on this task relative to sham rats, making more errors on the return component of the foraging trip. The spatial deficits produced by lesions to the PoS and the ADN, downstream structures in the HD cell circuit, are not as severe as those observed in earlier studies in rats with lesions to the dorsal tegmental nucleus.