Release from proactive interference by experimental amnesia: electroconvulsive shock improves radial-arm maze performance in rats

Rats were trained in an eight-arm radial maze and tested using a proactive interference (PI) procedure. Each test trial consisted of forced choices of four randomly selected arms followed, after a 2-h delay, by free choices among all eight arms. Normally, rats chose correctly during the free choices by entering and retrieving food from the four arms not yet visited during the test trial. Occasionally, an interference trial preceded the test trial by 1.5 or 3 h; interference trials consisted of forced choices of another four arms and an immediate test. The presence of an interference trial lowered test-trial performance (PI). Electroconvulsive shock (ECS) administered immediately after the interference trial had no effect; i.e., PI was still observed. When ECS was administered at the midpoint of the 3-h intertrial interval, performance increased to control (no ECS, no PI) levels. Such release from PI, however, was not obtained, and test-trial performance remained inaccurate when ECS was delivered immediately after the forced choices of the test trial (either 1.5 or 3 h after the interference trial).     

Repetition and alternation in rats

An experiment is reported in which rats were forced equally often to each arm of a T-maze, and, following each forced trial, were given a free trial. Group R were rewarded for repeating (i.e. choosing the side to which they had just been forced), group A for alternating. All rats began by alternating, but group R eventually learned to repeat. Various delays between free and forced trials were introduced, and the forgetting curve was found to be similar for the two groups. It is argued that these results show: (i) The decline in alternation with delay found in experiments on spontaneous and rewarded alternation is due to loss of information rather than a decline in the alternation tendency. (ii) The trace involved is sufficiently general to be regarded as a memory trace. The fact that rats can learn to repeat is inconsistent with the accounts of the memory trace suggested by the theories of Deutsch and Walker.     

Integration and representation in rats' serial pattern learning in the T-maze

Rats were exposed to three-trial series consisting of reinforced (R) trials and one nonreinforced (N) trial in a fixed order, RRN and RNR (Experiments 1 and 2) or NRR and RRN (Experiment 3), on extended visually distinct runways in a T-maze. When initially presented with the same sequence on each series in a session (separate presentations) with the same runway on all trials within a series (Experiments 1 and 3), all the rats developed slower running speeds on N than on R trials. When a runway was sometimes changed between the first and next two trials during separate presentations training (Experiment 2) or both sequences were later intermixed within each session in each experiment, only rats exposed to each sequence on a specific runway maintained these serial running patterns. Rats displayed serial running patterns on a test RNN sequence similar to that on the RNR sequence (Experiment 2), as would be predicted by an intertrial association model of serial pattern learning (Capaldi & Molina, 1979), but responded on test RRR and NRN sequences (Experiment 3) as would be predicted by an ordinal-trial-tag/intratrial association model (Burns, Wiley, & Payne, 1986). Results from test series of free-choice trials in Experiments 1 and 2 failed to support a prediction of the intratrial association model that these rats would integrate RRN and RNR sequences. Rather than always selecting a baited runway on both the second and the third free-choice trials, the rats only selected a baited runway on the third trial on the basis of their choice on the second trial, as would be predicted by the intertrial association model. Only after experiencing all possible outcome sequences during forced-choice training in Experiment 3 did these rats predominantly select a baited runway on every free-choice trial.     

Infantile handling and the facilitation of discrimination and reversal learning

One group of rats was removed from their home cage and received daily handling from Days 3-21, while the control group of litter mates remained in the cage and did not receive any treatment. On Day 22 all rats were weaned and they were housed in individual cages until they were 60 days old. After two pretraining days, subjects were given daily blocks of one free and three forced trials in the T-maze and were rewarded with food after making the correct response. Following 20 days of training and testing on black-white discrimination, subjects were given 10 days of reversal training with four daily trials. Results indicate that the handled animals showed faster running and a greater number of correct choices than the control rats during both the acquisition and reversal learning phases of the experiment.     

The role of exploration in win-shift and win-stay performance on a radial maze

Win-shift spatial memory tasks in a radial maze reinforce animals for avoiding previously visited rewarded arms; win-stay tasks reinforce them for returning to those arms. Win-shift tasks have generally been found much easier to perform, and this may be explained either in terms of foraging models which postulate avoidance of locations where food has been found, or in terms of the predominance of spontaneous alternation (exploration). Experiment 1 examined spontaneous alternation behavior in the radial maze as a function of whether the first visit to an arm had been rewarded or not, and showed that alternation was more probable after nonreward than after reward in both hungry and thirsty rats (a result which conflicts with the foraging account of the win-shift superiority). Experiment 2 replicated the finding that win-stay discrimination performance was inferior to win-shift. A manipulation (lengthening the delay between initial and test choices) which weakens spontaneous alternation, reduced, but did not reverse, the win-shift superiority. In Experiment 3, in order to eliminate the influence of spontaneous alternation, versions of the win-stay and win-shift tasks were devised in which, unlike the original task, all arms were familiar at the choice trial. Under those conditions win-stay was performed better than win-shift. It is concluded that spontaneous alternation plays a major role in many spatial memory tasks, and that the results can best be accounted for by combining principles of exploration and simple associative learning, without recourse to foraging models.     

Memory systems in the rat: effects of reward probability,context, and congruency between working and reference memory

The interaction of working and reference memory was studied in rats on an eight-arm radial maze. In two experiments, rats were trained to perform working memory and reference memory tasks. On working memory trials, they were allowed to enter four randomly chosen arms for reward in a study phase and then had to choose the unentered arms for reward in a test phase. On reference memory trials, they had to learn to visit the same four arms on the maze on every trial for reward. Retention was tested on working memory trials in which the interval between the study and test phase was 15 s, 15 min, or 30 min. At each retention interval, tests were performed in which the correct WM arms were either congruent or incongruent with the correct RM arms. Both experiments showed that congruency interacted with retention interval, yielding more forgetting at 30 min on incongruent trials than on congruent trials. The effect of reference memory strength on the congruency effect was examined in Experiment 1, and the effect of associating different contexts with working and reference memory on the congruency effect was studied in Experiment 2.     

Distinguishing between new and used traces: Differential effects of electroconvulsive shock on memories for places presented and places passed

In a radial maze test of spatial memory, rats enter relatively novel arms while avoiding locations visited a few hours earlier. Certainly, new memories are acquired for arms entered during the retention test. However, the mnemonic consequences of avoiding arms previously entered are not as clearly predicted; old memories might remain unchanged and yet guide behavior, or the use of old memories during a retention test might renew such memories. The possibilities were evaluated in two experiments in which rats performed in a 12-arm radial maze. Each day the arms were randomly sorted into three sets: A, B, C. Each trial began with forced choices of the 4 arms in Set A and ended after 4 hr in an 8-choice test in which the 4 arms not yet visited (Set B) contained food reward. When electroconvulsive shock (ECS) occurred immediately after Set A choices, accuracy during the test was high; when ECS was administered 2 hr after Set A, choices during the test were less accurate. Old memories therefore appear to be more susceptible than new memories to ECS-induced amnesia. In other trials, an extra retention test was given at the mid-point (2 hr) of the retention interval; this 8-choice test consisted of the remaining 4 arms (Set C) and the original 4 arms (Set A). When ECS was administered after the intermediate test, memory for arms in Set A was 2 hr old (but had just been used), while memory for arms in Set C was new (0 hr). The retention test 2 hr later (testing B vs. A or B vs. C) revealed that ECS had an amnestic effect on the recently used memory for arms in Set A but had no effect on the newly acquired memory for arms in Set C. With respect to ECS-induced amnesia, therefore, memories used in a retention test resemble memories that have aged more than memories that have been newly acquired.     

Memory for temporal order of new and familiar spatial location sequences: role of the medial prefrontal cortex

Rats with medial prefrontal cortex or sham control lesions were tested on an eight-arm radial maze task to examine memory for the temporal order of a variable and a constant sequence of spatial locations as a function of temporal distance. During the study phase of each trial, rats were allowed to visit each of eight arms once in an order that was randomly selected or fixed for that trial. The test phase required the rats to choose which of two arms occurred earlier in the sequence of arms visited during the study phase. The arms selected as test arms varied according to temporal distance (0, 2, 4, or 6) or the number of arms that occurred between the two test arms in the study phase. For the variable sequences based on new information, control rats showed an increasing temporal distance function. Relative to control rats, medial prefrontal cortex-lesioned rats displayed a temporal order memory deficit across all distances. For the constant sequence based on familiar information, control rats performed well across all distances. Relative to controls, the medial prefrontal cortex-lesioned rats displayed a performance deficit. The results support the idea that the medial prefrontal cortex contributes to mnemonic operations associated with temporal order for new and familiar spatial location information.     

The control of food-caching behavior by Western scrub-jays (Aphelocoma californica)

Western scrub-jays (Aphelocoma californica) did not show extinction when caching behavior was never rewarded and they had no choice of where to cache the food. However, when the jays had the choice of caching items in 2 different locations or during 2 successive episodes, and only 1 of each was always rewarded at recovery, they rapidly learned to cache in the rewarded location or episode. When the jays had learned during training trials that their caches were always moved to 1 of 2 locations they did not cache in, then on the test trial they cached in the location that had been previously rewarded. To test whether these jays avoided the location in which their caches had been pilfered or chose the rewarded location, the procedure was repeated to include a 3rd location that was never rewarded. The jays avoided the pilfered location but cached equally in the rewarded and nonrewarded locations.     

Memory persistence of rats in a radial maze varies with training procedure

Different estimations of the time-course of spatial working memory have been reported. Some authors found working memory in the radial maze to be relatively long lasting, while others found more rapid exponential decay. In the present experiments it was attempted to account for the conflicting results by investigating the effects of different training procedures. Two types of training were examined under the same circumstances. A group of seven rats was given a series of delayed trials (5, 20, 60, and 120 min). Every delay was repeated at least four times and the delays were presented in an ascending order. The number of errors decreased at every delay except the last one (120 min), where error levels were constant (.50 errors/trial). The good performance was not based on use of intramaze cues or response chaining. In another group of seven rats the same delays were introduced in a quasi-random order and alternated with uninterrupted trials. The number of errors increased exponentially with the length of the delay. However, when this procedure was repeated, the number of errors decreased. These results suggest that training with delayed trials is a major factor to account for the differences in reports of memory persistence in the radial maze.     

Spatial memory over long retention intervals: nonmemorial factors are not necessary for accurate performance on the radial-arm maze by rats

A. Markowska, O. Buresová, and J. Bures (1983, Behavioral and Neural Biology, 38, 97-112) argued that the apparent persistence of accurate spatial working memory over delays of several hours arises from the formation of response strategies and the use of olfactory stimuli that develop with extended training at long delays. To test this explanation rats with extensive prior training at long delays were forced to enter the first four arms in a random order. On test days, the maze was rotated 180 degrees during the 2-h retention interval to determine whether the rats were using intramaze or extramaze (i.e., spatial) cues to guide their choices. On both rotation and control days, postdelay choices were spatially guided, averaging over 90% correct. Accurate spatial working memory at long delays is a reproducible phenomenon and does not appear to result from nonmemorial artifacts.     

Acute Ethanol Administration Impairs Spatial Performance While Facilitating Nonspatial Performance in Rats

Acute ethanol administration produces learning and memory impairments similar to those found following lesions to the hippocampal system in rats. For example, both ethanol and hippocampal lesions impair performance on spatial learning and memory tasks while sparing performance on many nonspatial learning and memory tasks. Lesions to the hippocampal system can also alter the nature of the information that the animal uses to guide its behavior, from using spatial information to using individual cues. In the present experiment, rats were trained, while sober, to navigate on an eight-arm radial arm maze to a specific arm for food reward. During training, the rewarded arm was always in the same specific location and contained well-defined cues. After the rat learned the task, a memory test was conducted under different doses of ethanol (0.0 g/kg [saline control], 1.0, 1.5, or 2.0 g/kg, intraperitoneal). On the test day the maze was rotated so that the cued arm was 90 degrees to the right of its original position. During testing, intact rats showed a significant bias to approach the place where they had been previously rewarded, even though the cue was no longer located there. Acute ethanol administration dose dependently reduced approaches to the rewarded place. However, ethanol administration did not result in increases in random choices; rather, it resulted in a dose-dependent increase in approaches to the cued arm, now in a new location. These results extend previous research showing that acute ethanol administration and lesions to the hippocampal system produce similar effects on learning and memory in rats.     

Lesions of the caudate nucleus selectively impair "reference memory" acquisition in the radial maze

Groups of Long-Evans rats with bilateral lesions of the caudate nucleus, sham lesions, or no lesions were given one trial per day in an eight-arm radial maze. The same four maze arms were baited on each trial. The remaining four arms never contained food. Optimal performance required animals to enter each of the baited arms only once on each trial and to avoid entering the arms in the unbaited set. Rats with caudate lesions learned to enter each of the baited arms only once on each trial. However, these rats were severely impaired in learning to avoid entering the arms in the unbaited set. Implications for dual-memory theories are discussed.     

Errorless learning of a conditional temporal discrimination

In the present study we extended errorless learning to a conditional temporal discrimination. Pigeons' responses to a left-red key after a 2-s sample and to a right-green key after a 10-s sample were reinforced. There were two groups: One learned the discrimination through trial and error and the other through an errorless learning procedure. Then, both groups were presented with three types of tests. First, they were exposed to intermediate durations between 2 s and 10 s, and given a choice between both keys (stimulus generalization test). Second, a delay from 1 s to 16 s was included between the offset of the sample and the onset of the choice keys (delay test). Finally, pigeons learned a new discrimination in which the stimuli were switched (reversal test). Results showed that pigeons from the Errorless group made significantly fewer errors than those in the Trial-and-Error group. Both groups performed similarly during the stimulus generalization test and the reversal test, but results of the delay test suggested that, on long stimulus trials, responding in the errorless training group was less disrupted by delays.     

Effects of parafascicular excitotoxic lesions on two-way active avoidance and odor-discrimination

To investigate whether the parafascicular (PF) nucleus of the thalamus is involved in different learning and memory tasks, two experiments were carried out in adult male Wistar rats that were submitted to pre-training bilateral N-methyl-d-aspartate PF infusions (0.15M, pH 7.4; 1.2 microl/side, 0.2 microl/min). In Experiment 1, we evaluated the effects of PF lesions in two identical 30-trial training sessions, separated by a 24-h interval, of a two-way active avoidance conditioning. PF-lesioned rats exhibited impaired performance in both sessions, measured by number of avoidance responses. In Experiment 2, the effects of PF lesions were assessed in a training session (5 trials) and a 24-h retention test (2 retention trials and 2 relearning trials) of an odor-discrimination task. PF lesions did not significantly disrupt the acquisition or the first retention trial, which was not rewarded. However, lesioned animals' performance was clearly affected in subsequent trials, following the introduction of the single non-rewarded trial. Current data are discussed considering evidence that lesions of the PF nucleus affect learning and memory functions mediated by anatomically related areas of the frontal cortex and striatum.     

Radial maze analog for pigeons: Evidence for flexible coding strategies may result from faulty assumptions

Previous research with the radial maze has found evidence that rats can remember both places that they have already been (retrospective coding) and places they have yet to visit (prospective coding; Cook, R. G., Brown, M. F., & Riley, D. A. (1985). Flexible memory processing by rats: Use of prospective and retrospective information in the radial maze. Journal of Experimental Psychology: Animal Behaviour Processes, 11, 453-469). Such dual coding also has been found in pigeons using a radial maze analog in which insertion of a delay at different points during a trial affects performance differentially depending on where in the trial it is inserted. When a delay is interpolated either early or late in a trial, there is minimal disruption of performance compared with when it is interpolated in the middle of the trial. However, the analysis required with this procedure requires the assumption that if errors made on control trials are subtracted from errors made in delay trials, the remaining errors can be directly attributed to the delay. But errors may also be attributed to the changing criterion for making a response as the trial proceeds. Furthermore, the animal’s tendency to choose alternatives in a systematic order may also affect its need to remember the sequence of choices made (and yet to be made) on each trial. In the present research, we avoided having to make this assumption by giving the pigeons a two-alternative choice at the time of testing and by randomly determining for the pigeon the order of predelay choices on each trial. This change in procedure resulted in comparable performance as a function of where in the trial the test occurred on both control and delay trials. The effect of the delay was to produce a general decrement in performance independent of where it occurred in the trial.     

Strategy selection in a task with spatial and nonspatial components: effects of fimbria-fornix lesions in rats.

The main purpose of the present research was to investigate the ability of rats to learn a 12-arm radial maze task that requires the concurrent utilization of both spatial and intramaze cue information. The task involves in a single trial both place and cue learning as well as reference memory (RM) and working memory (WM). Since the animal can choose place and cue arms in any order, the strategies employed to learn the task can be studied as well as the kinds of memory errors that are made. The results of Experiment 1 showed that the number of errors made on the place and cue components of the task did not differ, and that more RM than WM errors were made early during learning. As the task was learned, the animals tended to choose the place arms before choosing the intramaze cue arms, thus suggesting that a spatial strategy was employed first followed by a cue strategy. In Experiment 2 lesions of the fimbria-fornix resulted in temporary impairments in both RM and WM that were especially apparent on the spatial component of the task. The lesioned rats also switched from choosing mostly place arms early during the trial to choosing more cue arms. While fimbria-fornix lesioned rats recovered from the memory impairments with training, the change in response strategy persisted throughout postoperative testing. The procedure of combining both spatial and non-spatial components concurrently in the same task should prove of value in studying response strategies in animals.     

Orientation in trapezoid-shaped enclosures: implications for theoretical accounts of geometry learning

Human participants learned to select 1 of 4 distinctively marked corners in a rectangular virtual enclosure. After training, control and test trials were interspersed with training trials. On control and test trials, all markers were equivalent in color, but only during test trials was the shape of the enclosure manipulated. Specifically, for each test trial, a single long wall or short wall of the enclosure increased twice as long as or decreased half as long as that present in the training enclosure. These manipulations produced 8 unique trapezoid-shaped enclosures. Participants were allowed to select 1 corner during control and test trials. Performance during control trials revealed that participants selected the correct and rotationally equivalent locations. Performance during test trials revealed that participants selected locations in trapezoid-shaped enclosures that were consistent with those predicted by global geometry (i.e., principal axis of space) but were inconsistent with those predicted by local geometry (i.e., proportion of rewarded training features present at a location). Results have implications for theoretical accounts of geometry learning.     

Variation in arm-choice strategies of rats in radial-maze tasks

Rats were trained with three delay-interpolated tasks in a radial-arm maze. The tasks differed in the post-delay bait conditions. When every arm was baited in the post-delay free choices (Task E8B), rats made adjacent-arm choices frequently. When only the four arms unvisited in the pre-delay forced choices were baited after delay (Task U4B), rats chose unvisited arms preferentially, with frequent arm investigations. When four quasi-randomly selected arms were baited after delay (Task R4B), rats did not choose adjacent arms as frequently as the rats in Task E8B did, and made fewer arm investigations. These results indicate that the rats developed different arm-choice strategies in accordance with tasks. As for the effects of rewarding, post-delay choice behavior was not affected by the pre-delay bait conditions.     

Testing pigeon memory in a change detection task

Six pigeons were trained in a change detection task with four colors. They were shown two colored circles on a sample array, followed by a test array with the color of one circle changed. The pigeons learned to choose the changed color and transferred their performance to four unfamiliar colors, suggesting that they had learned a generalized concept of color change. They also transferred performance to test delays several times their 50-msec training delay without prior delay training. The accurate delay performance of several seconds suggests that their change detection was memory based, as opposed to a perceptual attentional capture process. These experiments are the first to show that an animal species (pigeons, in this case) can learn a change detection task identical to ones used to test human memory, thereby providing the possibility of directly comparing short-term memory processing across species.