In 5 experiments rats were required to escape from a triangular shaped pool by swimming to a submerged platform. The principal group of interest in each experiment received training with a beacon attached to the platform. The purpose of the experiments was to assess if the beacon overshadowed (Experiments 1-4) or blocked (Experiment 5) learning about the position of the platform with reference to the shape of the pool. The platform was located in the center of the pool for the first 2 experiments and in a corner for the remaining experiments. Although there was an overshadowing effect in Experiment 1, the remaining experiments failed to reveal any disruptive influence of the beacon on learning based on the shape of the pool. Moreover, in Experiments 3-5 there was an indication that the beacon facilitated such learning. The results suggest that spatial learning based on the shape of a test environment may not take place in the same way as that based on more discrete landmarks.
In Experiments 1 and 2, rats received initial training in which two neutral events were presented as a serial compound (A->X). Subsequent training with A as a signal for shock was found to endow X with the ability to evoke the conditioned response of suppression. Experiment 2 also showed that responding to X was diminished if, prior to testing, Stimulus A underwent extinction. Two possible mechanisms for these findings are considered: (a) that X elicits responding through the associative chain X-A-shock, and (b) that A activates a representation of X that gains direct associative strength during conditioning with A and loses it during extinction of A. Experiment 3 demonstrated that an X-shock association established after initial A-»X training can be extinguished by nonreinforced presentations of A. These results suggest that associatively evoked representations of stimuli can enter into associations.In standard demonstrations of sensory preconditioning (e.g., Prewitt, 1967), subjects are given an initial phase of training with a serial compound event X-A, neither of the components of which has any marked motivational significance or response-eliciting power. In a second phase of training, one of the elements of the compound (A) undergoes standard Pavlovian training and, by virtue of its association with a motivationally significant unconditioned stimulus (US), comes to evoke an overt conditioned response (CR). In the final, test, phase of the procedure it is demonstrated that Stimulus X is also capable of evoking the CR. This result has been interpreted in terms of the formation of an association between X and A during the first phase of training. The second phase establishes an A-US association so that presentation of the X stimulus at test is able to contact the representation of the US (and thus evoke the CR) by way of the associative chain X-A-US.In addition, the effect can be found when A and X are presented as a simultaneous compound (e.g., Brogden, 1939;Rescorla & Freberg, 1978); in this case too, an excitatory X-A association could still be formed and might be responsible for the result observed. The remaining temporal arrangement (the backward case), in which A precedes the presentation of X, has been little studied; and to the extent that this procedure is less likely to generate the excitatory X-A association, it might be supposed that it would be unlikely to yield a sensory preconditioning effect. Indeed, with one exception, studies that have used backward pairings (i.e., A-X) in the first phase of training have failed to find any effect (
A novel procedure is described in which the floor temperatures (warm and cool) in an operant chamber are used as contextual cues in 2 experiments with rats. Experiment 1 demonstrated that rats learn the relationship between these thermal contexts and auditory stimuli that have been paired with them. Experiment 2 showed that thermal contexts can serve a conditional function that (a) reflects the operation of a mechanism that is common to conventional, visual contexts and (b) is bound to these contexts' ability to retrieve the nature of the relationship between the auditory stimuli and food. The experimental study of rodent behavior within psychology and neuroscience has been dominated by the use of the operant chamber. Even in its basic form, this apparatus provides the opportunity to present animals with a range of stimuli, manipulanda, and events of motivational significance. Moreover, the ambient or contextual cues that accompany being placed in such a chamber (its visual characteristics, odor, and so on) are also cues that animals readily learn about (see Bouton, 1991). As versatile as this apparatus is, it remains the case that the number of stimulus dimensions that are both available and readily controlled (principally, the auditory and visual dimensions) serves as a constraint on the experiments that can be conducted. For example, when an additional stimulus dimension is required, different odor cues have been used that require procedures designed to minimize contamination between them. These procedures can be either expensive or inconvenient (e.g., Hall & Honey, 1989). At a practical level, therefore, the study of rodent behavior in the laboratory would be facilitated by the development of a system that allows another dimension to be presented. Moreover, many of our conceptual models of animal learning and memory (e.g., Gluck & Myers, 1993; McLaren, Kaye, & Mackintosh, 1989; Pearce, 1994; Rescoda & Wagner, 1972) assume that the principles that underlie them will be as likely to operate in one (sensory) domain as another. Use of an additional dimension would provide a way of assessing the generality of those effects on which our understanding of learning and memory has developed. With these practical and theoretical considerations in mind, we describe the development and use of a novel addition to an operant
Rats were placed in 4 contexts (A, B, C, D) where they received 2 auditory stimuli (X, Y); in A and B, presentations of X were paired with food and those of Y were not, and in C and D, Y was paired with food and X was not. Rats then received combinations of contexts that had provided congruent (AB, CD) or incongruent (AD, CB) information about X and Y's relationship to food. Responding was more variable during congruent than incongruent trials (Experiment 1) and was systematically increased and decreased during congruent (relative to incongruent) trials by the presentation of food or no food, respectively (Experiment 2). These results support a connectionist approach to acquired changes in stimulus distinctiveness.
Neural manipulations were used to examine the mechanisms that underlie the acquired equivalence and distinctiveness of cues in rats. Control rats and those with excitotoxic lesions of either the hippocampus (HPC) or entorhinal cortex (EC) acquired the following conditional discrimination: In Contexts A and B, Stimulus X-->food and Stimulus Y-->no food, and in Contexts C and D, Y-->food and X-->no food. Rats then received many food pellets in A but not in C. After this treatment, control rats showed more magazine activity in B than in D--an acquired equivalence-distinctiveness effect. This effect was also evident in HPC rats but not in EC rats. These results indicate that changes in stimulus distinctiveness are dissociable from the process of conditional learning.
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