Animal Learning & Behavior

Five experiments were conducted to evaluate various aspects of stimulus preexposure effects on conditioned saccharin aversion in rats of three age groups: weanling (19–25 days), young-adult (92–170 days), and old-age (680–850 days). In Experiment 1, flavor neophobia was examined. Only the young-adult and old-age animals showed evidence of neophobia. Furthermore, habituation of the neophobic reaction differed for these two age groups. Using a brief to moderately long flavor-preexposure period and an intense US (Experiment 2), we demonstrated that the youngest age group was most likely to exhibit retarded conditioning as a result of preexposure to the flavor CS. Using a weaker US and a moderately long saccharin-preexposure period (Experiment 3), age differences in conditioning resulting from preexposure to the flavor CS were reduced. When animals were preexposed to saccharin continuously for 48 h (Experiment 4), age differences in the preexposure effect were not evident. In Experiment 5, the intensity of the US was reduced to determine whether floor effects in the previous experiments had masked age differences in the ability of nonpreexposed rats to acquire an aversion to saccharin. Results indicated that taste-aversion learning was directly related to age. Although open to other interpretations, the results support the notion that the flavor-preexposure effect is influenced by the initial level of flavor neophobia. They also suggest that systematic parametric variation is sometimes necessary to obtain an accurate description of age differences in learning.

Although an arbitrarily specified instrumental response may persist when free reinforcers are concurrently available, the interpretation that earned reinforcers are preferred is tenuous. The present advance-response procedure used both time allocation and advance response rates as indices of preference between free and earned water in rats. When multiple schedule components were two response-dependent schedules with different overall reinforcement rates, higher rates of reinforcement were preferred. However, when the multiple schedule consisted of response-dependent and response-independent components equated for overall rates of reinforcement, no consistent preference for free or earned reinforcers was evident. That a preference for free reinforcers was not obtained is difficult to reconcile with concepts of least effort.

Three experiments examined rats’ ability to discriminate a compound conditioned stimulus (CS) from the individual elements of that compound in a flavor aversion conditioning paradigm. In Experiment 1, presentations of a compound of sucrose and saline solutions were followed by lithium chloride injections, but presentations of those elements individually were nonrein-forced (positive patterning). Conversely, in Experiments 2 and 3, presentations of the individual elements were followed by lithium chloride injection, but compound presentations were non-reinforced (negative patterning). The discriminations were acquired in all three experiments. In addition, all three experiments investigated the effects of preexposure of the discriminative stimuli on subsequent acquisition of the patterned discriminations. In positive patterning, preexposure had no measurable effect on the acquisition of responding (suppression) to the reinforced compound stimulus, but slowed the loss of suppression to the nonreinforced elements. In negative patterning, preexposure slowed the acquisition of suppression to the reinforced elements but had little effect on the loss of suppression to the nonreinforced compound.

The effects of sodium pentobarbital on matching and oddity performance in pigeons were examined by employing a higher-order conditional discrimination paradigm. In this paradigm, the line orientation which was superimposed on all of the response keys signaled whether a response to the matching color or a response to the nonmatching color was correct. All pigeons had extensive previous training in this paradigm and were tested at each of three dosage levels: 5, 7.5, and 10 mg/kg. For all birds, a clear dose-related decrease in accuracy was observed; however, the effect was not differential for matching and oddity trials. Accuracy reductions were accompanied by an increase in position preference on both types of trials. The data are compatible with recent claims that physical identity of the sample and correct comparison stimulus need have no special status for pigeons.

The degree to which rats and monkeys base their discriminations of complex auditory stimuli (“tunes”) on frequency contours rather than on local features was investigated. In Experiment 1, groups of rats and monkeys trained with tunes as S+ and S− acquired a simple operant discrimination no faster than groups that received the same notes of each tune but in a new random order on each trial; neither did the groups differ on two transfer tests devised to detect learning of frequency contour in the tune-trained animals. Acquisition in the tune-trained and random-notes groups seemed to be based on the overall frequency difference between S+ and S−, which was about 1.5 octaves. In Experiment 2, S+ and S− were similar to each other with regard to overall frequency and individual notes, the most salient differentiating characteristic of the tunes being their tonal pattern. The tune-trained groups were clearly superior to the random-notes animals in acquisition, and an initial transfer test suggested that the former might have learned the discrimination on the basis of frequency contour. However, the detailed transfer tests of Experiment 3 strongly suggested that the tune-trained rats and monkeys based their discriminations primarily on local cues rather than on frequency contour. Based on the results of Experiment 4, the data of an earlier study that suggested frequency contour learning in monkeys and rats were reinterpreted in terms of control by local cues.

Pigeons and rats were exposed to a mixed variable-time extinction schedule of reinforcement. During the variable-time component of the schedule, response-independent food was delivered at either a left or a right feeder. The animals were allowed to perform observing responses to produce either stimuli paired with the component of the mixed schedule that was in effect (temporal information) or stimuli paired with the feeder that might deliver food (spatial information). Only stimuli conveying temporal information reinforced observing. This result contradicts a prediction of the “information hypothesis” of observing, but is consistent with various conditioned-reinforcement interpretations of observing.

Rats performed a new delayed matching-to-sample task—the continuous nonmatching-to-sample task. A variable number of trials with one stimulus alternated with trials with a second stimulus. A response on the trial following a stimulus change (nonmatch trial) was reinforced. Responses to repeated stimuli were never reinforced. Rats could maximize reinforcement by remembering across the intertriai interval which stimulus was presented on the previous trial. Sequential analysis indicated that interference from previous conflicting trials (proactive interference, PI) reduced response accuracy but did not affect retention: Accuracy was lower on trials following a nonmatch trial than on trials following repeated stimuli. Furthermore, accuracy increased as a function of the time between the to-be-remembered nonmatch trial and the previous interfering trial. However, neither time between trials nor the distance from a stimulus change affected the rate of decline in accuracy over the retention interval.

The open-field behavior of wild and domestic Norway rats was compared in 15-min tests administered over 5 successive days. Wild rats exhibited more ambulation, jumping, grooming, and time inactive than domestic rats and spent more time along the arena wall. Within- and between-trial changes in behavior were generally greater for wild rats. Factor analyses revealed major loadings on factors identified as “locomotor behavior” and “grooming.” Support was obtained for the hypothesis that domestication has raised the threshold for avoidance-escape behavior in response to a novel environment.

In three experiments that used appetitive preparations with rats, we examined the effects of reinforcing a compound consisting of two previously reinforced stimuli on subsequent responding to those stimuli. Experiment 1 showed that a Pavlovian conditioned stimulus given this treatment evoked fewer magazine entries when presented alone than did a reinforced stimulus that did not receive the compound treatment. Experiment 2 examined inhibition of delay and generalization decrement accounts for the results of Experiment 1. Experiment 3 extended this finding to an instrumental learning paradigm.