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In Experiment I rats were trained for 2% days under partial (PRF) or continuous reinforcement (CRF) conditions starting at 18, 22,28, or 36 days of age and were then subjected to immediate extinction. At all ages there was a strong partial reinforcement extinction effect (PREE), and absolute size of PREE was greatest in the youngest rats. Rate of extinction increased as a function of age following both CRF and PRF. In Experiment II the youngest and oldest age groups of Experiment I were run under the two reward conditions of Experiment I and in a third condition, PRF with number of rewards rather than trials equated to CRF (PRF-R). The PRF-R and PRF groups were not different in extinction, and both were more persistent than CRF. The youngest rats were again more persistent than the oldest, particularly after PRF training. In Experiment III it was shown that the well-known paradoxical effect, greater reward in CRF acquisition leads to faster extinction, operates in our youngest and oldest animals, but is more pronounced in the oldest. The results are discussed in terms of whether they require different explanations than those often applied to extinction data from adult rats.We seem to know much more about the ontogeny of aversive (fear) conditioning in rats than about appetitive learning, perhaps because it seems easier to equate motivational factors at different ages in aversive than in appetitive learning (Campbell, 1967). However, we do have a little information on how level of maturity in rats affects learning in appetitive situations. Weanling rats are reported to be inferior to adults in learning a barpress response for food reward and in forming a lightdark discrimination (Campbell, Jaynes, & Misanin, 1968). Similarly, juvenile rats (less than 50 days of age) do less well on a spatial discrimination than adults (Bronstein & Spear, 1972). The younger rats in the studies of Campbell and his colleagues also show poorer retention of both a learned barpress and light-dark discrimination (Campbell et al., 1968), unless there are periodic reexposures of the rats to the experimental task over the retention interval (Campbell & Jaynes, 1969).While these experiments tell us something about the ontogeny of appetitive learning, they fall short of being decisive in several respects. First, the number of ages investigated is small over the range of ages investigated. This makes it difficult to determine the age at which an adult pattern first appears, and says little about the abruptness of the transition. A second related point is that, when traimng sessions are conducted over an extended period of time, it is difficult to separate changes in performance related to developmental factors from changes resulting from amount or duration of training. Finally, experimental treatment of the youngest subjects in these studies on appetitive learning usually begins in the fourth postnatal week or later,

In two experiments conducted in a runway apparatus with young rats as subjects, a classic partial reinforcement acquisition (Goodrich) effect (PRAE) was obtained in rats trained from 30 to 45 days of age but not in those trained from 18 to 25 days of age. In an immediate extinction test for half of the subjects, a partial reinforcement extinction effect (FREE) was found in both age groups, but there were interesting differences in the extinction patterns. Following a 45-day retention interval and a continuous reinforcement reacquisition treatment for all groups, the FREE emerged clearly in adult animals which had not undergone previous extinction. In those animals which had previously been extinguished the FREE emerged again but was somewhat reduced. The results from the younger animals suggest that relatively permanent persistence in approach to food can be formed in the days that normally immediately precede and follow weaning in laboratory rats.

The effect of prenatal alcohol exposure on nipple attachment behavior was studied at a number of ages in rat pups whose mothers consumed liquid diets containing 35% or 0% ethanol-derived calories (EDC) or standard lab chow during pregnancy. In the first experiment, nondeprived pups were tested in groups of three (one from each of the prenatal treatment groups) for nipple attachment to an anesthetized lab chow dam. The second experiment examined the influence of the type of test dam (35% EDC, 0% EDC, or lab chow) on nipple attachment behavior. Finally, experiment three evaluated the possibility that the data from the preceeding experiments were the result of either testing the offspring in triads or alcohol-induced alterations in maternal behavior. Rat pups from all three groups reared by their biological mothers and a group of 35% EDC offspring fostered at birth to lab chow surrogate mothers were tested alone rather than in triads. The data from all three experiments clearly indicated an age-related pattern of nipple attachment behavior; the latency to attach was shortest in 6- to 12-day-old pups and increased rapidly from 13 to 21 days of age. Prenatal exposure to alcohol significantly increased the latency to attach to the nipple and the rank order of attachment at the younger ages (three to nine days of age) but not in the older animals. Also, the prenatal effect of alcohol was independent of the type of dam to which testing was conducted, the social interaction involved in testing triads, or fostering procedures. These results suggest that prenatal alcohol exposure can interfere with the development of normal suckling behavior, which might influence normal growth.

Two experiments show that, after taste-aversion conditioning, rats can use external retrieval cues to recall or anticipate the aversive taste solution and avoid its location without making contact with the flavor. They also show that the rat's avoidance of a conditioned aversive taste and its consumption of the aversive flavored solution can be attenuated by giving it prior runway training in which taste reward is given inconsistently on a partial reinforcement schedule.

In Experiment 1, six age groups of rats were trained to run an alley for either partial (PRF) or continuous reinforcement (CRF). Training was during a 2-day period starting at six different ages, and extinction was initiated about 12 hr later. There was a clear partial reinforcement extinction effect (FREE) at all ages and, particularly after PRF training, an inverse relationship between resistance to extinction and age. In Experiment 2, retention and durability of persistence as well as immediate persistence were tested following acquisition at three different ages. The immediate extinction tests confirmed the results of Experiment 1. In the delayed extinction tests, greater resistance to extinction following PRF than following CRF was found in all age groups after both the retention and durability manipulations. In neither test was there an effect of age in original acquisition on the magnitude of adult persistence. These experiments confirm earlier findings of remarkable persistence in rats trained and tested at weanling age, show that the persistence is even greater in preweanlings, and show that it is retained into young adulthood.

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