Four experiments point to the existence of a transitional period in neonatal rats for the learning of persistence. Partial reinforcement training with suckling on an anesthetized dam as reward induced greater persistence in extinction of the approach response than did continuous reinforcement in rat pups 14 days old but not in 11-day-old pups. Other aspects of this period of development are discussed.The partial reinforcement extinction effect (FREE) describes the relative persistence-increased resistance to extinction-that results from partial or intermittent reinforcement relative to the continuous reinforcement of an instrumental response. Research in our laboratory has demonstrated the occurrence of the FREE in preweanling rat pups (as young as 17-18 days of age) with dry food as reward and the retention of this learned persistence over a 50-day period ). In more recent experiments we have shown that appetitive instrumental learning and extinction is possible in previsual and preaudile (10-to 11-day-old) rat pups (Amsel, Burdette, & Letz, 1976;Amsel, Letz, & Burdette, 1977). There is, however, no evidence of a FREE in 11-day-old rat pups with nonnutritive suckling as reward (Amsel, Letz, & Burdette, 1977, Experiment 2).Consequently, in the present series of experiments, we attempted to identify a transitional period for the manifestation of the FREE. In Experiment 1, we developed a spaced-trial procedure and investigated the effect of the type of suckling experience on acquisition and extinction of an approach response of 11-and 14-day-old rat pups, hi Experiment 2, schedule of reward and type of suckling experience were manipulated in
The acquisition and extinction of approach behavior in 11-day-old albino rat pups was studied in three experiments. The reinforcement in Experiments 1 and 2 was 15 sec of nonnutritive suckling on an anesthetized lactating female. In Experiment 1, detention duration, 0-or 15-sec in the goal box on nonreward (N) trials, was studied in two groups whose acquisition training consisted of alternating blocks of rewarded and nonrewarded trials with a short (5-sec) intertrial interval. Experiment 2 combined an examination of the effect of detention length with whether or not the mother was physically present, though inaccessible, on N trials with partially or continuously rewarded groups. Detaining subjects on N trials slowed the rate of extinction when the mother was present but increased rate of extinction when she was absent. There was no evidence of a partial reinforcement extinction effect in any of the groups. The effectiveness of six types of reinforcement on promoting acquisition of approach behavior was assessed in Experiment 3. Equally good acquisition, but differences in extinction, was obtained with an adult conspecific as the reinforcer, independent of sex and suckling, but there was no acquisition of approach responding when a sibling or no conspecific was in the goal box. These data extend earlier findings of appetitive learning and extinction in infant rats. They do not support the view that inhibitory mechanisms are absent in the rat until the third or fourth week of life.
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,
The present studies examined the effects of intermeal interval and pellet magnitude on the development and maintenance of schedule-induced polydipsia using a between-subjects design as contrasted with earlier investigations, which have exclusively employed within-subject procedures. In Experiment 1, pellets were delivered on one of six fixed-time schedules (20, 40, 60, 120, 180, and 240 sec) to different groups of rats. Although the typical bitonic function relating water intake to intermeal interval was obtained, the function showed a wide plateau ranging from 60 to 180 sec. Experiment 2 demonstrated that both intermeal interval and pellet magnitude regulate schedule-induced polydipsia. While small-magnitude pellets could support drinking only at relatively short intermeal intervals, increasing pellet magnitude to medium or large resulted in drinking at progressively longer intermeal intervals. Indeed, we observed drinking on a fixed-time 720-sec schedule with large-magnitude pellets. These findings are derivable from the opponent-process theory of acquired motivation.
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