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The development of memory of an escape response in Swiss-Webster mice between 3-11 days of age was examined in four experiments. Mice were given 25 training trials in a straight-alley escape task and then retested at various retention intervals. Collectively, the results showed that 5-and 7day-old mice possess a retention capacity of less than 6 hr. At 9 days of age, however, retention capacity greatly increases to at least 96 hr., suggesting that a period of maturational development critical to long-term memory processes occurs at 9 days of age in the neonatal mouse.

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 flavorpreexposure 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 tasteaversion 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.Nonreinforced preexposure to a stimulus decreases the chances of that stimulus's becoming a conditioned stimulus (CS) when subsequently paired with an unconditioned stimulus (US). This preexposure effect has been demonstrated for a variety of stimuli, for a number of species, and in different learning situations (Lubow, 1973), and has been termed latent inhibition (Lubow & Moore, 1959). Recently, Misanin, Guanowsky, and Riccio (1983) have suggested that when the preexposed stimulus is flavor and the learning situation is taste-aversion conditioning, this effect might vary with age. More specifically, they suggested that the ease with which flavor is established as a latent inhibitor or "safe" stimulus (Kalat & Rozin, 1973) is inversely related to age. They found that three 30-min preexposures to saccharin prevented an aversion from being conditioned to saccharin in weanling rats but only retarded taste-aversion conditioning in young adult rats. They also found that weanling rats did not display neophobia when first exposed to a flavor, whereas young This research was supported by a summer research grant from Susquehanna University to James R. Misanin. Reprint requests should be sent to James R. Misanin, Department of Psychology, Susquehanna University, Selinsgrove, PA 17870. 69 adult rats did. Therefore, they suggested that, just as it is more difficult to convert a conditioned inhibitor than a neutral stimulus into a CS in excitatory learning, it should be mor...

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Weanling, young-adult, and aged rats were subjected to taste-aversion conditioning with a relatively weak US (1% body weight i.p. injection of .15M Lifll) at one of four CS-US intervals (0, 0.75, 1.5, and 3.0 h). Age differences occurred at all intervals, with aged rats showing superior conditioning. Increasing the CS-US interval was more disruptive the younger the rat. Although not unequivocal, the results suggest that the processes involved in associative memory during taste-aversion conditioning may involve different processes from those that mediate long-delay learning in more conventional learning tasks.There is ample evidence that both reference memory (i.e. , the long-term maintenance of acquired associations between events; e.g. , Gold & McGaugh , 1975) and working memory (i.e ., the memory process that permits correct discriminative behavior on the basis of trace stimuli ; Zometzer, Thompson , & Rogers , 1982) are impaired with aging in rats (Kubanis & Zometzer, 1981). In contrast, it is not clear whether the associative memory capability (i.e. , the memory capability that is necessary for good performance when there is a long delay between two events to be associated) of aged rats is impaired . Although Doty (1966) found aged rats to be significantly inferior to young-adult rats on a delayed avoidance task, Ingram and Peacock (1980) reported little evidence of significant age differences in saccharin aversions conditioned in mature rats with short (15-min) and long (1-to 4-h) conditioned stimulus-unconditioned stimulus (CS-US) intervals.Ingram and Peacock's (1980) failure to find age differences may have been due to their relatively intense US . The results of a recent study (Misanin, Blatt, & Hinderliter, 1985) indicate that age differences in conditioned taste aversion may be obscured by a US of even moderate intensity. Furthermore, Hinderliter and Misanin (1988), using a weaker US (.15 M LiCl) than that used by Ingram and Peacock (.4 M LiCl), found that both weanling and aged rats showed substantially less aversion to a novel flavor than did young adults when the illnessinducing US was delayed 1 h after access to the flavor CS. Hinderliter and Misanin, unlike Ingram and Peacock , however, did not systematically manipulate the CS-US interval. Thus , the purpose of the present study was to Thie research was supported by Grant HD21161 from the National Institute of Child Health and Human Development to the first author . Reprint requests are to be sent to James R. Misanin, Department of Psychology, Susquehanna University, Selinsgrove, PA 17870. use a weaker US intensity than that used by Ingram and Peacock (1980) and systematically manipulate the CS-US interval in the long-delay taste-aversion conditioning of weanling , young-adult, and aged rats to determine at what CS-US intervals age differences emerge. METHOD SubjectsThe rats used were 40 female Wistar albino rats from each of three age groups: weanling (21-24 days), young adult (76-% days), and aged (680-825 days). All animals were born in the uni...