A Practical Approach, Second Edition=Ronald D. Ho.pdf

1118 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONat 60, 90, 120, or 150 of age. Over time, all four groups demonstrated an increasing ability to solvethe delayed response task. However, Harlow did note that the capacity to solve delayed-responseproblems was better in animals 125 to 135 days of age compared to 60-90 days of age. In addition,half of the monkeys at 200 to 250 days of age had essentially flawless performance in this task. Itis clear that the capacity to solve delayed-response problems matures later than the capacity tosolve object-discrimination tasks.Two broad conclusions can be derived from the monkey data presented thus far. First, certainlearning abilities appear extremely early in life in the rhesus monkey, e.g., conditioned avoidanceor black-white discrimination. The ability to solve a problem may be present as early as 1 day ofage, and within a matter of days, performance will change from chance levels to nearly 100%accuracy. Second, the development of certain, more complex, learning abilities is more gradual andprogressively improves, e.g., delay-response learning and some discrimination learning tasks (e.g.,color, pattern or shape discrimination). Interestedly, extensive early training on complex tasks (e.g.,delayed response task) did not facilitate the solution, but rather success was clearly a function ofage, i.e., even when young animals were trained more extensively than the older animals, theirperformance did not equal that of older monkeys (Zimmerman and Torrey, 1965).Conclusions for Cognitive DevelopmentThere is evidence that rats, dogs, monkeys and human infants have learning capacities that changeduring development. In all species evaluated, the capacity to mediate reflexive behaviors emergedbefore learned behavioral reactions. Vogt and Rudy (Vogt and Rudy, 1984) suggested that thesequence of behavioral development reflects a sequential caudal to rostral maturation of components.That is, the lower centers of a sensory system are sufficient to support primitive reflexivebehaviors, and it is well established that subcortical structures are sufficient to support manyinstances of Pavlovian conditioning (Girden et al., 1936; LeDoux et al., 1984). However, highercenters of learning (e.g., hippocampus and prefrontal cortex) are necessary for the acquisition ofmore complex behaviors.Therefore, in all common laboratory species, evaluations of cognitive function during juveniletoxicity studies can provide insight into the ontogeny of the maturation of these systems. The timerequired to learn a particular task is often greater in neonates and infants compared to adults. Inaddition, the accuracy of learning that task is often not equal to an adult. This implies that eventhough a task can be completed at a fairly young age, it is by no means functionally mature, i.e.,at adult levels of performance. Finally, it is important to remember that learning and memory areintimately associated with the maturation of many of the sensory systems described above (Table 4).Ontogeny of CommunicationEarly in postnatal development, many species use some form of communication to express fundamentalneeds (e.g., hunger and fear). However, the complexity of the communication often undergoesa period of prolonged postnatal development.HumansInfants have many methods of preverbal communication, e.g., crying, smiling, clinging, and kissing(Illingworth, 1983; Morley, 1972; Murray and Murray, 1980; Nelson, 2000). At about 2 months,the infant listens to voices and coos in response, and around 3 months, she begins to make sounds(e.g., “aah”, “ngah”). At 4 months, laughing out loud occurs, and the infant may show displeasureif social contact is broken or excitement at the sight of food. At 8 months, preference for the motheremerges, the infant begins to babble with new complexity and inflections that mimic the nativelanguage. Around 9 to 10 months a critical step occurs in speech development, an infant begins to© 2006 by Taylor & Francis Group, LLC