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

POSTNATAL DEVELOPMENTAL MILESTONES 1111visual placing task by PND 17 (Smart and Dobbing, 1971). This suggests that although rats areborn with their eyes closed, the visual system of the rat may be at a developmental level roughlyequivalent to species born with their eyes open, including humans.DogsThe age when positive conditioned responses can first be seen is dependent on the modality of thestimulus, i.e., a conditioned response involving integration at a higher sensory level occurs later inlife than those at lower levels (e.g., rhinencephalic). The majority of the data available on theontogeny of sensory and reflexive development in dogs can be found in “Integrative Developmentof Brain and Behavior in the Dog” by Michael W. Fox (Fox, 1964a).Non-Human PrimatesA trend exists among primates for those species that reach higher levels of functioning to haveprolonged juvenile periods such that the duration for juvenile growth ranges from a period ofmonths in tree shrews to about 18 months in prosimians (e.g., slow loris), 2-3 years in monkeysand 8 years in great apes (Schultz, 1969).Primate infants demonstrate a rooting reflex at birth with a maximum response at PND 4, whichdisappears by PND 10 (Mowbray and Cadell, 1962). Other reflexes present at birth include handand foot grasping and clasping (Mowbray and Cadell, 1962). From birth, a rhesus monkey placeddorsal side down will quickly “right” by turning to achieve ventral contact (Hines, 1942). Squirrelmonkeys have the righting reflex during the first week post birth (Schusterman and Sjoberg, 1969),to orient up by 3 weeks of age (Elias, 1977) and to air-right at week 5 (King et al., 1974).Somatosensory systems are functional at birth; infant rhesus monkeys have been shown torespond to a pinprick at birth (Golub et al., 1991a). While these systems are functional at birth,they may still be refined during development. For example, macaques have been shown to discriminatetexture and size differences at the level of adult thresholds by postnatal week 10, indicatingthat tactile sensory systems, while functional at birth, continue to develop postnatally (Carlson,1984).In non-human primates, auditory responsiveness is observed at birth, indicating that a functionalauditory sensory system develops prenatally (Winter et al., 1973), and infant monkeys show a startleresponse to an abrupt sound on PND 10 (Mowbray and Cadell, 1962).Taste buds in the soft palate of the common marmoset increase in number after birth, reachinga maximum number at 2 months and then decrease until 9 years of age (Yamaguchi et al., 2001).Nonhuman primates can visually track their mothers at birth (Dolhinow and Murphy, 1982),can track a moving object within the first few days of birth (Mendelson, 21982), and orient to asmall object by PND 13 (Mowbray and Cadell, 1962). Optical quality (optical line spread function)for well-focused retinal images in nonhuman primates is very good at birth and improves rapidlyto adult levels by postnatal week 9 (Williams and Boothe, 1981). Visual acuity (argued to be afunction of maturation of neural elements) is low at birth and improves slowly during the firstpostnatal months (Neuringer et al., 1984) and reaches adult levels by about 1 year (Williams andBoothe, 1981).Sensory and Reflex ConclusionsThe sequence of development of sensory and reflex systems in rats, dogs and non-human primatescorrelate rather closely with those of the human infant. For example, the gustatory system innewborn animals is apparently important for selective ingestion, and olfaction and taste are amongthe first sensory systems to develop. Behavioral responses in humans and rats suggest the presenceof functional taste and olfaction systems at birth (Hudson and Distel, 1983; Teicher and Blass,© 2006 by Taylor & Francis Group, LLC