Developmental trajectories provide the empirical foundation for theories about change processes during development. However, the ability to distinguish among alternative trajectories depends on how frequently observations are sampled. This study used real behavioral data, with real patterns of variability, to examine the effects of sampling at different intervals on characterization of the underlying trajectory. Data were derived from a set of 32 infant motor skills indexed daily during the first 18 months. Larger sampling intervals (2-31 days) were simulated by systematically removing observations from the daily data and interpolating over the gaps. Infrequent sampling caused decreasing sensitivity to fluctuations in the daily data: Variable trajectories erroneously appeared as step-functions and estimates of onset ages were increasingly off target. Sensitivity to variation decreased as an inverse power function of sampling interval, resulting in severe degradation of the trajectory with intervals longer than 7 days. These findings suggest that sampling rates typically used by developmental researchers may be inadequate to accurately depict patterns of variability and the shape of developmental change. Inadequate sampling regimes therefore may seriously compromise theories of development.

A series of experiments provided evidence for the existence of a functional opioid system in the fetal rat near term. Application of a tactile probe to the perioral region of the fetus consistently evoked a stereotypic facial wiping response. Administration of low dosages of morphine to the fetus had little effect on nonevoked motor activity but reduced fetal responsiveness to cutaneous stimulation. Milk infused into the mouth of the fetus reduced fetal responsiveness to the tactile probe. Milk's effect on cutaneous responsiveness was reversed by injection of the nonspecific opioid antagonist naloxone. The effect of milk on fetal responsiveness to cutaneous stimulation was reversed by the kappa opioid antagonist nor-binaltorphimine, but not by the mu antagonist beta-funaltrexamine. Milk engages the endogenous opioid system of the fetal rat and affects fetal responsiveness by interacting with the kappa receptors of the opioid system.

This chapter provides a synthesis of recent research in motor development. Motor behavior encompasses everything that we do, and therefore is relevant to every branch of psychological science. Our aim is to address central concepts and methodological issues that continue to challenge developmental scientists, and to show how the study of motor behavior can yield fresh insights into the process of development. The chapter is organized into three broad themes that are emblematic of the state of current work on motor development: (1) movements are embodied, and are thus the product of a physical body acting in the real world; (2) behavior is embedded within a physical environment rich with sensory information, and requiring perception for effective action; (3) motor development is enculturated, shaped by cultural practices from caregiving to social norms. We illustrate 10 general developmental concepts within these broad themes using examples drawn from research on humans and animals, with an emphasis on early development, from prenatal through childhood.

Although born blind and deaf, newborn rats exhibit a remarkable capacity to recognize and gain access to the nipples of the lactating mother. However, it is well-known that full-term rat neonates will not attach to an artificial nipple. In the present study, an artificial nipple fashioned from soft vinyl was presented to rat fetuses from E17-E21 of gestation. Fetuses showed side-to-side head movements that resulted in oral capture of the nipple on E18 and exhibited a direct nipple-grasping response from E19 through term. Frame-by-frame analysis of videotape records of E21 rat fetuses revealed that tactile contact with the artificial nipple elicited mouthing, licking directed at the nipple, forelimb treadling, and grasping of the nipple. Fetuses also exhibited components of aversive behavior, including facial wiping and head turning, that appeared to terminate oral contact with the nipple. Morphine pretreatment reduced the expression of aversive responses and promoted licking and grasping of the artificial nipple. In addition to documenting the prenatal ontogeny of this important neonatal behavior, these findings imply a role for endogenous opioids in the newborn rat's first suckling episode.

The nativist-empiricist debate and the nativist commitment to the idea of core knowledge and endowments that exist without relevant postnatal experience continue to distract attention from the reality of developmental systems. The developmental systems approach embraces the concept of epigenesis, that is, the view that development emerges via cascades of interactions across multiple levels of causation, from genes to environments. This view is rooted in a broader interpretation of experience and an appreciation for the nonobvious nature of development. We illustrate this systems approach with examples from studies of imprinting, spatial cognition, and language development, revealing the inadequacies of the nativist-empiricist debate and the inconvenient truths of development. Developmental scientists should no longer abide the nativist-empiricist debate and nativists' ungrounded focus on origins. Rather, the future lies in grounding our science in contemporary theory and developmental process.3

We investigated sensory and behavioral responsiveness of the rat fetus. On Days 19, 20, or 21 of gestation, rat fetuses received intraoral infusions of a biologically important stimulus, milk, or a novel chemical stimulus, lemon. Using a technique to directly observe behavior in utero, we found that rat fetuses discriminate between intraoral infusions of milk and lemon, exhibiting different levels and patterns of overall activity after infusion. Milk was found to evoke a low magnitude, delayed increase in overall fetal activity from Day 19 through Day 21, whereas lemon evoked a high-magnitude, spiked pattern of activity that diminished from Day 19 to Day 21. Late in gestation these two stimuli elicited species-typical action patterns. Milk infusions elicited a stretch response much like the one shown by pups at the nipple; lemon infusions elicited face wiping typical of older pups and adults exposed to aversive gustatory stimuli. Fetal rats are responsive to changes that occur in their uterine environment during the last third of gestation. Behavioral evidence indicates that their overall activity is affected by physical restraint produced by crowding in the uterus (Smotherman & Robinson, 1986), by the application of various forms of tactile stimulation, and by exposure to chemical cues through injection into the amniotic Quid or direct infusion into the mouth (Smotherman & Robinson, 1985, in press). Compared to our understanding of postnatal behavior, however, relatively little is yet known about the sensory responsiveness and behavioral capabilities of the fetus.In contrast to the paucity of information about fetal behavior, the behavioral competence of newborn mammals is well documented. Even in an altricial species such as the rat (Rattus norvegicus), the pups, born deaf and blind, are capable of orientation in a thermal gradient (Alberts, 1978), location and attachment to the nipple (Blass & Teicher, 1980), instrumental learning (Johanson & Hall, 1979), and specific responsiveness to particular chemical stimuli, such as dimethyl disulfide(a component of pup saliva; Pederson & Blass, 1981) and milk (Hall & Rosenblatt, 1977). The expression of these behavioral abilities soon after birth suggests the existence of prenatal antecedents.In the present study, the sensory and behavioral responsiveness of the rat fetus were investigated. On the last 3 days of gestation, Days 19, 20, or 21, rat fetuses received intraoral infusions of a biologically important stimulus, milk, or a novel chemical cue, lemon. Using techniques that permit direct observation of fetal behavior, we found the response of fetuses to chemical infusions characterized in terms of (a)

Adults primarily walk to reach a new location, but why do infants walk? Do infants, like adults, walk to travel to a distant goal? We observed 30 13-month-old and 30 19-month-old infants during natural walking in a laboratory playroom. We characterized the bout structure of walking—when infants start and stop walking—to examine why infants start and stop walking. Locomotor activity was composed largely of brief spurts of walking. Half of 13-month-olds’ bouts and 41% of 19-month-olds’ bouts consisted of three or fewer steps—too few to carry infants to a distant goal. Most bouts ended in the middle of the floor, not at a recognizable goal. Survival analyses of the distribution of steps per bout indicated that the probability of continuing to walk was independent of the length of the ongoing bout; infants were just as likely to stop walking after 5 steps as after 50 steps and they showed no bias toward bouts long enough to carry them across the room to a goal. However, 13-month-olds showed an increased probability of stopping after 1-3 steps, and they did not initiate walking more frequently to compensate for their surfeit of short bouts. We propose that infants’ natural walking is not intentionally directed at distant goals; rather, it is a stochastic process that serves exploratory functions. Relations between the bout structure of walking and other measures of walking suggest that locomotor exploration is constrained by walking skill in younger infants, but not in older infants.

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