Motor Development: A new Synthesis

Fetuses were maintained in a supine posture throughout the experimental session to facilitate clear observation of all four limbs. Fetal behavior was quantified by scoring the movement of each limb, treating each movement as a point event entered into a computer using real-time event recording software. This data acquisition system preserved information about the limb involved in movement (left vs. right, fore vs. hind), the time the movement occurred (± 0.1 sec), and whether the limb movement was independent or conjugate (). Conjugate limb movements (CLM) occurred when the two limbs appeared to move as one, with both limbs initiating movement at the same time and following parallel trajectories with similar velocity. An example of CLM by an E20 fetus is depicted in (right), which shows drawings from three individual video frames separated by intervals of five frames (0.17 sec). CLM were distinctive during baseline and testing periods in yoked subjects, and were readily distinguished from passive dragging of one limb during yoke training, because the passive limb appeared to trail the active limb through a movement trajectory. Further, CLM involved movement of both limbs at an angle orthogonal to the line of the interlimb yoke, whereas passive movement involved one limb moving in the same direction as the line of the yoke. In such cases, the active movement of the leading limb was scored as an individual limb movement, but the passive movement of the trailing limb was not. This method of quantifying fetal movement has been used extensively in previous studies of fetal motor behavior and responsiveness to sensory stimuli (; ). Estimates of reliability have been assessed from repeatedly scoring the same session recorded on video; real-time scoring of basic categories of limb movement typically results in intra-rater reliability coefficients exceeding .95 and interrater reliabilities of at least .90.

Moving toward a grand theory of development: in memory of Esther Thelen.

The research perspective exemplified by Thelen in the domain of child development offers a rich set of conceptual and experimental tools that provide new insights into long-standing questions in developmental and behavioral neuroscience. Application of this research approach to nonhuman animals, such as the rat, presents many advantages to researchers interested in fundamental problems in motor development, most notably the ability to use experimental methods and technologies that are not feasible with human infants and children. This advantage is particularly salient in efforts to trace the developmental origins of behavior. For example, the newborn rat does not display adultlike motor skills, but does show organized motor behavior that is essential for its survival and subsequent development. The expression of organized motor behavior within minutes or hours of birth implies a developmental history antecedent to birth. Study of fetal behavior in the rodent model thus provides a unique window into the ontogenetic origins of motor behavior and the codeterminants of developmental change in the motor system (; ).


Developmental systems theorists like Esther Thelen ..

The research perspective exemplified by Thelen in the domain of child development offers a rich set of conceptual and experimental tools that provide new insights into long-standing questions in developmental and behavioral neuroscience. Application of this research approach to nonhuman animals, such as the rat, presents many advantages to researchers interested in fundamental problems in motor development, most notably the ability to use experimental methods and technologies that are not feasible with human infants and children. This advantage is particularly salient in efforts to trace the developmental origins of behavior. For example, the newborn rat does not display adultlike motor skills, but does show organized motor behavior that is essential for its survival and subsequent development. The expression of organized motor behavior within minutes or hours of birth implies a developmental history antecedent to birth. Study of fetal behavior in the rodent model thus provides a unique window into the ontogenetic origins of motor behavior and the codeterminants of developmental change in the motor system (; ).


Coupling Perception and Action in the Development of …

Growth of the fetus and changes in the intrauterine environment present substantial challenges to the fetal motor system as well as opportunities for motor experience (; ). The period of late gestation from postconception day 18 (E18) to E21 is a time of rapid growth and development in the rat. Physically, the rat fetus grows at a nearly exponential rate over this span, with crown–rump length increasing 182% (20.5 mm to 37.4 mm) and body mass 344% (1.6 g to 5.5 g; ; ). Although the earliest movements are expressed only 2 days earlier, on E16, general motor activity rises to a peak by E18 (; ; ). Over the next 3 days (E18–21), fetal movements are transformed quantitatively and qualitatively from a seemingly random assortment of jerks and twitches to a repertoire of coordinated action that foreshadows functional behavior of the neonate and adult (; ). Synchronous movement of the limbs—perhaps the earliest form of motor coordination—rises to a peak of expression on E19 in the forelimbs and E20 in the hind limbs (), and elements of suckling (; ), grooming (), head orientation (), postural control (), and locomotion (; ) can be expressed for the first time. In terms of motor development, the short span from E18 to E21 represents a crucial period in which the foundations of motor control and coordination are being established.