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Lecture 5

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Psychology 2410A/B
Adam Cohen

WEEK 5: THE ROAD TO WALKING: WHAT LEARNING TO WALK TELLS US ABOUT DEVELOPMENT Starting Point • Altering Leg Movement • Long before infants can walk, they move thei legs in an alternating pattern that shares the 50% phasing that is so characteristic of walking • Newborn infants cannot walk, of course, but when held upright with their feet on the floor or a tabletop, they sometimes alternate their legs in a slow motion facsimile of walking • McGraw accorded it the distinction of starting point in her stage-like depiction of walking and others have followed suit Supine kicking is relatively common and occurs when infants are mildly aroused • Upright steps typically disappear at about 8 weeks of age while supine kicks continue throughout the first year of life • Disappearance of newborn walking actually results form changes in the body, not just the brain • During the newborn period, infants legs increase in mass faster than they increase in strength • Alternating leg movements begin even before birth • 4-D ultrasound images suggest that walking in the womb appears during the first trimester • Leg alternation is emblematic of prototypical walking • Central pattern generators for walking localized in branchial and lumbosacral regions of the spinal cord and are activated by neural pathways descending from the brainstem and cerebral cortex One argument against fetal alternation as the starting point is that alternating leg movements do not appear in a developmental vacuum • Other Patterns, Other Dimensions • A second argument against considering fetal and neonatal alternation to be the core starting point of walking is that environmental factors influence the pattern of leg movements • Findings suggest that alternation is an adaptive response to moving legs in a physically demanding environment • A final argument for why alternation should not be considered the starting point for walking is that functional walking involved much more than generation of a stereotypic walking cycle • Real walking involves flexibility and diversity in patterns of interlimb coordination so as to speed up and slow down, steer and navigate through a cluttered environment, adjust to changes in footing, slope, and loads of being carried, and maintain balance through all of these actions Precursors • An iconic sequence fails to depict the diversity and variability of precursory forms • Variety of Means for Mobility • Mobility does not wait for walking • Pre-walking infants drag, pull, hoist, and propel their bodies in a wonderous variety of ways • Hands/feet crawling (on palms) and crab crawling as precursory stages of walking WEEK 5: THE ROAD TO WALKING: WHAT LEARNING TO WALK TELLS US ABOUT DEVELOPMENT • About half of infants belly crawl with their abdomens on the floor • Belly crawls include ‘commando’ styles with the abdomen continually dragging along the floor and bent arms pulling the body, ‘inchworm’ styles where the body raises momentarily onto knees before lurching forward onto the belly, leg-only crawls asymmetric one-armed and.or one legged crawls and so on • Belly crawling improves and infants limb movements become larger and faster • When ex-belly crawlers begin crawling on hands and knees or hands and feet, they are twice as proficient in terms of the size and speed of their movements as infants who skip belly crawling entirely • Most infants convince their parents to participate in their quest for upright mobility • Some infants discover that they can ‘walk’ on their knees keeping both hands in the air • Most infants discover that they can ‘walk’ on their knees keeping both hands in the air • Most infants ‘cruise’ by moving sideways while hanging onto furniture for support • Stages and Transient Forms • Precursory forms of mobility do not adhere to the criteria for universal developmental stages • Individual infants forge their own paths and the sequence of expression is variable • Postural development is often more likely an up and down roller coaster ride than a strictly uphill climb • Milestones are concurrent not discrete events • Infants continually discover new forms, but they don’t immediately drop the old ones • A final argument against a retinue of connected stages is that critical aspects of what infants learn while crawling and cruising do not transfer to walking • Over weeks of crawling, infants learn to perceive affordances for locomotion and judgements gradually gear in to the limits of their abilities • Learning must begin anew and the process is no faster the second or third time around • An alternative interpretation of precursory locomotor forms is that they represent transient expressions of an immature motor system that are functional in a particular context, but are not needed after more efficient methods of locomotion become available • Milestone Metaphor • Iconography and convenient labels can help to organize scientific thinking, but unfounded metaphors can misdirect our thinking • The actual process of development is typically nonlinear with more and less upright forms intermixed, multiple skills expressed concurrently and non of the forms obligatory prerequisites Onset • Child should walk around 12 months of age, give or take a few months • Changes in infants brains and bodies and external factors that affect infants brains and bodies determine the timing of walking onset • Onset Ages WEEK 5: THE ROAD TO WALKING: WHAT LEARNING TO WALK TELLS US ABOUT DEVELOPMENT •Developmental researchers typically rely on retrospective reports from parents and in prospective studies, use cross-sectional designs to compare various ages or longitudinal designs with assessments spaced months apart •The problem of widely spaced observations is two fold •First, estimates of an onset date are likely to be questimates mired in noise •Second, more serious problem is that widely spaced sampling intervals presumpose that the shape of the developmental trajectory is stage like •It may be stage like or not • Physical growth provides a dramatic illustration that developmental data require frequent sampling •Even small deviations from a daily sampling rate misrepresent the developmental trajectory •Accurate estimates of onset age or any other developmental benchmarks require frequent sampling • Experience •Onset ages are hugely affected by historical changes and cultural differences •Onset ages for the entire panoply of motor milestones have shifted to dramatically earlier ages form the norms established by the early pioneers of motor development •Accelerated motor development may reflect general improvements in nutrition and living conditions •In communities where walking onset is accelerated, mothers massage and exercise their infants beginning in the newborn period using ritualized daily routines performed expressly for the purpose of facilitating the onset of walking •How much mothers rub, shake, and bounce their infants makes a difference for motor development •The more frequent and consistent the stimulation the earlier infants begin walking •Back-sleepers achieve prone skills a few weeks later than prone sleepers •The timing of walking onset and other motor skilld i
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