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What factors influence movement or action?
• Biomechanical (e.g., size, shape, mass, strength, flexibility, coordination of body/body parts)
• Environmental (e.g., effects of gravity; surfaces that provide support for the body; objects toward which movements are directed)
• Both of these factors are continually changing
– Rapid and episodic changes in physical growth over the first two years
– Possibilities for interactions with the environment change with changes in infants’ growth and skills
Role of Perception in Action:
• Perception provides current information about both biomechanical and environmental constraints on action
– Allows actions to be planned (prospective)
• Feedback from prior movements used to anticipate the consequences of future actions
• Motor Development as a Perception-Action System
– How do infants detect and adapt to changes in their physical characteristics (biomechanical factors) and to changes in the environment?
• Vary perceptual information by manipulating infants’ physical characteristics (e.g., weighting infants down with backpacks) or by altering properties of the environment (e.g., changing surfaces)
Perceptual Control of Crawling
• Any action requires perceptual control of balance
– To maintain balance, need support to prevent falling
• Visual Cliff (Gibson & Walk, 1960)
– Provides visual depth cues that balance is not possible (past a certain point)
– Crawling experience is related to avoidance of the visual cliff
• Adolph (2000)
– Tested infants on a “real” cliff
• Visual and haptic information are consistent: Looks risky, feels risky
• Adjustable “gap”
• Can test infants in multiple trials
• Tested 9-month-old infants
– Average time sitting: 3 months– Average time crawling: 1 month
• Balance control depends on experience with specific postures
– Infants maintained balance while sitting but “fell” over the cliff while crawling
Walking
• Perceptual Control of Upright Balance
– Visual motion information from optic flow is important for balance control in standing and walking
• Locomotor experience is related to infants’ sensitivity to different types of optic flow
• Peripheral flow information is important for keeping balance– Infants with crawling experience react more strongly (show
more postural sway) to peripheral flow
• Prospective Control of Balance
– Optic flow information allows infants to control balance reactively
– Ideally, balance control should be prospective (i.e., adjustments occur before beginning to fall)
• Experience is related to prospective control of balance
– But experience in one posture (e.g., crawling) does not transfer to other postures (e.g., walking)
• What is critical for motor development?
– Infants must learn to discriminate actions that are possible from those that are not
– In some cases, infant learning seems to be posture-specific and does not transfer to novel contexts
• i.e., what infants learn about maintaining balance while sitting does not help them when they start crawling, crawling knowledge does not transfer to walking, etc.
– But in other cases, learning does transfer
• What are the processes underlying infants’ learning?
• Affordances: Possibilities for action
– Ex: a flat surface affords walking; a 90-degree slope does not afford walking (but may afford climbing)
• Affordances reflect the objective state of affairs regarding infants’ physical capabilities (biomechanical factors) and relevant features of the environment (environmental factors)
• A critical task in motor development is learning to perceive affordances prospectively
– Makes it possible for infants to select appropriate actions to meet goals
– But it is a difficult task, particularly because of the rapid changes in biomechanical and environmental factors for infants
• Infants’ bodies and skills and the environments to which they are exposed are constantly changing
• Experience with specific postures is critical for perceiving affordances prospectively (i.e., knowing what actions are possible or impossible)
• But what are infants learning from experience?
Possible Answers
• Learning that the experimenter will catch them? (i.e., in the “slopes” and “gaps” studies)– But infants in longitudinal studies became
more cautious over sessions– Similarly, infants in cross-sectional studies
who were “rescued” multiple times in their inexperienced posture avoided risky actions in their experienced posture
• Learning fear of heights?
– But infants often don’t show behavioral indices of fear when they avoid risky actions
• Facial expressions are primarily positive or neutral whether infants avoid the risky action or not
• Learning that sudden drop-offs, steep slopes, etc., are dangerous?
– But posture-specific learning suggests that infants are not simply learning facts about the environment that guide their actions
• e.g., infants always require a sturdy floor to support their bodies; a 50 degree slope or a 90-cm gap is risky for every infant in every posture
– But infants don’t seem to generalize these “facts” from one posture to another
– Also, infants often display flexibility within postures
• Ex: backpack study (Adolph & Avolio, 2000)
– Experienced walkers were able to adjust their perception of affordances from trial to trial
» Treated the same slope as risky while wearing lead-weighted shoulder packs and as safe while wearing feather-weighted packs
– Suggests that infants are not learning “static” facts about their physical capabilities
– In addition, infants show a variety of responses in studies in which infants are confronted with obstacles to locomotion
• Ex: Experienced walkers display a variety of strategies for negotiating risky slopes—e.g., crawling down on hands and knees, sliding in the “Superman” position, backing down, sliding in a sitting position, grabbing the experimenter, or avoiding the slope (Adolph, 1995, 1997)
– Individual infants use multiple strategies on the same slope on different trials within the same test session
– Suggests that infants are not learning simple stimulus-response associations or fixed patterns of responding
• Adolph & Eppler (2002):
– Novelty and variability of motor actions cannot be accounted for by simple association learning
– Harlow’s (1949, 1959) idea of “learning sets” is a better model for motor development
• Learners acquire a set of exploratory procedures and strategies for figuring out solutions to novel problems within a particular problem space
• Scope of transfer should be limited to similar problems/tasks
• Adolph & Eppler (2002)
– Infants assemble a repertoire of exploratory behaviors to generate the required perceptual information to perceive affordances
• Perceptual information specifies infants’ physical capabilities (biomechanical factors) and environmental factors
• To the extent that each posture functions as a separate perception-action system, perceptual information will fail to transfer (Adolph, 2002, 2005)
• To detect affordances, sensitivity to perceptual information is required
– Sensitivity to some perceptual information is present at birth, but is refined with experience
• Ex: postural sway in response to peripheral optic flow
• Exploratory behaviors help to generate perceptual information
– Exs: Crawling and walking infants (at all age levels and experience levels) show longer latencies to action and more looking and touching as they approach risky slopes compared with safe ones
• Walkers generate visual and mechanical information by standing with their feet straddling the brink and rocking back and forth over their ankles; crawlers lean forward with both hands on the slope and rock over their wrists
• Infants use perceptual information to perceive affordances and guide action