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This primer describes research on the development of motor behavior. We focus on infancy when basic action systems are acquired-posture, locomotion, manual actions, and facial actions-and we adopt a developmental systems perspective to understand the causes and consequences of developmental change. Experience facilitates improvements in motor behavior and infants accumulate immense amounts of varied everyday experience with all the basic action systems. At every point in development, perception guides behavior by providing feedback about the results of just prior movements and information about what to do next. Across development, new motor behaviors provide new inputs for perception. Thus, motor development opens up new opportunities for acquiring knowledge and acting on the world, instigating cascades of developmental changes in perceptual, cognitive, and social domains. This article is categorized under: Cognitive Biology > Cognitive Development Psychology > Motor Skill and Performance Neuroscience > Development.

Behavioral flexibility-the ability to tailor motor actions to changing body-environment relations-is critical for functional movement. Navigating the everyday environment requires the ability to generate a wide repertoire of actions, select the appropriate action for the current situation, and implement it quickly and accurately. We used a new, adjustable barrier paradigm to assess flexibility of motor actions in 20 17-month-old (eight girls, 12 boys) and 14 13-month-old (seven girls, eight boys) walking infants and a comparative sample of 14 adults (eight women, six men). Most participants were White, non-Hispanic, and middle class. Participants navigated under barriers normalized to their standing height (overhead, eye, chest, hip, and knee heights). Decreases in barrier height required lower postures for passage. Every participant altered their initial walking posture according to barrier height for every trial, and all but two 13-month-olds found solutions for passage. Compared to infants, adults displayed a wider variety of strategies (squat-walking, half-kneeling, etc.), found more appropriate solutions based on barrier height (ducked at eye height and low crawled at knee height), and implemented their solutions more quickly (within 4 s) and accurately (without bumping their heads against the barrier). Infants frequently crawled even when the barrier height did not warrant a low posture, displayed multiple postural shifts prior to passage and thus took longer to go, and often bumped their heads. Infants' improvements were related to age and walking experience. Thus, development of flexibility likely involves the contributions of multiple domains-motor, perception, and cognition-that facilitate strategy selection and implementation. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Infants learn nouns during object-naming events--moments when caregivers name the object of infants' play (e.g., ball as infant holds a ball). Do caregivers also label the actions of infants' play (e.g., roll as infant rolls a ball)? We investigated connections between mothers' verb inputs and infants' actions. We video-recorded 32 infant-mother dyads for 2 hr at home (13 month olds, n = 16; 18 month olds, n = 16; girls, n = 16; White, n = 23; Asian, n = 2; Black, n = 1; other, n = 1; multiple races, n = 5; Hispanic/Latinx, n = 2). Dyads were predominantly from middle-class to upper middle-class households. We identified each manual verb (e.g., press, shake) and whole-body verb (e.g., kick, go) that mothers directed to infants. We coded whether infants displayed manual and/or whole-body actions during a 6-s window surrounding the verb (i.e., 3 s prior and 3 s after the named verb). Mothers' verbs and infant actions were largely congruent: Whole-body verbs co-occurred with whole-body actions, and manual verbs co-occurred with manual actions. Moreover, half of mothers' verbs corresponded precisely to infants' concurrent action (e.g., infant pressed button as mother said, "Press the button"). In most instances, mothers commented on rather than instigated infants' actions. Findings suggest that verb learning is embodied, such that infants' motor actions offer powerful cues to verb meanings. Furthermore, our approach highlights the value of cross-domain research integrating infants' developing motor and language skills to understand word learning.

Object play yields enormous benefits for infant development. However, little is known about natural play at home where most object interactions occur. We conducted frame-by-frame video analyses of spontaneous activity in two 2-h home visits with 13-month-old crawling infants and 13-, 18-, and 23-month-old walking infants (N = 40; 21 boys; 75% White). Regardless of age, for every infant and time scale, across 10,015 object bouts, object interactions were short (median = 9.8 s) and varied (transitions among dozens of toys and non-toys) but consumed most of infants' time. We suggest that infant exuberant object play--immense amounts of brief, time-distributed, variable interactions with objects--may be conducive to learning object properties and functions, motor skill acquisition, and growth in cognitive, social, and language domains.

Open data sharing promises to accelerate the pace of discovery in the developmental and learning sciences, but significant technical, policy, and cultural barriers have limited its adoption. As a result, most research on learning and development remains shrouded in a culture of isolation. Data sharing is the rare exception (Gilmore, 2016). Many researchers who study teaching and learning in classroom, laboratory, museum, and home contexts use video as a primary source of raw research data. Unlike other measures, video captures the complexity, richness, and diversity of behavior. Moreover, because video is self-documenting, it presents significant potential for reuse. However, the potential for reuse goes largely unrealized because videos are rarely shared. Research videos contain information about participants' identities making the materials challenging to share. The large size of video files, diversity of formats, and incompatible software tools pose technical challenges. The Databrary (databrary.org) digital library enables researchers who study learning and development to store, share, stream, and annotate videos. In this article, we describe how Databrary has overcome barriers to sharing research videos and associated data and metadata. Databrary has developed solutions for respecting participants' privacy; for storing, streaming, and sharing videos; and for managing videos and associated metadata. The Databrary experience suggests ways that videos and other identifiable data collected in the context of educational research might be shared. Open data sharing enabled by Databrary can serve as a catalyst for a truly multidisciplinary science of learning.

Face-to-face interaction between infants and their caregivers is a mainstay of developmental research. However, common laboratory paradigms for studying dyadic interaction oversimplify the act of looking at the partner's face by seating infants and caregivers face to face in stationary positions. In less constrained conditions when both partners are freely mobile, infants and caregivers must move their heads and bodies to look at each other. We hypothesized that face looking and mutual gaze for each member of the dyad would decrease with increased motor costs of looking. To test this hypothesis, 12-month-old crawling and walking infants and their parents wore head-mounted eye trackers to record eye movements of each member of the dyad during locomotor free play in a large toy-filled playroom. Findings revealed that increased motor costs decreased face looking and mutual gaze: Each partner looked less at the other's face when their own posture or the other's posture required more motor effort to gain visual access to the other's face. Caregivers mirrored infants' posture by spending more time down on the ground when infants were prone, perhaps to facilitate face looking. Infants looked more at toys than at their caregiver's face, but caregivers looked at their infant's face and at toys in equal amounts. Furthermore, infants looked less at toys and faces compared to studies that used stationary tasks, suggesting that the attentional demands differ in an unconstrained locomotor task. Taken together, findings indicate that ever-changing motor constraints affect real-life social looking.

This article reviews research on the development of motor behavior from a developmental systems perspective. We focus on infancy when basic action systems are acquired. Posture provides a stable base for locomotion, manual actions, and facial actions. Experience facilitates improvements in motor behavior and infants accumulate immense amounts of experience with all of their basic action systems. At every point in development, perception guides motor behavior by providing feedback about the results of just prior movements and information about what to do next. Reciprocally, the development of motor behavior provides fodder for perception. More generally, motor development brings about new opportunities for acquiring knowledge about the world, and burgeoning motor skills can instigate cascades of developmental changes in perceptual, cognitive, and social domains. WIREs Cogn Sci 2017, 8:e1430. doi: 10.1002/wcs.1430 For further resources related to this article, please visit the WIREs website.

This review challenges the traditional interpretation of infants' and young children's responses to three types of potentially "fear-inducing" stimuli--snakes and spiders, heights, and strangers. The traditional account is that these stimuli are the objects of infants' earliest developing fears. We present evidence against the traditional account, and provide an alternative explanation of infants' behaviors toward each stimulus. Specifically, we propose that behaviors typically interpreted as "fearful" really reflect an array of stimulus-specific responses that are highly dependent on context, learning, and the perceptual features of the stimuli. We speculate about why researchers so commonly misinterpret these behaviors, and conclude with future directions for studying the development of fear in infants and young children.

The current study investigated age differences in free viewing gaze behavior. Adults and 6-, 9-, 12-, and 24-month-old infants watched a 60-s Sesame Street video clip while their eye movements were recorded. Adults displayed high inter-subject consistency in eye movements; they tended to fixate the same places at the same. Infants showed weaker consistency between observers and inter-subject consistency increased with age. Across age groups, the influence of both bottom-up features (fixating visually-salient areas) and top-down features (looking at faces) increased. Moreover, individual differences in fixating bottom-up and top-down features predicted whether infants' eye movements were consistent with those of adults, even when controlling for age. However, this relation was moderated by the number of faces available in the scene, suggesting that the development of adult-like viewing involves learning when to prioritize looking at bottom-up and top-down features.

Researchers can study complex developmental phenomena with all the inherent noise and complexity or simplify behaviors to hone in on the essential aspects of a phenomenon. We used the development of walking as a model system to compare the costs and benefits of simplifying a complex, noisy behavior. Traditionally, researchers simplify infant walking by recording gait measures as infants take continuous, forward steps along straight paths. Here, we compared the traditional straight-path task with spontaneous walking during 20 minutes of free play in 97 infants (10.75-19.99 months of age). We recorded infants' footfalls on an instrumented floor to calculate gait measures in the straight-path and free-play tasks. In addition, we scored videos for other critical aspects of spontaneous walking--steps per bout, shape of walking path, and step direction. Studying infant walking during free play incurred no cost compared with the straight-path task, but considerable benefits. Straight-path gait was highly correlated with spontaneous gait and both sets of measures improved with walking age, validating use of the straight-path task as an index of development. However, a large proportion of free-play bouts were too short to permit standard gait measures, and most bouts were curved with omnidirectional steps. The high prevalence of these "non-canonical" bouts was constant over development. We propose that a focus on spontaneous walking, the phenomenon we ostensibly wish to explain, yields important insights into the problems infants solve while learning to walk. Other areas of developmental research may also benefit from retaining the complexity of complex phenomena.

How does visual experience change over development? To investigate changes in visual input over the developmental transition from crawling to walking, thirty 13-month-olds crawled or walked down a straight path wearing a head-mounted eye tracker that recorded gaze direction and head-centered field of view. Thirteen additional infants wore a motion tracker that recorded head orientation. Compared to walkers, crawlers' field of view contained less walls and more floor. Walkers directed gaze straight ahead at caregivers, whereas crawlers looked down at the floor. Crawlers obtained visual information about targets at higher elevations-caregivers and toys-by craning their heads upward and sitting up to bring the room into view. Findings indicate that visual experiences are intimately tied to infants' posture.

Possibilities for action depend on the fit between the body and the environment. Perceiving what actions are possible is challenging, because the body and the environment are always changing. How do people adapt to changes in body size and compression? In Experiment 1, we tested pregnant women monthly over the course of pregnancy to determine whether they adapted to changing possibilities for squeezing through doorways. As women gained belly girth and weight, previously passable doorways were no longer passable, but women's decisions to attempt passage tracked their changing abilities. Moreover, their accuracy was equivalent to that of nonpregnant adults. In Experiment 2, nonpregnant adults wore a "pregnancy pack" that instantly increased the size of their bellies, and they judged whether doorways were passable. Accuracy in the "pregnant" participants was only marginally worse than that of actual pregnant women, suggesting that participants adapted to the prosthesis during the test session. In Experiment 3, participants wore the pregnancy pack and gauged passability before and after attempting passage. The judgments were grossly inaccurate prior to receiving feedback. These findings indicate that experience facilitates perceptual-motor recalibration for certain types of actions.

Perceiving possibilities for action-affordances-requires sensitivity, accuracy, and consistency. In the current study, we tested children of different ages (16-month-olds to 7-year-olds) and adults to examine the development of affordance perception for reaching through openings of various sizes. Using a psychophysical procedure, we estimated individual affordance functions to characterize participants' actual ability to fit their hand through openings and individual decision functions to characterize attempts to reach. Decisions were less accurate in younger children (16-month-olds to 5-year-olds); they were more likely to attempt impossible openings and to touch openings prior to refusing, suggesting a slow developmental trend in learning to perceive affordances for fitting through openings. However, analyses of multiple outcome measures revealed that the youngest participants were equally consistent in their decision making as the oldest ones and that every age group showed sensitivity to changes in the environment by scaling their attempts to opening size.

The valid assessment of vocabulary development in dual‐language‐learning infants is critical to developmental science. We developed the Dual Language Learners English‐Spanish (DLL‐ES) Inventories to measure vocabularies of U.S. English‐Spanish DLLs. The inventories provide translation equivalents for all Spanish and English items on Communicative Development Inventory (CDI) short forms; extended inventories based on CDI long forms; and Spanish language‐variety options. Item‐Response Theory analyses applied to Wordbank and Web‐CDI data (n = 2603, 12–18 months; n = 6722, 16–36 months; half female; 1% Asian, 3% Black, 2% Hispanic, 30% White, 64% unknown) showed near‐perfect associations between DLL‐ES and CDI long‐form scores. Interviews with 10 Hispanic mothers of 18‐ to 24‐month‐olds (2 White, 1 Black, 7 multi‐racial; 6 female) provide a proof of concept for the value of the DLL‐ES for assessing the vocabularies of DLLs. [ABSTRACT FROM AUTHOR]

What is the optimal penalty for errors in infant skill learning? Behavioral analyses indicate that errors are frequent but trivial as infants acquire foundational skills. In learning to walk, for example, falling is commonplace but appears to incur only a negligible penalty. Behavioral data, however, cannot reveal whether a low penalty for falling is beneficial for learning to walk. Here, we used a simulated bipedal robot as an embodied model to test the optimal penalty for errors in learning to walk. We trained the robot to walk using 12,500 independent simulations on walking paths produced by infants during free play and systematically varied the penalty for falling—a level of precision, control, and magnitude impossible with real infants. When trained with lower penalties for falling, the robot learned to walk farther and better on familiar, trained paths and better generalized its learning to novel, untrained paths. Indeed, zero penalty for errors led to the best performance for both learning and generalization. Moreover, the beneficial effects of a low penalty were stronger for generalization than for learning. Robot simulations corroborate prior behavioral data and suggest that a low penalty for errors helps infants learn foundational skills (e.g., walking, talking, and social interactions) that require immense flexibility, creativity, and adaptability. Research Highlights: During infant skill acquisition, errors are commonplace but appear to incur a low penalty; when learning to walk, for example, falls are frequent but trivial.To test the optimal penalty for errors, we trained a simulated robot to walk using real infant paths and systematically manipulated the penalty for falling.Lower penalties in training led to better performance on familiar, trained paths and on novel untrained paths, and zero penalty was most beneficial.Benefits of a low penalty were stronger for untrained than for trained paths, suggesting that discounting errors facilitates acquiring skills that require immense flexibility and generalization. [ABSTRACT FROM AUTHOR]

How do infants plan and guide locomotion under challenging conditions? This experiment investigated the real-time process of visual and haptic exploration in 14-month-old infants as they decided whether and how to walk over challenging terrain--a series of bridges varying in width. Infants' direction of gaze was recorded with a head-mounted eye tracker and their haptic exploration and locomotor actions were captured on video. Infants' exploration was an organized, efficient sequence of visual, haptic, and locomotor behaviors. They used visual exploration from a distance as an initial assessment on nearly every bridge. Visual information subsequently prompted gait modifications while approaching narrow bridges and haptic exploration at the edge of the bridge. Results confirm predictions about the sequential, ramping-up process of exploration and the distinct roles of vision and touch. Exploration, however, was not a guarantee of adaptive decisions. With walking experience, exploratory behaviors became increasingly efficient and infants were better able to interpret the resulting perceptual information in terms of whether it was safe to walk.

Some grips on the handle of a tool can be planned on the basis of information directly available in the scene. Other grips, however, must be planned on the basis of the final position of the hand. "End-state comfort" grips require an awkward or uncomfortable initial grip so as to later implement the action comfortably and efficiently. From a cognitive perspective, planning for end-state comfort requires a consistent representation of the entire action sequence, including the latter part, which is not based on information directly available in the scene. Many investigators have found that young children fail to demonstrate planning for end-state comfort and that adultlike performance does not appear until about 12 years of age. In 2 experiments, we used a hammering task that engaged children in a goal-directed action with multiple steps. We assessed end-state-comfort planning in novel ways by measuring children's hand choice, grip choice, and tool implementation over multiple trials. The hammering task also uniquely allowed us to assess the efficiency of implementation. We replicated the previous developmental trend in 4-, 8-, and 12-year-old children with our novel task. Most important, our data revealed that 4-year-olds are in a transitional stage during which several competing strategies were exhibited during a single session. Preschoolers changed their grip within trials and across trials, indicating awareness of errors and a willingness to sacrifice speed for more efficient implementation. The end-state-comfort grip initially competes as one grip type among many but gradually displaces all others. Children's sensitivity to costs and drive for efficiency may motivate this change.