Walking and falling: Using robot simulations to model the role of errors in infant walking.

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.

Comparing apples to manzanas and oranges to naranjas: A new measure of English-Spanish vocabulary for dual language learners.

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.

What we don't know about what babies know: Reconsidering psychophysics, exploration, and infant behavior.

Researchers must infer "what babies know" based on what babies do. Thus, to maximize information from doing, researchers should use tasks and tools that capture the richness of infants' behaviors. We clarify Gibson's views about the richness of infants' behavior and their exploration in the service of guiding action - what Gibson called "learning about affordances."

"Go, go, go!" Mothers' verbs align with infants' locomotion.

Caregivers often tailor their language to infants' ongoing actions (e.g., "are you stacking the blocks?"). When infants develop new motor skills, do caregivers show concomitant changes in their language input? We tested whether the use of verbs that refer to locomotor actions (e.g., "come," "bring," "walk") differed for mothers of 13-month-old crawling (N = 16) and walking infants (N = 16), and mothers of 18-month-old experienced walkers (N = 16). Mothers directed twice as many locomotor verbs to walkers compared to same-age crawlers, but mothers' locomotor verbs were similar for younger and older walkers. In real-time, mothers' use of locomotor verbs was dense when infants were locomoting, and sparse when infants were stationary, regardless of infants' crawler/walker status. Consequently, infants who spent more time in motion received more locomotor verbs compared to infants who moved less frequently. Findings indicate that infants' motor skills guide their in-the-moment behaviors, which in turn shape the language they receive from caregivers. RESEARCH HIGHLIGHTS: Infants' motor skills guide their in-the-moment behaviors, which in turn shape the language they receive from caregivers. Mothers directed more frequent and diverse verbs that referenced locomotion (e.g., "come," "go," "bring") to walking infants compared to same-aged crawling infants. Mothers' locomotor verbs were temporally dense when infants locomoted and sparse when infants were stationary, regardless of whether infants could walk or only crawl.

Gahvora cradling in Tajikistan: Cultural practices and associations with motor development.

In Tajikistan, infants are bound supine in a "gahvora" cradle that severely restricts movement. Does cradling affect motor development and body growth? In three studies (2013-2018), we investigated associations between time in the gahvora (within days and across age) and motor skills and flattened head dimensions in 8-24-month-old Tajik infants (N = 269, 133 girls, 136 boys)) and 4.3-5.1-year-old children (N = 91, 53 girls, 38 boys). Infants had later motor onset ages relative to World Health Organization standards and pronounced brachycephaly; cradling predicted walk onset age and the proficiency of sitting, crawling, and walking. By 4-5 years, children's motor skills were comparable with US norms. Cultural differences in early experiences offer a unique lens onto developmental processes and equifinality in development.

How mothers help children learn to use everyday objects.

Children must learn specific motor actions to use everyday objects as their designers intended. However, designed actions are not obvious to children and often are difficult to implement. Children must know what actions to do and how to execute them. Previous work identified a protracted developmental progression in learning designed actions-from nondesigned exploratory actions, to display of the designed action, to successful implementation. Presumably, caregivers can help children to overcome the challenges in discovering and implementing designed actions. Mothers of 12-, 18- to 24-, and 30- to 36-month-olds (N = 74) were asked to teach their children to open containers with twist-off or pull-off lids. Mothers' manual and verbal input aligned with the developmental progression and with children's actions in the moment, pointing to the role of attuned social information in helping children learn to use objects for activities of daily living. However, mothers sometimes "overhelped" by implementing designed actions for children instead of getting children to do it themselves, highlighting the challenges of teaching novices difficult motor actions.

Infant exuberant object play at home: Immense amounts of time-distributed, variable practice.

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.

The process of learning the designed actions of toys.

Many everyday objects require "hidden" affordances to use as designed (e.g., twist open a water bottle). Previous work found a reliable developmental progression in children's learning of designed actions with adult objects such as containers and zippers-from non-designed exploratory actions, to the basics of the designed action, to successful implementation. Many objects designed for children (e.g., toys) also entail designed actions (e.g., interlocking bricks) but might not require a protracted period of discovery and implementation. We encouraged 12- to 60-month-old children (n = 91) and a comparative sample of 20 adults to play with six Duplo bricks to test whether the developmental progression identified for children's learning of adult objects with hidden affordances holds for a popular toy expressly designed for children. We also examined whether children's moment-to-moment behaviors with Duplo bricks inform on general processes involved in discovery and implementation of hidden affordances. With age, children progressed from non-designed exploratory actions, to attempts to interlock, to success, suggesting that the three-step developmental progression revealed with everyday adult objects broadly applies to learning hidden affordances regardless of object type. Detailing the process of learning (the type and timing of children's non-designed actions and attempts to interlock) revealed that the degree of lag between steps of the progression depends on the transparency of the required actions, the availability of perceptual feedback, and the difficulty of the perceptual-motor requirements. Findings provide insights into factors that help or hinder learning of hidden affordances.

The impact of errors in infant development: Falling like a baby.

What is the role of errors in infants' acquisition of basic skills such as walking, skills that require immense amounts of practice to become flexible and generative? Do infants change their behaviors based on negative feedback from errors, as suggested by "reinforcement learning" in artificial intelligence, or do errors go largely unmarked so that learning relies on positive feedback? We used falling as a model system to examine the impact of errors in infant development. We examined fall severity based on parent reports of prior falls and videos of 563 falls incurred by 138 13- to 19-month-old infants during free play in a laboratory playroom. Parent reports of notable falls were limited to 33% of infants and medical attention was limited to 2% of infants. Video-recorded falls were typically low-impact events. After falling during free play in the laboratory, infants rarely fussed (4% of falls), caregivers rarely showed concern (8% of falls), and infants were back at play within seconds. Impact forces were mitigated by infants' effective reactive behaviors, quick arrest of the fall before torso or head impact, and small body size. Moreover, falling did not alter infants' subsequent behavior. Infants were not deterred from locomotion or from interacting with the objects and elevations implicated in their falls. We propose that a system that discounts the impact of errors in early stages of development encourages infants to practice basic skills such as walking to the point of mastery.

"Dancing" Together: Infant-Mother Locomotor Synchrony.

Pre-mobile infants and caregivers spontaneously engage in a sequence of contingent facial expressions and vocalizations that researchers have referred to as a social "dance." Does this dance continue when both partners are free to move across the floor? Locomotor synchrony was assessed in 13- to 19-month-old infant-mother dyads (N = 30) by tracking each partner's step-to-step location during free play. Although infants moved more than mothers, dyads spontaneously synchronized their locomotor activity. For 27 dyads, the spatiotemporal path of one partner uniquely identified the path of the other. Clustering analyses revealed two patterns of synchrony (mother-follow and yo-yo), and infants were more likely than mothers to lead the dance. Like face-to-face synchrony, locomotor synchrony scaffolds infants' interactions with the outside world.

Children's use of everyday artifacts: Learning the hidden affordance of zipping.

The everyday world is populated with artifacts that require specific motor actions to use objects as their designers intended. But researchers know little about how children learn to use everyday artifacts. We encouraged forty-four 12- to 60-month-old children to unzip a vinyl pouch during a single 60-s trial. Although unzipping a pouch may seem simple, it is not. Unzipping requires precise role-differentiated bimanual actions-one hand must stabilize the pouch while the other hand applies a pulling force on the tab. Moreover, kinematic data from six adults showed that the tolerance limits for applying the forces are relatively narrow (pulling the tab within 63° of the zipper teeth while stabilizing the pouch within 4 cm of the slider). Children showed an age-related progression for the unzipping action. The youngest children did not display the designed pulling action; children at intermediate ages pulled the tab but applied forces outside the tolerance limits (pulled in the wrong direction, failed to stabilize the pouch in the correct location), and the oldest children successfully implemented the designed action. Findings highlight the perceptual-motor requirements in children's discovery and implementation of the hidden affordances of everyday artifacts.

Practice and proficiency: Factors that facilitate infant walking skill.

Infant walking skill improves with practice-crudely estimated by elapsed time since walk onset. However, despite the robust relation between elapsed time (months walking) and skill, practice is likely constrained and facilitated by infants' home environments, sociodemographic influences, and spontaneous activity. Individual pathways are tremendously diverse in the timing of walk onset and the trajectory of improvement, and presumably, in the amount and type of practice. So, what factors affect the development of walking skill? We examined the role of months walking, walk onset age, spontaneous locomotor activity, body dimensions, and environmental factors on the development of walking skill in two sociodemographically distinct samples (ns = 38 and 44) of 13-, 15-, and 19-month-old infants. Months walking best predicted how well infants walked, but environmental factors and spontaneous activity explained additional variance in walking skill. Specifically, less crowded homes, a larger percentage of time in spontaneous walking, and a smaller percentage of short walking bouts predicted more mature walking. Walk onset age differed by sample but did not affect walking skill. Findings indicate that elapsed time since walk onset remains a robust predictor of walking skill, but environmental factors and spontaneous activity also contribute to infants' practice, thereby affecting walking skill.

Motor Development: Embodied, Embedded, Enculturated, and Enabling.

Motor development and psychological development are fundamentally related, but researchers typically consider them separately. In this review, we present four key features of infant motor development and show that motor skill acquisition both requires and reflects basic psychological functions. ( a) Motor development is embodied: Opportunities for action depend on the current status of the body. ( b) Motor development is embedded: Variations in the environment create and constrain possibilities for action. ( c) Motor development is enculturated: Social and cultural influences shape motor behaviors. ( d) Motor development is enabling: New motor skills create new opportunities for exploration and learning that instigate cascades of development across diverse psychological domains. For each of these key features, we show that changes in infants' bodies, environments, and experiences entail behavioral flexibility and are thus essential to psychology. Moreover, we suggest that motor development is an ideal model system for the study of psychological development.

Where Infants Go: Real-Time Dynamics of Locomotor Exploration in Crawling and Walking Infants.

Where do infants go? A longstanding assumption is that infants primarily crawl or walk to reach destinations viewed while stationary. However, many bouts of spontaneous locomotion do not end at new people, places, or things. Study 1 showed that half of 10- and 13-month-old crawlers' (N = 29) bouts end at destinations-more than previously found with walkers. Study 2 confirmed that, although infants do not commonly go to destinations, 12-month-old crawlers go to proportionally more destinations than age-matched walkers (N = 16). Head-mounted eye tracking revealed that crawlers and walkers mostly take steps in place while fixating something within reach. When infants do go to a destination, they take straight, short paths to a target fixated while stationary.

Look before you fit: The real-time planning cascade in children and adults.

Goal-directed actions involve problem solving-how to coordinate perception and action to get the job done. Whereas previous work focused on the ages at which children succeed in problem solving, we focused on how children solve motor problems in real time. We used object fitting as a model system to understand how perception and action unfold from moment to moment. Preschoolers (N = 25) and adults (N = 24) inserted three-dimensional objects into their corresponding openings in a "shape-sorting" box. We applied a new combination of real-time methods to the problem of object fitting-head-mounted eye tracking to record looking behaviors, video microcoding to record adjustments in object orientation between reach and insertion, and real-time analysis techniques (recurrent quantification analysis and Granger causality) to test the timing relations between visual and manual actions. Children, like adults, solved the problem successfully. However, adults outperformed children in terms of their speed of fitting, and speed depended on when adjustments of object orientation occurred. Adults adjusted object orientation during transport, whereas children adjusted object orientation after arriving at the box. Children's delays in adjustment resulted from delays in looking at the target shape and its corresponding aperture. Findings show that planning is a real-time cascade of perception and action, and looking provides the basis for planning actions prospectively. We suggest that developmental improvements in problem solving are driven by real-time changes in the instigation of the planning cascade and the timing of its components.

Oh, Behave!: PRESIDENTIAL ADDRESS, XXth International Conference on Infant Studies New Orleans, LA, US May 2016.

Behavior is essential for understanding infant learning and development. Although behavior is transient and ephemeral, we have the technology to make it tangible and enduring. Video uniquely captures and preserves the details of behavior and the surrounding context. By sharing videos for documentation and data reuse, we can exploit the tremendous opportuni-ties provided by infancy research and overcome the important challenges in studying behavior. The Datavyu video coding software and Databrary digital video library provide tools and infrastructure for mining and sharing the richness of video. This article is based on my Presidential Address to the International Congress on Infant Studies in New Orleans, May 22, 2016 (Video 1 at https://www.databrary.org/volume/955/slot/39352/-?asset=190106. Given that the article de-scribes the power of video for understanding behavior, I use video clips rather than static images to illustrate most of my points, and the videos are shared on the Databrary library.

Missing in action: Tool use is action based.

In this commentary on Osiurak and Reynaud's target article, we argue that action is largely missing in their account of the ascendance of human technological culture. We propose that an action-based developmental account can help to bridge the cognitive-sociocultural divide in explanations of the discovery, production, and cultural transmission of human tool use.

It's the journey, not the destination: Locomotor exploration in infants.

What incites infant locomotion? Recent research suggests that locomotor exploration is not primarily directed toward distant people, places, or things. However, this question has not been addressed experimentally. In the current study, we asked whether a room filled with toys designed to encourage locomotion (stroller, ball, etc.) elicits different quantities or patterns of exploration than a room with no toys. Caregivers were present but did not interact with infants. Although most walking bouts in the toy-filled room involved toys, to our surprise, 15-month-olds in both rooms produced the same quantity of locomotion. This finding suggests that mere space to move is sufficient to elicit locomotion. However, infants' patterns of locomotor exploration differed: Infants in the toy-filled room spent a smaller percent of the session within arms' reach of their caregiver and explored more locations in the room. Real-time analyses show that infants in the toy-filled room took an increasing number of steps per bout and covered more area as the session continued, whereas infants in the no-toy room took fewer and fewer steps per bout and traveled repeatedly over the same ground. Although not required to elicit locomotion, moving with toys encouraged infants to travel farther from their caregivers and to explore new areas.

Postural, Visual, and Manual Coordination in the Development of Prehension.

We investigated the real-time cascade of postural, visual, and manual actions for object prehension in 38 6- to 12-month-old infants (all independent sitters) and eight adults. Participants' task was to retrieve a target as they spun past it at different speeds on a motorized chair. A head-mounted eye tracker recorded visual actions and video captured postural and manual actions. Prehension played out in a coordinated sequence of postural-visual-manual behaviors starting with turning the head and trunk to bring the toy into view, which in turn instigated the start of the reach. Visually fixating the toy to locate its position guided the hand for toy contact and retrieval. Prehension performance decreased at faster speeds, but quick planning and implementation of actions predicted better performance.

Infants plan prehension while pivoting.

Skilled object retrieval requires coordination of the perceptual and motor systems. Coordination is especially challenging when body position is changing and visual search is required to locate the target. In three experiments, we used a "pivot paradigm" to induce changes in body position: Participants were passively pivoted 180° toward a target placed at varied locations to the left and right of the center of a reaching board. Experiment 1 showed that 6- to 15-month-old infants (n = 41) plan prehension so quickly that they retrieve targets mid-turn and scale their reaches to target location relative to turn direction. Experiment 2 characterized planning mid-turn reaching in 6- to 8-month-olds (n = 5) wearing a head-mounted eye tracker. Reach planning depended on when the target appeared in the field of view-not on target fixation. Experiment 3 used head-mounted eye tracking and motion tracking to assess perceptual-motor coordination in adults (n = 13). Adults displayed more mid-turn reaching than infants. But like infants, adults scaled reaching to target location relative to turn direction, and contact time depended on when the target came into view-not on target fixation. Findings show that fast, efficient perceptual-motor coordination supports flexibility in infant prehension, and constraints on coordination are similar across the lifespan.