Sleight of hand: role-differentiated bimanual manipulation speed across infancy.

Role-differentiated bimanual manipulation (RDBM) is a complex behaviour requiring the complementary movement of two hands to achieve a common goal. We investigated the relation of RDBM speed (time to complete a successful RDBM) with a hand preference for acquiring objects (early right, late right, left, no preference), toy type (simple/difficult), age (9-14 months), and hand (right/left) used to perform the RDBM. Changes in RDBM speed across age were examined across different hand preference groups for RDBMs performed on simple toys using the right hand. The analysis revealed that early-right preference infants had a steeper slope than the no preference/left-preference infants. The same was true for right-preference infants (early- and late-) for RDBMs performed on difficult toys using the right hand. A mixed ANOVA revealed that there were decreases in RDBM times across age, therefore infants are faster at performing RDBMs over time, regardless of toy type, hand used, or hand preference. The results of the present study suggest that when exploring the development of hand preference, we should consider the influence of age, hand preference, and hand used.

Early, concurrent, and consistent hand preferences predict stacking in toddlerhood.

Stacking is a hallmark of fine motor skill development and requires skilled hand use. One mechanism for children to gain manual proficiency involves establishing a hand preference that creates practice differences between the hands as the preferred hand is used more often and in different ways than the other. Prior work found that stacking skill emerged earlier for infants with an identifiable hand preference. However, it is not known how hand preference relates to later toddler stacking performance. This study examined the effects of early hand preference (infant pattern), concurrent hand preference (toddler pattern), and consistent hand preference (infant to toddler pattern) on toddler stacking skills. Sixty-one toddlers, whose hand preferences as infants were known, were assessed for their toddler hand preference and their stacking skill across 7 monthly visits from 18 to 24 months of age. Using multilevel Poisson longitudinal analysis, children with hand preferences that were consistent across both infancy and toddlerhood were more successful at stacking, compared to those with inconsistent preferences across the infant and toddler periods. Thus, consistency of hand preferences during the first 2 years likely contributes toward individual differences in the development of fine motor skills.

Preschool language ability is predicted by toddler hand preference trajectories.

Prior work has found links between consistency in toddler handedness for the fine motor skill role-differentiated bimanual manipulation (RDBM), and language development at 2 and 3 years of age. The current study investigated whether consistency in handedness from 18 to 24 months (N = 90) for RDBM predicts receptive and expressive language abilities assessed using the Preschool Language Scales 5th edition (PLS-5) at 5 years old. Latent class growth analyses identified 3 stable RDBM hand preference trajectories: a left hand preference with moderate right hand use (left-moderate right), a right hand preference with moderate left hand use (right-moderate left), and a right hand preference with only mild left hand use (right-mild left). At 5 years of age, children with a right-mild left handedness trajectory as toddlers scored significantly higher on receptive and expressive language abilities compared to children with a left-moderate right hand preference. Children with a right-mild left hand preference for RDBM also scored significantly higher on receptive language abilities compared to children with a right-moderate left RDBM hand preference. Children with left-moderate right and children with a right-moderate left hand preference for RDBM as toddlers did not differ in receptive or expressive language abilities at 5 years. Results indicate that individual differences in hand preference consistency for fine motor skill in toddlerhood have cascading effects on language outcomes into the preschool years. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

The Development of Object Construction from Infancy through Toddlerhood.

Early in development, children explore and combine objects in increasingly complex ways. One manual skill, object construction, represents a major shift in how objects are explored relative to other objects. Despite recent connections with cognitive functioning such as spatial skills, the development of object construction ability has rarely been studied and its trajectory has not been adequately described. The purpose of this study was to describe the development of three types of object construction (stacking, nesting, affixing) across 5 monthly infant visits and 7 monthly toddler visits using a longitudinal design and a large sample size. Infants (10-14 months, n=131) and toddlers drawn from the infant sample (18-24 months, n=65) were given sets of age-appropriate toys each of which elicited at least one type of constructive play. We described success at different construction tasks and identified trends for construction skill for infant and toddler development using multilevel modeling. We found that each of the three construction actions developed in unique ways across the 10-24-month period. Infant construction skill predicted the development of toddler skill, but toddler construction skill diverged from infant trajectories. We discuss the role of combination strategies in the development of object construction skill and how object construction could be related to other domains of development.

Evolution and development of handedness: An Evo-Devo approach.

Hand preference is a sensorimotor skill whose development both reflects and promotes the development of hemispheric lateralization for manual and cognitive functions. Extensive comparative, crosscultural, and paleoanthropological evidence demonstrates the prevalence of limb lateralized preferences across vertebrate species and the prevalence of right-handedness within hominid evolution. Many reviews of the evolution and development of human handedness have proposed adaptive explanations for its evolution. However, during the last 3 decades a new approach to understanding evolution (the Extended Evolutionary Synthesis-EES) provided a persuasive alternative to the conventional (Neo-Darwinian Synthetic Theory-ST) evolutionary and developmental accounts. EES combines modern evolutionary and developmental research (Evo-Devo) in ways that alter understanding of natural selection, adaptation, and the role of genes in development and evolution. These changes make obsolete all past accounts of the evolution and development of lateralization and handedness because EES/Evo-Devo requires new study designs. The developmental trajectories of any structural or functional trait must be specified so that it may be related to variations in the developmental trajectories of other traits. First, we describe how the EES/Evo-Devo differs from the conventional ST, particularly for understanding of how traits develop. Then, we apply Evo-Devo to the study of handedness development in infancy and its relation to the development of other cognitive functions. Finally, we argue that identifying the development of atypical traits would benefit from knowledge of the range of individual differences in typical developmental trajectories of hand-use preference and their relation to variations in the developmental trajectories of cognitive functions.

The development of neuromotor skills and hand preference during infancy.

Assessing infant handedness has been controversial. Different assessment techniques and theoretical approaches produce different results. Evidence from a dynamic systems perspective showed that the development of postural control during infancy affects the expression of an infant's handedness. However, others found that developmental changes in postural control influenced the amount of symmetrical (bimanual) reaching during infancy, but not hand preference. Since most studies of infant handedness use age to assess development, perhaps measures of an infant's developing neuromotor control, irrespective of age, would better predict changes in an infant's hand preference. To assess neuromotor development, items from [Touwen's (1976) Neurological development in infancy. Lavenham, Suffolk: The Lavenham Press, LTD]. "Group III" indices were used. These items assess developmental changes in neuromotor abilities throughout the 6-14-month age period. Hand preference for acquiring objects was measured during these same months. Group Based Trajectory Models (GBTM) of 380 infants identified four different groups of infants according to the trajectory of the development of their hand preferences (32% Early Right, 12% Early Left, 25% Late Right, 30% No Preference). A multilevel model was used to compare these four developmental trajectories according to age and neuromotor development. Age, not neuromotor development, is a better predictor of differences in developmental trajectories of the four hand preference groups. However, Late Right infants are significantly less developed at 6 months than No Preference, Early Right and Left infants and both Early Right and Left infants are most advanced at 6 months. All groups exhibit similar rates of neuromotor development indicating no "catch-up" by the Late Right infants. Thus, any assessment of infant handedness will incorporate necessarily four groups of infants with differently developing hand preferences and neuromotor abilities.

Toddler hand preference trajectories predict 3-year language outcome.

A growing body of work suggests that early motor experience affects development in unexpected domains. In the current study, children's hand preference for role-differentiated bimanual manipulation (RDBM) was measured at monthly intervals from 18 to 24 months of age (N = 90). At 3 years of age, children's language ability was assessed using the Preschool Language Scales 5th edition (PLS™-5). Three distinct RDBM hand preference trajectories were identified using latent class growth analysis: (1) children with a left hand preference but a moderate amount of right hand use; (2) children with a right hand preference but a moderate amount of left hand use; and (3) children with a right hand preference and only a mild amount of left hand use. Stability over time within all three trajectories indicated that children did not change hand use patterns from 18 to 24 months. Children with the greatest amount of preferred (i.e., right) hand use demonstrated higher expressive language scores compared to children in both trajectories with moderate levels of non-preferred hand use. Children with the greatest amount of right hand use also had higher scores for receptive language compared to children with a right hand preference but moderate left hand use. Results support that consistency in handedness as measured by the amount of preferred hand use is related to distal language outcomes in development.

Do hand preferences predict stacking skill during infancy?

The cascade theory of handedness suggests that hand preferences develop from a history of cascading and sequentially developing manual asymmetries for a variety of actions. Infants who consistently use their preferred hand for a variety of actions likely would gain proficiency using that preferred hand and, consequently, perform more proficiently on other challenging manual tasks. One such task is object stacking, which has been linked with a number of cognitive abilities. If infant hand preference facilitates the development of stacking skill, then this could provide a link by which early hand preference might affect the development of cognition. From a sample of 380 infants assessed for an acquisition hand preference across 6-14 months, 131 infants were assessed for stacking skill from 10 to 14 months at monthly visits. Four unique handedness sub-groups were identified from the 380-infant sample: left, trending right, stable right, or no hand preference. Each of the four hand preference groups exhibited different trajectories in the development of their stacking skills. Left- and stable right-handers stacked more items than infants with no preference by 14 months, whereas infants with a trending right preference did not. The proportion of preferred hand use (right and left) from 6 to 9 months also predicted an earlier initial onset of stacking skill, whereas the proportion of only right hand use did not. Thus, the development of a hand preference predicts an earlier emergence of stacking skill and may have implications for other domains of infant cognitive development.

Infant Hand Preference and the Development of Cognitive Abilities.

Hand preference develops in the first two postnatal years with nearly half of infants exhibiting a consistent early preference for acquiring objects. Others exhibit a more variable developmental trajectory but by the end of their second postnatal year, most exhibit a consistent hand preference for role-differentiated bimanual manipulation. According to some forms of embodiment theory, these differences in hand use patterns should influence the way children interact with their environments, which, in turn, should affect the structure and function of brain development. Such early differences in brain development should result in different trajectories of psychological development. We present evidence that children with consistent early hand preferences exhibit advanced patterns of cognitive development as compared to children who develop a hand preference later. Differences in the developmental trajectory of hand preference are predictive of developmental differences in language, object management skills, and tool-use skills. As predicted by Casasanto's body-specificity hypothesis, infants with different hand preferences proceed along different developmental pathways of cognitive functioning.

The influence of a hand preference for acquiring objects on the development of a hand preference for unimanual manipulation from 6 to 14 months.

Development of hand preferences for unimanual manipulation of objects was explored in 90 infants (57 males) tested monthly from 6 to 14 months. From a larger sample of 380 infants, 30 infants with a consistent left hand preference for acquiring objects were matched for sex and development of locomotion skills with 30 infants with a consistent right hand preference for acquisition and 30 with no preference. Although frequency of unimanual manipulations increased during 6-14 month period, infants with a hand preference for acquisition did more object manipulations than those without a preference for acquisition. Multilevel modeling of unimanual manipulation trajectories for the three hand-preference groups revealed that hand preferences for unimanual manipulation become more distinctive with age, and the preference is predicted by the hand preference for object acquisition. Infants with a right and left hand preference for object acquisition develop a right and left (respectively) hand preference for unimanual manipulation. However, the majority of infants at each month do not exhibit hand preferences for unimanual manipulation that are unlikely to occur by chance, even by 14 months. The results are consistent with a cascading theory of handedness development in which early preferences (i.e., for acquisition) are transferred to later developing preferences (i.e., for unimanual manipulation).

Different assessment tasks produce different estimates of handedness stability during the eight to 14 month age period.

Using 150 infants (57% males), two common tasks for assessing infant hand-use preferences for acquiring objects were compared for their ability to detect stable preferences during the age period of eight to 14 months. One task assesses the preference using nine presentations of objects; the other uses 32 presentations. Monthly classifications of hand preference for each task were determined by either a commonly used a decision criterion in which one hand is used 50% more often than the other or a criterion based on proportion of hand-use difference that exceeds a conventional alpha probability of 0.05. The seven monthly assessments provided by the two tasks also were examined for latent classes in their developmental trajectories. The two tasks were significantly different for both their identification of latent classes and their monthly classification of the infant's hand-use preference. The 32 presentations yielded three developmental trajectories (45% right preferring, 5% left preferring, and 50% no clear preference) whereas the nine presentations revealed only two trajectories (70% right, 30% no preference). The nine presentations task, with the 50% proportion decision criterion, was very generous in classifying right and left-preferring infants at each month but produced greater fluctuations across months compared to the 32 presentation task with an alpha decision criterion. Both tasks revealed that a large proportion of infants are still developing a hand-use preference during this age period. Recommendations are made for examining the development of hand-use preferences and their relation to the development of other neuropsychological functions.

Latent classes in the developmental trajectories of infant handedness.

Handedness for acquiring objects was assessed monthly from 6 to 14 months in 328 infants (182 males). A group based trajectory model identified 3 latent groups with different developmental trajectories: those with an identifiable right preference (38%) or left preference (14%) and those without an identifiable preference (48%) but with a significant trend toward right-handedness. Each group exhibited significant quadratic trends: Those with a right preference increased to asymptote at about 10 months and began decreasing thereafter; those with a left preference increased to asymptote at about 11 months; those without a preference exhibited increasing right-hand use. Since adult handedness reflects different patterns of neural organization which relate to differences in psychological functioning, the observed differences in infant handedness development may relate to differences in the development of infant neurobehavioral organization and functioning. Several methods were used to explore the relation of latent classes to more conventional ways of classifying infant handedness. Classification into handedness groups according to either a monthly z-score or a combination of 4 or fewer months for a handedness index failed to provide reliable estimates of handedness identified by the trajectory analysis. If identified trajectories of handedness development relate to the development of the infant's neurobehavioral organization, researchers who assess infant handedness only once in order to relate it to cognitive, social and emotional functioning may risk misclassifying the handedness of as many as 37-45% of infants.

Postural influences on the development of infant lateralized and symmetric hand-use.

Within-individual variability is such an apparent characteristic of infant handedness that handedness is believed to consolidate only in childhood. Research showed that manifest handedness is influenced by emerging postural skills (sitting, crawling, and walking). In this investigation, it was proposed that symmetric hand-use (tendency to acquire objects bimanually), rather than lateralized hand-use (the use of one hand more than the other), may be influenced by postural changes. Trajectories of lateralized and symmetric hand-use for object acquisition were examined in 275 infants tested monthly from 6 to 14 months. Multilevel modeling revealed that change in lateralized hand-use is unrelated to developmental transitions in infant posture, whereas the trajectory of symmetric hand-use changes significantly with the development of postural skills.

Early handedness in infancy predicts language ability in toddlers.

Researchers have long been interested in the relationship between handedness and language in development. However, traditional handedness studies using single age groups, small samples, or too few measurement time points have not capitalized on individual variability and may have masked 2 recently identified patterns in infants: those with a consistent hand-use preference and those with an inconsistent preference. In this study, we asked whether a consistent infant hand-use preference is related to later language ability. We assessed handedness in 38 children at monthly intervals from 6-14 months (infant visits) and again from 18-24 months (toddler visits). We found that consistent right-handedness during infancy was associated with advanced language skills at 24 months, as measured by the Bayley Scales of Infant and Toddler Development (Bayley-III; Bayley, 2006). Children who were not lateralized as infants but who became right-handed or left-handed as toddlers had typical language scores. Neither timing nor direction of lateralization was related to cognitive or general motor skills. This study builds on previous literature linking right-handedness and language during the first 2 years of life.

Multiple trajectories in the developmental psychobiology of human handedness.

We show that handedness is a product of a multifaceted biosocial developmental process that begins prenatally and continues into adulthood. Although right-handedness predominates, handedness varies continuously across the population. Therefore, our phrase "multiple trajectories"refers to both differences in developmental pathways that can lead to similarities in handedness and similarities in pathways that can lead to differences in handedness. The task for the researcher is to identify how, when, and for what actions the trajectory of handedness development can be maintained or changed for an individual. Given the complexity of these developmental pathways, it is likely that the asymmetric sensorimotor activity that occurs during the development of handedness influences other hemispheric variations in neural processing. Indeed, researchers have investigated how handedness relates to cognitive, social, and emotional functioning because handedness represents different patterns of hemispheric specialization. Although the story of handedness development is not complete, it is well worth pursuing because it makes the development of brain-behavior relations more transparent, especially for hemispheric differences in function.

How the development of handedness could contribute to the development of language.

We propose a developmental process which may link the development of handedness with the development of hemispheric specialization for speech processing. Using Arbib's proposed sequence of sensorimotor development of manual skills and gestures (that he considers to be the basis of speech gestures and proto-language), we show how the development of hand-use preferences in proto-reaching skills concatenate into object acquisition skills and eventually into role-differentiated bimanual manipulation skills (that reflect interhemispheric communication and coordination). These latter sensorimotor skills might facilitate the development of speech processing via their influence on the development of tool-using and object management abilities.

Unimanual to bimanual: tracking the development of handedness from 6 to 24 months.

Manual skills change dramatically over the first two years of life, creating an interesting challenge for researchers studying the development of handedness. A vast body of work to date has focused on unimanual skills during the period from the onset of reaching to walking. The current study sought to connect such early unimanual hand use to later role-differentiated bimanual manipulation (RDBM), in which one hand stabilizes the object for the other hand's action. We examined hand use in 38 children over 16 monthly visits using a validated measure for assessing hand preference for acquiring objects when children were 6-14 months old. We also developed a new measure for assessing RDBM preference presented when children were 18-24 months old. The new measure reliably elicited RDBM actions in both toddlers and an adult control group (N = 15). Results revealed that some children show preferences for acquiring objects as infants; these preferences are stable and persist into their second year as new skills appear. Moreover, children with no hand preference during infancy shifted to left or right lateralized hand use as toddlers. Despite a higher incidence of left-handedness compared to adult norms, the majority of children were right-handed by 2 years of age.