Why are some people left-handed? Are there two (right- and non-right-handed), three (right-, left-, and mixed-handed) categories or is handedness a quantitative trait with relatively arbitrarily defined categories? With right-handedness dominating in the population, left-handedness and mixed handedness become atypical. Why, then, is atypical handedness associated with atypical hemispheric specialization for language and other psychological functions? Why is atypical handedness a marker for the development of various forms of socio-emotional and cognitive disorders? The proposed project provides crucial information about the early development of handedness that will help form the foundation for the finding answers to these questions. During the age of 6 to 14 months infants develop proficiency for apprehending objects, manipulating them, and employing complex bimanual manipulations (with action differences between the hands) that facilitate tool use, construction of artifacts, and solution to physical problems. Hand-use preferences for these skills appear during this period and play an important role in the distinction of what makes the actions skillful. The proposed project will chart the development of hand-use preferences for each of three manual skills (prehension, unimanual manipulation, and role differentiated bimanual manipulation), the relations among these skills, and their relation to the infant's age, neuromotor development, genotype and development of tool-use and object construction skills. Using a large sample and sophisticated quantitative techniques, different patterns in the development (as measured by age and/or neuromotor maturity) of lateralization of manual skill may be identified that can be compared to the infant's genotype, and tool-use and construction abilities.
The characteristics of handedness have been important for neurology (especially because it affects recovery of function after brain damage), cognitive neuroscience (especially because it influences how the two halves of the brain specialize for different psychological abilities), neuropsychology (especially because it affects how the brain accomplishes cognitive and social functioning), developmental psychology (especially because it relates to atypical language, cognitive, and social development), anthropology (especially because it relates to the evolution of human intelligence and language), and medicine (especially because it relates to specific problems of cerebral palsy, hemiplegia, intraventricular hemorrhages and other forms of early brain damage). Unfortunately, handedness is too often haphazardly defined in research (producing frequently contradictory or confusing results) such that many have considered it to be a trivial aspect of human functioning. Yet, no culture is without a distinct right-bias in handedness within the population and people consider it an intrinsic aspect of their identity, much like their gender. With this grant, we applied a statistically defensible definition of infant handedness and sophisticated quantitative measures to identify its developmental pathway during infancy. We could only do this with a large sample size, assessed monthly from 6 to 14 months. Thus, we discovered that infants develop along three different paths that affect how handedness contributes to the specialization of brain functions, language and other forms of cognitive development, and social/emotional regulation. This project improves our understanding the development of handedness and can provide insight into the evolution of handedness and its relation to human language and intelligence. The primary aim of this grant was to provide a detailed account of the development of handedness (for three different types of infant hand-use – acquiring objects, manipulating them with one hand, and role differentiated bimanual manipulation – "RDBM") during the period from 6 to 14 months-of-age. Then, differences in the pattern of handedness development (e.g., left- vs. right-handers; infants with a hand-use preference vs. those without) were to be related to the development of other manually dependent skills (tool-use, artifact construction, etc.). The most significant result of the project was the discovery that there are three types of infant development that is made apparent by the development of a hand-use preference. About 38% of infants manifest a right hand-use preference beginning at about 6 to 7 months of age which increases to a plateau at about 10 to 12 months of age. Another 16 % of infants develop a left hand-use preference that seems to plateau at about 13 to 14 months of age. However, the plurality of infants (46%) exhibits no handedness until 10 to 11 months they begin to exhibit a preference to use their right hand. Although many other studies report individual differences in development, unlike our study, their sample sizes are too small to capture the types of underlying patterns in the development of those individual differences. Consequently, there have been no studies examining the developmental consequences of these different patterns of development during infancy. We need to determine whether these three patterns of development affect other kinds of cognitive or social/emotional abilities. We have found that those three developmental pathways for a hand-use preference to acquire objects result in a similar hand-use preferences for manipulating objects by 9 months of age. Also, although a hand-use preference for role-differentiated bimanual manipulation (RDBM) of objects (one hand supports the fine motor actions of the other hand as the object is manipulated and explored) does not occur until 13 months of age, RDBM handedness is the result of the handedness for manipulating objects with one hand, which, in turn, is a result of the hand-use preference for acquiring objects. Thus, the three developmental pathways exhibited by a hand-use preference for acquiring objects affect the subsequent development of hand-use preferences for manipulating them. Indeed, by the end of infancy, most children have already established a preference in their hand-use that continues to build into the adult handedness. We have found that the infantâ€™s hand-use preference for acquiring objects also predicts the hand-use preference for RDBM during the toddler period (18 to 24 months) and the infantâ€™s language skills (measured by Bayley scales) at 24 months. Moreover, a stable hand-use preference for acquiring objects during infancy is associated with faster development of the skill for managing multiple objects. This object management skill reflects a form of knowledge that is relevant for acquiring skill in tool-use and artifact construction (assembling a new object from other objects, e.g., stacking objects, adhering objects). Thus, the three patterns of development identified by the infantâ€™s hand-use for acquiring objects seem to affect the development of hand-use preferences in the manipulation of objects and their management. These, in turn, contribute to successful tool-use and artifact construction and may facilitate the development of language. Handedness development matters because it influences the development of other abilities.