Temporal characteristics of interlimb coordination were examined in adolescents and young adults with developmental dyslexia, matched normal control subjects, and matched learning disabled adolescent students without reading difficulties. Subjects were asked to tap in time to an entraining metronome at each of 3 prescribed rates by moving the index fingers of both hands in unison, in rhythmical alternation, or in more complex bimanual patterns. Dyslexic subjects showed significant deficits of timing precision on bimanual tasks that required the integration of asynchronous responses, but not when they moved the fingers in unison. Findings are discussed in terms of both the hypothesis that impaired temporal resolution in dyslexia reflects an underlying deficit of lefthemisphere function and an alternative hypothesis that functional deficits in developmental dyslexia are associated with impaired interhemispheric communication.Learning to read builds on the speech processes of the child at many levels (Denckla, 1979;Mann, 1986), and language impairment is one of the most common behavioral correlates of developmental dyslexia (Doehring, Trites, Patel, & Fiedorowicz, 1981;Jorm, 1979;Vellutino, 1978). Current dyslexia research therefore has emphasized the linguistic analysis of language deficits and has brought converging evidence to the effect that phonological processing deficits, for example, play a critical role in reading impairment (Mann, 1986; Wagner & Torgensen, 1987).However, neuropsychological investigations have identified various biological and neurological correlates of developmental dyslexia including abnormal patterns of hemispheric specialization and genetic variations (Denckla, 1979;Hiscock & Kinsbourne, 1982; Smith, Goldgar, Pennington, Kimberling, & Lubs, 1986), the functional significance of which cannot be clarified by a linguistic analysis alone (Ellis, 1985;Tallal, Stark, & Mellits, 1985). An alternative research strategy has therefore emerged that recognizes the importance of language disabilities as proximal causes, but focuses on underlying "physiologically plausible" processes (Hammond, 1982) that may account both for the nonlinguistic, noncognitive behavioral correlates and the language impairment in developmental dyslexia.Lashley's classic hypothesis that speech and skilled movements share neural mechanisms of timing precision and serial order control (

Abstract:Most newborn infants (65 percent) preferred to lie with their heads turned to the right, whereas 15 percent showed a distinct preference for the left. Orientation preference is maintained for at least 2 months and predicts preferential hand use in reaching tasks at both 16 and 22 weeks. Right head-orientation preference in early infancy may contribute to the early development of right-handedness. Article:Right-and left-handedness are associated with individual differences in a wide variety of psychological phenomena from cognitive styles to recovery of function after brain damage (1), but the causes of these associations are unknown. Studying the development of hand preference may disclose some causes of these functional associations. The two characteristics to be explained in human handedness are individual variability, which is common to humans and other mammalian species, and the predominance of right-handedness, which is unique to humans. In nonhuman species, limb preference is randomly distributed among right-, mixed-, and left-limb use (2).Although individual variability could be genetic in origin, the evidence from breeding studies in mice and ontogenetic studies of monkeys favors a nongenetic origin (3). Collins reported that mice reared under conditions favoring the use of the right limb develop a right-limb preference, whereas mice reared under conditions favoring the use of the left limb develop a left-limb preference (4). If environmental circumstances can bias the distribution of limb preferences of mice, then, since every human culture provides some pressure in favor of the right hand (5), culture might constitute the bias responsible for the predominance of righthandedness in humans. If so, however, this would leave unanswered why all cultures should favor righthandedness. Annett has argued that cultural pressures can maintain and increase the bias toward the right hand, but they probably cannot explain its initial occurrence and universal prevalence (6). Therefore, she concluded that the source of the right bias must be genetic.Of the various genetic models of human handedness that have been proposed, only Annett's accounts for all of the genealogical data, particularly the proportion of right-handedness among the offspring of two left-handed parents (7). Her model postulates a single allele, which, when present, superimposes a dextral bias on individual variability in handedness, but, when absent-as in the offspring of two left-handed parents-permits individual handedness to assort in the random proportions typical of mammals. Her model also implies that this allele may not affect handedness directly, but instead may produce asymmetries for other functions, which, in turn, produce the dextral bias.Most newborn infants orient their heads toward their right sides while supine (8). This right bias has been thought to contribute to the development of the right bias in handedness by producing lateral asymmetries in visual experience of the hands and differences between the hands in neuromoto...

P. F. MacNeilage, M. G. Studdert-Kennedy, and B. Lindblom (1987) proposed a progression for handedness in primates that was supposed to account for the evolution of a right bias in human handedness. To test this proposal, the authors performed meta-analyses on 62 studies that provided individual data (representing 31 species: 9 prosimians, 6 New World monkeys, 10 Old World monkeys, 2 lesser apes, and 4 greater apes), of the 118 studies of primate handedness published since 1987. Although evidence of a population-level left-handed bias for prosimians and Old World monkeys supports P. F. MacNeilage et al., the data from apes, New World monkeys, and individual species of prosimians and New World monkeys do not. Something other than primate handedness may have been the evolutionary precursor of the right bias in hand-use distribution among hominids.

Ten newborn infants who preferred to lie with their heads turned rightward and ten who preferred to lie with their heads turned leftward had their hand-use preferences for reaching assessed at 8 different ages during the period 12-74 weeks postpartum. Eighteen infants maintained stable hand-use preferences throughout this period and the direction of their neonatal head orientation preference predicted the hand they preferred to use. The neonatal head-turn preference was maintained through the first 2 months and induced lateral asymmetries in visual regard and motor control of the hands. These lateral asymmetries are plausible contributors to mechanisms linking neonatal headturn preference to infant hand-use preference. Thus, the dextral bias in handedness may be derived, in part, from the rightward bias in neonatal head-turn preference.

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 to 14 months old. We also developed a new measure for assessing RDBM preference presented when children were 18 to 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.

Role-differentiated bimanual manipulation (RDBM) is a complementary movement of both hands that requires differentiation between actions of the hands. Previous research showed that RDBM can be observed in infants as early as 7 months. However, RDBM could be considered a skill only when its frequency, duration, and use is appropriate for the type of manual task, and there is some evidence of intentionality in use. Twenty-four normally developing infants were studied longitudinally at 7, 9, 11, and 13 months to assess the frequency and duration of five clearly different types of RDBM with three "single-part" and three "two-part" toys as they emerge during development. Also, the sequences of actions that lead to RDBM were examined for evidence of "intentionality." The results show that although the each type of RDBM appears early in infancy, RDBM only begins to exhibit the characters of a skill by 13 months. Moreover, the type of toy influences not only the likelihood of eliciting role differentiation, but also the type of RDBM behavior and the organization of the sequence of actions that lead to RDBM. Some useful criteria for defining an infant sensorimotor skill are provided in discussion.

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 two 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-III®. Children who were not lateralized as infants, but 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.

Role-differentiated bimanual manipulation requires each hand to perform different, but complementary, actions on one or more objects. It is usually considered to be a late-developing high-level motor and cognitive skill involving the coordination of the two hemispheres of the brain. The frequency of role-differentiated bimanual manipulations was recorded in a longitudinal sample of 24 infants tested at 7, 9, 11, and 13 months during play with 10 different toys. Role differentiation was observed as early as 7 months, and its frequency was unaffected by toy characteristics. Role-differentiated bimanual actions increased with age, and the type of toy did influence the likelihood of eliciting role differentiation between 9 and 13 months. Article: Role-differentiated hand use can be defined as a bimanual manipulation skill in which each hand performs a different action, but these actions coalesce in the manipulation of an object. The different actions of the two hands have complementary functions; one hand has a supporting, or stabilizing, role as the other manipulates or explores the object. Although Gesell (1939) found very little bimanual activity around 12 to 14 months when he presented children with blocks, role-differentiated bimanual actions have been observed during toy play as early as 1 year of age, and they have been labeled role differentiation (