Ontogenetic constraints on the evolution of right-handedness.

Ontogenetic factors constrain the evolution of species-typical traits. Because human infants are born “prematurely” relative to other pri- mates, the development of handedness during infancy can reveal impor- tant ontogenetic influences on handedness that may have contributed to the evolution of the human species-typical trait of a population-level right- hand dominance

Concordance of Handedness Between Teacher and Student Facilitates Learning Manual Skills

Eighty-six left- and right-handed male and female adults received demonstrations of the manual actions involved in tying three different knots from either left- or right-handed female instructors. Learning was greatly facilitated by concordance of handedness between teacher and student, Sex of subject had no effect, nor were any interaction effects significant. Therefore, it is conceivable that observation learning of manual skill, which accompanied the hominid evolution of tool-use and tool-making skills, could have provided selective pressure for concordance (the right-bias) in human handedness

Intermanual transfer of tactile discrimination.

Little is known about infant tactile discrimination, even though most sensory and motor innervation of each and is ,restricted to the contralateral hemisphere

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

Infant interest expressions as coordinative motor structures

Two opposing facial actions, raised and knit (contracted} brows, hove been considered expressions of the unitary emotion of interest. We examined differential relationships between these brow actions and accompanying head, eye, arm, and other facial movements in 5- and 7-month-old infants who were videotaped as toys were presented above or below eye level. Raised-brow movements significantly co-occurred with head-up and/or eyes-up movements for both ages. Knit-brows co-occurred with eyes-down at 5 months and head-down at 7 months. Frequency of arm movements was not systematically related to head, eye, or brow movements. Muscles that move the brows con be recruited when young infants move their head and/or eyes. Therefore, converging sources of evidence are needed before interest con be inferred from the brow actions of infants

Experience and progesterone in ring dove incubation.

Doves were tested for progesterone-induced incubation after they had acquired previous experience with from one to five different phases of their initial breeding cycle. A sixth group had no previous breeding experience. Previous breeding experience inclusive of at least the nestbuilding phase of the cycle is a significant facilitator of incubation behaviour induced by injected progesterone, but previous experience with only the courtship phase was not sufficient. Experience with phases additional to nestbuilding increased overall nest-responsiveness without significantly increasing the number of birds incubating. The nature of a ring dove's previous experience is therefore an important influence in its behavioural response to exogenous progesterone

A meta-analysis of primate hand preference for reaching and other hand-use preferences.

Humans, as do most vertebrate species studied, exhibit a limb preference for unimanual activities. However, two characteristics of the human limb preference are thought to distinguish it from that of other vertebrates: (a) The preference is the same across a variety of manual tasks that have few task demands or motor skills in common (handedness consistency); and (b) the handedness consistency is unevenly distributed in the population with a distinct right-handed skew. Thus, depending on the criteria used to define a preference, 70%–90% of humans exhibit a consistent right-hand preference for manual activities (Annett, 1985). This sharp population bias in the distribution of hand preference has been prevalent for much of the natural history of humans (Corballis, 1991) and is present in all cultures (Annett, 1985). Anthropological evidence suggests a population bias toward right-handedness in the hominid ancestors of humans that dates back at least 1.8 million years (McManus, 2002; Toth, 1985). The evidence shows both a right-hand dominance in the construction of tools and an asymmetry in form of tools such that their use would be much more manageable with the right hand. Thus, the right bias in human handedness seems to be an evolutionary extension of a right bias in hominid handedness

Rate and timing precision of motor coordination in Developmental Dyslexia.

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)

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

The effect of certain task characteristics on performance on two neuropsychological tests of spatial abilites.

Certain neuropsychological assessments of spatial ability assume that the processing of diagonality and nondiagonality of patterns is equivalent and that processing 2-D representations is equivalent to processing 3-D objects. The Stick Test and the Locomotor Maze Test also assume that successful performance requires the use of mental rotation. Normal adult subjects received either 2-D or 3-D versions of the Stick Test (n = 45) or an unlabeled or labeled version of the maze test (n = 25). Both tests used either nondiagonal or diagonal patterns. More errors were made on 2-D representations of sticks than on the 3-D sticks. Also, more errors occurred with the diagonal than with the orthogonal patterns on both tasks. When maze paths were labeled, fewer subjects made errors on the orthogonal paths than on the diagonal paths. Few subjects reported using mental rotation to perform these tasks. The performance of normal adults may violate the assumptions usually made about a test,s measure of spatial ability