Incidence of nondextrality in the NAS/NRC Twin Registry

Genetic theories still flounder on the fact that similarity of hand preference is the same in monozygotic (MZ) and dizygotic (DZ) twins. Lateral preference on a well-designed set of 5 activities was obtained from 2,131 male pairs. On item analysis, only "throw" discriminated zygosity, attributable to "excess" nondextral MZ pairs. This item is remarkably free of the intense cultural bias against sinistrality.

Rhythms of dialogue in infancy: coordinated timing in development

Although theories of early social development emphasize the advantage of mother-infant rhythmic coupling and bidirectional coordination, empirical demonstrations remain sparse. We therefore test the hypothesis that vocal rhythm coordination at age 4 months predicts attachment and cognition at age 12 months. Partner and site novelty were studied by recording mother-infant, stranger-infant, and mother-stranger face-to-face interactions in both home and laboratory sites for 88 4-month-old infants, for a total of 410 recordings. An automated dialogic coding scheme, appropriate to the nonperiodic rhythms of our data, implemented a systems concept of every action as jointly produced by both partners. Adult-infant coordination at age 4 months indeed predicted both outcomes at age 12 months, but midrange degree of mother-infant and stranger-infant coordination was optimal for attachment (Strange Situation), whereas high ("tight") stranger-infant coordination in the lab was optimal for cognition (Bayley Scales). Thus, high coordination can index more or less optimal outcomes, as a function of outcome measure, partner, and site. Bidirectional coordination patterns were salient in both attachment and cognition predictions. Comparison of mother-infant and stranger-infant interactions was particularly informative, suggesting the dynamics of infants' early differentiation from mothers. Stranger and infant showed different patterns of vocal rhythm activity level, were more bidirectional, accounted for 8 times more variance in Bayley scores, predicted attachment just as well as mother and infant, and revealed more varied contingency structures and a wider range of attachment outcomes. To explain why vocal timing measures at age 4 months predict outcomes at age 12 months, our dialogue model was construed as containing procedures for regulating the pragmatics of proto-conversation. The timing patterns of the 4-month-olds were seen as procedural or performance knowledge, and as precursors of various kinesic patterns in the outcomes of 12-month-olds. Thus, our work further defines a fundamental dyadic timing matrix--a system that guides the trajectory of relatedness, informing all relational theories of development.

Mathematical models for coordinated interpersonal timing in mother-infant interactions in the first year of life

Two mathematical models of social contingency are examined in terms of their development over the first year of life. The interactions of 53 mothers and their infants were recorded at 6 weeks, 4 months, and 12 months. The infants' gazes at 6 weeks, the mothers' vocal behavior at 6 weeks, and the vocal behavior of the mother and infant at 4 and 12 months were automatically coded in terms of four states. The conditional dependence model and the response effects model were computed for each interaction at each age, and the coefficients of the models were examined as a function of age. The relative success of the models as estimates of moment-to-moment contingency as well as their variations with age are discussed.

Postnatal development of the suprachiasmatic hypothalamic nucleus of the rat

Light and electron microscopy of newborn, four day, one, two, three and five week old rats revealed principally a progressive increase in the diversity and number of synaptic contacts in the suprachiasmatic nucleus (SCN). The major increase in synaptic diversity occurred between four days and one week of age. Correlation between this finding and the adult synaptic morphology of SCN (Gülder, 1976) on the one hand, and the ontogeny of circadian rhythms on the other were made. This suggested that the retinal afferents arriving on day four form asymmetrical contacts with dendrites. While increase in synaptic number was progressive, it was most marked between three and five weeks of age. By five weeks, most features of the adult SCN were present. No significant morphological effects were evident as a result of neonatal retinal lesions.

A simple apparatus for controlled pressure perfusion fixation

An apparatus is described for vascular perfusion of small animals with fixative solutions which while simple and inexpensive allows quantitative and reproducible pressure control. Direct measurement of outflow pressure demonstrated that a sphygmomanometer used to supply external pressure to a plastic blood transfer pack achieved these features. Reproducibility of pressure was maintained by filling the bag with air between perfusions, or when smaller volumes of fixative solution were needed. Rodent brains from fetal to adult ages, and other animals of similar size, have been reliably well fixed by this method.