Kunitake Suzuki

The Tokyo Twin Cohort Project: Overview and initial findings

The Tokyo Twin Cohort Project (ToTCoP) is a large-scale longitudinal study of 5 years based on 1619 pairs of infant twins reared together. The purpose of the study is to construct a population-based twin registry in Japan and to investigate human growth and development and twin themselves. It covers behavioral, neurological, physical and environmental variables measured by questionnaire, home visiting and brain imaging technology. The full registry contains over 47,000 multiple births collected from the Basic Resident Register, and the targeted population is 3070 probable twins of 0 to 2 years old. Preliminary analysis of the entry questionnaire data showed no serious sampling biases. Descriptive statistics of parental characteristics (parental age, gestation age, parity and placentation, maternal weight, parenting stress) and childrens characteristics (body size at birth, 4 and 10 months of age, milk consumption, and sleeping and social behavior) and their correlations, genetic and environmental contributions and correlations are reported.

Effects of nonconscious perception on motor response

The present study reviews the literature on the empirical evidence for the dissociation between perception and action. We first review several key studies on brain-damaged patients, such as those suffering from blindsight and visual/tactile agnosia, and on experimental findings examining pointing movements in normal people in response to a nonconsciously perceived stimulus. We then describe three experiments we conducted using simple reaction time (RT) tasks with backward masking, in which the first (weak) and second (strong) electric stimuli were consecutively presented with a 40-ms interstimulus interval (ISI). First, we compared simple RTs for three stimulus conditions: weak alone, strong alone, and double, i.e., weak plus strong (Experiment 1); then, we manipulated the intensity of the first stimulus from the threshold (T) to 1.2T and 2T, with the second stimulus at 4T (Experiment 2); finally, we tested three different ISIs (20, 40, and 60 ms) with the stimulus intensities at 1.2T and 4T for the first and second stimuli (Experiment 3). These experiments showed that simple RTs were shorter for the double condition than for the strong-alone condition, indicating that motor processes under the double condition may be triggered by sensory inputs arising from the first stimulus. Our results also showed that the first stimulus was perceived without conscious awareness. These findings suggested that motor processes may be dissociated from conscious perceptual processes and that these two processes may not take place in a series but, rather, in parallel. We discussed the likely mechanisms underlying nonconscious perception and motor response to a nonconsciously perceived stimulus.

Genetic and environmental structure of individual differences in hand, foot, and ear preferences: A twin study

Recent studies with large sample size have reported moderate heritability of hand preference. However, little is known about genetic and environmental factors for lateral preference. We examined the genetic and environmental factors for hand, foot, and ear preferences using a twin design study. A lateral preference questionnaire was administered to twin participants (N=956). Phenotypic correlation matrices of lateral preferences were computed for monozygotic and dizygotic twin pairs, and were subjected to Cholesky decomposition to compute additive genetic and unique environmental correlation matrices. Promax rotation factor analysis of each genetic and environmental correlation matrix yielded six genetic and four environmental factors. Factor-loading patterns for these factors indicated that hand and foot lateral activities were affected by different genetic factors. By contrast, each of the four environmental factors was mainly associated with hand, foot, or ear preference. These results suggest that the genetic structure for lateral preference may be more complex than the environmental structure. In particular, hand preference may be multidimensional in terms of genetic factors.

Genetic and environmental sex differences in mental rotation ability: A Japanese twin study

Sex differences in mental rotation ability have been observed in many countries. A previous study of Finnish participants reported that genetic and environmental influences on mental rotation ability differ between sexes. In this study, we assessed genetic and environmental influences on variance in mental rotation ability in 649 Japanese twins using a mental rotation test. To explain the influence of sex on variance in mental rotation ability, we applied genetic analysis using the sex limitation model. The following two factors explained variance in mental rotation ability: (1) the additive genetic factor, which reflects the accumulated small influence of many genes, and (2) the unique environmental factor, which is a type of environmental factor that differs between co-twins. The shared environmental factor, a type of environmental factor common for co-twins, could not explain the variance in mental rotation ability. Furthermore, the additive genetic factor was the same between sexes (i.e., not qualitative sex differences for the additive genetic factor), indicating that the same genes affect mental rotation ability in both sexes. Despite this observation, the additive genetic influence was greater in males than in females. In contrast, the unique environmental influence was not different between sexes. Considering the current results and those of a previous study, the quantitative sex difference for the additive genetic influences in mental rotation ability may be universal, while the unique environmental differences may depend on the characteristics of specific populations.

Two cohort and three independent anonymous twin projects at the Keio Twin Research Center (KoTReC)

The Keio Twin Research Center has conducted two longitudinal twin cohort projects and has collected three independent and anonymous twin data sets for studies of phenotypes related to psychological, socio-economic, and mental health factors. The Keio Twin Study has examined adolescent and adult cohorts, with a total of over 2,400 pairs of twins and their parents. DNA samples are available for approximately 600 of these twin pairs. The Tokyo Twin Cohort Project has followed a total of 1,600 twin pairs from infancy to early childhood. The large-scale cross-sectional twin study (CROSS) has collected data from over 4,000 twin pairs, from 3 to 26 years of age, and from two high school twin cohorts containing a total of 1,000 pairs of twins. These data sets of anonymous twin studies have mainly targeted academic performance, attitude, and social environment. The present article introduces the research designs and major findings of our center, such as genetic structures of cognitive abilities, personality traits, and academic performances, developmental effects of genes and environment on attitude, socio-cognitive ability and parenting, genes x environment interaction on attitude and conduct problem, and statistical methodological challenges and so on. We discuss the challenges in conducting twin research in Japan.

Genetic effects on infant handedness under spatial constraint conditions

Previous studies have reported a genetic influence on the individual differences in adult handedness; however, relatively little is known about genetic influences on the development of infant hand selection. In the current study, we examined whether genetic influences on handedness are expressed in various spatial locations in infants aged 18 months using the twin method. Infants were asked to respond to targets positioned in left, middle, and right locations using grasping movements. Results showed that similarities in hand selection within monozygotic twin pairs was more than two times higher than that of the dizygotic twin pairs in the middle location. In the left location, similarities in hand selection within monozygotic and dizygotic twin pairs were low. In addition, low individual differences in hand selection in the right location were also observed. These results suggest a non-additive genetic influence on handedness, and that spatial constraint is a crucial factor for the expression of genetic effects on handedness in infants.

Relationships among Visual Awareness, Reaction Time, and Lateralized Readiness Potential in a Simple Reaction Time Task under the Backward Masking Paradigm

The goal of the present study was to examine whether a backward masking paradigm, in which a prime and a mask stimuli were consecutively presented with a short stimulus onset asynchrony affected the time needed for either the perceptual or motor stages of processing and the simple reaction times. The times needed for the perceptual and motor stages were evaluated by measuring the stimulus-locked and response-locked lateralized readiness potentials. The results showed that the onset of the stimulus-locked lateralized readiness potentials under the backward masking paradigm took place earlier than it did under the condition of a mask stimulus presented alone, whereas the onset of the response-locked lateralized readiness potentials did not significantly differ under different stimulus conditions. These results suggested that the participants responded to the masked prime stimulus despite being unaware of the prime stimulus. This may have been mediated by facilitation of the perceptual rather than motor stages.

Genetic and environmental relationships between head circumference growth in the first year of life and sociocognitive development in the second year: a longitudinal twin study.

Although growth in head circumference (HC) during infancy is known to predict later childhood outcomes, the mechanisms underlying this association with later sociocognitive abilities remain undetermined. Thus, using a sample of 241 pairs of normally developing Japanese twins, this study investigated the underpinnings of the association between HC growth (difference between HC at birth and at 10 months) and sociocognitive abilities at 19 months as measured by 10 items from the M-CHAT. Phenotypic correlations between HC at birth and sociocognitive abilities and between HC growth and sociocognitive abilities were marginal and not significant. However, multivariate genetic analyses using Cholesky decomposition revealed that genetic influences on HC growth and those on sociocognitive abilities were negatively associated. On the other hand, shared and nonshared environmental influences on HC growth were positively associated with influences on sociocognitive abilities. Genetic and environmental influences on HC at birth were not significantly associated with influences on sociocognitive abilities. These results help to clarify the role of brain growth during infancy in the subsequent development of sociocognitive abilities and highlight the importance of examining the different roles of genetic and environmental influences in studies of these areas.

The involvement of cognitive processing in a perceptual-motor process examined with EEG time–frequency analysis

OBJECTIVE For motor activities, visual information is crucial for organizing a movement with respect to a given situation. The present study investigates how cognitive information processing is associated with this visuomotor process. METHODS Brain dynamics in executing two perceptual-motor tasks were examined in terms of event-related synchronization (ERS) and event-related desynchronization (ERD) of EEG. Those tasks were (1) reaching toward and grasping a visual object with a pinch grip, and (2) matching the pinch grip size with respect to the perceived object size. RESULTS According to the aperture size in the task execution, both the tasks were affected by the perceived object size inducing the Ebbinghaus illusion. The alpha-ERD patterns were associated with the movement execution and appeared to be identical in both the tasks, whilst the gamma-ERS appeared only for the grasping motion. CONCLUSIONS These results suggest that cognitive processing was involved not only in the matching task but also in the grasping task. These ERD/ERS patterns are thought to reflect the similarity and difference in the perceptual-motor processes between the two tasks. SIGNIFICANCE The analysis of ERD/ERS can provide insight on the qualitative feature in a visuomotor process associated with the involvement of cognitive processing.