Lateral specialization and verbal/spatial ability in preschool children: age, sex and familial handedness differences.

One hundred and fifty-five preschool children between 2 1/2 and 5 1/2 yr of age were administered two dichotic tests (digit and nonverbal sound) and three Wechsler Intelligence subtests (vocabulary, geometric design and block design). The familial dextral (FD) children around 4 yr and older demonstrated a greater degree of absolute ear laterality than the FD children under 4 yr of age. There were also age-related changes in the relationship between children's ability scores and their ear laterality scores. For older FD children a greater right-ear advantage for verbal stimuli and a left-ear advantage for nonverbal stimuli was associated with higher vocabulary scores. Under 4 yr of age, however, a strong right-ear advantage for verbal stimuli was associated with relatively higher spatial than vocabulary ability. For these younger FD children, less marked laterality predicted higher vocabulary scores. It is hypothesized that a cortical organizational transition is occurring around 4 yr of age and that this transition involves a shift from reliance on subcortical to cortical mediation, accompanied by increasing callosally mediated inhibition and integration of hemispheric function. Furthermore, there were sex differences observed in the relationship between lateral specialization and spatial ability. As young as 2 1/2 yr of age, the FD boys' spatial ability scores were positively associated with having a left-ear advantage for nonverbal sound stimuli. The FD girls' spatial ability scores were not significantly related to their ear advantage scores for nonverbal sounds but were associated with their ear advantage scores for verbal stimuli. In addition, around 4 yr of age a shift was observed in the relationship between the boys' absolute laterality scores and the difference between their spatial and vocabulary ability scores which was not found for the girls' scores. It it suggested that the maturation of right hemispheric spatial mediation may be delayed or inhibited in girls.