Lateralization of sensory and motor functions in human neonates.

Lateral differences in some motor and sensory functions were examined in 89 full-term normal newborns, 2 to 4 days old. They were divided in two groups, according to the cerebral dominance in the family: Group A (70 subjects), offspring of right-handed parents and siblings, and Group B (19 subjects), with at least one parent or sibling left-handed or ambidextrous. Only subjects of Group A showed a marked tendency to spend more time with the head to right as opposed to the left, to turn right after release from the midline position, to have their heads right 5 min. later, to lead with the right leg on the placing response. The two groups were significantly different from each other on these tests. Differences were not observed for stepping, tactile responses, asymmetrical tonic neck reactions in either of the two groups. The results confirm the existence of lateral differentiations of neurological functions in the newborn and stress the presence of genetic factors in these phenomena.