The postnatal development of the motor speech area: a preliminary study.
This preliminary study follows the maturation of motor speech areas and their adjacent orofacial motor zones in the right and left hemispheres of the human infant from 3 months to 72 months of age. Quantitative studies of basilar dendrite patterns of layer 5 cortical pyramids are reported in 17 age-graded subjects. The sequence of developmental changes is characterized by early (3 month) prominence of orofacial motor zones over motor speech areas and of the right hemisphere over the left as expressed in total basilar dendritic length and the length of proximal order dendritic segments. The complex series of changes which follow appear to involve increase in number and marked increase in length of the developing distal order segments and possible shortening of the proximal segments. During a variable sequence of apparent growth spurts and resorptions, dendrite arborizations of the motor speech areas overtake and exceed those of orofacial musculature and the total length of dendrite systems on the left finally exceeds those on the right, although even in the most mature group, (42-72 months), distal segment length in the right motor speech area still exceeds that of the left. The early structural primacy of right-sided dendrite systems and the progressive shift to left-sided primacy is considered in the light of the presence of a long phylogenetic and ontogenetic history of gross anatomical asymmetry favoring the left side. The possibility that developmental patterns of the cortical macroenvironment and microenvironment may be dissociated with the microenvironment depending heavily on epigenetic factors receives support from recent data suggesting the existence of a time window or critical period during which primary language proficiency must be attained if normal lateralization of language function is to occur.