THE HUMAN BRAIN AGE 7-11 YEARS - A VOLUMETRIC-ANALYSIS BASED ON MAGNETIC-RESONANCE IMAGES

Volumetric magnetic resonance image (MRI)-based morphometry was perfor med on the brains of 30 normal children (15 males and 15 males) with a mean age of 9 years (range 7-11 years). This age range lies in a late hut critical phase of brain growth where net volumetric increment wil l be small hut when the details of brain circuity are being fine-tuned to support the operations of the adult brain. The brain at this age i s 95% the volume of the adult brain. The brain of the female child is 93% the volume of the male child. For more than 95% of brain structure s, the volumetric differences in male and female child brain are unifo rmly scaled to the volume difference of the total brain in the two sex es. Exceptions to this pattern of uniform scaling are the caudate, hip pocampus and pallidum, which are disproportionately larger in female t han male child brain, and the amygdala, which is disproportionately sm aller in the female child brain. The patterns of uniform scaling are g enerally sustained during the final volumetric increment in overall br ain size between age 7-11 and adulthood. There are exceptions to this uniform scaling of child to adult brain, and certain of these exceptio ns are sexually dimorphic. Thus, with respect to major brain regions, the cerebellum in the female but not the male child is already at adul t volume while the brainstem in both sexes must enlarge more than the brain as a whole. The collective subcortical gray matter structures of the forebrain of the female child are already at their adult volumes. The volumes of these same structures in the male child, by contrast, are greater than their adult volumes and, by implication, must regress in volume before adulthood. The volume of the central white matter, o n the other hand, is disproportionately smaller in female than male ch ild brain with respect to the adult volumes of cerebral central white matter. By implication, relative volumetric increase of cerebral centr al white matter by adulthood must be greater in the female than male b rain. The juxtaposed progressive and regressive patterns of growth of brain structures implied by these observations in the human brain have a soundly established precedent in the developing rhesus brain. There is emerging evidence that sexually dimorphic abnormal regulation of t hese terminal patterns of brain development are associated with gravel y disabling human disorders of obscure etiology.