The Human Brain Age 7-11 Years: A Volumetric Analysis Based on Magnetic Resonance Images

Volumetric magnetic resonance image (MRI)-based morphometrywas performed on the brains of 30 normal children (15 malesand 15 males) with a mean age of 9 years (range 7–11 years).This age range lies in a late but critical phase of brain growthwhere net volumetric increment will be small but when the detailsof brain circuit are being fine-tuned to support the operationsof the adult brain. The brain at this age is 95% the volumeof the adult brain. The brain of the female child is 93% thevolume of the male child. For more than 95% of brain structures,the volumetric differences in male and female child brain areuniformly scaled to the volume difference of the total brainin the two sexes. Exceptions to this pattern of uniform scalingare the caudate, hippocampus and pallidum, which are disproportionatelylarger in female than male child brain, and the amygdala, whichis disproportionately smaller in the female child brain. Thepatterns of uniform scaling are generally sustained during thefinal volumetric increment in overall brain size between age1–11 and adulthood. There are exceptions to this uniformscaling of child to adult brain, and certain of these exceptionsare sexually dimorphic. Thus, with respect to major brain regions,the cerebellum in the female but not the male child is alreadyat adult volume while the brainstem in both sexes must enlargemore than the brain as a whole. The collective subcortical graymatter structures of the forebrain of the female child are alreadyat their adult volumes. The volumes of these same structuresin the male child, by contrast, are greater than their adultvolumes and, by implication, must regress in volume before adulthood.The volume of the central white matter, on the other hand, isdisproportionately smaller in female than male child brain withrespect to the adult volumes c cerebral central white matter.By implication, relative volumetric increase of cerebral centralwhite matter by adulthood must be greater in the female thanmale brain. The juxtaposed progressive and regressive patternsof growth of brain structures implied by these observationsin the human brain have a soundly established precedent in thedeveloping rhesus brain. There is emerging evidence that sexuallydimorphic abnormal regulation of these terminal patterns ofbrain development are associated with gravely disabling humandisorders of obscure etiology.