THE ONTOGENY OF HUMAN GYRIFICATION

During development the human cortex changes from a smooth lissencephal ic structure to one that is highly convoluted. Increases in the degree of cortical folding are associated with brain size only for the first part of brain growth; during the second half, differences in cortical folding match those of brain size, resulting in no change in the degr ee of folding. When the degree of cortical folding is studied as a fun ction of age, a brief postnatal overshoot, an effect of brain size, is observed. The analysis suggests that the mechanical hypothesis of cor tical buckling can best explain the degree of cortical folding, but th at other hypotheses, like gyrogenesis, are required to explain the pla cement and orientation of sulci. The adult asymptote in degree of cort ical folding is associated with the onset and disappearance of single subplate lamina, suggesting that sulphate:cortical plate associations should be examined as causal for gyrification. Areas whose sulci diffe r in length between the two hemispheres have similar degrees of convol utedness, supporting interpretations that the sizes of gyri are asymme tric in the two hemispheres. The ontogenetic data support the thesis t hat human cortical proportions evolved when the brain enlarged in size and that the process was not one of neoteny.