cortical$age   ( age)
cortical$group ( Control, Schizophrenia)
cortical$sex   ( 1 = male, 2 = female)
cortical$symm  ( a symmetry measure 
                 from the original 3D cortical surface )
cortical$x     (500 x , y  coordinates of an axial slice through the
                cortical surface intersecting the anterior and posterior 
                commissures)
cortical$r     (500 radii from equal angular polar coordinates )