docs: def _calcDepths(self):
          '''
          return the cortical depth of each subpopulation
          '''
          depths = self.layerBoundaries.mean(axis=1)[1:]
  
          depth_y = []
          for y in self.y:
              if y in ['p23', 'b23', 'nb23']:
                  depth_y = np.r_[depth_y, depths[0]]
              elif y in ['p4', 'ss4(L23)', 'ss4(L4)', 'b4', 'nb4']:
                  depth_y = np.r_[depth_y, depths[1]]
              elif y in ['p5(L23)', 'p5(L56)', 'b5', 'nb5']:
                  depth_y = np.r_[depth_y, depths[2]]
              elif y in ['p6(L4)', 'p6(L56)', 'b6', 'nb6']:
                  depth_y = np.r_[depth_y, depths[3]]
              else:
                  raise Exception, 'this aint right'
                  
          return depth_y