Fig. (7) The Associated Nonlinear Functions (ANFs) of the three global PDMs for subject #4. The ANF of the 1^st PDM (left) is almost linear and much larger than the other two ANFs (i.e. its contribution to the model output is larger). The positive slope of the 1^st PDM is consistent with passive fluid dynamics, but the small negative slope of the 2^nd PDM (middle) is not consistent with passive fluid dynamics and suggests the presence of an active mechanism of counter-regulation. The ANF of the 3^rd PDM (right) has a positive slope and exhibits a nonlinear characteristic.