A generalization of the “clamping” method is described and its application to the pupil light-reflex servomechanism illustrated. The relationship between this environmental clamping and more classical input-output analysis is demonstrated by the accuracy of predictions from the transfer function and is discussed theoretically. Separation of linear and nonlinear factors in system behavior in the oscillatory model has interesting experimental consequence.
LAWRENCE STARK, "Environmental Clamping of Biological Systems: Pupil Servomechanism," J. Opt. Soc. Am. 52, 925-929 (1962)