Light and target distance stimuli were presented to normal human subjects, and their pupillary responses were measured. A homeomorphic computer model of the pupillary control system is presented in which the form of interaction of controller signals due to light and target distance was investigated. The error remaining when model parameters were optimized to fit experimental pupil size (area or diameter) was smaller for the linear interaction hypothesis than for either power law or logical law interaction. A generalized second-order nonlinear model with six parameters (vs. 3 for each of the other models) yielded somewhat lower residual error. With the use of a modified Akaike information criterion, the value (in an information theoretic sense) of the improved fit afforded by the three additional parameters in the generalized nonlinear model was shown to be small, and thus the generalized second-order nonlinear model was rejected in favor of the simpler and more parsimonious linear model.