Three developments in the bioengineering approach to a neurological control system, the pupillary response to light and accommodation are presented: (i)Wiener kernels(ii)noise analysis(iii)neurophysiological dissectionThe first two Wiener kernels, and a crosssection of the second kernel are shown in Figure 4. The first and second order kernels are of opposite sign and thus represent the early terms of a scale compression nonlinearity. That the first order kernel and the main diagonal slice of the second order kernel resemble the pupil pulse response suggests that the nonlinearity interaction occurs before the basic quasi-linear time shaping mechanism. The width of the second order kernel off the main diagonal is less than 1/2 second, indicating that there is no second order nonlinear interaction longer than this time. Similarly, the time delay, or non-minimum phase element is shown to be independent of the nonlinear interaction.Noise, a significant feature in the pupillary system, makes measurement of the deterministic characteristics difficult. However, the very nature of the noise itself may be independently studied to reveal aspects of the system otherwise inaccessible. This approach has enabled us to define the distribution function and frequency characteristics of pupil noise which describe a multiplicative noise generating mechanism. Ensemble characteristics displayed in the “noise plane” reveal an additional lag element in the noise pathway of a parallel path model. A similar parallel path is required in terms of a switching operation in off-responses. Other accommodation and light input experiments, and further, the strong correlation between noise in both eyes, suggest the Edinger-Westphal nucleus as the noise source or noise entry point.Neurophysiological studies involve comparison of stimulation by light with electrical stimulation of the various nuclei within the brain and to the nerves to the iris musculature. These have confirmed the locus of limiting dynamics to be in the neuromuscular plant. Microelectrode recording from single neurons in the brain stem suggest interesting interactions between different portions of the neurological subsystems controlling pupillary size. Several models of possible noise generating neuronal mechanisms within the brain, constrained to be compatible with the noise features described above, are being used to guide experimentation which may decide which model is valid.These three contemporary experimental approaches have been shown to present complementary views of the pupillary system. In addition to obtaining scientific description and deeper understanding of the pupil as a neurological control system, we are also obtaining further insight into the different number of engineering control-theoretic approaches.