The development of a visual display system that takes advantage of the physiological process of vision requires a detailed knowledge of the parameters effected. During the development of the ESPRIT eye-tracked visual display, tests were run on allowable system lag, on the trade-off between the size of high resolution area and resolution, and on other parameters. The results indicated that some parameters could effect performance even though the viewer might not be aware of it. One of the parameters, eye-track lag, was found to have some unexpected results.