In the study of perception during reading, the use of eye movement contingent control of the stimulus display has proved to be a useful research technique. With such a system, it is possible to experimentally manipulate, in real time, the characteristics of the stimulus display that is present on selected fixations as reading is in progress and to observe the effects of the manipulation on the eye movement pattern. The technique has been used in a situation where the display must be changed during the period of a saccadic eye movement and when it was necessary to make a change at some time after the beginning of a fixation. Among the instrumentation concerns that are important to consider when setting up a system to do this type of research are the following: (1) the eye movement equipment must have fast throughput and low noise, and yield new information with high frequency; (2) the program must sample the eye position with a high frequency, detect saccade onset and make the decision about whether to initiate a display change as early as the study permits, and minimize the amount of computation involved in changing the image; and (3) the display device must be capable of being rapidly refreshed and of initiating the presentation of a new image part way through the refresh cycle. (FL)