Topographical reproducibility of small scotoma using computerised dynamic fixation target.

The computer assisted dynamic fixation technique uses the patient's eye movements to locate the test stimulus in the central visual field in relation to a randomly meandering fixation target. The patient looks at the moving fixation target on a high resolution monitor and tries to keep it inside a circle using a joystick. The stimuli are presented at predetermined locations in a seemingly random manner and the awareness of the stimuli is registered by the patient pressing the joystick button. That novel fixation maintenance technique has been combined with suprathreshold static light offset (dark-on-bright-light decrement) stimuli created on a cathode ray tube and used in mapping of the physiological blind spot in 10 healthy eyes. Each eye was examined twice with the same test program in order to document the repeatability of the results. On two consective tests, the physiological blind spot measured an average of 5.7 degrees horizontally and 6.4 degrees vertically, using 15% (17 dB) contrast, 16 mm2, single intensity offset stimuli. The moving eye method and the light offset stimuli, on repeated testing, yielded an average topographical reproducibility rate of 73% in mapping of the scotomas, with an average of 1.4 degrees and 0.6 degree variability in the vertical and horizontal dimensions of the blind spot respectively.