Stability of oculomotor fixation as a function of target contrast and blur

Previous studies have shown that changes in target parameters, such as luminance, color, size, and shape exert little effect on the stability of fixation, measured as the standard deviation of eye positions over several seconds of fixation. In this study, we investigated the effect of target contrast and dioptric blur on the stability of fixation in five normal subjects. The horizontal positions of both eyes were assessed by infrared limbal tracking while the subjects fixated binocularly for 20-s intervals on a cross-shaped target at 2 m. Across subjects, the mean stability of fixation on a high contrast target worsened significantly from 8.9 to 12.0 min arc with fixation through increasing amounts (0 to 4 D) of dioptric blur. However, without blur there was no statistically significant effect on fixation stability of varying the target contrast from 7 to 84%. The variability of vergence was not significantly influenced in either condition. Like other target parameters investigated in previous studies, dioptric blur and contrast have relatively inconsequential effects on fixation stability, indicating that the signals used to control fixation are largely independent of stimulus parameters. Practically, the findings suggest that when patients' eye movements are recorded for clinical reasons, the results are unlikely to be affected seriously by the presence of residual refractive error.