Vergence errors: some hitherto unreported aspects of fixation disparity

Measurement of the monocular components of fixation disparity indicated a higher prevalence of asymmetric contributions to the total deviation than previously reported. Furthermore, the exact proportion varied from moment to moment. Two of six subjects showed significant changes in fixation disparity over a period of 1 week. For all six subjects the changes in fixation deviation of one eye were virtually independent of those changes occurring in the other eye. In other words, these monocular variances were uncorrelated. Settings of the monocular components of binocular fixation disparity were accomplished at an accuracy close to that achieved by using a monocular vernier technique. The remaining differences appeared to be due to occasional instabilities during binocular viewing. The usual method of clinical measurement in which only one element is moved is not always equivalent to that determined by summing the two monocular components. The principal process measured by subjective fixation disparity appears to be either oculomotor or localized directional shifts of a monocular nature.