ABSTRACT Two examples of nonlinear, forced vertical vergence, fixation disparity curves are presented. These curves have a flattened portion similar to the sigmoid forced horizontal vergence fixation disparity curve described as “type I.” The existence of sigmoid, forced vertical vergence, fixation disparity curves suggests that the horizontal and vertical vergence mechanisms may operate in similar fashion. Possible clinical implications of this finding relative to prismatic prescriptions for vertical vergence anomalies are also discussed.
G. Timothy Petito (Sat,) studied this question.