[Behavior of the other eye in measuring fixation disparity]

In the subjective measurement of FD, the proband fuses contours presented in the peripheral macular areas of both eyes ('fusion lock'). The position of both eyes relative to each other is monitored by means of two haploscopically seen vertical lines presented in the central visual field, one above and one below a binocularly seen horizontal line. The subject is instructed to shift one of the vertical lines horizontally until the two are aligned, while fixating their intersection with the horizontal line. It has recently been questioned whether the foveolae really are pointed at the perceived intersection. To investigate this matter, we registered the position of one eye while intermittently covering the fellow eye, while the subject maintained fixation of the intersection. We found slight differences in monocular eye position depending on the presence or absence of fusion in the macular periphery, confirming earlier findings in part. These differences were more pronounced in the nondominant eye.