Effects of nonius line and fusion lock parameters on fixation disparity

Clinical devices for fixation disparity vary in the size and position of the nonius test-lines and in the location of the binocular fusion lock, and published results of studies comparing fixation disparity devices also vary. This study examined how varying stimulus parameters affects the magnitude of horizontal fixation disparity and the precision of nonius alignment in normal observers.

Targets were bright vertical lines presented for 150 ms on an oscilloscope and viewed in the dark from 3.95 m through a mirror haploscope. The fusion stimulus was either central, peripheral, or central-plus-peripheral. Fixation disparity was measured for nonius lines with a fixed vertical separation and variable lengths, using the method of constant stimuli. In a second experiment, fixation disparity was assessed for vergence demands of 6delta base-in to 12delta base-out using nonius lines of fixed length and variable vertical separations. For comparison, binocular Vernier thresholds and constant errors were also assessed.

Mean values of fixation disparity are invariant with the length and separation of the nonius lines and proximity to the fusion lock, over the range of values tested. When measured with a central fusion lock, the precision of binocular nonius alignment (vergence variability) worsens if the separation between the nonius lines increases beyond approximately 20 min arc. Vergence variability is larger with a peripheral fusion lock for small nonius-line separations and approximately the same as with the central lock for large line separations. Fixation disparity and precision were smallest with a central-plus-peripheral fusion-lock combination. Similar effects of line length and separation were found for binocular Vernier.

The finding of invariance of fixation disparity with line length for nonius targets < 1degrees extends those of earlier findings for larger targets. Because the precision of binocular nonius alignment is poorer for large nonius-line separations or with a peripheral fusion lock, fixation disparity should be most repeatable when measured with a central fusion lock and nonius lines separated by 20 min arc or less. However, a peripheral fusion lock and small nonius-line separation may provide the most effective diagnostic combination for fixation disparity.