Fixation disparity
1, the blue lines and characters illustrate the situation of optimal binocular vision: the extra-ocular muscles adjust the vergence angle between the two visual axes so that the fixation target X is projected in each eye onto the centre of the fovea, i.e. the location on the retina with the highest spatial resolution.
This mechanism of sensory fusion with normal retinal correspondence operates within a certain limit of disparity, referred to as Panum’s area.
For testing the state of binocular vision, Hofmann and Bielschowsky[9] included an additional fusion stimulus to the two eyes and still found a perceived offset of scale and arrow; they referred to this offset as “Disparitätsrest” (in German), which means “residual disparity”.
In case of a deviating vergence state, the dichoptic targets need to have a certain physical horizontal offset in order to be perceived in line.
These subjective measures agree with objective recordings with eye trackers,[12] if no fusion stimulus is involved.
For measuring subjective fixation disparity, researchers as Ogle,[11] Sheedy and Saladin,[13] Mallett,[14] Wesson[15] constructed test instrumentation including fusion targets and dichoptic targets using cross-polarized filters in front of the eyes; some of these devices are commercially available.
This seemed to be justified by the first objective recording of fixation disparity made in 1960 by Hebbard[16] with an eye tracking method based on small mirrors fixed onto contact lenses: he found agreement between the two measures (in the one observer tested).
2a) and the optimal vergence state when a target is projected in each eye onto the center of the foveola (V0=2 arc tan ((pd)/2)/D), blue line in Fig.
· Subjective fixation disparity (sFD) is defined as the angular amount of the offset between dichoptic targets that need to be adjusted to a certain offset d so that the observer perceives the dichoptic targets in alignment (see the pair of nonius lines in Fig.
These prisms force the eyes to change the vergence angle while the viewing distance remains unchanged.
The following conditions of subjective fixation disparity tend to be more prevalent in observers with eye strain.
3b), meaning that the binocular system is not able to reach a small fixation disparity when vergence is forced by prisms in the base-in and base-out direction.
[13] The proximity FD-curve (measured subjectively as a function of viewing distance) tends to be steeper, meaning that the binocular system is not able to keep the fixation disparity small, if a target is shifted closer in the range of about 100 to 20 cm.
[39][6] All the above measures in studies of eye strain refer to the subjective fixation disparity, because the procedure with dichoptic targets is technically easy and therefore can conveniently be applied in the clinical setting with some commercial test devices.
Some of the cited studies found, that measures of subjective fixation disparity are a better diagnostic criterion for eye strain than the heterophoria, i.e. the vergence state without a fusion stimulus.
Eye glasses with an included prism power is the optical method to reduce a fixation disparity.
This test procedure is typically made in near vision of 40 cm, e.g. with the Mallett-unit, the Disparometer, or the Wesson card (see above).
[42][43] The usefulness of prism eye glasses has been criticized since the initial fixation disparity may reappear again after some time due to the adaptability of the vergence system.
The effectiveness has been confirmed both in terms of alleviation of visual symptoms and in better physiological conditions, e.g. the prism-FD curves became more flat.
