1. Fixation Disparity

2. Fixation Disparity
Fixation disparity is a small error in the visual system.
Occurs as a result of Panum’s fusional area
Small error-minutes of arc
6 to 12 minutes of arc

4. Fixation Disparity
Depends of type of stimulus, larger for lower spatial frequencies.
Acts as stimulus for the vergence system
Balance between the convergence and divergence
Compensation for phoria
Accommodative inputs

5. Measuring Fixation Disaparity
Calculate fixation disparity
FD = 2a/2.9 x 10-4 x b/b
2a=PD
b=target distance
b=difference between target distance and convergence distance
2.9 x 10-4 = conversion factor from radians to min of arc

6. Example

7. Can be measured at distance and near
Horizontal deviation
Vertical deviation
Uses nonius lines with a fusion lock
Takes advantage of vernier acuity

8. How Tests Work Diagrams of Exo, Eso and Vertical
Change nonius lines to compensate for FD
Use prism to compensate for FD

9. Sheedy Disparometer

10. Mallet Unit

11. Wesson Card

13. Saladin Card

14. Saladin Card

15. Saladin Card

16. Generating a forced vergence disparity curve
Put prism in place and measure the amount of fixation disparity
Associated phoria is the amount of prism needed to eliminate the fixation disparity
Y-axis is the amount of fixation disparity
X-axis is the amount of prism

17. Effect of Prism on FD
Base In prism
Base Out prism
Vergence movement

18. Types of fixation disparity curves
Type I
Type II
Type III
Type IV

19. Type I

20. Type II

21. Type III

22. Type IV

24. What do the curves tell us?
Measure of vergence adaptation
Fast or disparity vergence system
Slow or vergence adaptation system

25. Vertical fixation disparity
Very little adaptation in this system

26. Spherical lenses
Moves curve up and down

27. Calculating AC/A ratio
For each lens power find the corresponding prism power that produces the same fixation disparity value
Plot each lens/prism pair of points

28. Analysis
Quadrant 1
Quadrant 2
Quadrant 3
Quadrant 4

29. Characteristics of fixation disparity
Fixation distance

30. Saladin Vs Wesson Card Ngan et al OVS (2005)
Compared the FD and associated phoria in adults using the Saladin and Wesson cards
Compared the y-intercept (FD) and x-intercept x-intercept.

31. Saladin Vs Wesson Test Y-intercept X-intercept Saladin Card -0.2 0.4
Wesson Cards
-10.2
-2.9

32. Clinical application Relationship to symptoms Prism prescriptions
Effect of orthoptics