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Fixation Disparity

Fixation Disparity. 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. Fixation Disparity. Depends of type of stimulus, larger for lower spatial frequencies.

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Fixation Disparity

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  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

  3. 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

  4. 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

  5. Example

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

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

  8. Sheedy Disparometer

  9. Mallet Unit

  10. Wesson Card

  11. Saladin Card

  12. Saladin Card

  13. Saladin Card

  14. 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

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

  16. Types of fixation disparity curves • Type I • Type II • Type III • Type IV

  17. Type I

  18. Type II

  19. Type III

  20. Type IV

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

  22. Vertical fixation disparity • Very little adaptation in this system

  23. Spherical lenses • Moves curve up and down

  24. 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

  25. Analysis • Quadrant 1 • Quadrant 2 • Quadrant 3 • Quadrant 4

  26. Characteristics of fixation disparity • Fixation distance

  27. 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.

  28. Saladin Vs Wesson

  29. Clinical application • Relationship to symptoms • Prism prescriptions • Effect of orthoptics

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