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Fixation Disparity

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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

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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

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**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

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Example

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**Can be measured at distance and near**

Horizontal deviation Vertical deviation Uses nonius lines with a fusion lock Takes advantage of vernier acuity

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**How Tests Work Diagrams of Exo, Eso and Vertical**

Change nonius lines to compensate for FD Use prism to compensate for FD

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Sheedy Disparometer

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Mallet Unit

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Wesson Card

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Saladin Card

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Saladin Card

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Saladin Card

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**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

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

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**Types of fixation disparity curves**

Type I Type II Type III Type IV

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Type I

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Type II

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Type III

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Type IV

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**What do the curves tell us?**

Measure of vergence adaptation Fast or disparity vergence system Slow or vergence adaptation system

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**Vertical fixation disparity**

Very little adaptation in this system

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Spherical lenses Moves curve up and down

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**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

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Analysis Quadrant 1 Quadrant 2 Quadrant 3 Quadrant 4

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**Characteristics of fixation disparity**

Fixation distance

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**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.

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**Saladin Vs Wesson Test Y-intercept X-intercept Saladin Card -0.2 0.4**

Wesson Cards -10.2 -2.9

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**Clinical application Relationship to symptoms Prism prescriptions**

Effect of orthoptics

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