Fixation Disparity. Fixation disparity is a small error in the visual system. Occurs as a result of Panums fusional area Small error-minutes of arc 6.

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.

Acts as stimulus for the vergence system Balance between the convergence and

divergence Compensation for phoria Accommodative inputs

Measuring Fixation Disaparity

Calculate fixation disparityFD = 2a/2.9 x 10-4 x b/b2a=PDb=target distance b=difference between target distance and

convergence distance2.9 x 10-4 = conversion factor from radians to

min of arc

Example

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

How Tests Work

Diagrams of Exo, Eso and Vertical Change nonius lines to compensate for FD Use prism to compensate for FD

Sheedy Disparometer

Mallet Unit

Wesson Card

Saladin Card

Saladin Card

Saladin Card

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

Effect of Prism on FD

Base In prism Base Out prism Vergence movement

Types of fixation disparity curves

Type I Type II Type III Type IV

Type I

Type II

Type III

Type IV

What do the curves tell us?

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

Vertical fixation disparity

Very little adaptation in this system

Spherical lenses

Moves curve up and down

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

Analysis

Quadrant 1 Quadrant 2 Quadrant 3 Quadrant 4

Characteristics of fixation disparity

Fixation distance

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.

Saladin Vs Wesson

Test Y-intercept X-intercept

Saladin Card -0.2 0.4

Wesson Cards -10.2 -2.9

Clinical application

Relationship to symptoms Prism prescriptions Effect of orthoptics