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 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
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 xintercept 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
