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 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