Which fixation disparity curve is seen in a sigmoidal shape and present in patients that are asymptomatic?

• A. Type 1
• B. Type 2
• C. Type 3
• D. Type 4

Answer: (A)

The key idea is how fixation disparity responds to prism demand. A sigmoid, or S-shaped, curve means the binocular system holds fixation disparity near zero across a wide range of prism powers, showing strong fusional vergence and no symptoms. This pattern reflects a well-compensated binocular system in people who don’t notice any diplopia or strain when their eyes are challenged with prism. Among the four types of fixation disparity curves, the one that shows this sigmoid shape in asymptomatic patients is the first type. The other patterns tend to be associated with symptoms or different adaptation behaviors, so they don’t match the asymptomatic, sigmoidal presentation.