Which fixation disparity curve is seen is associated with unstable binocular vision and will often have symptoms of aniseikonia and poor sensory fusion?

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

Answer: (D)

Fixation disparity curves show how small misalignments between the eyes change with different vergence demands. When the binocular system is stable and well fused, the curve is smooth and predictable, with small disparities that shift gradually as prism is added or removed. If the system is unstable, the curve becomes irregular and nonlinear, with abrupt shifts or large disparities, reflecting poor vergence control. This instability is often accompanied by symptoms such as aniseikonia and weak sensory fusion, because the brain struggles to maintain consistent alignment and fuse the images across changing demands. The most unstable pattern—Type 4—captures this combination of pronounced irregularity and fusion problems, making it the best match for unstable binocular vision with aniseikonia and poor sensory fusion. Types 1–3 tend to be more stable and predictable, lacking the marked instability seen in Type 4.