Which stimulus primarily drives fusional vergence when viewing a target?

• A. Proximal Cue
• B. Blur
• C. Accommodative Demand
• D. Retinal Disparity

Answer: (D)

Fusional vergence is the eye movement that corrects small misalignments to achieve single binocular vision, and the trigger for this correction is retinal disparity—the difference in the image position between the two retinas. When the two images don’t line up, the brain detects this disparity and drives vergence to bring the eyes so that corresponding retinal points view the same target. Proximal cues can influence vergence in near tasks, and accommodative demand links accommodation to convergence, but those cues modulate vergence for focusing or distance cues rather than driving the fusion correction itself. The direct cue that prompts the fusional adjustment to fuse two images is retinal disparity.