What is the main stimulus that initiates fusional vergence?

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

Answer: (D)

Disparity between the images seen by the two eyes is the primary signal that triggers fusional vergence. When the two eyes would project the image to different retinal locations, the brain detects this misalignment and commands the eye muscles to rotate the eyes so the images fall on corresponding retinal points, restoring single, fused vision. This adjustment can be convergence or divergence depending on the direction of misalignment.

Other cues—like how near the target seems (proximal cues), blur affecting clarity, or the amount of accommodative effort needed—can influence vergence through other pathways or by modulating the response, but they do not initiate fusional vergence the way retinal disparity does.