Which prism type tests negative fusional vergence (NFV)?

Which prism type tests negative fusional vergence (NFV)?

• A. BI prism
• B. BO prism
• C. Plus lenses
• D. Minus lenses

Answer: (A)

Negative fusional vergence is the eye’s ability to diverge to maintain single vision when the image alignment is pushed inward. In practice, this is tested with a base-in prism. The base-in prism shifts the image toward the nose, creating a situation where the eyes must diverge to fuse the images. The point at which fusion is broken or the amount of prism that can be neutralized represents the negative fusional vergence amplitude. Base-out prisms, by contrast, create a convergence demand and thus measure positive fusional vergence, while lenses aren’t used to isolate NFV in the standard vergence test.

Negative fusional vergence is the eye’s ability to diverge to maintain single vision when the image alignment is pushed inward. In practice, this is tested with a base-in prism. The base-in prism shifts the image toward the nose, creating a situation where the eyes must diverge to fuse the images. The point at which fusion is broken or the amount of prism that can be neutralized represents the negative fusional vergence amplitude. Base-out prisms, by contrast, create a convergence demand and thus measure positive fusional vergence, while lenses aren’t used to isolate NFV in the standard vergence test.