Which prism type tests positive fusional vergence (PFV)?

Which prism type tests positive fusional vergence (PFV)?

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

Answer: (B)

Positive fusional vergence is the extra convergence the eyes can produce beyond what the object’s position demands, in order to maintain single vision. Testing with a base-out prism directly creates an outward displacement of the image, so the eyes must converge more to fuse the two images. The point at which diplopia occurs (and single vision is lost) defines the PFV limit, making this method the standard way to assess PFV. Base-in prisms would push the eyes toward divergence (negative fusional vergence), and plus/minus lenses mainly alter accommodative demand and overall vergence indirectly rather than isolating PFV with a direct convergence demand.

Positive fusional vergence is the extra convergence the eyes can produce beyond what the object’s position demands, in order to maintain single vision. Testing with a base-out prism directly creates an outward displacement of the image, so the eyes must converge more to fuse the two images. The point at which diplopia occurs (and single vision is lost) defines the PFV limit, making this method the standard way to assess PFV. Base-in prisms would push the eyes toward divergence (negative fusional vergence), and plus/minus lenses mainly alter accommodative demand and overall vergence indirectly rather than isolating PFV with a direct convergence demand.