PFV is the vergence response that serves to overcome which type of prism demand, and it is tested with which method?

• A. Base-in prism testing
• B. Base-out demand
• C. Base-out prism testing
• D. Vergence ranges

Answer: (C)

Positive fusional vergence is the inward turning of the eyes that counters a base-out prism displacement to maintain single vision. When a base-out prism is placed before the eyes, the image shifts outward, and the eyes must converge more to fuse. This convergence reserve is what PFV measures. The method used to assess it is base-out prism testing, where gradually increasing base-out prism powers are presented and you note the break point (loss of fusion) and recovery point as the prism is removed. This specifically isolates the positive fusional vergence response. Base-in prism testing would tap negative fusional vergence (divergence), and the other options don’t describe the testing method for PFV.