Which criterion states that the smaller fusional vergence reserve should be at least half the greater fusional vergence reserve?

• A. Sheard's criterion
• B. Percival's criterion
• C. Both criteria
• D. Neither criterion

Answer: (B)

This topic is about how the two directions of fusional vergence balance to keep single vision. You have convergent (positive) and divergent (negative) fusional reserves that you can recruit to offset a misalignment (phoria). Percival's criterion says the smaller of these two reserves should be at least half the size of the larger one. This ensures both directions have enough reserve so small changes in demand don’t push you into diplopia or eyestrain.

For example, if your larger reserve is 6 prism diopters, the smaller should be at least 3 prism diopters. If the smaller reserve were only 2, the ratio would fall short of the criterion, making the system more prone to decompensation with everyday tasks.

This criterion is different from Sheard's, which relates the phoria magnitude to the opposing fusional reserve rather than comparing the two reserves directly.