Base-out prisms affect vergence by increasing which demand?

Base-out prisms affect vergence by increasing which demand?

• A. Increases divergence demand (NFV)
• B. Increases convergence demand (PFV)
• C. Increases accommodation demand
• D. Alters pupil size

Answer: (B)

Base-out prisms create an outward shift of the image seen by each eye, so the visual system must compensate by turning the eyes inward to fuse the images. That inward turning is an increased convergence response, i.e., a higher convergence demand. In vergence terms, this is positive fusional vergence (PFV). The prism itself doesn’t inherently raise accommodation or change pupil size; its main effect is on vergence, testing the ability to converge to maintain single vision when the image is displaced outward.

Base-out prisms create an outward shift of the image seen by each eye, so the visual system must compensate by turning the eyes inward to fuse the images. That inward turning is an increased convergence response, i.e., a higher convergence demand. In vergence terms, this is positive fusional vergence (PFV). The prism itself doesn’t inherently raise accommodation or change pupil size; its main effect is on vergence, testing the ability to converge to maintain single vision when the image is displaced outward.