The prism required to neutralize fixation disparity is known as ______ phoria.

• A. Associated phoria
• B. Compensating phoria
• C. Secondary phoria
• D. Tertiary phoria

Answer: (A)

Fixation disparity is a small misalignment of the eyes that persists even when fusion is present. To measure how much misalignment the visual system must compensate for during normal viewing, you add a prism until the disparity is neutralized and fusion is maintained. The amount of prism needed to achieve this neutralization is the associated phoria. In other words, associated phoria is the prism value that "neutralizes" fixation disparity, reflecting the prism the vergence system relies on during binocular fusion. Other phoria terms describe misalignment under dissociation or different contexts, but they do not represent the prism required to neutralize fixation disparity.