According to the phi phenomenon, during a subjective cover test, a patient with an exo deviation would view the paddle motion in which direction relative to the deviation?

• A. Same direction
• B. Opposite direction
• C. No movement
• D. Upward

Answer: (A)

The phi phenomenon explains why a patient perceives motion of a stationary target during a dissociated cover test. When one eye is briefly covered and then uncovered, the eye that re-fixates moves to its new position. The brain interprets the changing retinal input as motion of the target in the direction of that eye movement.

In an exo deviation, the eye tends to drift outward. As the eye re-fixates, the motion is outward, so the patient perceives the paddle moving in the same direction as the deviation. This illusion of motion in the same direction helps indicate an outward (exo) misalignment.