Which phenomenon primarily explains why the secondary deviation will always be greater than the primary deviation for a patient with a non-comitant deviation?

• A. Troxler effect
• B. Donder's law
• C. Hering's law
• D. Sherrington's law

Answer: (C)

Hering's law of equal innervation to yoked muscles explains why the secondary deviation is larger in noncomitant deviations. When the brain directs gaze, it sends similar neural commands to the pair of muscles that move the eyes together. In a noncomitant (direction-dependent) deviation, the two eyes’ muscles don’t respond identically in every direction due to mechanical or innervational differences. So, with fixation in one eye, the misalignment observed in the other—the primary deviation—reflects this shared innervation. If fixation is switched so the other eye fixes, the same innervation now acts on a different muscle pairing, and because the muscles don’t pull equally in that direction, the deviation of the previously fixating eye—the secondary deviation—becomes larger. The idea that yoked muscles receive equal innervation and that the mechanical response differs between directions accounts for why the secondary deviation outstrips the primary.