Up-down asymmetry of saccadic contrapulsion in lateral medullary syndrome.

We report greater amplitude of horizontal contrapulsion on upward vs. downward vertical saccades in a patient with saccadic contrapulsion as a result of a focal demyelinating lesion of the left dorsolateral medulla. The vertical asymmetry in horizontal saccadic contrapulsion likely relates to an imbalance of fastigial oculomotor inputs on horizontal burst neurons during vertical saccades as reported in nonhuman primates. Our finding suggests that such asymmetry is also present in humans and provides insight into the pathophysiology of saccade generation.