Upward/Downward asymmetries of vertical vestibulo-ocular reflex

Asymmetries were present at the level of the vestibulo-collic reflex in the upright as well as in side-down position during stimulation about pitch axis. Indeed the lead of extensor neck muscle activation peak was greater than that of the inhibition peak; this relationship was inverted for the flexor muscles of the neck. Asymmetry was found in both upright and side-down position indicating that the origin of the vertical response asymmetry is from semicircular canals and the otolithic input does not suppress it. Interestingly, in the frontal plane the electromyographic response was much more symmetric. We suggest that the activation of both anterior and or inhibition of both posterior semicircular canals during downward head pitch causes greater phase lead in the electromyographic responses than that observed during upward head pitch. Conversely, the combined stimulation of both anterior and posterior canals (roll stimulation) gives rise to more symmetric responses.