Vestibular and optokinetic asymmetries in the ocular and cervical reflexes

We showed that asymmetry in the vestibular and optokinetic systems was present in the vertical vestibulo-collic reflex and optocollic reflex in the upright position, this indicating that it is operative in most common conditions. In fact, during sagittal stimulation in the upright position the electromyographic (EMG) activation peak of extensor head muscles was leading the EMG inhibition peak. An opposite phase pattern was observed in flexor head muscles. Asymmetry did not originate from otolithic receptors because it was not abolished when changing the animal's position with respect to gravity. We suggest that the upward preponderance of the head reflexes is necessary to oppose the force of gravity, which acts unidirectionally in the sagittal plane. Thus, the activation of extensor head muscles is anticipated in time to oppose the force of gravity. This hypothesis is corroborated by the absence of muscle activation in the horizontal and torsional planes in which the force of gravity does not act unidirectionally.