Tonic cervical stimulation: does it influence eye position and eye movements in man?

In healthy subjects eye movements were analysed during body rotation, during trunk torsion either with the head passively held stationary in space or with the head voluntarily stabilized in space, and during voluntary head movements. Trapezoidal movements around the vertical axis were performed (+/- 40 degrees, plateau 10 s, duration of ramp 1 or 4 s). Moreover the influence of a tonic head deviation up to 40 degrees on optokinetic nystagmus and on vestibulo-ocular reflex during sinusoidal turning was examined. Eye movements were recorded by DC-electrooculography. Saccadic and slow components of eye movements and the shift of eye position during the plateau of the trapezoidal stimulus were analysed. For all modes of stimulation during the plateau no nystagmus occurred. At the end of the dynamic phase of the stimulus relatively frequent eye deviations--mostly in the direction of the head deviation--were observed, not only after turning the trunk with the head stabilized in space (cervical stimulation) but also after turning head and trunk together. The fact that such eye deviations are thus observed even in the absence of any tonic, especially cervical stimulus, supports the assumption that they cannot be attributed to a tonic stimulus but merely to an effect of the preceding phasic stimulus which outlasts them. Also amplitude and direction of eye shifts during the plateau do not depend on a tonic stimulus, but merely on the eye deviation reached at the end of the dynamic phase of stimulation. Optokinetic nystagmus and vestibulo-ocular reflex are not influenced by an additional tonic cervical stimulus.