Cervico-vestibular and visuo-vestibular interaction. Self-motion perception, nystagmus, and gaze shift

In 8 healthy subjects we studied self-motion perception and nystagmus due to sinusoidal stimulation (amplitude 90 degrees peak to peak, frequency 0.05 Hz) of the horizontal semicircular canals, the cervical proprioceptors, and the retina. We used an electrically driven rotatory chair and optokinetic drum combination. For cervical stimulation the subject's head was placed in a clamp, attached to the drum. Eye movements were recorded by means of electrooculography, d.c. amplification. Subjects signalled the estimated head position by means of a 'joystick'. In the present series of experiments the vestibular and cervical informations were played off against each other in combined stimulation conditions with an interstimulus phase lag of 0 to 315 degrees, in steps of 45 degrees. Similarly, the vestibular and visual informations were played off against each other. Concerning estimated head position, our main finding is that both the visually and the cervically induced illusion of head rotation overrule the vestibular sensation of head motion. The ocular response to combined vestibular plus cervical stimulation shows that both nystagmus slow phases and saccades of the cervical and the vestibular responses add up by vectorial summation.