Direction and angle of active head tilts influencing the Purkinje effect and the inhibition of postrotatory nystagmus I and II.

Postrotatory nystagmus I and II (P I, P II) were evoked in four normal humans by velocity steps (prior velocity of rotating chair 90 degrees/s). 4 s after the stop the head was actively tilted by the subject 90 degrees forwards, backwards, to the shoulder of the previous direction of rotation -'ipsilaterally', or to the other shoulder-'contralaterally'. In control trials, the head was kept in the previous erect position. Compared with the control experiments, P I was significantly reduced by all head tilts. Inhibition of P I was strongest with forward and weakest with backward tilts. This difference is explained by the inclination of the utricular base by 30 degrees backward with respect to the horizontal of the skull and by the elastic properties of the sensory matrix. A smaller amplitude (45 degrees) of head tilt about the roll axis leads to a weaker inhibition (28.5%) than a 90 degree tilt, which corresponds to the difference of the sine of the tilt angle and thereby reflects the mechanical force acting on the receptor layer.