Optokinetic torsion: dynamics and relation to circularvection.

Continuous records of optokinetic torsion to sinusoidal inputs were obtained using the electromagnetic scleral search-coil technique. We measured the gain and phase lag of optokinetic torsion in response to a spherical visual display rotating steadily at various angular velocities and sinusoidally at frequencies from 0.2 to 2.0 Hz and at amplitudes from 10 to 80 deg. Gain (peak slow-phase eye velocity over stimulus angular velocity) of up to 0.12 were obtained with stimulus frequencies of 0.2 Hz and declined to an average value of about 0.02 at a frequency of 2.0 Hz. Phase lag was virtually zero at a frequency of 0.2 Hz and increased to over 80 deg at 2.0 Hz. The records from the sinusoidal stimuli show very few quick phases. With increasing stimulus amplitudes, the amplitude of the response increased but its gain declined. We found no evidence of torsional after-nystagmus nor any relation between the torsional response and reports of vection or sensation of body tilt induced by the rotating display. Torsional optokinetic nystagmus is most suited to compensate for low-amplitude, low-frequency stimulus rotation and normally supplements torsion induced by head tilt.