The angle of visual roll motion determines displacement of subjective visual vertical

The effect of full-field sinusoidal visual roll motion stimuli of various frequencies and peak velocities upon the orientation of subjective visual vertical (SV) was studied. The angle of the optokinetically induced displacement of SV was found to be a linear function of the logarithm of the stimulus oscillation angle. Interindividual slopes of this function varied between 2 and 9. The logarithmic function is independent of stimulus frequency within the range of .02 Hz to .5 Hz and of peak stimulus velocity from 7.5°/sec to 170°/sec. It holds for oscillation angles up to 100°–140°. With larger rotational angles, saturation is reached. With small stimulus angles, a surprisingly high threshold (5°-8°) was observed in our experiments. This may reflect the unphysiological combination of visual roll stimuli without corroborating vestibular and proprioceptive inputs normally present when body sway produces visual stimulation. Under natural conditions, the visual feedback about spontaneous sway stabilizes body posture by integrating rotational velocity over stimulus duration which is equal to rotational angle.