Abstract
Torsional eye movements are usually induced by a tilt of the head. However, torsion may be also elicited while observing a tilted image with the head upright in a phenomenon known as optostatic torsion (OST). This effect is small and usually tested while fixating somewhere in the image. Here we studied the OST effect in subjects looking at images tilted 30 degrees to the left or to the right. We compared OST while subjects either fixated on a fixation spot at the center of the image or free viewed the image at will. We found that the effect more than doubled during free viewing going from 0.21±0.03 deg to 0.55±0.08 deg (paired t-test, p=0.0003). These results suggest that actively engaging with the scene enhances the reflex driving optostatic torsion and that many past estimates of the amount of torsion induced by tilted images may have been an underestimation of their real effect in vision.