Abstract
Looking at a tilted scene induces small rotations of the eye around the line of sight, a phenomenon called optostatic torsion (OST). To better understand how the vestibular ocular reflex (VOR) integrates visual spatial information, we tested if an upright image, with perceived but illusory tilt, could also induce OST. Participants sat in a dark room and fixated on a small target at the center of a landscape, illusory, and control stimulus, while torsional eye movements were recorded with an eye tracker. The illusory stimulus was based on the well-known Café Wall illusion which consists of slightly offset rows of alternating black and white tiles separated by “mortar” lines that appear tilted. The illusory stimulus utilized this tiled pattern on four walls to produce the illusion of a tilted room, mirror images provided both CW (+) and CCW (-) configurations. The tile shift was then modified to create a symmetric control image with a similar abstract pattern but no illusory effect. The natural scene was presented at ±4° and ±30° tilt, the control stimulus at ±4° tilt, and both configurations of the upright (0° tilt) illusory stimulus were presented. The magnitude of small tilt (4°) was selected to match the perceived tilt reported in a 2AFC task measuring the amount of tilt required to cancel the effect. The CCW and CW configurations of the illusory stimuli were perceived as vertical with 3.67±0.57° and -3.80±0.93° tilt, respectively. Both the landscape (30° tilt: 0.54±0.14, p<0.01; 4° tilt: 0.40±0.14, p=0.021) and control stimulus (0.21±0.09, p=0.046) induced a significant amount of OST, while the illusory stimulus did not (0.11±0.07, p=0.151). However, comparison of the OST induced by the control and illusory stimulus did not statistically differ (p=0.423). This may suggest a dissociation between reflexive torsional eye movement response and perception of tilt pathways.