Rik Hendrix Supervision: Maaike de Vrijer Jan van Gisbergen Bachelor internship Biomedical sciences, main course: human movement sciences Department of biophysics, Radboud University Nijmegen, March 3 rd July 4 th Subjective Body Tilt (SBT): Verbal estimate of body tilt negligible errors at all tilt angles Subjective Visual Vertical (SVV): Aligning a line with gravity systematic tilt undercompensation at large tilt angles Optokinetic stimulation: stimulation with moving images Dichgans (1974) showed that optokinetic stimulation affects: Subjective body tilt (SBT) Subjective visual vertical (SVV) However: Dichgans never directly compared these effects! Causes tilt illusion opposite to rotation direction Can optokinetic effects on SVV be explained by changes in perceived body tilt (SBT)? 8 Subjects (6 male, 2 female; 5 nave) Vestibular chair 3 optokinetic conditions: Stationary (control) Clockwise (CW) Counterclockwise (CCW) Adjustable visual line Tasks: Subjective Body Tilt (SBT) Subjective Visual Vertical (SVV) Tilt angles between -135 and 135 with 15 intervals Rotation to randomly chosen tilt angle in the dark Continuous optokinetic stimulation Verbal estimate of body tilt angle after 30 seconds Rotation back to upright. 30 Seconds rest period (lights on) Tilt angles between -120 and 120 with 30 intervals Rotation to randomly chosen tilt angle in the dark Continuous optokinetic stimulation Align visual line with gravity (3 trials) Rotation back to upright 30 Seconds rest period (lights on) CW: Rather constant effect, overestimation when tilted to the left, underestimation when tilted to the right CCW: Rather constant effect, opposite to CW Stationary: negligible errors at all body tilt angles CW: constant effect when tilted to the right, increasing effect when tilted to the left. Asymmetry! CCW: constant effect when tilted to the left, increasing effect when tilted to the right. Asymmetry! Stationary: negligible errors at small tilt angles (30) and systematic errors at large tilt angles (60) Stationary: Negligible errors 30 Systematic errors 60 Counterclockwise: Constant effect when tilted to the right Increasing effect when tilted to the left Clockwise: Constant effect when tilted to the left Increasing when tilted to the right Effect constant Effect increasing with tilt angle SBT SVV SBT SVV SBT task: effects of tilt increasing and decreasing stimuli are roughly equal No asymmetry SVV task: effect of tilt increasing stimulus larger than effect of tilt decreasing stimulus Asymmetry Tilt increasing stimulus has a different effect on the SVV than a tilt decreasing stimulus Analysis of the research question for both tilt increasing and tilt decreasing stimuli Two comparisons: SVV vs SBT SVV compensation vs SBT SBT SVV Comparison of optokinetic effects in SBT and SVV: Tilt decreasing stimulus: SVV = SBT Tilt increasing stimulus: SVV > SBT SVV vs SBT SVV compensation vs SBT Stationary stimulus: SBT > SVV compensation angle Tilt decreasing stimulus: SBT >> SVV compensation angle Tilt increasing stimulus: SBT = SVV compensation angle SVV compensation vs SBT Even if different combinations of physical tilt and optokinetic stimulation produce the same tilt percept (SBT), the percept of verticality (SVV) varies. Can optokinetic effects on SVV be explained by changes in perceived body tilt (SBT)? Ambiguous answer: sometimes effects on SVV can be explained by changes in SBT, but not all the time. Can optokinetic effects on SVV be explained by changes in perceived body tilt (SBT)? 1. Yes, but the relation is different for tilt-increasing and tilt-decreasing stimulation 2. The question remains whether this can be explained by current models