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 naïve)  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

Δ SBT Δ 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: ◦ Δ SVV vs Δ SBT ◦ SVV compensation vs SBT Δ SBT Δ 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