Verticality perception during body rotation in roll Rens Vingerhoets Pieter Medendorp Jan van Gisbergen NICI & Department of Biophysics Nici-Juniorendag, 3 mei 2007

Introduction

Perceptual updating Introduction The brain has to combine retinal information and information on body orientation to maintain a stable percept of the world

Perceptual updating Introduction Information on body orientation from: Visual panoramic cues Somatosensory cues Vestibular system: Semicircular Canals Otoliths

Introduction Vestibular System

Semicircular canals Introduction Detect angular acceleration Response to constant velocity decays

Otoliths fa g a Introduction Respond to gravito-inertial force (GIF) Cannot discriminate between tilt and translation (ambiguity problem) fa g F=G-A a

Otoliths Introduction Ambiguity problem: Neural strategy for otolith disambiguation: Canal-otolith interaction

Canal-otolith interaction model Introduction Canal-otolith interaction model b w Subjective vertical GIF + Otoliths Internal Model Linear acceleration Angular velocity Angular velocity Canals Head tilt leads to canal signal, acceleration does not

Canal-otolith interaction model Introduction Canal-otolith interaction model b Bias mechanism w Subjective vertical GIF + g Otoliths Internal Model Linear acceleration Angular velocity Angular velocity Canals Bias mechanism based on static verticality estimates Now test this model for dynamic verticality perception during roll rotation

Roll-rotation Introduction Sideward rotation about an axis through the nose

g = Error in verticality percept Introduction Model predictions g = Error in verticality percept g 90 180 270 360 SVV Error,  (deg) Tilt angle (deg) -180 -90 90L 90R Static g Preceding rotation (deg) 180 360 540 720 900 1080 -180 -360 -540 -720 -900 -1080 90L 90R Dynamic

Questions Introduction Are systematic errors under dynamic conditions similar to static errors? Evidence for phase delay in the verticality percept as predicted by the internal model?

Methods

Methods Vestibular Chair

Experiments Methods Subjective visual vertical (SVV) Roll rotation at 30 deg/s, alternating CW and CCW Measurements at 15 deg intervals Scaling method using flashed lines Static: 0 – 360 deg 10 measurements 30 s after rotation stop, every 2 s Dynamic: 0 – 1080 deg = 3 complete cycles measurements during rotation, every 2 s Subjective body tilt (SBT) Verbal estimation of body tilt at random times during rotation

Scaling method Methods Flash! 1 minute past the hour! Error in SVV = real orientation (30o) – estimated orientation (6o) = 24o Response error equals error in verticality percept (g)

Results

Static CW Results Systematic errors 90R 180 90L 180 Systematic errors 0 –150o & 240 – 360o verticality percept biased towards the head Bias toward feet near 180o 90 Error in SVV (deg) -90 -180 90 180 270 360 Tilt angle (deg)

Preceding rotation, Dr (deg) Results All subjects Error in SVV, g (deg) Preceding rotation, Dr (deg)

Comparison Static & Dynamic

Preceding rotation, Dr (deg) Results Static vs Dynamic Error in SVV, g (deg) Preceding rotation, Dr (deg)

Preceding rotation, Dr (deg) Results Static vs Dynamic Error in SVV, g (deg) Preceding rotation, Dr (deg)

Static vs Dynamic Results Dynamic data qualitatively similar but: Larger errors in red zone More inter-subject variability Green zone is broader Bistability

Dynamic: complete rotation

Dynamic: complete rotation Results Dynamic: complete rotation Error in SVV, g (deg) Analyse per proefpersoon is essentieel. Populatie gemiddelde is onzin. Preceding rotation, Dr (deg)

Dynamic: complete rotation Results Dynamic: complete rotation Error in SVV, g (deg) Analyse per proefpersoon is essentieel. Populatie gemiddelde is onzin. Preceding rotation, Dr (deg)

Dynamic: complete rotation Results Dynamic: complete rotation Error pattern repeats in successive cycles No phase delay

Dynamic Subjective Body Tilt

Results Dynamic SBT Error in SBT,  (deg) Preceding rotation, Dr (deg)

Results Dynamic SBT Error in SBT,  (deg) Preceding rotation, Dr (deg)

Dynamic SBT Results Smaller errors than in SVV No bistability Errors in SVV are not caused by errors in SBT

Dissociation between SVV & SBT Results Dissociation between SVV & SBT Errors in verticality estimates not caused by misjudgment of body tilt SVV ideal subject SVV real subject Errors based on SBT

Model Fits

Model Fits Results SVV in red zone biased toward head  w>0 SVV in green zone biased toward feet  w<0 Dynamic errors larger than static wdyn > wstat

Preceding rotation, Dr (deg) Results Static Fits Error in SVV, g (deg) Preceding rotation, Dr (deg)

Preceding rotation, Dr (deg) Results Dynamic Fits Error in SVV, g (deg) Preceding rotation, Dr (deg)

Model Fits Results Model can describe SVV responses if: W-values differ for static and dynamic W-values differ in red and green zone

Analysis of phase shifts Results Analysis of phase shifts Instead of focusing on the error g, we can also look at the amount of compensation for tilt. We refer to this angle as b. = compensation for rotation (b = tilt - g) Perfect task execution: b = body tilt angle g b

Analysis of phase shift Introduction Analysis of phase shift Model prediction for b Dynamic CW 360 Actual tilt 315 b 270 225 Tilt Angle (deg) 180 135 90 Phase shift: = 0 when Dr ≈ 370 & Dr ≈ 735 45 180 360 540 720 900 1080 Preceding rotation, Dr (deg)

Analysis of phase shifts Results Analysis of phase shifts 1 Subject  (deg) 50o -720 o -360 o o 360 o 720 o 50o Model  (deg) 50o 50o All subjects Preceding rotation, Dr (deg) Preceding rotation, Dr (deg)

Analysis of phase shifts Results Analysis of phase shifts No clear evidence for phase lag If anything, it is a lead rather than lag

Conclusions

Conclusions Conclusions We have shown that: Errors in verticality perception are not caused by misjudgments of body tilt The egocentric bias is larger under dynamic conditions than under static conditions Verticality judgments are biased toward the feet around 180o tilt. Both statically and dynamically There is no evidence for a phase delay