the percept of visual verticality during combined roll-pitch tilt maurice dahmen student medical...
TRANSCRIPT
The percept of visual verticality during combined roll-pitch tilt
Maurice Dahmen Student medical biologyDecember 2006-July 2007
Supervisors: Maaike de Vrijer & Jan van Gisbergen
Contents
Introduction Vestibular system: Otoliths Research objective Subjective visual vertical
Bayesian model Methods Results
Experimental data + model fits Discussion Summary and conclusions
Vestibular system
Introduction
Vestibular system:otoliths
Introduction
Pitched orientation
Sensitivity for roll and pitch
Introduction
What is the role of the vestibular system in spatial perception?
The otoliths can measure head tilt with respect to gravity.
Research objective
Subjective visual vertical
Systematic errors during SVV adjustment task Subjects in roll tilt set a luminous line parallel to the earth vertical
Under- and overestimation of tilt (A- and E-effect ) Magnitude of errors differs among subjects
Introduction
Errors in SVV-task (example)
E-effect
E-effect A-effect
A-effect
Introduction
Bayesian interpretations of A-effect
Sensory tilt signal is noisy A priori information: head is mostly near upright Brain combines sensory information and prior to obtain
optimal tilt estimate
Model
De Vrijer et al., 2007
tilt (ρ)
Bayesian model
Adapted from Carandini, 2006
tilt (ρ) tilt (ρ)
Model
Fit Bayesian model
Model
Fits of the Bayesian model to SVV data of 8 subjects were very accurate
Further test of the Bayesian model
Model predicts that a noisier tilt signal leads to a more biased SVV (larger A-effect)
We used pitch tilt to modulate the noise in the roll tilt signal
Model
How does pitch-tilt affect the pattern of systematic errors in SVV during roll tilt?
Larger A-effect ? Normal A-effect Smaller A-effect ?
Research question
Vestibular chair: pitch
Methods
-45° 0° 45°
Vestibular chair: roll
Methods
Experimental setup 8 subjects (6 male, 2 female) In same pitch position during entire session Tilted to the various roll angles in complete
darkness Roll-tilt varied from –90 to +90 at 15 degree intervals
20 seconds waiting time to extinguish canal signals
SVH adjustment task Back to upright position Room lights on
Methods
SVH adjustment task
Subjects used a joystick to adjust the orientation of the line
The line was polarized by a bright dot at one end. Subjects were instructed to set the line parallel to
the virtual horizon with the dot pointing rightward A period of 12 seconds was available for each
adjustment There were 10 adjustments during a run
Methods
Estimation of orientation of utricle plane
Reid’s plane:The plane passing through the inferior margin of the ocular orbits and the center of the external auditory canals .Angle between Reid’s plane and utricle plane: 25º
(Blanks, Curthoys and Markham, 1975)
Methods
Subject 1
Results
Results as expected
Subject 2
Results
Results as expected
Subject 3
Results
Large E-effect, not expected
Pooled data
Results
The Mittelstaedt-model
Problem: E-effects cannot be explained by Bayesian model!
Alternative: The Mittelstaedt-model
Discussion
Introducing the Mittelstaedt-model
Discussion
Fitting the M-model
Parameters•S: 0.42•M: 0.70
Discussion
Fitting the M-model
Parameters•S: 0.36•M: 0.24
Discussion
Fitting the M-model
Discussion
Summary and conclusions
The Bayesian model cannot fit our data (because of E-effect)
A-effects become larger when subject is in backward pitch
Mittelstaedt-model can fit our data Further explorations are essential to fully
understand the model.
Questions?