Transcript Spatial representations for perception and action in the

The Role Of Visual And Vestibular Tilt
Cues In Human Verticality Perception
Pieter Medendorp, Rens Vingerhoets,
Maaike de Vrijer, Jan Van Gisbergen
What is vertical?
1. How is body posture
detected?
2. How can we determine the
orientation of visual objects
relative to the direction of
gravity when we are tilted?
3. What is the role of the
vestibular system?
4. What is the role of panoramic
cues?
Maintaining Visual Stability
Brain must combine visual and vestibular information to preserve a
stable percept of the world when we make head movements
Measuring Head Tilt
Visual system (panorama)
Tactile system (pressure cues)
Muscle sensors
Vestibular system:
- semicircular canals
- otoliths
What is the role of visual and vestibular tilt cues in human
verticality perception?
Subjective Visual Vertical (SVV)
Aubert (1861) was first to observe substantial errors in the
perception of world-centered orientation of visual lines during
body tilt
Errors of undercompensation
Subjective Visual Vertical (SVV)
2AFC: ”judge
orientation of
line re gravity”
De Vrijer et al. J. Neurophysiol. 2008;
De Vrijer et al. in progress
Subjective Visual Vertical (SVV)
70
Error (deg)
Pooled (n=8 subs)
35
0
-35
-70
SD (deg)
10
2AFC: ”judge
orientation of
line re gravity”
5
0
-120
0
Tilt Angle (deg)
Is tilt angle
underestimated?
De Vrijer et al. J. Neurophysiol. 2008;
De Vrijer et al. in progress
30
60
120
Subjective Body Tilt (SBT)
Tilt Estimates
Response Error [deg]
Subjective Vertical
Tilt Angle [deg]
•
Subjects know quite well how they are tilted (SBT)
•
Yet, their line settings undercompensate for tilt (SVV)
Van Beuzekom, Medendorp & Van Gisbergen, Vision Res, 2001
Precision vs Accuracy (SVV vs SBT)
0.30
S1
SVV
less accuracy
but better
precision in
SVV
SBT
0.15
Probability
S2
0
0.30
S3
S4
0.15
0
0
30
60
90
Tilt Angle (deg)
De Vrijer et al. J. Neurophysiol. 2008;
De Vrijer et al. in progress
0
30
60
90
Effect of visual tilt cues
Rod-and-Frame effect (Witkin
and Asch 1948)
(Carpenter and Blakemore 1973;
Li and Matin 2005).
Frame and tilt cues on the SVV
40
20
One subject
Tilt 0º
0
-20
Error in SVV (deg)
-40
40
20
Tilt 60º
0
-20
-40
40
20
0
”adjust the line to the
direction of gravity”
Vingerhoets et al. under revision
-20
-40
-90
Tilt 120º
-45
0
45
90
Frame line orientation in space, θw (deg)
Frame and tilt cues on the SVV
40
20
One subject
Tilt 0º
0
-20
Frame lines parallel or orthogonal to
“dark” SVV have no effect: 90º
periodical SVV modulation.
Frame effect is asymmetric.
-40
Error in SVV (deg)
SVV depends on orientation of the
frame line and degree of body tilt
40
20
Tilt 60º
0
-20
-40
40
20
0
-20
-40
-90
Tilt 120º
-45
0
45
90
Frame line orientation in space, θw (deg)
Vingerhoets et al. under revision
Frame and tilt cues on the SVV
30 Population
Tilt 0º
15
0
Frame lines parallel or orthogonal to
“dark” SVV have no effect: 90º
periodical SVV modulation.
Frame effect is asymmetric.
-15
Error in SVV (deg)
SVV depends on orientation of the
frame line and degree of body tilt
30
Tilt 60º
15
0
-15
30
15
0
-15
-90
Tilt 120º
-45
0
45
90
Frame line orientation in space, θw (deg)
Vingerhoets et al. under revision
Summary of results
• In darkness, the subjective visual vertical (SVV) undercompensates for tilt.
• Errors in the tilt signal cannot be held accountable since
body tilt perception is almost veridical.
• Frame cues induce periodic SVV modulations, which
become stronger in tilt.
How to explain verticality perception if body tilt signal and
retinal signal are unbiased?
Optimal observer theory (Bayes)
Optimal observer theory (Bayes)
Prior Likelihood
1)
The incoming (noisy) tilt signal may
have been caused by a range of
possible tilt angles (likelihood
function)
2)
On a priori grounds, not all tilt
angles are equally probable: mostly
the head is near upright (prior)
3)
The most likely tilt angle is the
product of likelihood en prior
(posterior)
Posterior
Percept
0º
Stimulus
MacNeilage et al. Exp Br Res, 2007;
De Vrijer et al. J. Neurophysiol. 2008
Bayesian model
mt
δ
0º
Orientation on retina φr
-180º -90º
0º
90º 180º
~
φr
Orientation on retina φr

β
-180º -90º
0º
90º 180º
Head tilt ρ
Assumptions:
•Head tilt signal is
unbiased but noisy
•Noise increases
with tilt angle
0º
Head tilt ρ
90º
~
φs
0º
Orientation
in space
-90º
-180º
-180º -90º
0º
90º 180º
δ
Probability
Probability
Head tilt
mr
Tilt Likelihood
SVV
180º
Head tilt ρ
SBT
Probability
ρ
Measurement mt (n.u.)
Tilt Sensors
~
Orientation in space φs
Orientation
on retina
Test line Likelihood
Probability
φr
Measurement mr (n.u.)
Focal Vision
Result: less noise but
biased signal
0º
Head tilt ρ
Prior
0º
Head tilt ρ
De Vrijer et al. J. Neurophysiol. 2008
Model fits (Error)
Model fits (Error)
<0
Model fits (SD)
Model fits (SD)
Bayesian model
mt
δ
0º
Orientation on retina φr
-180º -90º
0º
90º 180º
~
φr
Orientation on retina φr

β
-180º -90º
0º
90º 180º
Head tilt ρ
0º
Head tilt ρ
90º
~
φs
0º
Orientation
in space
-90º
-180º
-180º -90º
0º
90º 180º
δ
Probability
Probability
Head tilt
mr
Tilt Likelihood
SVV
180º
Head tilt ρ
SBT
Probability
ρ
Measurement mt (n.u.)
Tilt Sensors
~
Orientation in space φs
Orientation
on retina
Test line Likelihood
Probability
φr
Measurement mr (n.u.)
Focal Vision
0º
Head tilt ρ
Prior
0º
Head tilt ρ
De Vrijer et al. J. Neurophysiol. 2008
Bayesian model
Orientation
on retina
Test line Likelihood
mt
Probability
φr
Measurement mr (n.u.)
Focal Vision
0º
Orientation on retina φr
-180º -90º
0º
90º 180º
Orientation on retina φr
~
φr

β
~
Orientation in space φs
δ
SVV
180º
90º
~
φs
0º
Orientation
in space
-90º
-180º
-180º -90º
0º
90º 180º
Head tilt ρ
δ
-180º -90º
0º
90º 180º
Head tilt ρ
Tilt Likelihood
0º
Head tilt ρ
Probability
mr
Probability
Head tilt
SBT
Probability
ρ
Measurement mt (n.u.)
Tilt Sensors
0º
Head tilt ρ
Prior
0º
Head tilt ρ
Vingerhoets et al. under revision
Bayesian model
mt
0º
Orientation on retina φr
-180º -90º
0º
90º 180º
δ
Frame line
orientation
~
φr
Frame line Likelihood
mf
Probability
θr
Measurement mf (n.u.)
Global Vision

0º
Orientation on retina φr
-180º -90º
0º
β
90º 180º
0º
Orientation
in space
-90º
0º
90º 180º
Tilt Likelihood
0º
Head tilt ρ
Probability
Probability
SBT
Probability
Measurement mt (n.u.)
0º
Head tilt ρ
~
φs
δ
mr
-180º -90º
90º
Head tilt ρ
Tilt Sensors
Head tilt
SVV
180º
-180º
-180º -90º
90º 180º
Orientation on retina Өr
ρ
~
Orientation in space φs
Orientation
on retina
Test line Likelihood
Probability
φr
Measurement mr (n.u.)
Focal Vision
0º
Head tilt ρ
Prior
0º
Head tilt ρ
Vingerhoets et al. under revision
Model fits
40
20
One subject
Tilt 0º
0
-20
Error in SVV (deg)
-40
40
20
Tilt 60º
0
-20
-40
40
20
0
-20
-40
-90
Tilt 120º
-45
0
45
90
Frame line orientation in space, θw (deg)
Vingerhoets et al. under revision
Model fits
S1
S2
S3
S4
S5
S6
Tilt 120º
30
Error in SVV (deg)
0
-30
30
0
-30
30
0
All subjects
-30
-90
-45
0
45
90
-90
-45
0
Frame line orientation in space, θw (deg)
Vingerhoets et al. under revision
45
90
Conclusions
Tilted subjects make large errors when adjusting the line
to gravity, but can estimate their tilt angle quite accurately.
Optic frame cues further modulate these lines settings,
with stronger modulations in tilt.
Bayesian strategy accounts for these findings, suggesting
that verticality perception is based on an optimal fusion of
visual, vestibular and egocentric references.