Transcript Body Reference in the Sense of Verticality

Manifestation of
Body Reference in the
Sense of Verticality
Ronald Kaptein
October 6, 2003
Introduction
• Classical experimental results
• Mittelstaedt’s model
• Unresolved issues
– Hysteresis
– Bistability
• Objectives of present study
Introduction
Classical studies
Paradox in classical studies
When tilted in the dark
• Subjects make no systematic errors in estimating
their body orientation
• Subjects make systematic errors in estimating the
direction of vertical
Tilt dependent pattern of errors
(rear view)
Introduction
Mittelstaedt’s model
Assumptions
• The gravity signal is derived from the otoliths
Assumptions
• Errors would occur if no corrections are
made for unequal sizes of the otolith organs
 sin( ) 

ˆ  atan
 S  cos( ) 
S  0.6
S is the ratio of the
gains of the saccule
and the utricle
Idiotropic vector
• A constant head-fixed bias signal (idiotropic
vector) solves this problem for small tilts
• But it increases the error for large tilts
Mittelstaedt model
• This head-fixed bias can be seen as a
strategy to decrease errors in the daily
encountered tilt range
sin( )


N
ˆ  at an
 S  cos( )  M

N

S  0.6
M  0 .4






N  (sin( ))2  ( S cos( ))2
Introduction
Unresolved issues
Hysteresis
•
Visual vertical settings
for CW and CCW
rotations to same tilt
angle are different
Indicates involvement of
dynamic factors, which
conflicts with Mittelstaedt
model
Udo de Haes & Schöne (1970)
Bistability
•
Anecdotal reports of bistable visual-vertical
settings at large tilts.
Anecdotally reported setting
Classical & predicted setting
Fischer (1930)
Udo de Haes and Schöne (1970)
Objectives of present study
•
•
•
Quantative verification of hysteresis and
bistability.
Check possible connection between
hysteresis and bistability.
Check if hysteresis and bistability are also
present in body-tilt estimations
Method
Vestibular roll rotation
• Subjects are rotated to an angle between 0 and 360º,
clockwise (CW) or counterclockwise (CCW).
Testing begins 30 s after stop.
Paradigms
• Visual vertical paradigm
– Subjects have to indicate the vertical by
adjusting a polarized luminous line (6 subjects,
3 naive)
• Body tilt paradigm
– Subjects have to verbally indicate their
perceived body orientation using a clock scale
(4 subjects, 1 naive)
Results
• Visual vertical
• Body tilt
• Summary main findings
Results
Visual-vertical settings
Results visual-vertical settings
• Deviation from Mittelstaedt prediction and
classical data at large tilts.
Expected visual-vertical settings
• Expected results according to Mittelstaedt
model:
Results of typical subject
• Bistable settings and major departure from
Mittelstaedt prediction at large tilts (gray zone).
 CW
Results of typical subject
• Hysteresis negligible
 CW
o CCW
Results of all subjects
• 5 of the 6 subjects show bistability
- CW
- CCW
Mean results of visual-vertical
settings
• Hysteresis also negligible in overal mean
- CW
- CCW
Pictorial illustration of bistability
Results
Body-tilt estimates
Results body-tilt estimates of
typical subject
• No bistability at large tilts
 CW
Results body-tilt estimates of
typical subject
• Weak signs of hysteresis
 CW
o CCW
Body-tilt estimates of all subjects
• None of the subjects shows bistability
- CW
- CCW
Mean body-tilt estimate
• Overall means show clear hysteresis:
- CW
- CCW
Main results
• Bistable response patterns are robust in the
visual-vertical task, but absent in the bodytilt task
• Weak hysteresis in body-tilt estimates, none
in visual-vertical results.
Discussion
• Comparison of visual vertical and body tilt
• Hysteresis
• Modelling bistability
Discussion
Comparison of visual vertical and
body tilt
Comparison of performance in
the two tasks
• No correlation between subjective visual
vertical and subjective body tilt
CW
SVV
SBT
--------
CCW
Errors in visual vertical do not
result from wrong tilt estimates
• Correlation not significant (R=-0.03)
CW
 CCW
Discussion
Hysteresis
No hysteresis in visual vertical
• Hysteresis in body-tilt but not in visualvertical results
- CW
- CCW
Hysteresis
• Hysteresis in body-tilt percept
– May indicate that estimated body-tilt is partly based on
path integration of canals, which will adapt during
constant velocity rotation.
• No hysteresis in visual-vertical settings
– The results of Udo de Haes & Schone are not
confirmed. Mittelstaedt’s assumption that the final tilt
angle is the important variable is supported.
Discussion
Modelling bistability
Bistability
• The bistable transition near 135º is a robust
finding in nearly all subjects.
• The anecdotal reports of bistability (Fischer
(1930), Udo de Haes &Schöne (1970)) are
confirmed and quantified.
Manifestation of body reference
• All data can be described by the influence of a
body reference, which is head- or feet-directed.
Mittelstaedt model cannot
account for all data
• Fitting Mittelstaedt on all data clearly fails:
M=0.2±0.2
S=0.97±0.05
R²=0.26
Mittelstaedt can account for
small and medium tilt data
• Fitting Mittelstaedt on white zone does not
account for the gray zone:
M=0.32±0.02
S=0.61±0.04
R²=0.70
Descriptive model
• Allowing the idiotropic to be different in the
two tilt zones works:
M1= 0.33±0.02
M2= -1.5±0.4
switch = 133±1
S= 0.60±0.03
R²= 0.68
Descriptive model
• Different idiotropics for the two tilt regions
can fit the data:
Head-directed
idiotropic:
Feet-directed
Idiotropic:
Possible mechanisms underlying
bistability
• Why? Reports from subjects about the
nature of the task gives an indication:
– For small and medium tilts the task is easy and
more or less automatic.
– For large tilts the task is difficult and subjects
try to use every cue availabe, making the task
more cognitive.
• The brain may use different strategies
(systems) in the two tilt zones.
Possible mechanisms underlying
bistability
• Default brainstem mechanism
– Operates on assumption that tilt is in normal
working range (head-directed idiotropic,
Mittelstaedt model)
• Cognitive system
– Takes over when tilt is beyond normal working
range.
Cognitive system uses perceived
body-tilt signal
CW
SVV
SBT
--------
CCW
What determines the transition
angle?
• Transition near =90º
2
1
What determines the transition
angle?
• If it makes sense to switch the body
reference from head to feet directed, one
would expect this to happen when the
(perceived) tilt exceeds 90º.
• But it happens when  exceeds 90º.
What determines the transition
angle?
• When the default visual vertical starts to point
towards the subject’s feet (>90), the brain
changes strategy and uses the feet as reference.
 > 90
(egocentric
reference
frame)
Conclusions
Experimental conclusions
• No correlation between visual-vertical and
body-tilt data.
• Hysteresis in body-tilt but not in visualvertical data.
• Collapse and bistability of visual-vertical
settings at large tilts (>135º).
Modelling conclusions
• All settings show influence of a body
reference. Head-directed for small and
medium tilts, feet-directed for large tilts.
• Two different systems might be used: a
default brainstem system and a cognitive
system.
• The change of system might be related to
the line setting in egocentric coordinates.
The End
Extras
Mochten er vragen of opmerkingen
over komen.
Eggert
• Eggert comes to the same mathematical
formulation as Mittelstaedt using a different
approach:
– The brain works according to Bayes rule.
– The brain uses prior knowledge stating that
small tilt angles are more likely to occur then
large ones.
– The utricle and the saccule have different
Signal-to-Noise ratios.
Why has bistability never been
found in classical studies?
• Why was this transition not seen in earlier
experiments?
– Most earlier experiments only used tilt ranges up
to 180º, thus coupling tilt position and rotation
direction. Our large tilt range may have limited
the possibility of using this prior knowledge.
– It is striking to note that the two earlier reports
that reported bistability also used a large tilt
range.