Manifestation of Body Reference in the

Sense of VerticalityRonald 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 studiesWhen 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

6.0

)cos(

)sin(atanˆ

S

S

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

22 ))cos(())(sin(

4.0

6.0

)cos(

)sin(

atanˆ

SN

M

S

MN

SN

Introduction

Unresolved issues

Hysteresis

• Visual vertical settings for CW and CCW rotations to same tilt angle are different

Udo de Haes & Schöne (1970)

Indicates involvement of dynamic factors, which conflicts with Mittelstaedt model

Bistability

• Anecdotal reports of bistable visual-vertical settings at large tilts.

Fischer (1930)Udo de Haes and Schöne (1970)

Classical & predicted setting

Anecdotally reported setting

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

CWo 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

CWo 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 body-tilt 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

SVV

SBT--------

CW 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 visual-vertical 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±0.2S=0.97±0.05R²=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±0.02S=0.61±0.04R²=0.70

Descriptive model

• Allowing the idiotropic to be different in the two tilt zones works:

M1= 0.33±0.02M2= -1.5±0.4switch = 133±1 S= 0.60±0.03R²= 0.68

Descriptive model

• Different idiotropics for the two tilt regions can fit the data:

Head-directed idiotropic:

Feet-directedIdiotropic:

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

SVV

SBT--------

CW CCW

What determines the transition angle?

• Transition near =90º

12

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(egocentricreferenceframe)

Conclusions

Experimental conclusions

• No correlation between visual-vertical and body-tilt data.

• Hysteresis in body-tilt but not in visual-vertical 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.