Manifestation of Body Reference in the Sense of Verticality

1. Manifestation of Body Reference in the Sense of Verticality Ronald Kaptein October 6, 2003

2. Introduction • Classical experimental results • Mittelstaedt’s model • Unresolved issues • Hysteresis • Bistability • Objectives of present study

3. Introduction Classical studies

4. 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

5. Tilt dependent pattern of errors (rear view)

6. Introduction Mittelstaedt’s model

7. Assumptions • The gravity signal is derived from the otoliths

8. Assumptions • Errors would occur if no corrections are made for unequal sizes of the otolith organs S is the ratio of the gains of the saccule and the utricle

9. Idiotropic vector • A constant head-fixed bias signal (idiotropic vector) solves this problem for small tilts • But it increases the error for large tilts

10. Mittelstaedt model • This head-fixed bias can be seen as a strategy to decrease errors in the daily encountered tilt range

11. Introduction Unresolved issues

12. 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)

13. 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)

14. 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

15. Method

16. Vestibular roll rotation • Subjects are rotated to an angle between 0 and 360º, clockwise (CW) or counterclockwise (CCW). Testing begins 30 s after stop.

17. 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)

18. Results • Visual vertical • Body tilt • Summary main findings

19. Results Visual-vertical settings

20. Results visual-vertical settings • Deviation from Mittelstaedt prediction and classical data at large tilts.

21. Expected visual-vertical settings • Expected results according to Mittelstaedt model:

22. Results of typical subject • Bistable settings and major departure from Mittelstaedt prediction at large tilts (gray zone).  CW

23. Results of typical subject • Hysteresis negligible  CW o CCW

24. Results of all subjects • 5 of the 6 subjects show bistability - CW - CCW

25. Mean results of visual-vertical settings • Hysteresis also negligible in overal mean - CW - CCW

26. Pictorial illustration of bistability

27. Results Body-tilt estimates

28. Results body-tilt estimates of typical subject • No bistability at large tilts  CW

29. Results body-tilt estimates of typical subject • Weak signs of hysteresis  CW o CCW

30. Body-tilt estimates of all subjects • None of the subjects shows bistability - CW - CCW

31. Mean body-tilt estimate • Overall means show clear hysteresis: - CW - CCW

32. 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.

33. Discussion • Comparison of visual vertical and body tilt • Hysteresis • Modelling bistability

34. Discussion Comparison of visual vertical and body tilt

35. Comparison of performance in the two tasks • No correlation between subjective visual vertical and subjective body tilt CCW CW SVV SBT --------

36. Errors in visual vertical do not result from wrong tilt estimates • Correlation not significant (R=-0.03) CW CCW

37. Discussion Hysteresis

38. No hysteresis in visual vertical • Hysteresis in body-tilt but not in visual-vertical results - CW - CCW

39. 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.

40. Discussion Modelling bistability

41. 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.

42. Manifestation of body reference • All data can be described by the influence of a body reference, which is head- or feet-directed.

43. 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

44. 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

45. 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

46. Descriptive model • Different idiotropics for the two tilt regions can fit the data: Head-directed idiotropic: Feet-directed Idiotropic:

47. 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.

48. 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.

49. Cognitive system uses perceived body-tilt signal CCW CW SVV SBT --------

50. What determines the transition angle? • Transition near =90º 2 1