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 Udo de Haes & Schöne (1970) Indicates involvement of dynamic factors, which conflicts with Mittelstaedt model

13. 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

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 SVV SBT -------- CW CCW

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: M 1 = 0.33±0.02 M 2 = -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 SVV SBT -------- CW CCW

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

51. 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.

52. 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)

53. Conclusions

54. 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).

55. 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.

56. The End

57. Extras Mochten er vragen of opmerkingen over komen.

58. 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.

59. 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.