UNVEILING THE HIDDEN SENSE

1. UNVEILING THE HIDDEN SENSE Farewell lecture May 30, 2008

2. VESTIBULAR CONTRIBUTION TO SPATIAL AWARENESS detection of self motion sensing body orientation in space visual perception in earth-centric coordinates

3. SCOPE • Vestibular sensors • Spatial orientation in dynamic conditions • Spatial vision in tilted observers • Bayesian model

4. VESTIBULAR SENSORS canals otoliths

5. CANALS DETECT ROTATION high-pass filter insensitive to constant velocity rotation nerve fibers code head velocity

6. CONSTANT ROTATION IN DARKNESS • rotation percept decays • after stop, percept of rotation in opposite direction • reflects cupular mechanics

7. OTOLITHS sensitive to tilt and translation

8. OTOLITH SIGNAL IS AMBIGUOUS hair cellscannot distinguish tilt and translation

9. AMBIGUITY PROBLEM inverse problem • otolith signal may have various causes: • translation (a) • force of gravity due to tilt (g) • combination of a and g • How can the brain resolve this ambiguity ?

10. CANAL- OTOLITH INTERACTION MODEL • canals detect rotation during tilt changes • their signal helps to decompose otolith signal Angelaki et al. (1999)

11. CANAL–OTOLITH INTERACTION MODEL tilt angle linear acceleration angular velocity • basic principle: • tilt stimulates otoliths AND canals • - translation stimulates only otoliths Merfeld and Zupan (2002) J. Neurophysiology

12. TESTING THE MODEL percepts during rotation about a tilted axis (OVAR) Vingerhoets et al. (2006) J. Neurophysiol. Vingerhoets et al. (2007) J. Neurophysiol.

13. THE ACTUAL MOTION - rotation about tilted axis - in darkness - constant velocity

14. MODEL PREDICTIONS rotation signal decays gradually wrong interpretation otolith signal: illusory translation percept

15. SCHEMATIC SUMMARY OF RESULTS Actual motion: rotation percept Percept: translation percept confirms prediction

16. TRANSLATION AND ROTATION PERCEPT DATA rotation percept translation percept

17. SPATIAL PERCEPTION IN STATIC TILT

18. SENSING THE DIRECTION OF GRAVITY experiments in darkness • Two different tasks: • Set line to vertical (SVV) • Estimate your body tilt (SBT) Van Beuzekom & Van Gisbergen (2000) J. Neurophysiol. Van Beuzekom et al. (2001) Vision Res. Kaptein & Van Gisbergen (2004, 2005) J. Neurophysiol. De Vrijer et al. (2008) J. Neurophysiol.

19. ACCURACY vs PRECISION Accuracy: How close is the response to the true value? Precision: How reproducible is the response? darts analogy:

20. ACCURACY AND PRECISION IN LINE TASK (SVV) accuracy precision De Vrijer et al. (2008) J. Neurophysiol. De Vrijer et al. (2008) in progress

21. ACCURACY IN LINE TASK due to underestimation of body tilt?

22. NO UNDERESTIMATION OF BODY TILT SVV SBT • Subjects know quite well how they are tilted (SBT) • Yet, their line settings undercompensate for tilt (SVV) Van Beuzekom et al. (2001) Vision Res. Kaptein and Van Gisbergen (2004) J. Neurophysiol.

23. PRECISION IN LINE TASK is scatter in SVV simply reflection of noise in body tilt signal? De Vrijer et al. (2008) J. Neurophysiol. De Vrijer et al. (2008) in progress

24. SVV LESS NOISY THAN SBT psychometric experiments at 0o and 90o tilt: De Vrijer et al. in progress

25. SVV LESS NOISY THAN SBT

26. SUMMARY SBT AND SVV DATA estimate body tilt (SBT) adjust line to vertical (SVV) • Two paradoxical findings: • subject knows tilt angle, yet makes biased line settings • more certain about line setting than about body tilt

27. SBT DATA SHOW: • An unbiased head tilt signal is available • Noise increases with tilt angle

28. SIGNALS REQUIRED FOR SPATIAL VISION retinal signal head-tilt signal to compute line in space (Ls), brain must combine info about line orientation on retina (LR) and head tilt (HS)

29. SIMPLY USING RAW TILT SIGNAL … would not explain SVV bias !! spatial vision would be accurate, but noisy raw tilt signal

30. A BAYESIAN PERSPECTIVE IDEAL OBSERVER MODEL

31. IDEAL OBSERVER STRATEGY • Use sensory data: noisy tilt signal suggests range of possible tilt angles (likelihood) • Use prior knowledge: we know that large tilt angles are very uncommon (prior) • Most likely tilt angle (posterior) is product of likelihood and prior Eggert (1998) PhD Thesis, Munich MacNeilage et al. (2007) Exp. Brain Res. De Vrijer et al. (2008) J. Neurophysiol.

32. IDEAL OBSERVER STRATEGY • Tilt prior has 2 effects on SVV: • Less noise • Bias at large tilt

33. WHY WOULD THIS MAKE SENSE? • Less noise in spatial vision • Downside: bias at large tilts • Average performance improves (large tilts are rare)

34. DEMO BIAS EFFECT INCREASES WITH TILT

35. no bias no bias De Vrijer et al. (2008) J. Neurophysiology

37. small bias small bias

38. largebias

39. largebias

40. MODEL PARAMETERS • 1) head tilt noise level in upright • 2) increase of head tilt noise with tilt • 3) prior width • 4) eye torsion amplitude

41. MODEL FITS: SVV ACCURACY

42. MODEL FITS: SVV ACCURACY < 0

43. MODEL EXPLANATION OF NOISE LEVELS:SVV vs SBT PRECISION • SVV is less noisy than the SBT (remarkable, but explained by model) • SBT becomes more noisy at larger tilt (supports model assumption) • SBT noise levels compatible with head-tilt fit results

44. CONCLUSION • Accuracy-precision trade-off in spatial vision: • Bayesian strategy reduces noise at small tilts • causes systematic errors at large tilts

45. ACKNOWLEDGMENTS

46. COWORKERS OCULOMOTOR CONTROL David Robinson Stan Gielen Fenno Ottes John van Opstal Arend Smit André Minken Karin Krommenhoek Bart Melis Vivek Chaturvedi Lo Bour DIck Stegeman Klaus Kopec Hubert Misslisch

47. COWORKERS SPATIAL AWARENESS Anton Van Beuzekom Ronald Kaptein Rens Vingerhoets Stan Van Pelt Maaike De Vrijer Pieter Medendorp

48. TECHNICAL SUPPORT Ger van Lingen Victor Langeveld Günter Windau Hans Kleijnen Ton van Dreumel Stijn Martens Wil Corbeek, Harrie van Brakel, Arno Engels, Jaap Nieboer (TD-FNWI) Fred Philipsen, Theo Arts (CDL)

49. FOR THIS SYMPOSIUM: • THE SPEAKERS: • Dora Angelaki • Bernhard Hess • Daniel Merfeld • Casper Erkelens • Wolfgang Becker • Jos Eggermont • THE ORGANIZERS: • Pieter Medendorp • John van Opstal • Stan Gielen • Margiet van Pelt

50. FOR MORAL SUPPORT