1. SPATIAL AWARENESS DEMO 20 juni 2008 detection of self motion sensing body orientation in space visual perception in earth-centric coordinates

2. VESTIBULAR SENSORS canals otoliths

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

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

5. OTOLITHS sensitive to tilt and translation

6. OTOLITH SIGNAL IS AMBIGUOUS hair cells cannot distinguish tilt and translation

7. SPATIAL ORIENTATION ILLUSION pilot is upright, but feels tilted

8. AMBIGUITY 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 ? inverse problem

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

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

11. OVAR Vingerhoets et al. (2006) J. Neurophysiol. Vingerhoets et al. (2007) J. Neurophysiol. TESTING THE MODEL

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

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

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

15. TRANSLATION AND ROTATION PERCEPT DATA rotation percept translation percept

16. SPATIAL PERCEPTION IN STATIC TILT

17. SENSING THE DIRECTION OF GRAVITY Two different tasks: 1.Set line to vertical (SVV) 2.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. experiments in darkness

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

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

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

21. 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.

22. 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

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

24. SVV LESS NOISY THAN SBT

25. SUMMARY SBT AND SVV DATA Two paradoxical findings: 1.subject knows tilt angle, yet makes biased line settings 2.more certain about line setting than about body tilt estimate body tilt (SBT) adjust line to vertical (SVV)

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

27. SIGNALS REQUIRED FOR SPATIAL VISION retinal signal to compute line in space (Ls), brain must combine info about line orientation on retina (L R ) and head tilt (H S ) head-tilt signal

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

29. A BAYESIAN PERSPECTIVE IDEAL OBSERVER MODEL

30. IDEAL OBSERVER STRATEGY 1)Use sensory data: noisy tilt signal suggests range of possible tilt angles (likelihood) 2)Use prior knowledge: we know that large tilt angles are very uncommon (prior) 3)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.

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

32. WHY WOULD THIS MAKE SENSE? 1)Less noise in spatial vision 2)Downside: bias at large tilts 3)Average performance improves (large tilts are rare)

33. DEMO BIAS EFFECT INCREASES WITH TILT

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

36. small bias

37. large bias

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

40. MODEL FITS: SVV ACCURACY

41. < 0

42. 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

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

44. ACTIVE-TILT RESULTS ARE SIMILAR body tilt estimates are quite accurate but large errors in line task line taskbody tilt estimate Van Beuzekom et al. (2001) Vision Res.

45. psychometrische curve van 0 o tilt percept proefpersoon wordt vaak in allerlei standen rond 0 o gekanteld beoordeelt elke stand als links of rechts geen scherpe drempel door ruis in tiltsignaal

46. Ruis tiltsignaal bij 0 0 psychometrische kromme ruis in tiltsignaal

47. Ruis tiltsignaal bij 0 en 90 o meer ruis bij 90 o

48. Ruis tiltsignaal bij 0 en 90 o resultaten 5 proefpersonen meer ruis bij 90 o