A BAYESIAN PERSPECTIVE ON SPATIAL PERCEPTION Maaike de Vrijer Jan van Gisbergen February 20, 2008

VISUAL STABILITY Brain must combine visual and vestibular information to preserve a stable percept of the world when we make head movements

SEMICIRCULAR CANALS Three set of mutually perpendicular canals measure rotations in three dimensions High-pass properties

FUNCTION OF THE OTOLITHS Sensitive to tilt and translation Nerve fibers code deflection of the hair cell cilia (gravito- inertial force)

EXPERIMENTS

PERCEPTION OF VERTICALITY How well can a tilted subject adjust a line to the direction of gravity? Van Beuzekom & Van Gisbergen (2000) J. Neurophysiol. Kaptein & Van Gisbergen (2004) J. Neurophysiol.

PASSIVE TILT EXPERIMENTS

RESULTS LINE ADJUSTMENT ”adjust the line to the direction of gravity” De Vrijer et al. (2008)

SYSTEMATIC ERRORS LINE TASK is tilt angle underestimated?

ACTIVE TILT RESULTS REVEAL PARADOX tilt estimates are quite accurate but large errors in line task line taskbody tilt estimate

SPATIAL PERCEPTION TESTS Paradox: subject knows tilt angle but has biased line settings estimate body tilt adjust line to direction of gravity

BAYESIAN STRATEGY

BAYESIAN MODEL

OPTIMAL OBSERVER THEORY De Vrijer et al. (2008) Noisy otolith signal would spoil precise visual signal Most of the time, the head is near upright (prior knowledge) Brain takes prior knowledge into account when using the sensory tilt signal

BAYES MODEL OPTIMAL OBSERVER 1)The incoming (noisy) otolith signal may have been caused by a range of possible tilt angles (likelihood function) 2)On a priori grounds, not all tilt angles are equally probable: mostly the head is near upright (prior) 3)The most likely tilt angle is the product of likelihood en prior (posterior)

BAYESIAN MODEL Noisy tilt signal causes uncertainty about the actual tilt angle (likelihood) Brain applies correction, based on experience of which tilt angles are most common (prior) Result is a weighted compromise (posterior)

BAYESIAN MODEL FITS De Vrijer et al. (2008) tilt compensation error

MODEL ASSUMPTIONS 1.Prior has a fixed width, independent of tilt angle 2.Noise in tilt signal increases with tilt angle: ………true?

MEASURING UNCERTAINTY: PSYCHOPHYSICAL EXPERIMENTS De Vrijer & Van Wamel (2007)

PSYCHOMETRIC CURVE OF 0 o BODY TILT PERCEPT Subject is tilted at various angles around 0 o Makes right or left judgment (forced-choice) No sharply defined threshold, due to noise in tilt signal

NOISE IN TILT SIGNAL AT 0 0 Psychometric curve Noise in tilt signal

NOISE BODY TILT SIGNAL AT 0 o AND 90 o more noise at 90 o

NOISE AT 0 o AND 90 o results 5 subjects more noise at 90 o

VISUAL VERTICAL AND BODY TILT SIGNAL AT 90 o less accuracy but better precision in visual vertical

VISUAL VERTICAL AND BODY TILT SIGNAL visual vertical body tilt

Rechtop is de perceptie van lijnoriëntatie bijna foutloos gekantelde proefpersoon ziet de visuele wereld geroteerd klokschaal methode

Effect van sensor ruis prior heeft des te meer effect op het percept naarmate de sensorische meting ruiziger is

Bayes model visuele verticaal de Vrijer et al. (2007)

Oculomotor verticaal Van Beuzekom & Van Gisbergen (2000) fouten in visuele en oculomotor verticaal zijn vergelijkbaar

RENS VINGERHOETS

AMBIGUITY PROBLEM OTOLITHS Detect gravito-inertial force (GIF) No distinction between tilt and translation tilt translation

models vestibular signal processing

SOLVING THE AMBIGUITY PROBLEM: TWO MODELS Two putative strategies for decomposition of the otolith signal: frequency-filtering model canal-otolith interaction model inverse problem

CANAL-OTOLITH INTERACTION canals respond to rotation during tilt brain can use their signal to decompose otolith signal

SPATIAL ORIENTATION ILLUSIONS

PILOT AIRCRAFT CARRIER pilot is upright during launch in the dark, but feels tilted backward

ROTATION IN DARKNESS rotation percept decays slowly; after stop subject feels rotation in opposite direction reflection of cupula mechanics

EXPERIMENTS

Percepts during rotation about a tilted axis Vingerhoets et al. (2006) J. Neurophysiol. Vingerhoets et al. (2007) J. Neurophysiol.

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

PERCEIVED MOTION Results support canal-otolith interaction model rotation percept translation percept

TRANSLATION AND ROTATION PERCEPT rotation percept translation percept

MODEL SIMULATION otolith signal, actually caused by tilt changes, is partially ascribed to translation when the rotation signal dies out

SPATIAL PERCEPTION TESTS Paradox: subject knows tilt angle but has biased line settings estimate body tilt adjust line to direction of gravity