1. Self-motion perception during off-vertical axis yaw rotation Rens Vingerhoets 1,2, Pieter Medendorp 2, Stan Gielen 1 and Jan van Gisbergen 1 1 Dept. of Biophysics, 2 Nijmegen Institute for Cognition and Information, Radboud University Nijmegen, The Netherlands INTRODUCTION Rotation about an axis tilted relative to gravity generates striking illusory motion percepts. As the initial percept of body rotation gradually subsides, a feeling of being translated along a cone emerges. The direction of the illusory conical motion is opposite to the actual rotation. Left Ear Down (LED) Right Ear Down (RED) Nose Up (NU) Nose Down (ND) Real motion Illusory motion RESULTS 1: Matching velocity reflects rotation and translation During vertical axis rotation perceived egomotion velocity decays exponentially. Rotation about a tilted axis led to a similar decay of the sense of rotation. The gradually emerging translation percept, opposite in NU and ND phase, showed up as a bifurcation into two different velocity levels. Mean of 6 subjects. 2: Decomposition of matching velocity Time (s) 1.The canal-otolith interaction model (Merfeld et al. 2005) 2.The frequency-segregation model (Paige and Tomko 1991) CONCLUSIONS Results were most consistent with the canal-otolith interaction model. Fits required parameter values that differed from the original proposal. Leaky integration of acceleration signal of otoliths is necessary. 0306090120 -15 0 15 30 Time (s) Stimulus Velocity (deg/s) First run Second run 05101520 Run Stimuli in first and second run Stimuli in fourth trial Tilt 0Tilt 15 Tilt 30 0306090120 0 5 10 Translation (cm/s) 0306090120 0 20 40 60 Rotation (deg/s) Tilt series Speed series Angular velocity (deg/s) C1C1 F1F1 Translation velocity (cm/s) Time (s) -20 0 20 40 60 Matching Velocity (deg/s) 0306090120 -20 0 20 40 60 0306090120 Time (s) Matching Velocity (cm/s) -10 0 10 20 30 0 6090120 Speed 20Speed 40Speed 50 NU ND Tilt 0 Tilt 15 Tilt 30 Speed 20 Speed 40 Speed 50 3: Model fits Model simulations for 30 deg/s and 15 deg tilt, show that data can be matched by the canal-otolith interaction model. This required extension with a leaky integration stage of acceleration and use of a new set of parameters. LED ND NU RED Internal Model Tilt Angular velocity GIF Otoliths Low Pass High Pass Canals Angular velocity Rotation Otoliths Canals Rotation   a ^ ^ kfkf kfkf ω ^ g a ^ ^ ω ^ g Translation kk kaka It has been suggested that the illusion is caused by improper interpretation of the ambiguous otolith signal (Denise et al. 1988). Two models for otolith disambiguation have been proposed. Here we tested whether the percept has a time course and magnitude that would favor one of these two hypotheses. Canal-otolith interaction model (C-model) METHODS OVAR stimulus: Subjects (n=6) were rotated in yaw about an off-vertical axis in darkness. - Tilt series ( 0, 15 and 30 deg tilt at 30 deg/s ) - Speed series ( 20, 30, 40, 50 deg/s at 15 deg tilt ) 2 AFC-task: during each run, subjects indicated at regular intervals (NU/ND) whether a dot presented for 350ms in front of them moved faster or slower than their perceived self-motion. Percept: the time course of the matching velocity (in cm/s or deg/s) was determined across runs. Illustration of adaptive staircase procedure The response curves can be decomposed into a rotation (R) and translation (T) component, based on the assumption that R follows the same time course in both phases and that T has opposite signs in both phases R follows a decaying exponential with a time constant of ~ 20 s. T follows a delayed increasing exponential with a time constant of ~ 15 s. The magnitude of perceived translation increases with increasing rotation speed and tilt angle, but remains < 14 cm/s. Frequency segregation model (F-model) This model exploits the canal signal to determine the changes in in the otolith signal resulting from tilt and attributes the remaining part to linear acceleration. This model filters the otolith signal in two parallel pathways, such that high-frequency otolith signals are linked to translation while low-frequency input is interpreted as due to tilt. Stimuli presented in subsequent trials in the first and second run of a session. Changes in stimulus velocity across runs in the fourth trial. We simulated various versions of the C and F model. We tested the models as published with perfect (not shown) and leaky integration (C1, F1). We also modified the feedback parameters of the canal-otolith interaction model (C2). C2C2 168.2 0 15 30 45 030 60 90120 5 10 Data -10 0 10 20 30 1515 0 15