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Abstract:
During eccentric yaw rotations around an Earth-vertical axis the semi-circular canals are
stimulated (rotational acceleration) as well as the otoliths (tangential acceleration). Most
likely the brain uses both sensory signals, the canal and the otolith signal, when faced with a
rotation direction detection task. Keeping the rotational acceleration profile unchanged and
increasing the radius of the eccentric rotation the tangential acceleration increases. Therefore,
we hypothesized that thresholds would decrease with increasing radius of rotation. The
threshold was defined as the peak acceleration needed to detect the correct direction of motion
in 75 of the trials. Ten participants were tested in seven conditions (150 trials each): a
head-centered rotation, a translation and five eccentric rotations with varying radii (R=0.1,
0.2, 0.3, 0.5, 0.8 m). The motion had 1s duration and consisted of a single cycle sinusoidal
acceleration. Participants were blindfolded, heard white noise and their head was kept in
place with a neck brace. The results show a significant decrease of thresholds with increasing
radius. It can be seen that the detection process for eccentric rotations is not exclusively
based on either the canal or the otolith signal, but that both signals are integrated. A model
able to predict the thresholds of the eccentric rotations is proposed, which is solely based on
the thresholds for the head-centered rotation and the translational motion. For small radii the
detection processes is mainly based on the canal signal whereas for large radii it is dominated
by the otolith signal. For intermediate radii the reduction in threshold due to the sensory
combination is largest compared to using only one of the two sensors. One additional participant
suffered from occasional vertigo after an ear infection indicating vestibular problems.
She showed unusually high thresholds for translational motions, but normal thresholds for
head-centered rotations. Interestingly, her thresholds for eccentric rotations were higher than
her threshold for the head-centered rotations suggesting that she did not only use the rotational
signal, but instead had a problem integrating the two sensory signals. These findings
indicate that signals from the otolith and the semi-circular canals are not used independently,
but are integrated in order to solve a direction detection task.
