1 INTRODUCTION
Spatial orientation refers to the fact that the brain integratively
recognizes its own position, posture and motion in space through several
sensory systems. Vestibular, visual and somatosensory inputs with
various motion are constantly integrated in central nervous system to
determine the spatial orientation.1
Among the various parameters of spatial orientation, the measurement of
tilt perception for gravity was first done by Grahe in 1922, when he
measured the vertical position with a measuring
apparatus.2 He first tilted it manually and returned
the subject to the vertical position gradually. He made volunteers with
their eyes closed make a sign at the moment they accurately perceived
the vertical position. The results were from 2° to
3°, thus indicating that they perceived it very
accurately.
Later, Israel et al had volunteers sit inside an opaque fiberglass
sphere.3,4 The sphere was mounted within two
motor-driven rings. The subjects were secured onto the chair with three
belts, and their head was fixed with a helmet to keep them in the same
position. The chair position was adjusted so that the head was at the
centre of rotation. Two push-buttons were fixed on the right side of the
chair, with which the subjects could control the position of the sphere.
The subjects were first required to position their body at an angle of
90°, 180° or 360°,
right or left, with the push-buttons, and then to rotate back to the
initial position after 3-6 s. The results were as follows; the mean
rotation from 0°→ 90° was
93.2°, for 0°→
180° was 169°, 0°→
360° was 313°,
90°→ 0° was 92.6°,
180°→ 0° was
157°and 360°→ 0°was 293°. These results confirmed that subjects
accurately perceived their vertical position.
However, both of these previous studies were presented by analogue
displays.
Berthoz et al studied the memory of body linear
displacement.5 Volunteers were seated on a robot,
their head was fixed in place, and their eyes were covered. After
undergoing a displacement of 2, 4, 6, 8, or 10 m, they reproduced, as
accurately as possible, the distance that was imposed. Even when the
stimulus acceleration (range 0.06 to 1 m/s2) was
changed or the stimulus duration was kept constant (16 s) over the
different distances, they could reproduce the distance very accurately.
Although these studies indicate the remarkable ability of the body for
special awareness, there has been no study about tilt perception and the
memory of tilt perception. We hypothesized that patients who have
vestibular dysfunction, especially bilateral vestibular dysfunction with
the Jumbling phenomenon, would have disorders in tilt perception and
memory.
Therefore, we developed an electric goniometer that enabled digital
display of the tilt angle for easy analysis of the results and we
studied the influence of vestibular disorders on the tilt perception and
short term memory of tilt perception in normal volunteers and patients
with no bilateral response to the caloric test.