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.