Illusory self-motion perception evoked by caloric vestibular stimulation in sitting versus supine body positions

The purpose of this study was to determine (i) if a change in the body position that alters the somatosensory afferentation and thus the signal integrated by sensory interaction influences the illusory self-motion perception evoked by cold calorics. If yes, (ii) is the direction of the provoked nystagmus also changed? The vestibular system in 47 healthy subjects was stimulated calorically with 20°C water while in supine and sitting positions but with the head fixed. After each procedure the subjects were asked to describe their self-motion experience, and the provoked nystagmus was analyzed. In 45.7% of these subjects a sensation of yaw rotation was reported while in the sitting position, whereas only 9.6% had this sensation while in the supine position. However, when in the supine position the experience of roll rotation dominated, i.e., 52.1% compared to 5.3% while in the sitting position. Pitch rotation was felt only in the sitting position by 4.3%. There was no such sensation in the supine position. The perception of a full-cycle rotation dominates in the sitting position. In the sitting position 20% of the subjects reported eccentrical head rotation along the circumference of a cone-the top of the cone was located in the neck region. Linear self-motion sensations did not differ in the two positions. The evoked nystagmus in both positions was only horizontal. In conclusion, a change in body position with respect to the gravity vector, while head position is fixed, causes a change in the somatosensory afferentation and modifies the integrated sensory signal by sensory interaction. In turn it influences the self-motion perception evoked by calorics. A change in body position does not affect the direction of nystagmus.