Contribution of tactile and interoceptive cues to the perception of the direction of gravity.

Without relevant visual cues, the Subjective Visual Vertical (SVV) is biased in roll tilted subjects toward the body axis (Aubert or A-effect). This effect is generally ascribed to changes in the vestibular and somatosensory inputs following a body tilt. This study focused on the contribution of interoception and tactile cues in the SVV. The body-cast technology and gastric fullness were used to obtain a diffuse tactile stimulation and an overload stomach stimulation, respectively. Fifteen subjects placed in a tilt-chair were rolled sideways from 0 degrees to 90 degrees. They were asked to adjust a luminous line to the vertical under two body restriction conditions (strapped vs. body-cast) and two stomach load conditions (empty vs. full). Results showed (1) an improvement in the SVV judgment when somaesthetic cues were available in the full stomach condition (p < 0.001), (2) an increased A-effect for the higher body tilt values in the body-cast condition (beyond 45 degrees, p < 0.001), and (3) a smaller disrupting effect of the body-cast in the SVV judgment in the full stomach condition (p < 0.05). Since the vestibular system produced the same gravity response in all conditions, it can be stated that somaesthetic cues are involved in the SVV. Tactile mechanoreceptors may have contributed by detecting the changing pattern of pressures generated on the skin that results from changes in body orientation. The stomach load may act through the inertial forces exerted against the gravity load when the stomach is full by the mechanoreceptors in the fundus. Thus, the somaesthetic system can indicate the direction of gravity via patterns of pressure within and at the surface of the body.