On the significance of somatosensation for spatial orientation during exposure to increased Gforces

The significance of the somatosensory system for the vestibular-induced G-excess illusion was studied during centrifugation with test subjects immersed in water. Metod: The visually perceived eye level (VPEL) was measured in darkness by means of an adjustable laser dot. Measurements were performed in the 1-g environment as well as during centrifugation (2G, 10 minutes). The gondola of the centrifuge was equipped with an immersion tank in which the subject was sitting, facing forwards. The subjects (n=10) were tested in two conditions, "dry" and "wet" (with water up to the level of the manubrium sterni). Results: At 1g the deviations of the VPEL were small (dry: 0.2±1.8°; wet: -0.1± 3.7°). At 2G there was a depression of the VPEL, which exponentially approached an asymptotic value (dry: -27.6± 10.3°; wet: -27.3±9.0°). The time constant was: 92±45s (dry): 73±44s (wet). Immersion in water had no significant influence. There was a considerable inter-individual variability. The asymptote ranged from -15 to -45 degrees but for each individual there was virtually no difference between the two conditions. However, in two subjects the time constant was considerably smaller during immersion. Conclusion: Somatosensory input does not reduce the magnitude of the fully developed G-excess illusion. However, it cannot be excluded that, in some cases, somatosensory information has an influence on the time course for the development of the illusion.