BACKGROUND: The somatogravic illusion (SGI) is a kind of spatial disorientation caused by a linear sustained acceleration. Pilots believe that visual cues, such as a visible horizon or texture flow, reduce this illusion. This study was performed to evaluate the influence of visual stimuli on the SGI using the Advanced Spatial Disorientation Demonstrator (ASDD). METHODS: There were eight healthy males who were exposed to a 0.58 g x axis linear acceleration on the ASDD, where the direction of the resultant gravitoinertial force was equivalent to 30 degrees pitch-up. One of the following visual stimuli was presented during each acceleration: BLANK (no visual cues); HORIZON (a visible horizon without motion); and TEXTURE (vertical lines moving toward the subject evoking vection). The subjective magnitude of the SGI in ordinal scale was observed; and in interval scale, the deviation of the moving point kept at the subjective horizon was observed. The differences among visual stimuli were analyzed. RESULTS: The subjective magnitude of the SGI (p < 0.01) and the deviation of the moving point (p < 0.05) were significantly smaller in HORIZON than in BLANK and TEXTURE. No difference was demonstrated between BLANK and TEXTURE. The linear vection produced by the TEXTURE stimulus did not affect the SGI. CONCLUSION: The data indicated that the presence of a visible horizon reduced the magnitude of the SGI. On the other hand, the presence of a vection stimulus did not influence the magnitude of the SGI.
BACKGROUND: The somatogravic illusion (SGI) is a kind of spatial disorientation caused by a linear sustained acceleration. Pilots believe that visual cues, such as a visible horizon or texture flow, reduce this illusion. This study was performed to evaluate the influence of visual stimuli on the SGI using the Advanced Spatial Disorientation Demonstrator (ASDD). METHODS: There were eight healthy males who were exposed to a 0.58 g x axis linear acceleration on the ASDD, where the direction of the resultant gravitoinertial force was equivalent to 30 degrees pitch-up. One of the following visual stimuli was presented during each acceleration: BLANK (no visual cues); HORIZON (a visible horizon without motion); and TEXTURE (vertical lines moving toward the subject evoking vection). The subjective magnitude of the SGI in ordinal scale was observed; and in interval scale, the deviation of the moving point kept at the subjective horizon was observed. The differences among visual stimuli were analyzed. RESULTS: The subjective magnitude of the SGI (p < 0.01) and the deviation of the moving point (p < 0.05) were significantly smaller in HORIZON than in BLANK and TEXTURE. No difference was demonstrated between BLANK and TEXTURE. The linear vection produced by the TEXTURE stimulus did not affect the SGI. CONCLUSION: The data indicated that the presence of a visible horizon reduced the magnitude of the SGI. On the other hand, the presence of a vection stimulus did not influence the magnitude of the SGI.