title: Visual flow scene effects on the somatogravic illusion in non-pilots.
abstract: BACKGROUND
  The somatogravic illusion (SGI) is easily broken when the pilot looks out the aircraft window during daylight flight, but it has proven difficult to break or even reduce the SGI in non-pilots in simulators using synthetic visual scenes. Could visual-flow scenes that accommodate compensatory head movement reduce the SGI in naive subjects?
  
  
  METHODS
  We investigated the effects of visual cues on the SGI induced by a human centrifuge. The subject was equipped with a head-tracked, head-mounted display (HMD) and was seated in a fixed gondola facing the center of rotation. The angular velocity of the centrifuge increased from near zero until a 0.57-G centripetal acceleration was attained, resulting in a tilt of the gravitoinertial force vector, corresponding to a pitch-up of 30 degrees. The subject indicated perceived horizontal continuously by means of a manual adjustable-plate system. We performed two experiments with within-subjects designs. In Experiment 1, the subjects (N = 13) viewed a darkened HMD and a presentation of simple visual flow beneath a horizon. In Experiment 2, the subjects (N = 12) viewed a darkened HMD, a scene including symbology superimposed on simple visual flow and horizon, and this scene without visual flow (static).
  
  
  RESULTS
  In Experiment 1, visual flow reduced the SGI from 12.4 +/- 1.4 degrees (mean +/- SE) to 8.7 +/- 1.5 degrees. In Experiment 2, the SGI was smaller in the visual flow condition (9.3 +/- 1.8 degrees) than with the static scene (13.3 +/- 1.7 degrees) and without HMD presentation (14.5 +/- 2.3 degrees), respectively.
  
  
  CONCLUSIONS
  It is possible to reduce the SGI in non-pilots by means of a synthetic horizon and simple visual flow conveyed by a head-tracked HMD. This may reflect the power of a more intuitive display for reducing the SGI.
authors.affiliation: Swedish Defence Research Agency, FOI, Linköping/Stockholm, Sweden, and Uppsala University, Uppsala, Sweden. lars.eriksson@foi.se
authors.forename: Lars
authors.initials: L
authors.lastname: Eriksson
authors.forename: Claes
authors.initials: C
authors.lastname: von Hofsten
authors.forename: Arne
authors.initials: A
authors.lastname: Tribukait
authors.forename: Ola
authors.initials: O
authors.lastname: Eiken
authors.forename: Peter
authors.initials: P
authors.lastname: Andersson
authors.forename: Johan
authors.initials: J
authors.lastname: Hedström
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D003867:Depth Perception; D005260:Female; D006801:Humans; D018471:Hypergravity; D008297:Male; D009903:Optical Illusions / Q000502:physiology*; D009949:Orientation / Q000502:physiology*; D010865:Pilot Projects; D013026:Space Flight; D014787:Vision Tests; D014785:Vision, Ocular / Q000502:physiology*