Oman York Conference(2001 in press)11/2/01 corresponding v, P, and M vectors. (Mittelstaedt has adopted the opposite convention ). The idiotropic vector is denoted"M" in recognition of Mittelstaedt's many contributions(Young et al,1986) The Sv in complete darkness(sometimes called the postural vertical) is determined only by the G and B vectors. The Sv of gravitationally horizontal observers who have a headward gravireceptor bias is tilted slightly in a headward direction, i.e. they report feeling tilted slightly head down, and conversely. Measurement of the postural vertical provides a convenient way to assess a persons gravireceptor bias B-at least in one G The"idiotropic"tendency M affects all judgements of sV when any visual cues are present. The idiotropic effect a usually stronger than gravireceptor bias, even when the latter is in a headward direction. Hence the Sv of a horizontally recumbent subject is deviated footward. When no F or Pcues are present, the resultant of M and B deviates the Sv footward. Hence an observer perceives a dimly lit gravitationally vertical line as rotated in the opposite direction to body tilt the well known aubert illusion Figure 5 shows a horizontally recumbent observer viewing the interior of a tilted, barnlike room in 1-G. The major and minor axes of symmetry of the visual environment are depicted with the array of bidirectional vectors F. Since the room interior has a familiar shape, and readily distinguishable ceiling(top) and floor(bottom), it is also said to possess visual polarity, depicted by the vector P. The visual vertical v lies along one of the major the axes of symmetry in a direction closest to P and M. Here V points in the direction of the true floor, so it is subjectively perceived as a floor. The direction of the subjective vertical SV is determined by a nonlinear interaction of the visual V and gravireceptor(G+B)vectors. How the vectors combine depend on the orientation of the subject. For relatively small static tilts of the subject or the environment as shown in the figure -up to a limit of perhaps 45 degrees-the SV lies in a direction intermediate between V and(G+B) However, if the subject is not in the normal erect position, but instead recumbent, supine or prone with respect to gravity, and V aligns with M, the SV can be"captured by (i.e. align with )the F B V and M vectors. Thus a supine subject feels gravitationally upright if the environment is tilted so p and v align with the body axis 3.2 Extending the model to o-g: how shown in Figure 6. The physical stimulus to the body's gravireceptors G is absent, but a headward or footward bias b remains. As in 1-G. the direction of the visual vertical v True Ceilin Subjective Floor Figure 6. Model for 0-G Visual Reorientation Illusion. Crewmember inverted in a Spacelab module feels right side uOman York Conference (2001 in press) 11/2/01 Page 8 corresponding V, P, and M vectors. (Mittelstaedt has adopted the opposite convention). The idiotropic vector is denoted “M” in recognition of Mittelstaedt’s many contributions (Young et al, 1986). The SV in complete darkness (sometimes called the postural vertical) is determined only by the G and B vectors. The SV of gravitationally horizontal observers who have a headward gravireceptor bias is tilted slightly in a headward direction, i.e. they report feeling tilted slightly head down, and conversely. Measurement of the postural vertical provides a convenient way to assess a person’s gravireceptor bias B – at least in one G. The “idiotropic” tendency M affects all judgements of SV when any visual cues are present. The idiotropic effect a usually stronger than gravireceptor bias, even when the latter is in a headward direction. Hence the SV of a horizontally recumbent subject is deviated footward. When no F or P cues are present, the resultant of M and B deviates the SV footward. Hence an observer perceives a dimly lit gravitationally vertical line as rotated in the opposite direction to body tilt – the well known Aubert illusion. Figure 5 shows a horizontally recumbent observer viewing the interior of a tilted, barnlike room in 1-G. The major and minor axes of symmetry of the visual environment are depicted with the array of bidirectional vectors F. Since the room interior has a familiar shape, and readily distinguishable ceiling (top) and floor (bottom), it is also said to possess visual polarity, depicted by the vector P. The visual vertical V lies along one of the major the axes of symmetry in a direction closest to P and M. Here V points in the direction of the true floor, so it is subjectively perceived as a floor. The direction of the subjective vertical SV is determined by a nonlinear interaction of the visual V and gravireceptor (G+B) vectors. How the vectors combine depends on the orientation of the subject. For relatively small static tilts of the subject or the environment as shown in the figure – up to a limit of perhaps 45 degrees – the SV lies in a direction intermediate between V and (G+B). However, if the subject is not in the normal erect position, but instead recumbent, supine or prone with respect to gravity, and V aligns with M, the SV can be “captured”by (i.e. align with) the V and M vectors. Thus a supine subject feels gravitationally upright if the environment is tilted so P and V align with the body axis M. 3.2 Extending the model to 0-G: How the model applies in weightlessness is shown in Figure 6. The physical stimulus to the body’s gravireceptors G is absent, but a headward or footward bias B remains. As in 1-G, the direction of the visual vertical V is Figure 6. Model for 0-G Visual Reorientation Illusion. Crewmember inverted in a Spacelab module feels right side up