Oman York Conference(2001 in press)11/2/01 more potent than in the foreground. It was also confirmed that non-polarized objects can inherit form of"extrinsic"polarity if they appear to be lying on or hanging from other object A second type of extrinsic" polarity derives from the conventional location of certain types of objects. For example, doors and simple window and picture frames are often up-down symmetrical. However their placement relative to adjacent surfaces provides extrinsic polarity cues for surfaces in a vertical plane. We do not expect to see a door in the middle of a wall, or a icture frame positioned close to a floor. It makes sense to think that object polarity depends little on the relative orientation of the object and observer. We must only recognize"what type of object it is. Relative orientation is probably more important for distinguishing details that allow an observer to distinguish "which"specific member of a class an object is 4. 4 Interaction between Gravity, Polarity, Frame, and Idiotropic Cues. The"rules"describing how G. f. P and m cues in various directions are combined under 1-G conditions have been defined in experiments where observers have been viewed the interior of tilted furnished room The interiors were fitted with anchored tables, desks, bookshelves, and other props etc so as to provide strong frame and polarity cues. Probably for practical reasons, most of the testing has been done with the observers and rooms tilted less than 30 degrees from the gravitational vertical (Kleint, 1936; Asch and Witkin, 1948; Singer, et al, 1970). As in the rod and Frame experiments, the indicated subjective vertical represents a compromise between the gravitational and frame/polarity directions. Howard and Childerson(1994)tested at larger room tilt angles and found that the sv was deviated toward the floor -ceiling -wall closest to being beneath their feet, but not to the diagonals(as in their frame experiments described in Sect. 4.2 above). The subjects were not asked whether the subjective identities of the floor-wall-ceiling surfaces exchanged as the room rotated into various positions, but in retrospect, and after trying it ourselves, lan and i are almost certain they did, and thus experienced vris analogous to those of astronauts. Subsequently, Howard and Hu(2000)also tested at the 90 and 180 degree extremes of body tilt. We knew from earlier experiments(e.g. Young, Oman and Dichgans, 1975 )that pitch and roll angular self-motion illusions(vection) was enhanced when the observers head and body were supine or inverted. But we were still surprised to discover that when Howard and Hus observers were gravitationally supine or inverted, and the room polarity vector was aligned with their body axis M vector, a substantial fraction felt gravitationally upright in the motionless room! It was as if gravireceptor information was being discounted when the head-body axis was not in the familiar gravitationally upright position. The subjective vertical seemed to be closely aligned with the coaligned idiotropic, visual polarity, and visual frame axes. This sort"capture was reminiscent of what we think happens to the astronauts. Not all subjects felt this, of course Some still felt oriented with respect to gravity, and others said their perceptions seemed to switch back and forth in a confusing way between the two rival interpretations. It was also amusing that if gravitationally supine but subjectively upright observers extended their arms gravitationally upward, the arms felt oddly levitated, as if floating. It felt different than extending your arms while lying supine in bed at home in a gravitationally upright visual environment. lan refers to this special sensation as a"levitation"illusion. Howard, Jenkin and Hu(2000)also showed that the incidence of "levitation" "illusion increases as a function of age We cannot be sure whether the latter is due to increased experience with polarity cues as one ages, senescent loss of vestibular receptor sensitivity, or bothOman York Conference (2001 in press) 11/2/01 Page 13 more potent than in the foreground. It was also confirmed that non-polarized objects can inherit a form of “extrinsic” polarity if they appear to be lying on or hanging from other objects. A second type of “extrinsic” polarity derives from the conventional location of certain types of objects. For example, doors and simple window and picture frames are often up-down symmetrical. However their placement relative to adjacent surfaces provides extrinsic polarity cues for surfaces in a vertical plane. We do not expect to see a door in the middle of a wall, or a picture frame positioned close to a floor. It makes sense to think that object polarity depends little on the relative orientation of the object and observer. We must only recognize “what” type of object it is. Relative orientation is probably more important for distinguishing details that allow an observer to distinguish “which” specific member of a class an object is. 4.4 Interaction between Gravity, Polarity, Frame, and Idiotropic Cues. The “rules” describing how G, F, P, and M cues in various directions are combined under 1-G conditions have been defined in experiments where observers have been viewed the interior of tilted furnished rooms. The interiors were fitted with anchored tables, desks, bookshelves, and other props etc. so as to provide strong frame and polarity cues. Probably for practical reasons, most of the testing has been done with the observers and rooms tilted less than 30 degrees from the gravitational vertical (Kleint, 1936; Asch and Witkin, 1948; Singer, et al, 1970). As in the Rod and Frame experiments, the indicated subjective vertical represents a compromise between the gravitational and frame/polarity directions. Howard and Childerson (1994) tested at larger room tilt angles, and found that the SV was deviated toward the floor-ceiling-wall closest to being beneath their feet, but not to the diagonals (as in their frame experiments described in Sect. 4.2 above). The subjects were not asked whether the subjective identities of the floor-wall-ceiling surfaces exchanged as the room rotated into various positions, but in retrospect, and after trying it ourselves, Ian and I are almost certain they did, and thus experienced VRIs analogous to those of astronauts. Subsequently, Howard and Hu (2000) also tested at the 90 and 180 degree extremes of body tilt. We knew from earlier experiments (e.g. Young, Oman and Dichgans, 1975) that pitch and roll angular self-motion illusions (vection) was enhanced when the observer’s head and body were supine or inverted. But we were still surprised to discover that when Howard and Hu’s observers were gravitationally supine or inverted, and the room polarity vector was aligned with their body axis M vector, a substantial fraction felt gravitationally upright in the motionless room ! It was as if gravireceptor information was being discounted when the head-body axis was not in the familiar gravitationally upright position. The subjective vertical seemed to be closely aligned with the coaligned idiotropic, visual polarity, and visual frame axes. This sort “capture” was reminiscent of what we think happens to the astronauts. Not all subjects felt this, of course. Some still felt oriented with respect to gravity, and others said their perceptions seemed to switch back and forth in a confusing way between the two rival interpretations. It was also amusing that if gravitationally supine but subjectively upright observers extended their arms gravitationally upward, the arms felt oddly levitated, as if floating. It felt different than extending your arms while lying supine in bed at home in a gravitationally upright visual environment. Ian refers to this special sensation as a “levitation” illusion. Howard, Jenkin and Hu (2000) also showed that the incidence of “levitation” illusion increases as a function of age. We cannot be sure whether the latter is due to increased experience with polarity cues as one ages, senescent loss of vestibular receptor sensitivity, or both