Oman York Conference(2001 in press)11/2/01 Page 14 4.5 Animal and Human Visual Orientation Experiments in Weightlessness. Many astronauts have asked us"isnt it strange that we still have a vertical in weightlessness, even though dropped objects don't fall Certainly, but since an astronauts job requires knowing whether they are facing forward or aft, port or starboard in the spacecraft, everyone maintains an exocentric(allocentric)reference frame. This frame is the anchor for our hierarchically organized set of knowedge and visual memories for where things are- the latter is sometimes called a"spatial framework". The framework lets us remember where things are, look and reach for things in the correct direction, and mentally visualize unseen parts of the vehicle in correct relative orientation. Based on recordings from place and direction cells in the limbic system of animals on Earth(O Keefe 1976; Taube et al, 1990) believe that the human sense of place and direction is neurally coded in a gravitationally horizontal plane. Taube showed that prominent visual landmarks can reorient our sense of direction within this horizontal plane. Normally, the orientation of this plane is anchored by gravity. We (taube, et al, 1999, 2000)recently monitored rat head direction cells in parabolic flight, and Knierim, et al(2000)studied place cell behavior in orbital flight. Both experiments confirmed that place and direction cells usually continue to maintain allocentric place and directional coding when the animals walk on the floor or walls of the test chamber. However in both experiments, there was evidence that the allocentric reference frames sometimes-but not always-reoriented onto the surface the animal was walking on. Apparently humans are not the only animals who experience VRIs in weightlessness These animal experiments strongly support the notion that the human CNs also maintains allocentric reference direction at the neural level, represented by the sv direction in the present model. It makes sense to think that the CNS uses this sv direction to determine the perceptual identity of ambiguous nonpolarized surfaces in the visual surround. However, since the SV direction is not"anchored" by gravity, idiotropic and gravireceptor bias and visual polarity cues can cause the orientation of the horizontal reference plane to suddenly shift. Depending on the individual, down" is either along the body axis, or perpendicular to the subjective floor(Figure 6). However, if gravireceptor bias is strongly headward, in conflict with the visual vertical V, the observer experiences a O-G Inversion Illusion(Figure 8) by assuming that the Sv is no lor associated with the local visual vertical v but with an unseen outside coordinate frame and describes himself as right side up in an upside down vehicl In 1998 we had the opportunity to quantify how frame and polarity cues affected the sv in fou astronauts on the sts-90 Neurolab mission(Oman, et al, 2000). For practical reasons, we could not use real tilted visual environments so instead our observers wore a wide field of view(65 deg. x 48 deg. color stereo head mounted display Figure 13. Neurolab crewmember wearing head Mounted display and spring harnessOman York Conference (2001 in press) 11/2/01 Page 14 4.5 Animal and Human Visual Orientation Experiments in Weightlessness. Many astronauts have asked us “isn’t it strange that we still have a vertical in weightlessness, even though dropped objects don’t fall ?” Certainly, but since an astronaut’s job requires knowing whether they are facing forward or aft, port or starboard in the spacecraft, everyone maintains an exocentric (allocentric) reference frame. This frame is the anchor for our hierarchically organized set of knowedge and visual memories for where things are – the latter is sometimes called a “spatial framework”. The framework lets us remember where things are, look and reach for things in the correct direction, and mentally visualize unseen parts of the vehicle in correct relative orientation. Based on recordings from place and direction cells in the limbic system of animals on Earth (O’Keefe 1976; Taube et al, 1990) believe that the human sense of place and direction is neurally coded in a gravitationally horizontal plane. Taube showed that prominent visual landmarks can reorient our sense of direction within this horizontal plane. Normally, the orientation of this plane is anchored by gravity. We (Taube, et al, 1999, 2000) recently monitored rat head direction cells in parabolic flight, and Knierim, et al (2000) studied place cell behavior in orbital flight. Both experiments confirmed that place and direction cells usually continue to maintain allocentric place and directional coding when the animals walk on the floor or walls of the test chamber. However in both experiments, there was evidence that the allocentric reference frames sometimes – but not always - reoriented onto the surface the animal was walking on. Apparently humans are not the only animals who experience VRIs in weightlessness. These animal experiments strongly support the notion that the human CNS also maintains an allocentric reference direction at the neural level, represented by the SV direction in the present model. It makes sense to think that the CNS uses this SV direction to determine the perceptual identity of ambiguous nonpolarized surfaces in the visual surround. However, since the SV direction is not “anchored” by gravity, idiotropic and gravireceptor bias and visual polarity cues can cause the orientation of the horizontal reference plane to suddenly shift. Depending on the individual, “down” is either along the body axis, or perpendicular to the subjective floor (Figure 6). However, if gravireceptor bias is strongly headward, in conflict with the visual vertical V, the observer experiences a 0-G Inversion Illusion (Figure 8) by assuming that the SV is no longer associated with the local visual vertical V, but with an unseen outside coordinate frame, and describes himself as right side up in an upside down vehicle. In 1998 we had the opportunity to quantify how frame and polarity cues affected the SV in four astronauts on the STS-90 Neurolab mission (Oman, et al, 2000). For practical reasons, we could not use real tilted visual environments, so instead our observers wore a wide field of view (65 deg. x 48 deg.), color stereo head mounted display Figure 13. Neurolab crewmember wearing head Mounted display and spring harness