Orientation and Gravity

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Orientation and Gravity

• Seth Bachelier
• Vestibular Classics
• January 5, 2007

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Space and Spatial Extension

• Orientation Perception of motion and position
  of an object with respect to frame of reference
• Kant - A sensory system property
• Gathering and Organization of information from
  our senses

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Reference Frames

• Egocentric relative to ones own body or head
• Allocentric (exocentric) defined by gravity and
  landmarks in the surroundings

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Egocentric Co-ordinate System
Planes X-frontal, Y-sagittal, Z-transversal
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Judging Object Position

• Dependent on retinal information
• Extra-ocular muscles (efference copies)
• Good for rapid changes in gaze direction
• Eye-muscle proprioception
• Valuable in maintaining resting position of the
  eye in the absence of visual cues (darkness)

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The Tilt Chair
Visually perceived eye level - VPEL Subjective
straight out from the face SSFF VPEL and SSFF
in upright position and 30 degree backward tilt
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When gravitoinertial force is increased, a shear
force acts posteriorly in the plane of the
utricle. This causes an illusion of a backward
head tilt reflected in a depression of the VPEL
and SSFF relative to the gravitoinertial
horizontal.
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Gondola Centrifugation
During centrifugation, the long axis (z-axis) is
aligned with the resultant force vector from
gravity and centrifugal force. Data was obtained
after at least 10 minutes to exclude influence of
semicircular canal stimulation.
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Results Tilt Chair (1G)
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Results - Centrifuge
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Discussion - Tilt

• In upright posture, VPEL and SSFF coincide and
  are close to GH
• At 30 degree backward tilt, VPEL remains close to
  GH while SSFF remains fixed relative to the head
• The egocentric frame and sense of eye position
  are not influenced by head tilt at 1G
• There is no vestibulo-ocular response (VOR), it
  would change the SSFF with respect to the head
• Determining VPEL during backward tilt is a
  combination of reflexive eye movements AND
  awareness of gravity and compensation of backward
  head tilt

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Discussion - Centrifuge

• SSFF at 2G 12 degree downward shift represents
  a shift in the egocentric frame
• 2 possibilities
• Otolith signals change the resting position of
  the eye and downward gaze is not accounted for by
  efference copies or proprioception, so when the
  egocentric frame is changed, both egocentric and
  allocentric data are required to properly
  determine orientation in space
• In hypergravity, abdominal graviceptors can be
  more accurate than the utricle and are perhaps
  combined with vestibular information to determine
  SSFF

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Study 2 Perceiving Up

• The force of gravity is determined by vectorially
  summing several cues
• Touch receptors
• Intrinsic vision cues
• Extrinsic vision cues
• Environmental cues

The direction up will oppose the perceived
direction of the pull of gravity.
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Separating Cues

Viewing the image through the shroud prevents
visual cues of the spacecrafts orientation.
And the body could be upright or placed on its
left side.
The image could be aligned with or orthogonal to
the body axis.
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Conditions

• Upright with upright vision
• Upright with visually defined top to the right
• Left side down with upright vision aligned with
  gravity
• Left side down with vision to the left aligned
  with the body (not run in flight)

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Results Normal Gravity
This data is the same as control trials made on
Earth.
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Results - Microgravity
Data is closely aligned with the body axis in all
cases. Similar results in panels 2 and 3 indicate
physical cues had no effect on vision. This is
also recognized in the center panel where
although vision is to the right, there are twice
as many responses opposite the visual field.
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Results - Hypergravity
Comparisons of normal and hypergravity shows that
under hypergravity responses are more aligned
with the body axis.
Hypergravity
Normal Gravity
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Discussion

• The data under normal gravity conditions fit the
  expected model
• Under microgravity, ONLY the body vector
  generated a perceived direction of upvisual cues
  were ignored
• Hypergravity vector did not outweigh the body
  vector
• 2 explanations
• Ignoring the visual cues could be a product of
  the distracting environment, experimental
  conditions
• There were only 4 possible choices for the most
  convex shape, finer gradations could be used

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Discussion

• Ultimately, under unusual gravity conditions and
  intense distraction, subjects tended to use their
  body as the primary reference frame
• The need for improved visual cues to rely on
• larger field
• Higher contrast
• Stronger orientation cues
• More depth