Orientation and GravityOrientation and Gravity

Seth BachelierSeth Bachelier

Vestibular ClassicsVestibular Classics

January 5, 2007January 5, 2007

Space and Spatial ExtensionSpace and Spatial Extension

Orientation – Perception of motion Orientation – Perception of motion and position of an object with respect and position of an object with respect to frame of referenceto frame of reference

Kant - A sensory system propertyKant - A sensory system property• Gathering and Organization of Gathering and Organization of

information from our sensesinformation from our senses

Reference FramesReference Frames

Egocentric – relative to one’s own Egocentric – relative to one’s own body or headbody or head

Allocentric (exocentric) – defined by Allocentric (exocentric) – defined by gravity and landmarks in the gravity and landmarks in the surroundingssurroundings

Egocentric Co-ordinate System

Planes: X-frontal, Y-sagittal, Z-transversal

Judging Object PositionJudging Object Position

Dependent on retinal informationDependent on retinal information• Extra-ocular muscles (efference copies)Extra-ocular muscles (efference copies)

Good for rapid changes in gaze directionGood for rapid changes in gaze direction

• Eye-muscle proprioceptionEye-muscle proprioception Valuable in maintaining resting position of Valuable in maintaining resting position of

the eye in the absence of visual cues the eye in the absence of visual cues (darkness)(darkness)

Visually perceived eye level - VPEL

Subjective straight out from the face – SSFF

VPEL and SSFF in upright position and 30 degree backward tilt

The Tilt Chair

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.

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.

Results – Tilt Chair (1G)Results – Tilt Chair (1G)

Results - CentrifugeResults - Centrifuge

Discussion - TiltDiscussion - Tilt In upright posture, VPEL and SSFF coincide and are In upright posture, VPEL and SSFF coincide and are

close to GHclose to GH At 30 degree backward tilt, VPEL remains close to GH At 30 degree backward tilt, VPEL remains close to GH

while SSFF remains fixed relative to the headwhile SSFF remains fixed relative to the head The egocentric frame and sense of eye position are not The egocentric frame and sense of eye position are not

influenced by head tilt at 1Ginfluenced by head tilt at 1G There is no vestibulo-ocular response (VOR), it would There is no vestibulo-ocular response (VOR), it would

change the SSFF with respect to the headchange the SSFF with respect to the head Determining VPEL during backward tilt is a combination Determining VPEL during backward tilt is a combination

of reflexive eye movements AND awareness of gravity of reflexive eye movements AND awareness of gravity and compensation of backward head tiltand compensation of backward head tilt

Discussion - CentrifugeDiscussion - Centrifuge SSFF at 2G –SSFF at 2G – 12 degree downward shift 12 degree downward shift

represents a shift in the egocentric framerepresents a shift in the egocentric frame 2 possibilities2 possibilities Otolith signals change the resting position of Otolith signals change the resting position of

the eye and downward gaze is not accounted the eye and downward gaze is not accounted for by efference copies or proprioception, so for by efference copies or proprioception, so when the egocentric frame is changed, both when the egocentric frame is changed, both egocentric and allocentric data are required to egocentric and allocentric data are required to properly determine orientation in spaceproperly determine orientation in space

In hypergravity, abdominal graviceptors can In hypergravity, abdominal graviceptors can be more accurate than the utricle and are be more accurate than the utricle and are perhaps combined with vestibular information perhaps combined with vestibular information to determine SSFF to determine SSFF

Study #2 – Perceiving “Up”Study #2 – Perceiving “Up”

The force of gravity is determined by The force of gravity is determined by vectorially summing several cuesvectorially summing several cues• Touch receptorsTouch receptors• Intrinsic vision cuesIntrinsic vision cues• Extrinsic vision cuesExtrinsic vision cues• Environmental cuesEnvironmental cues

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

Separating CuesSeparating Cues

Viewing the image through the shroud prevents visual cues of the spacecraft’s orientation.

The image could be aligned with or orthogonal to the body axis.

And the body could be upright or placed on its left side.

ConditionsConditions

Upright with upright visionUpright with upright vision Upright with visually defined top to Upright with visually defined top to

the rightthe right Left side down with upright vision Left side down with upright vision

aligned with gravityaligned with gravity Left side down with vision to the left Left side down with vision to the left

aligned with the body (not run in aligned with the body (not run in flight)flight)

Results – Normal GravityResults – Normal Gravity

This data is the same as control trials made on Earth.

Results - MicrogravityResults - 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.

Results - HypergravityResults - Hypergravity

Hypergravity Normal Gravity

Comparisons of normal and hypergravity shows that under hypergravity responses are more aligned with the body axis.

DiscussionDiscussion The data under normal gravity conditions fit The data under normal gravity conditions fit

the expected modelthe expected model Under microgravity, ONLY the body vector Under microgravity, ONLY the body vector

generated a perceived direction of up…visual generated a perceived direction of up…visual cues were ignoredcues were ignored

Hypergravity vector did not outweigh the body Hypergravity vector did not outweigh the body vectorvector• 2 explanations2 explanations• Ignoring the visual cues could be a product Ignoring the visual cues could be a product

of the distracting environment, of the distracting environment, experimental conditionsexperimental conditions

• There were only 4 possible choices for the There were only 4 possible choices for the most convex shape, finer gradations could most convex shape, finer gradations could be usedbe used

DiscussionDiscussion Ultimately, under unusual gravity conditions Ultimately, under unusual gravity conditions

and intense distraction, subjects tended to use and intense distraction, subjects tended to use their body as the primary reference frametheir body as the primary reference frame

The need for improved visual cues to rely onThe need for improved visual cues to rely on• larger fieldlarger field• Higher contrastHigher contrast• Stronger orientation cuesStronger orientation cues• More depthMore depth