Orientation and Gravity Seth Bachelier Vestibular Classics January 5, 2007

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

Reference Frames Egocentric – relative to one’s own body or head Egocentric – relative to one’s own body or head Allocentric (exocentric) – defined by gravity and landmarks in the surroundings Allocentric (exocentric) – defined by gravity and landmarks in the surroundings

Egocentric Co-ordinate System Planes: X-frontal, Y-sagittal, Z-transversal

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

Discussion - Tilt In upright posture, VPEL and SSFF coincide and are close to GH 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 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 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 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 Determining VPEL during backward tilt is a combination of reflexive eye movements AND awareness of gravity and compensation of backward head tilt

Discussion - Centrifuge SSFF at 2G – 12 degree downward shift represents a shift in the egocentric frame SSFF at 2G – 12 degree downward shift represents a shift in the egocentric frame 2 possibilities 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 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 In hypergravity, abdominal graviceptors can be more accurate than the utricle and are perhaps combined with vestibular information to determine SSFF

Study #2 – Perceiving “Up” The force of gravity is determined by vectorially summing several cues The force of gravity is determined by vectorially 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 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.

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

Results – Normal Gravity This data is the same as control trials made on Earth.

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.

Results - Hypergravity HypergravityNormal Gravity Comparisons of normal and hypergravity shows that under hypergravity responses are more aligned with the body axis.

Discussion The data under normal gravity conditions fit the expected model The data under normal gravity conditions fit the expected model Under microgravity, ONLY the body vector generated a perceived direction of up…visual cues were ignored Under microgravity, ONLY the body vector generated a perceived direction of up…visual cues were ignored Hypergravity vector did not outweigh the body vector Hypergravity vector did not outweigh the body vector 2 explanations2 explanations Ignoring the visual cues could be a product of the distracting environment, experimental conditionsIgnoring 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 usedThere were only 4 possible choices for the most convex shape, finer gradations could be used

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