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FIGURE 5.2 A. Schematic illustration of a parabolic flight maneuver (above) and a typical trace of the gravitoinertial force level (below). Each parabola contains approximately 20 s of steady 0 g and 1.8 g. B. In the free-fall phase of parabolic flight, the aircraft and passengers are accelerating toward earth at the same rate so it is possible to float. Lack of contact leads to loss of orientation relative to the aircraft if the eyes are closed. C. A sense of orientation is restored and subjects feel upside down if pressure is applied to their head (arrow) while they are free floating, eyes closed.

individuals experience face-up or face-down orbital motion.* The direction of experienced orbital motion is opposite the direction of actual body rotation. In addition, for the face-up and face-down experienced orientations there is a 180° phase shift in relation to actual body position for the two experienced orientations. Figure 5.3B shows the relationship between experienced orbital motion and the actual rotary position. For the face-up orbital motion, the person feels at the top of the orbit when actually face down; in the left quadrant, when turning onto the left side; at the bottom, when physically face-up, and in the right quadrant, when actually right-side down. By contrast, for face-up motion, the subject is physically face up when at the top of the orbital path. Most subjects can shift their apparent orientation between face-up and face-down by straining in the apparatus and applying pressure to the front or back of their head. Pressure cues can also affect the apparent diameter of the orbital motion. By increasing the actual pressure on the front of their head as they rotate, subjects can make their apparent orbits increase in diameter by several feet.

Putting pressure on the soles of their feet or the top of the head can cause changes in apparent orientation in relation to the test chamber. Foot pressure makes the

* Occasionally, a subject will feel as if he/she is undergoing orbital motion in a horizontal plane while always face-up or face-down.

FIGURE 5.3 A. A subject restrained in the Z-axis recumbent rotation apparatus. B. During actual counterclockwise rotation in the apparatus, subjects experience clockwise orbital rotation (broken arrows) with a constant heading, face-up is illustrated. The actual and perceived patterns are both consistent with the rotation of contact cues around the body (solid arrows). C. Increasing plantar pressure by straining against the restraints makes subjects feel upright, pivoting in a cone about their feet while maintaining a constant heading.

FIGURE 5.3 A. A subject restrained in the Z-axis recumbent rotation apparatus. B. During actual counterclockwise rotation in the apparatus, subjects experience clockwise orbital rotation (broken arrows) with a constant heading, face-up is illustrated. The actual and perceived patterns are both consistent with the rotation of contact cues around the body (solid arrows). C. Increasing plantar pressure by straining against the restraints makes subjects feel upright, pivoting in a cone about their feet while maintaining a constant heading.

subjects feel upright while describing their orbital path (see Figure 5.3C), and pressure on the top of the head makes them feel upside down. Thus, the overall pattern of touch and pressure cues impinging on the body dramatically influences experienced orientation. The transition from horizontal to vertical orbital motion is not experienced as a physical rotation from horizontal to vertical, but rather as a gradual feeling of being less and less in one orientation and then progressively more compelling in the new orientation, until it is completely vivid. The absence of a sense of spatial displacement from horizontal to vertical is likely related to the absence of the semicircular canal and otolith activity that would accompany such a change. A similar fading-out, fading-in of experienced orientation occurs in the orientation illusions experienced in parabolic flight described above.

The ZARR studies emphasize the importance of touch and pressure cues for orientation and indicate that dynamic patterns of somatosensory stimulation can supplement and even supplant vestibular contributions to perceived orientation. This is especially obvious in parabolic flight experiments involving the ZARR where background force level varies between 0 and 1.8 g. When blindfolded subjects ride in the ZARR at 30 rpm during straight and level flight, they experience the patterns illustrated in Figure 5.3 with comparable orbital diameters. However, as the aircraft pulls "g's" and increases the subject's effective weight, the forces exerted on the subject's body by the apparatus increase and the diameter of the apparent orbit rapidly becomes larger and larger. It peaks and remains constant as the g force levels off at its peak and can seem as much as 20 feet in diameter. As the g force begins to decline, the orbit diameter shrinks more and more until in weightlessness no orbital motion whatsoever is experienced. Even though the subject is rotating at 30 rpm, no motion is experienced whatsoever; the subject feels totally stationary.6 In this circumstance, at constant velocity rotation, there is no differential touch and pressure stimulation of the body surface because the subject is weightless. If the subject is allowed unrestricted vision during rotation, then in the high force periods he or she will experience continuous body rotation while simultaneously undergoing orbital motion in the opposite direction. The orbit diameter is more than 50% smaller when the eyes are open. In 0 g, some subjects experience constant velocity rotation about their long body axis, while others feel completely stationary and perceive the aircraft to be rotating about them.

The importance of tactile cues for orientation can be demonstrated under normal terrestrial conditions as well. If the soles of a blindfolded seated subject's feet are brushed by a smooth rotating surface while the feet are prevented from moving by a yoke, illusory self-rotation will be elicited (see Figure 5.4). The subject, within 10-20 seconds, will perceive the rotating platform under his feet as stationary and experience his body as rotating in the direction opposite that of the platform. Simultaneously, the subject's eyes will exhibit a nystagmoid pattern of eye movements with slow phase direction compensatory for the direction of apparent body displacement and with velocity scaled to that of experienced velocity.10 Even though subjects know the platform can rotate under their feet, they still come to perceive it to be stationary and themselves as turning.

5.3 "TOP-DOWN" COGNITIVE INFLUENCES ON ORIENTATION

Contact cues from the feet can also override vestibular signals to orientation. It has been known for hundreds of years that if a seated person is passively rotated in the dark at constant velocity, then he or she will soon come to feel stationary. The adequate stimulus for the semicircular canals is angular acceleration and canal afferent discharge returns to resting levels during constant velocity turning, thus the canals cannot discriminate rest from constant velocity. If an individual is rapidly decelerated to rest from constant velocity rotation, body rotation in the opposite direction will be perceived. This occurs because during the rapid deceleration the cupula in each horizontal semicircular canal becomes deviated in the direction opposite that during initial acceleration to constant velocity. During constant velocity rotation, the canals gradually re-equilibrate and during deceleration, opposite direction rotation is sensed. Importantly, if immediately post-rotation the subject lowers his feet from the footrest of the chair to the floor of the experimental chamber, then there will be a truncation or a reversal of the post-rotation motion aftereffect. The subject may perceive his direction of motion reverse so that it is in the same direction

FIGURE 5.4 Illusory leftward self-rotation (dashed arrow) and compensatory eye movements (left beating nystagmus) are elicited when the "floor" moves rightward (solid arrows) under the soles of the yoked feet of the stationary, blindfolded observer. The observer perceives the moving surface to be stationary under his feet.

FIGURE 5.4 Illusory leftward self-rotation (dashed arrow) and compensatory eye movements (left beating nystagmus) are elicited when the "floor" moves rightward (solid arrows) under the soles of the yoked feet of the stationary, blindfolded observer. The observer perceives the moving surface to be stationary under his feet.

as per-rotation. Lifting and lowering the feet will cause the aftereffect to alternately change direction and to dissipate faster than the usual 20-30 seconds. With stops from higher velocities of rotation, the duration of the aftereffect is increased. It is important to note that when the subject's feet remain on the footrest of the rotating chair, he experiences his feet as stationary in relation to the footrest and himself and the entire chair as turning. On lowering his feet to touch the floor, he experiences it to be stationary and his body and the chair to be turning in relation to it. This pattern means that contact of the feet with different reference frames — chair vs. floor — greatly influences the apparent motion of the body. Such an influence points to "top-down" cognitive effects on orientation rather than a "bottom-up" integration of sensory signals.

Similar top-down influences can be demonstrated for hand and arm movement control. When a subject, in darkness, is accelerated in a rotating chair nystagmus is elicited. The nystagmus has a slow phase that is opposite the direction of rotation. If a target light is attached to a boom on the chair and fixed relative to the observer, during angular acceleration, the target will be seen to change its position relative to the subject. The subject will see it displace in the direction of acceleration. After reaching a peak subject-relative displacement, the target will still seem to be moving relative to the observer but no longer increasing its subject-relative displacement.

Simultaneously, the observer will perceive rotation of self and target together relative to the unseen experimental chamber. This phenomenon is known as the oculogyral illusion.11

If the subject is allowed to maintain hand contact with the target light, then when the chair is accelerated, the apparent displacement and motion of the target relative to the subject will be suppressed.12 If the target light is extinguished and the subject maintains hand contact with the target light mount and attempts to fixate it, the vestibular nystagmus normally evoked by chair acceleration will be greatly attenuated.12 In both the target light and the darkness conditions, suppression is contingent on the subject's hand making physical contact with the target or target holder. Having the hand spatially even a few millimeters away has no suppressing effect. These results demonstrate that somatosensory plus proprioceptive information about target location influences visual localization and oculomotor control, and can suppress visual mislocalizations induced by vestibular stimulation.

Hand contact can also suppress other forms of illusory visual motion. The autokinetic effect — the apparent visual motion of a stationary target light in an otherwise dark room — has been one of the classic illusions studied by psychologists and physiologists. This illusion was found to be greatly suppressed in magnitude and total time of occurrence if the subject grasped the target light holder with his or her index finger next to the light.13 In addition, the amplitude of involuntary losses of fixation of the target light was diminished as well. These results point to a proprioceptive and somatosensory contribution to stabilization of visual direction and oculomotor control. A subsequent study using a scleral search coil technique to evaluate the quantitative reduction of fixation instability provided by holding vs. not holding the target light failed to show any differences.14 It turns out, however, that having a contact lens in the eye or having a piece of surgical tape against the lower eyelid provides somatosensory cues about eye position that are as effective as holding the target light in suppressing autokinesis.15 Together these observations emphasize the importance of somatosensory and proprioceptive cues — distributed across the body — in influencing sensory localization and oculomotor control and the broad range of cues the body can use to enhance accuracy.

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