Introduction

The perception of body orientation is influenced by multiple sensory and motor systems. Vision provides information about body orientation relative to the external environment. The semicircular canals of the inner ear are responsive to angular acceleration of the head and the otolith organs are sensitive to linear acceleration, including gravity. Somatosensory receptors provide information about body contact and orientation in relation to the ground or support surface. Proprioceptive receptors, Golgi tendon organs, and muscle spindles and joint receptors provide signals that, when interrelated with motor signals, provide information about body configuration. Audition can also provide spatially relevant signals about body orientation. Normally the information provided by these systems is congruent and redundant.

It is well known that unusual patterns of sensory stimulation can elicit errors in the representation of ongoing orientation. Visually induced illusions of self-motion are perhaps the most familiar. A person viewing a wide-field display of objects or stripes moving at constant velocity will soon experience the display to be stationary and her- or himself to be moving in the direction opposite the display motion (cf. Dichgans & Brandt, 1972). Rotary visual motion can evoke illusions of body tilt (Held, Dichgans, & Bauer, 1975). In the absence of vision, constant motion transduced by nearly any sensory modality can elicit illusory self-motion. A rotating auditory field can elicit illusory self-displacement and a pattern of compensatory motion of the eyes like that which occurs during actual body motion (Lackner, 1977). Moving tactile stimulation of the body surface can elicit apparent body motion (Lackner & DiZio, 1984). Vibrating skeletal muscles of the body to activate muscle spindle receptors can elicit various proprioceptive illusions in which the apparent configuration of the body is modified or illusory motion of the whole body relative to the environment is experienced (Lackner, 1988). Motor-based illusions of apparent body motion can also be generated when a seated, stationary, blindfolded individual pedals a platform under his or her feet. If it has the same inertia as the individual, compelling illusory self-motion will be evoked (Lackner & DiZio, 1984).

The situations described represent perceptual “good fits” with the available patterns of sensory and motor stimulation. For example, illusions of constant-velocity self-motion evoked by moving visual, auditory, or somatosensory stimulation patterns are consistent with a nonchanging vestibular input from the semicircular canals and otolith organs. Voluntary tilting movements of the head can often suppress the elicitation of sensory-induced illusions of body rotation, because the patterns of vestibular and proprioceptive feedback elicited by the head movements are inconsistent with those that would be generated during actual body rotation. The mechanisms and processes underlying the perception of self-motion and orientation are complex and not fully understood (cf. Lackner, 1978).

Exposure to unusual force conditions also affects the normal patterning of sensory stimulation and has profound influences on spatial orientation and the control of body movements. The otolith organs of the inner ear respond to linear acceleration. Consequently, their effective stimulus is gravitoinertial force, which is the result of gravity and imposed inertial forces arising from self-motion or from transport in a vehicle of some sort. Figure 25.1 illustrates how gravitoinertial force (gif) affects the otolith organs. Exposure to altered levels and directions of gravitoinertial force can elicit postural illusions, orientation illusions, and errors in sensory localization. In addition, it affects the control of body movements, because the effective weight of the body changes in altered force environments. Vestibulospinal reflexes are also modulated and alter the effective tonus of the antigravity musculature of the body (Lackner, Dizio, & Fisk, 1992; Watt, Money, & Tomi, 1986; Wilson & Peterson, 1978). This overall pattern of changes means that to bring about any posture or body movement relative to the immediate surroundings requires different patterns of muscle innervation than those employed prior to the change in gravitoinertial force.

Figure 25.1.  

A side view of simplified otolith organs stimulated by head tilt relative to gravitational acceleration, by acceleration relative to an inertial reference frame, and by centripetal force. The black arrows represent the contact force of support opposing gravity (g), the force that must be applied in order to accelerate the body relative to inertial space (a), centripetal force (c), and the vector resulting from contact forces (gif). These forces are applied to the body surface and are transmitted through the skeleton to the saccular (red) and utricular (blue) surfaces, which are represented as idealized orthogonal planes pitched up about 30 degrees relative to the naso-occipital axis of the head. When the head is in a natural upright posture (upper left), the saccular and utricular otoconial masses are displaced from their viscoelastic equilibria and deflect their hair cells in proportion to the components (dotted lines) of gif parallel to their respective planes. Static 30-degree pitch forward of the head (lower left) relative to gravity alters the ratio of saccular to utricular deflection relative to that in upright stationary conditions (upper left). Backward acceleration relative to space (upper right) and centrifugation while facing away from the rotation axis (lower right) can rotate the gif 30 degrees backward relative to the head and produce the same ratio of saccular to utricular deflection as a 30-degree forward tilt.

Centrifuges and slow-rotation rooms have been a valuable way of studying alterations in spatial orientation and sensory localization associated with unusual force conditions. In these devices, alterations in gravitoinerital force result from the centripetal force generated by rotation. The object on the rotating vehicle moves in a circular path because the centripetal force applied by the vehicle deflects it accordingly; otherwise the object would move in a straight path. Centripetal force is proprotional to the square of the velocity of rotation (in radians) times the radius of rotation. Consequently, the farther the object or person from the center of rotation and the faster the rate of rotation, the greater will be the increase in the resulting gravitoinertial force (gravity and centripetal force combined).

Rotating vehicles are of special interest because of the possibility of using them to generate artificial gravity in long-duration space flight. Humans in weightless conditions undergo progressive loss of bone and muscle strength. Approximately 1% of bone mineral content is lost per month of weightlessness in space flight (Holick, 1992, 1997). This degree of bone mineral loss is acceptable for brief missions but is a severe problem for very long-duration missions because of the possibility of bone fractures and severe skeletal-muscular control problems on return to Earth. The centripetal force or artificial gravity associated with rotation could in principle be substituted for Earth's gravity in space flight. One complication of rotating artificial gravity vehicles is the generation of Coriolis forces by body movements in relation to the vehicle. For example, if an object translates in the plane of rotation, as shown in Figure 25.2, a transient Coriolis force will be generated on it. This force is proportional to the velocity of rotation of the vehicle (ω) and the velocity of linear motion of the object (v): F_Cor = −2m(ω × v), where m is the mass of the object.

Figure 25.2.  

Illustration of Coriolis force (F_Cor) on a projectile (black circle) whose mass is m and whose instantaneous linear velocity is v relative to an environment rotating with angular velocity equal to ω. The dashed arrow indicates the object's trajectory.

Coriolis forces are generated by any linear body movement and by tilting movements of the head as well. The latter movements can evoke motion sickness because of the unusual pattern of vestibular stimulation generated. Early studies of rotating environments suggested that 3–4 rpm would be the highest feasible rotation velocity for an artificial gravity vehicle in space flight because it was thought that astronauts would become severely motion sick and disoriented at higher velocities (Graybiel, Clark, & Zarriello, 1960; Graybiel et al., 1965; Guedry, Kennedy, Harris, & Graybiel, 1964; Kennedy & Graybiel, 1962). In fact, as will be discussed later, quite high Coriolis forces are generated during our natural everyday movements when simultaneous torso and limb motions occur, so it is not unusual for humans to experience Coriolis forces.

The emphasis in this chapter will be on human orientation and movement control in unusual force environments, on adaptive changes in motor control that can occur in such environments, and on how static and dynamic patterns of somatosensory and haptic stimulation can affect apparent orientation and postural control. Throughout we will highlight the constant interplay and interaction of multiple sensory and motor influences in determining orientation, sensory localization, and movement control.