Biomedical subjects
Publications and source records attributed to J R Lackner.
On Earth, the functional stretch reflex is an important component in the maintenance of posture and muscle tone. In parabolic flight experiments, we evaluated whether the functional stretch reflex, as reflected in the tonic vibration reflex, adjusts appropriately for changes in background gravitoinertial force level. Virtually immediate alterations of appropriate sign occurred.
Illusions of body inversion and of vehicle inversion can be evoked by exposure to weightlessness in the microgravity conditions of orbital and parabolic flight. Such illusions can involve all possible combinations of self-inversion and vehicle inversion. In the absence of any patterns of external stimulation, individuals may lose all sense of body orientation to their surroundings while retaining a sense of their overall body configuration and cognitive awareness of their actual position. Touch and pressure cues provide a perceptual 'down' in the absence of visual input. When vision is allowed, apparent orientation is influenced by a variety of factors including the direction of gaze, the architectural layout of the vehicle, and sight of the body. The relative importance of the various factors affecting orientation changes with repeated exposure. The virtual absence of sensations of falling during exposure to free-fall emphasizes the role of cognitive factors in experienced orientation.
Human sensory-motor control and orientation involve the correlation of sensory information from many modalities with motor information about ongoing patterns of voluntary and reflexive activation of the body musculature. The vestibular system represents only one of the acceleration-sensitive receptor systems of the body conveying spatial information. Touch- and pressure-dependent receptors, somatosensory and interoceptive, as well as proprioceptive receptors contribute, along with visual and auditory signals specifying relative motion between self and surround. Control of body movement and orientation is dynamically adapted to the 1G force background of Earth. Exposure to non-1G environments such as in space travel produces a variety of sensory-motor disturbances, and often motion sickness, until adaptation is achieved. Exposure to virtual environments in which body movements are not accompanied by normal patterns of inertial and sensory feedback can also lead to control errors and elicit motion sickness.
Space motion sickness is a significant operational concern in the American and Soviet space programs. Nearly 70% of all astronauts and cosmonauts are affected to some degree during their first several days of flight. It is now beginning to appear that space motion sickness like terrestrial motion sickness is the consequence of multiple etiological factors. As we come to understand basic mechanisms of spatial orientation and sensory-motor adaptation we can begin to predict etiological factors in different motion environments. Individuals vary greatly in the extent to which they are susceptible to these different factors. However, individuals seem to be relatively self-consistent in terms of their rates of adaptation to provocative stimulation and their retention of adaptation. Attempts to relate susceptibility to motion sickness during the microgravity phases of parabolic flight maneuvers to vestibular function under 1G and 0G test conditions are described.
Exposure to nonterrestrial force levels affects the activity of gravitoinertial force sensitive receptors of the body, both of labyrinthine and nonlabyrinthine origin. It also disrupts the normal patterning of motor control of body orientation and movement. The patterns and levels of muscle innervation necessary to achieve particular body configurations and to bring about particular body movements are greatly affected by background force level and body orientation relative to the force vector. The present studies demonstrate that such altered sensorimotor control of head and body posture along with altered vestibulomotor control are evocative of motion sickness. This observation has explanatory significance both for space motion sickness and the re-entry disturbances that occur after prolonged spaceflight.
Head and body movements made in microgravity tend to bring on symptoms of motion sickness. Such head movements, relative to comparable ones made on Earth, are accompanied by unusual combinations of semicircular canal and otolith activity owing to the unloading of the otoliths in OG. Head movements also bring on symptoms of motion sickness during exposure to visual inversion (or reversal) on Earth because the vestibulo-ocular reflex is rendered anti-compensatory. Here, we present evidence that susceptibility to motion sickness during exposure to visual inversion is decreased in a 0G relative to a 1G force background. This difference in susceptibility appears related to the alteration in otolith function in 0G. Some implications of this finding for the etiology of space motion sickness are described.
In parabolic flight experiments, we have found post-rotary nystagmus to be differentially suppressed in free fall (OG) and in a high gravitoinertial force (1.8G) background relative to 1G. In addition, the influence of postrotary head movements on nystagmus suppression was found to be contingent on G level. The nature of this pattern indicated a G-dependency of the velocity storage and dumping mechanisms. Here, we have rank-correlated susceptibility to motion sickness during head movements in OG and 1.8G with the following: a) the decay time constant of the slow phase velocity of post-rotary nystagmus under 1G, no head movement, baseline conditions, b) the extent of time constant reduction elicited in OG and 1.8G; c) the extent of time constant reduction elicited by head tilts in 1G; and d) changes in the extent of time constant reduction in OG and 1.8G over repeated tests. Susceptibility was significantly correlated with the extent to which a head movement reduced the time constant in 1G, was weakly correlated with the baseline time constant, but was not correlated with the extent of reduction in OG or 1.8G. This pattern suggests a link between mechanisms evoking symptoms of space motion sickness and the mechanisms of velocity storage and dumping. Experimental means of evaluating this link are described.
We studied suppression of oculomotor and perceptual after-responses by post-rotary head movements in high (1.8 G), low (0 G), and normal (1 G) gravitoinertial force backgrounds in parabolic flight. Our aim was to identify what sensory and motor signals are critical for triggering suppression. In a prior experiment (DiZio and Lackner 1988), we found suppression using 40 degrees post-rotary head tilts in 1 G and 1.8 G but not 0 G force backgrounds. However, in free fall even without head tilts there was a significant suppression of nystagmus relative to 1 G and 1.8 G force backgrounds, thus potentially masking an effect of head tilt on suppression in 0 G. We have retested four of the original subjects with 90 degrees head tilts to maximize the likelihood of detecting suppression in 0 G. Although nystagmus and illusory after-rotation were suppressed by post-rotary head tilts in normal and high gravitoinertial force environments, there was still no evidence of suppression in free fall. We present evidence that the lack of suppression in 0 G is not attributable to post-rotary responses already being at a "basement" level, but rather that suppression depends on the registration of a change in head position relative to a significant level of gravitoinertial force.
We compared the slow-phase eye velocity elicited by sudden cessation of prolonged, constant velocity, vertical z-axis rotation of the body or the visual surround in 10 healthy college-age and over-65 individuals. The step gain of vestibular post-rotary nystagmus did not differ across age groups, but the time constant of slow-phase velocity decay was longer and more asymmetrical in the older group. The slow-phase velocity of optokinetic nystagmus attained the same initial levels for both age groups; it declined significantly during 60 s of stimulation for the older but not the younger group. The decay rate of optokinetic afternystagmus was quicker for the older subjects. This pattern of results may be related to already identified structural changes in the vestibular system and suggests the existence of yet unidentified changes in central vestibular and visual processing.
We measured the effects of postrotary head tilts on the perceived duration and the apparent axis of illusory self-rotation experienced following counterclockwise body rotation in high (1.8 G), normal (1 G), and low (0 G) gravitoinertial force environments. In the absence of head movements, the duration of illusory afterrotation was shorter in 0 G and 1.8 G than in 1 G, and it was further shortened by 40 degrees pitch-back head movements in 1 G and 1.8 G. Clockwise illusory afterrotation about the torso's vertical z-axis was always experienced in trials without postrotary head tilts. In trials with head movements, half the subjects experienced no change in this pattern; however, half experienced transient rightward roll of the torso's z-axis, which remained the rotation axis. The duration and extent of apparent roll were greater in 0 G and smaller in 1.8 G than in 1 G. We provide a functional explanation for the tendency for perceived self-rotation to be determined relative to the torso and to the gravitoinertial vertical rather than solely in relation to head position and head-fixed angular velocity sensors.
Mechanical unloading during head movements in weightlessness may be an etiological factor in space-motion sickness. We simulated altered head loading on Earth without affecting vestibular stimulation by having subjects wear a weighted helmet. Eight subjects were exposed to constant velocity rotation about a vertical axis with direction reversals every 60 sec. for eight reversals with the head loaded and eight with the head unloaded. The severity of motion sickness elicited was significantly higher when the head was loaded. This suggests that altered sensory-motor control of the head is also an etiological factor in space-motion sickness.
Mechanical unloading during head movements in weightlessness may be an etiological factor in space-motion sickness. We simulated altered head loading on Earth without affecting vestibular stimulation by having subjects wear a weighted helmet. Eight subjects were exposed to constant velocity rotation about a vertical axis with direction reversals every 60 sec. for eight reversals with the head loaded and eight with the head unloaded. The severity of motion sickness elicited was significantly higher when the head was loaded. This suggests that altered sensory-motor control of the head is also an etiological factor in space-motion sickness.
The effect of Coriolis, cross-coupled stimulation on the vestibuloocular reflex and the elicitation of motion sickness depends on background gravitoinertial force level (DiZio et al. 1986, 1987; Graybiel et al. 1977; Lackner and Graybiel 1984, 1986). We have explored whether this response dependency is related to the unusual patterns of sensorimotor activity present during exposure to non-terrestrial gravitoinertial force levels, to alterations in the encoding of head movements in different gravitoinertial force environments, or to some combination thereof. Blindfolded subjects were exposed to sudden stops after constant velocity, vertical z-axis rotation, sometimes with and sometimes without post-rotational head movements, in the 0 G, 1 G, and 1.8 G force phases of parabolic flight. After sudden stops without head movements, the time constant of decay of post-rotational nystagmus was significantly lower in 0 G than in 1 G and lower to a smaller extent in 1.8 G. Post-rotational head movements decreased the decay time constants in 1 G and in 1.8 G, but not in free fall. The same pattern emerged for the duration of illusory after-rotation. Systematic changes were not found in the peak slow phase velocity of nystagmus. These results suggest that tonic levels of otolithic and somatosensory activity in combination with canalicular, cervical, and motor activity regulate the velocity storage mechanism of the horizontal vestibuloocular reflex (Cohen et al. 1977; Raphan et al. 1979) and sensations of after-rotation. These same factors are likely to be important etiological elements in space motion sickness.
When a limb is used for locomotion, patterns of afferent and efferent activity related to its own motion are present as well as visual, vestibular, and other proprioceptive information about motion of the whole body. A study is reported in which it was asked whether visual stimulation present during whole-body motion can influence the perception of the leg movements propelling the body. Subjects were tested in conditions in which the stepping movements they made were identical but the amount of body displacement relative to inertial space and to the visual surround varied. These test conditions were created by getting the subjects to walk on a rotatable platform centered inside a large, independently rotatable, optokinetic drum. In each test condition, subjects, without looking at their legs, compared, against a standard condition in which the floor and drum were both stationary, their speed of body motion, their stride length and stepping rate, the direction of their steps, and the perceived force they exerted during stepping. When visual surround motion was incompatible with the motion normally associated with the stepping movements being made, changes in apparent body motion and in the awareness of the frequency, extent, and direction of the voluntary stepping movements resulted.
Perception of the surface contour of the body is generally thought to depend on topographically organized neural maps of somatosensation in the thalamus and cortex. Recent neurophysiological studies indicate that these maps are potentially modifiable through alterations in their sensory input. We present evidence that the apparent shape and orientation of the body can be changed within seconds by using muscle vibration to generate proprioceptive misinformation about limb position. Depending on the position of the hands or feet in relation to the rest of the body and to the test chamber, it is possible to generate systematic perceptual distortions of the body and changes in the apparent orientation of the body. Some implications of these observations for the maintenance of an accurate body schema, for spatial orientation, and for the encoding of ocular position are described.
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