![Astronaut at the end of the Canadian robotic arm [8]](https://s3-us-west-2.amazonaws.com/secure.notion-static.com/b9282024-2400-44d9-a2d3-4c7ca6e9cc0d/Untitled.png)
May 2004
NASA defines vertigo as: the sensation that the outer world is revolving about the patient (objective vertigo) or that he himself is moving in space (subjective vertigo) [9]. Or, in other words, it is an illusion that the subject is moving or that their surroundings are moving. Stedman’s Medical Dictionary [10] defines vertigo as:
1. A sensation of spinning or whirling motion. Vertigo implies a definite sensation of rotation of the subject (subjective vertigo) or of objects about the subject (objective vertigo) in any plane. 2. Imprecisely used as a general term to describe dizziness. [L. vertigo (vertigin-), dizziness, fr. verto, to turn]
More specifically, Stedman defines ‘height’ vertigo as: “dizziness experienced when looking down from a great height or in looking up at a high building or cliff.” This definition is similar to many stereotypes about height vertigo in that they attribute the phenomenon to some sort of feeling such as dizziness. Vertigo has also been ascribed to psychopathological processes such as neurotic acrophobia (fear of heights) or psychologically induced dizziness [1], but these seem to be symptoms of an underlying mechanism that serves as a signal to warn of instability.
Brandt et al (1980) believed that height vertigo was not simply representative of a purposeless phenomenon caused by a specific visual circumstance (e.g. standing at the edge of a cliff), but that it is a warning signal in the body to withdraw from an unstable situation. Moreover, it is sometimes coupled with fear or a phobia of heights, but fear is not a requisite [1] as many have concluded.
Perhaps you have experienced height vertigo when going up in a glass elevator, climbing a tree as a kid, ascending a ladder, looking down from a skyscraper, or peering over the edge of a rocky cliff. These sensations may or may not include fear, but they probably included some sort of feeling of uneasiness or sense of instability came over you. Why does this happen? What is the cause? Do visual cues somehow provoke height vertigo?
Height vertigo is sometimes experienced in space during extra-vehicular activities (EVAs) or spacewalks. While astronauts are outside of the shuttle carrying out various operations, they will occasionally encounter circumstances where they will suddenly feel as though they are plummeting toward the earth and will cling to objects for dear life. The following is an account of height vertigo experienced during EVAs in Richards et al:
“… nothing had prepared me for the terror that came over me when dangling from the end of a telescoping pole outside the confines of the space station during my spacewalk.”
“I was dangling on the end of a wavering pole, getting pushed out farther and farther from the space station. … Suddenly, it hit me: the feeling of speed … faster than anything that I … had ever experienced in my life. Accompanying this overpowering sense of speed was the overwhelming sensation of falling. I felt as if I were falling off the station and catapulting toward the earth. Furthermore, … it felt as if the space station itself were plummeting earthward with me clinging to its surface… I wanted to close my eyes in an effort to escape this dreadful and persistent sensation of falling. … I reasoned that this instantaneous sense and surge of orbital speed occurred because I had lost the stabilizing visual perspective provided by the protective interior walls of the space station. … deprived from any strong visual clues of containment, I felt as if I were falling off a cliff that just kept falling away from me. … I tried concentrating on the surface of the station far in front of me, attempting to use visual input to mask the reality…. Despite my efforts to block the reality out, speed and falling persisted.”
The astronaut attempts to explain the causes of this vertigo:
“To my knowledge, this sensation of falling rarely occurs during shuttle spacewalks, probably for two reasons. First, and probably most important, almost all shuttle spacewalks are conducted within the three-wall confines of the shuttle’s payload bay. The concave surface surrounding the spacewalker is probably sufficient to maintain the illusion of being contained, of being surrounded by a stable frame of visual reference. Second, whenever the spacewalker moves out of this envelope of containment, he or she is usually firmly attached by footholds to either the end of the robotic arm or to the edge of the payload bay. As I found on Mir, whenever I was able to wiggle my bulky, boot-laden toes under a handrail, the stability provided lessened the sensation of falling.”
“My friend admitted to me that … he, too, had experienced the same frightening phenomenon. Feet securely in place on the robotic arm, he was being conveyed to the far end of the [satellite].., well out in the confines of the payload bay. … in order to begin his repair work, he had to extricate himself from the stabilizing foot-loops and cross over to the [satellite]. Feet now free and reaching across the two-foot gap to the end of the [satellite], he unexpectedly and overwhelmingly felt the sensation of falling off the edge of the world! Stunned by the unexpected sensation, he bear-hugged the convex surface … and hung on for dear life! Once he was stabilized on the end of the [telescopic arm], the sensation eased and he was able to continue working. His space-walking partner experienced the same phenomenon at precisely the same location. His reaction was the same: to cling on to the [satellite].”
The vertigo experienced by astronauts is clearly an illusion, but Brandt refers to a dizziness or instability that does not necessarily include fear. The two are probably related, but the latter is due to a conflict between visual and vestibular cues.
Astronaut at the end of the Canadian robotic arm [8]
![Possible scenario where the visual cues of the earth might cause the astronaut to experience height vertigo as though he and the shuttle were falling to the earth [8]](https://s3-us-west-2.amazonaws.com/secure.notion-static.com/6fa4a89e-ee6d-430d-9c36-1d5d9fe64a21/Untitled.png)
Possible scenario where the visual cues of the earth might cause the astronaut to experience height vertigo as though he and the shuttle were falling to the earth [8]
The visual cue is ambiguous during EVAs because the earth is too distant for motion to be visually detected. In addition, the lack of gravitational cue is equivalent to the cue when falling; therefore, the nervous system may interpret the two cues and conclude that the astronaut is falling. Perhaps the only resolving cue is proprioceptive. This could explain why the onset of height vertigo did not occur until the feet were free, as in the above accounts.
Moreover, height vertigo did not occur until astronauts lost a visual reference cue (e.g. the shuttle) below their feet or until they left the three wall confines of the shuttle bay. Perhaps the walls are within some range for postural motion perception, whereas the earth is too far. What causes height vertigo during EVAs? What might be done to prevent it?
Height vertigo may be termed more specifically as ‘distance vertigo’ [1], because it was found that as the distance from objects increases, the necessary visual cues that indicate posture decrease and thus cause vertigo. Brandt et al (1980) proposed that man uses the visual senses to maintain balance. Various visual cues indicate the postural sway to the neural system, and the body adjusts to maintain balance. There is a retinal shift, α, which corresponds to head movements. However, the retinal shift must exceed a threshold due to a limited retinal resolution, Δ, before motion can be detected. As the distance from visual objects increases, the retinal shift gets smaller, and the body must sway farther to attain threshold, see Figure 2.