Up-down cues are human
sensory cues built into an environment to indicate which
direction is "up", even if "up" is arbitrary.
For example, a cluttered desk has a number of unattached objects on one surface but not on the others. We can assume that the objects are being held to that surface by gravity, and that the surface in question is the "top" of the desk. This establishes the up and down directions. In humans, the visual sensory system can overpower other sources of environmental information, such as the
vestibular system
The vestibular system, in vertebrates, is a sensory system that creates the sense of balance and spatial orientation for the purpose of coordinating movement with balance. Together with the cochlea, a part of the auditory system, it constitutes ...
. Conflicting signals are resolved by deference to the information gathered by the eyes. For example,
astronaut
An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a human spaceflight program to serve as a commander or crew member aboard a spacecraft. Although generally r ...
s in a
Space Shuttle
The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...
may have experienced
space sickness
Space adaptation syndrome (SAS) or space sickness is a condition experienced by as many as half of all space travelers during their adaptation to weightlessness once in orbit. It is the opposite of terrestrial motion sickness since it occurs when ...
when their vestibular systems are indicating
free-fall
In Newtonian physics, free fall is any motion of a body where gravity is the only force acting upon it. In the context of general relativity, where gravitation is reduced to a space-time curvature, a body in free fall has no force acting on i ...
and their visual sensory systems are indicating no movement. Building visual "hints" into the environment allows the visual system to override other senses and provide a solid sense of orientation and motion.
Sources
The Architecture of Artificial Gravity: Theory, Form, and Function in the High Frontier*
Perception
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