GRAVITY INFORMATION AND 
ORIENTATION CONSTANCY    The
obvious answer is that we 
obtain sensory information 
about the direction of gravity 
and that this information, 
although itself not visual, 
enables us to perceive the 
visual orientation of things in 
the environment. Two 
experiments show that this is 
true. In the first, the observer 
enters a small, dark room that 
revolves in a circular track, as 
in an astronaut’s centrifuge. 
Inside the room is a luminous 
rod that the observer is asked to 
place in a vertical position. The 
centrifugal force adds a 
horizontal component to the 
existing vertical force of 
gravity so that the resultant 
force on the observer is now in 
an oblique direction. On the 
average, observers place the rod 
in this oblique direction, 
perceiving it to be vertical.

    A similar conclusion can be 
drawn from a simpler 
experiment. Suppose an 
observer in a dark room is tilted 
with respect to the vertical 
direction of gravity as well as 
the vertical walls of the room. 
If asked to indicate when a rod 
is vertical, the observer can do 
so with reasonable accuracy. 
Under these conditions, the 
vertical rod does not generate 
an image that is vertical with 
respect to the retina. Thus, the 
perceptual system achieves 
constancy of perceived 
orientation.