Just as size constancy 
depends upon taking into 
account sensory information 
about distance, constancy here 
would seem to be based upon 
taking into account 
information about body 
orientation. For example, if 
observers are tilted 30 degrees 
clockwise, they will perceive 
the rod to be vertical only if its 
image is tilted 30 degrees 
counterclockwise from the 
vertical meridian on the 
retina. The perceptual system 
takes body orientation into 
account in inferring the 
environmental orientation of 
objects from their retinal-
image orientations. The same 
inference process undoubtedly 
underlies perception in the 
revolving-room experiment. As 
a result of the centrifugal 
force, the observer feels tilted 
and thus perceives the true 
vertical orientation as tilted 
and a tilted orientation as 
vertical.

    The direction of gravity is 
detected by the sensory system 
in various ways. The pressure 
on the feet, or on whatever 
parts of the body take its 
weight, is a cue. Muscular 
reactions to maintain balance 
may also provide cues. But the 
major source of information 
about gravity derives from 
sensory mechanisms inside the 
inner ear, in the so-called 
vestibular apparatus. Hair cells 
in two structures of the inner 
ear, the utricle and saccule, 
are embedded in a gelatinous 
substance. These hair cells 
bend with the pull of gravity, 
causing nerve fibers attached 
to them to discharge, the rate of 
discharge being a function of 
the degree to which the head is 
tilted. We are completely 
unaware of this source of 
information because no 
conscious sensations emanate 
from this part of the inner ear.