The perceived orientation of 
things in the environment—–
that is, whether they are 
vertical, oblique, or 
horizontal—–remains constant 
regardless of our own 
orientation. Even when we tilt 
our heads, which results in a 
tilting of the retinal image of 
contours in the scene, for 
example, we continue to 
perceive veridically how things 
are oriented. In the case of 
orientation constancy, the 
relevant fact that our 
perceptual system takes into 
account is our own position. It 
determines this by sensing the 
direction of gravity. However, 
there is good reason to believe 
that stimulus relations are also 
important in orientation 
constancy, as we shall see in 
Chapter 8.