The dents in the side of this 
battleship create an effect 
based on our perception of their 
shadows.

    The ubiquity and importance 
of shadow in depth perception 
can be gathered by simply 
looking around a room in 
which you are sitting or by 
considering the effect of shade 
and shadow in photographs and 
paintings. There is a curious 
ambiguity about the meaning of 
shading, however. A depression 
and an elevation in a surface 
will both be shaded on one side, 
since the light comes 
predominantly from a 
particular direction. How, 
then, can we tell the 
difference? If we are aware of 
the direction from which the 
illumination comes, we could, 
in principle, infer whether the 
region was elevated (e.g., a 
mound, bump, or bas relief) or 
depressed (e.g., a concave 
hollow, hole, or intaglio). If it 
were a mound, the shading 
would be on the side opposite to 
the source of light, whereas if 
it were a hollow, the shading 
would be on the same side.

    But what do we perceive 
when we do not know the 
direction of the source of light, 
as often we do not, particularly 
when viewing pictures? 
Although the stimulus is 
ambiguous, we tend to perceive 
a region immediately as 
elevated or depressed. In one
photograph shown here, 
we tend to see raised regions or 
"bumps," whereas in the 
other one, we tend to see 
indentations or "holes," even 
though it is simply the first one 
turned upside down. 
Apparently, the perceptual 
system, in the absence of 
contradictory information, 
makes the assumption that the 
light is coming from above.