Although most pictorial cues 
may be learned, responsiveness 
to the cue of cast shadow 
appears to be innate, at least in 
chickens. Wayne Hershberger 
at Northern Illinois University 
trained two groups of chicks, 
one group to peck at mounds 
rather than at depressions, and 
the other group the reverse, by 
rewarding them with food 
when they made the correct 
choice. The mounds and 
depressions were devoid of any 
shadows; thus, the correct 
perception of their depth 
depended entirely upon 
binocular or motion cues. 
Following this learning, the 
chicks were presented with 
pictures of mounds or 
depressions in which shadow 
was the only cue. If a chick had 
learned to peck at mounds, it 
should peck at the photograph 
it perceived as a mound—–
presumably the photograph 
with shadow at the bottom, 
which would look like a mound 
to us. If neither photograph 
looked more like a mound than 
the other, the chick should 
peck randomly or not peck at 
the pictures at all.