If, however, a constancy is 
based on an inference process, 
as is probably the case in size 
perception, experience may 
play a role in its genesis. To 
determine whether or not the 
constancy of a perception is 
innate, experiments must be 
constructed that rule out 
experience as a factor. One way 
of doing so is to test animals or 
infants at birth or shortly 
thereafter; a more drastic way 
is to test more mature animals 
that have been prevented from 
learning about the visual 
world, usually by eliminating 
their vision or pattern vision at 
birth. Both methods have 
inherent difficulties: One 
might think that infants and 
many animals are not mature 
enough to test at birth, since 
they are not capable of 
locomotion or of learning to 
discriminate between objects of 
differing sizes or lightnesses, 
while early deprivation of 
vision may cause severe 
deterioration of the visual 
nervous system and prevent 
normal maturation.