In the critical test, also 
conducted in the dark, the 
larger circle was placed farther 
back so that it subtended a 
visual angle equal to the 
smaller circle. If the rats 
already possessed the ability to 
perceive constancy of size, the 
larger circle, farther away, 
should look larger. However, 
the rats equally often ran to the 
small circle as to the large 
circle. In another variation, 
the larger circle was moved so 
far away that its visual angle 
was the smaller one; in these 
trials, the rats chose the other, 
objectively smaller circle (the 
visual angle of which was 
larger).

    A control group of rats reared 
in daylight and then given the 
same training and test in the 
dark consistently chose the 
objectively larger circle. These 
results indicate that the rats 
reared in daylight had already 
achieved constancy and that 
the rats raised in the dark did 
not misperceive size because of 
inadequate distance cues in the 
dark alleys. Finally, the 
experimental animals were 
placed back in lighted cages for 
a week. They were then 
retested in the experimental 
alleys and found to behave like 
the normally reared rats: They 
displayed constancy. Thus, 
during the single week of 
relatively restricted 
locomotion in a small cage (and 
perhaps during a much shorter 
period of time), something 
happened to alter perception 
from nonconstancy to 
constancy of size. This 
experiment suggests two things: 
Size constancy--at least in 
rats--is a consequence of 
experience, and size constancy 
may be a result of exposure to 
dynamic change of visual 
angle, that is, experience of 
objects moving toward and away 
from them and experience of 
their own motion toward and 
away from objects.