Size Constancy: The Stimulus-
Relation Explanation

Despite the increasing or 
decreasing size of the retinal 
image (or visual angle) of an 
object as a function of distance, 
its apparent size remains more 
or less the same. If all the brain 
had to go on in determining the 
size of the object was its visual 
angle, constancy would never 
be achieved. To the contrary, 
size perception would vary with 
every change in an object’s 
distance.

    Normally, however, we 
see objects not alone but in the 
context of other objects and 
against some background. 
According to the stimulus-
relation theory, size perception 
and size constancy can be 
explained by the ratio of the 
visual angle of one object to 
that of other objects. For 
example, if we see a man 
standing next to a house, his 
height bears a definite size 
relation to the height of the 
house. That relation will not 
change no matter from how far 
away we view the man. Or 
consider the size relation of an 
object to a uniformly textured 
background. An object on a 
lawn will cover a given number 
of units of the grass texture. 
Viewed from a different 
distance, the object will cover, 
or occlude, the same number of 
units. In instances such as 
this, the late James Gibson 
argued, constancy can be 
explained by the unvarying size 
ratio of objects in a scene, 
without having to refer to 
distance at all.