Another interpretation of 
constancy is similar to the 
simple stimulus theory--
namely, that ratios and other 
stimulus relations govern 
perception. However, they are 
believed to do so because 
interactions in the brain are 
the neural processes that 
determine perception. Here we 
see an example of the thinking 
of the Gestalt psychologists. In 
other words, both approaches 
would regard ratios, 
proportions, and other stimulus 
relations as the basis of 
constancy. But whereas the 
stimulus theory has it that 
these are the higher-order 
stimulus correlates of 
perception, the Gestalt theory 
has it that such stimulus 
relations yield constancy 
percepts because of the neural 
interaction to which they give 
rise.

    Another approach, stemming 
from the thinking of 
Helmholtz’s contemporary 
Ewald Hering, also emphasizes 
the role of neural interactions. 
It is a fact of the visual nervous 
system that the discharging of 
impulses in one neuron 
inhibits the discharging of 
impulses in adjacent neurons. 
This lateral inhibition, as it is 
called, might be thought to 
explain contrast, the 
inhibition causing one region 
to look darker when an 
adjacent region is "bright." 
Lateral inhibition has also 
been invoked to explain 
lightness constancy, although 
the explanation is more 
complicated. The reduction of 
the ratio principle to a 
physiological mechanism has 
given it great appeal to many 
investigators. Despite the 
underlying difference in the 
view of the mind in these 
approaches, they share the 
belief that constancy is based 
on such stimulus relations as 
ratios.