PERIPHERALISTS AND 
CENTRALISTS    Apart from 
particular theoretical 
perspectives, investigation in 
visual perception is 
characterized by dispute over 
the question of whether a 
particular phenomenon can be 
explained in terms of a 
peripheral mechanism or 
requires a more central 
explanation. By "peripheral" is 
meant a mechanism that can be 
localized in the eye or the 
movements of the eye; by 
"central" is meant a process 
that must occur deeper in the 
brain and thus often cannot 
even be specified in "hardware 
language" because we do not 
know enough about the brain.

    For example, suppose the 
phenomenon in question is the 
constancy of lightness of a 
surface despite variations in 
illumination on the surface. 
Ewald Hering, a contemporary 
and a theoretical opponent of 
Helmholtz, suggested several 
possible explanations. 
Although Hering advanced 
other, more central 
explanations, one was based on 
the well-known fact that the 
iris surrounding the pupillary 
opening of the eye 
automatically alters the pupil’s 
diameter as a function of the 
amount of light reaching the 
eye. If the illumination on a 
surface increases, the 
pupillary opening decreases, 
and this has the effect of 
offsetting the increased level of 
illumination. Thus, the 
intensity of light reaching the 
retina from a surface of a given 
reflectance could conceivably 
remain constant. If it did, 
perceptual constancy would be 
explained. This peripheral 
explanation turns out to be 
incorrect because, among other 
reasons, while the area of the 
pupil can change only by a 
factor of about 17, illumination 
can change by a factor of 
100,000. By way of contrast, 
Helmholtz’s view that the 
perceptual system takes 
account of the level of 
illumination and allows for it 
in judging sensations of 
intensity is a central theory. As 
has been seen, however, it is 
quite probable that this theory 
is also incorrect.