We can think of Hering's 
yellow-blue and red-green 
processes as separate channels 
in the nervous system, whose 
outputs can be represented as 
two meters, like old-fashioned 
voltmeters, with the indicator 
of one meter swinging to the 
left of zero to register yellow 
and to the right to register blue 
and the other meter doing the 
same for red versus green. The 
color of an object can then be 
described in terms of the two 
readings. Hering's third 
antagonistic process (you can 
think of it as a third voltmeter) 
registered black versus white. 
He realized that black and gray 
are not produced simply by 
absence of light coming from an 
object or surface but arise when 
and only when the light from 
the object is less than the 
average of the light coming 
from the surrounding regions. 
White arises only when the 
surround is darker and when no 
hue is present. (I have already 
discussed this in Chapter 3, 
with examples such as the 
turned-off television set.) In 
Hering's theory, the black-
white process requires a spatial 
comparison, or subtraction of 
reflectances, whereas his 
yellow-blue and red-green 
processes represent something 
occurring in one particular 
place in the visual field, 
without regard to the 
surrounds. (Hering was 
certainly aware that 
neighboring colors interact, 
but his color theory as 
enunciated in his latest work 
does not encompass those 
phenomena.) We have already 
seen that black versus white is 
indeed represented in the 
retina and brain by spatially 
opponent excitatory and 
inhibitory (on versus off) 
processes that are literally 
antagonistic.