MECHANISTS AND NONMECHANISTS 
If there is a real controversy 
today in the field of perception, 
it is not so much among 
followers of different theories 
as it is between those who hold 
a different philosophy about 
explanation in perception. 
Some believe that we have 
already reached a point where 
explanation of perceptual 
phenomena is possible in terms 
of known neural mechanisms. 
More particularly, I am 
referring to such mechanisms 
as the activity of neurons in 
the retina or the brain that 
respond to the presence of 
certain features of the stimulus 
on the retina, described by 
what is now known as feature-
detector theory. Another 
example would be lateral-
inhibition theory. The guiding 
philosophy of these approaches 
is that, once the appropriate 
stimulus impinges on the 
appropriate region of the 
retina, the appropriate cell or 
cells in the brain will be 
triggered to discharge. These 
explanations can be thought of 
as mechanistic, which simply 
means that a mechanism is 
posited that will lead 
automatically and inexorably 
to a particular perception. The 
process is generally assumed to 
be bottom-up. There is thus no 
need to invoke explanations 
that entail past experience, 
"hypotheses," "decisions," 
"inference," or "problem-
solving." Therefore, effects 
such as those of preferred 
perceptions, of holistic 
organization, and those based 
on attention pose difficulties 
for this approach. For a 
nonmechanistic theory, these 
very processes and effects are 
often central to the 
explanation. Here there is some 
degree of flexibility. The 
process is often in part top-
down.

    Feature-detector theory holds 
that we can explain the 
perception of object properties 
or events by the discharging of 
neurons in the brain sensitive 
to (tuned to) certain 
characteristics of the stimulus 
on the retina. We have 
discussed contour or edge 
detectors (Chapters 5 and 6), 
motion detectors (Chapter 7), 
and disparity-depth detectors 
(Chapter 3). It is of interest to 
note that, for this theory, the 
critical sensory information is 
peripheral: the orientation or 
motion of an image on the 
retina. The brain cell that does 
the detecting is simply 
responsive to that peripheral 
fact. However, because the 
determining physiological 
event is the discharging of a 
neuron in the brain, it would 
not be appropriate to designate 
this theory as peripheral.