We do not have to shine our 
light directly into the retina. It 
is usually easier and more 
natural to project our stimuli 
onto a screen a few meters away 
from the animal. The eye then 
produces on the retina a well-
focused image of the screen and 
the stimulus. We can now go 
ahead and determine the 
position, on the screen, of the 
receptive field's projection. If 
we wish, we can think of the 
receptive field as the part of the 
animal's visual world--in this 
case, the screen--that is seen 
by the cell we are recording 
from.

    We soon learn that cells can 
be choosy, and usually are. It 
may take some time and groping 
before we succeed in finding a 
stimulus that produces a really 
vigorous response from the cell. 
At first we may have difficulty 
even finding the receptive field 
on the screen, although at 
early stages, such as in the 
geniculate, we may locate it 
easily. Cells in the geniculate 
are choosy as to the size of a 
spot they will respond to or as to 
whether it is black on a white 
background or white on black. 
At higher levels in the brain, 
an edge (the line produced by a 
light-dark boundary) may be 
required to evoke a response 
from some cells, in which case 
the cells are likely to be fussy 
about the orientation of the 
edge--whether it is vertical, 
horizontal, or oblique. It may be 
important whether the 
stimulus is stationary or moves 
across the retina (or screen), or 
whether it is colored or white. 
If both eyes are looking at the 
screen, the exact screen 
distance may be crucial. 
Different cells, even within 
the same structure, may differ 
greatly in the stimuli to which 
they respond. We learn 
everything we can think to ask 
about a cell, and then move the 
electrode forward a fraction of a 
millimeter to the next cell, 
where we start testing all over 
again.

    From any one structure, we 
typically record from hundreds 
of cells, in experiments that 
take hours or days. Sooner or 
later we begin to form a general 
idea of what the cells in that 
structure have in common, and 
the ways in which they differ. 
Since each of these structures 
has millions of cells, we can 
sample only a small fraction of 
the population, but luckily 
there are not millions of kinds 
of cells, and sooner or later we 
stop finding new varieties. 
When we are satisfied, we take 
a deep breath and go on to the 
next level--going, for example, 
from the lateral geniculate body 
to the striate cortex--and there 
we repeat the whole procedure. 
The behavior of cells at the 
next stage will usually be more 
complicated than the behavior 
of cells at the previous level: 
the difference can be slight or 
it can be dramatic. By 
comparing successive levels, 
we begin to understand what 
each level is contributing to 
the analysis of our visual 
world--what operation each 
structure is performing on the 
information it receives so that 
it can extract from the 
environment information that 
is biologically useful to the 
animal.