To speak, as I do here, of 
separate stages immediately 
raises our problem with 
genealogy. In the retina, as we 
will see in Chapter 3, the 
minimum number of stages 
between receptors and the 
output is certainly three, but 
because of two other kinds of 
cells, some information takes a 
more diverted course, with four 
or five stages from input to 
output. For the sake of 
convenience, the diagram 
ignores these detours despite 
their importance, and makes 
the wiring look simpler than it 
really is. When I speak of the 
retinal ganglion cells as "stage 
3 or 4", it's not that I have 
forgotten how many there are.

    To appreciate the kind of 
transfer of information that 
takes place in a network of this 
kind, we may begin by 
considering the behavior of a 
single retinal ganglion cell. We 
know from its anatomy that 
such a cell gets input from 
many bipolar cells--perhaps 12, 
100, or 1000--and that each of 
these cells is in turn fed by a 
similar number of receptors. As 
a general rule, all the cells 
feeding into a single cell at a 
given stage, such as the bipolar 
cells that feed into a single 
retinal ganglion cell, are 
grouped closely together. In the 
case of the retina, the cells 
connected to any one cell at the 
next stage occupy an area 1 to 2 
millimeters in diameter; they 
are certainly not peppered all 
over the retina. Another way of 
putting this is that none of the 
connections within the retina 
are longer than about 1 to 2 
millimeters.

    If we had a detailed 
description of all the 
connections in such a 
structure and knew enough 
about the cellular physiology--
for example, which 
connections were excitatory 
and which inhibitory--we 
should in principle be able to 
deduce the nature of the 
operation on the information. 
In the case of the retina and 
the cortex, the knowledge 
available is nowhere near what 
we require. So far, the most 
efficient way to tackle the 
problem has been to record from 
the cells with microelectrodes 
and compare their inputs and 
outputs. In the visual system, 
this amounts to asking what 
happens in a cell such as a 
retinal ganglion cell or a 
cortical cell when the eye is 
exposed to a visual image.