It is easy to conceive of a high 
degree of specificity in the 
brain when you remember that 
it contains one million million 
cells.  If you lined up one 
million million periods of the 
size shown here next to one 
another, the line would stretch 
around the circumference of 
the earth almost twelve and a 
half times.

    After hearing about simple 
and complex cells, people often 
complain that the analysis of 
every tiny part of our visual 
field--for all possible 
orientations and for dark lines, 
light lines, and edges--must 
surely require an astronomic 
number of cells. The answer is 
yes, certainly. But that fits 
perfectly, because an 
astronomic number of cells is 
just what the cortex has. Today 
we can say what the cells in 
this part of the brain are doing, 
at least in response to many 
simple, everyday visual stimuli. 
I suspect that no two striate 
cortical cells do exactly the 
same thing, because whenever 
a microelectrode tip succeeds 
in recording from two cells at a 
time, the two show slight 
differences--in exact receptive 
field position, directional 
selectivity, strength of 
response, or some other 
attribute. In short, there seems 
to be little redundancy in this 
part of the brain.

    How sure can we be that 
these cells are not wired up to 
respond to some other stimulus 
besides straight line segments? 
It is not as though we and 
others have not tried many 
other stimuli, including faces, 
Cosmopolitan covers, and 
waving our hands. Experience 
shows that we would be foolish 
to think that we had exhausted 
the list of possibilities. In the 
early 1960s, just when we felt 
satisfied with the striate cortex 
and wanted to move on to the 
next area (in fact, we had 
moved on), we happened to 
record from a sluggishly 
responding cell in the striate 
cortex and, by making the slit 
shorter, found that this very 
cell was anything but a sluggish 
responder. In this way we 
stumbled on end stopping. And 
it took almost twenty years' 
work with the monkey striate 
cortex before we became aware 
of blobs--pockets of cells 
specialized for color, described 
in Chapter 8. Having expressed 
these reservations, I should add 
that I have no doubt at all that 
some of the findings, such as 
orientation selectivity, are 
genuine properties of these 
cells. There is too much 
collateral evidence, such as the 
functional anatomy, described 
in Chapter 5, to allow for much 
scepticism.