In many cells, perhaps one-
fifth of the population, moving 
the stimulus brings out another 
kind of specific response. 
Instead of firing equally well to 
both movements, back and 
forth, many cells will 
consistently respond better to 
one of the two directions. One 
movement may even produce a 
strong response and the reverse 
movement none or almost 
none, as illustrated in the 
figure to the left.

    In a single experiment we 
can test the responses of 200 to 
300 cells simply by learning all 
about one cell and then 
pushing the electrode ahead to 
the next cell to study it. 
Because once you have inserted 
the delicate electrode you 
obviously can't move it 
sideways without destroying it 
or the even more delicate 
cortex, this technique limits 
your examination to cells lying 
in a straight line. Fifty cells per 
millimeter of penetration is 
about the maximum we can get 
with present methods. When 
the orientation preferences of a 
few hundred or a thousand cells 
are examined, all orientations 
turn out to be about equally 
represented--vertical, 
horizontal, and every possible 
oblique. Considering the nature 
of the world we look at, 
containing as it does trees and 
horizons, the question arises 
whether any particular 
orientations, such as vertical 
and horizontal, are better 
represented than the others. 
Answers differ with different 
laboratory results, but everyone 
agrees that if biases do exist, 
they must be small--small 
enough to require statistics to 
discern them, which may mean 
they are negligible!

    In the monkey striate 
cortex, about 70 to 80 percent of 
cells have this property of 
orientation specificity. In the 
cat, all cortical cells seem to be 
orientation selective, even 
those with direct geniculate 
input.

    We find striking differences 
among orientation-specific 
cells, not just in optimum 
stimulus orientation or in the 
position of the receptive field 
on the retina, but in the way 
cells behave. The most useful 
distinction is between two 
classes of cells: simple and 
complex. As their names 
suggest, the two types differ in 
the complexity of their 
behavior, and we make the 
reasonable assumption that the 
cells with the simpler behavior 
are closer in the circuit to the 
input of the cortex.