VARIATIONS IN COMPLEXITY
A rough indication of 
physiological cell types found 
in the different layers of the 
striate cortex.

    As we would expect, cells 
near the input end of the 
cortex, in layer 4, show less 
complicated behavior than 
cells near the output. In the 
monkey, as noted in this 
chapter, cells in layer 4Cß, 
which receive input from the 
upper four (parvocellular) 
geniculate layers, all seem to 
have center-surround 
properties, without orientation 
selectivity. In layer 4Ca, whose 
input is from the ventral 
(magnocellular) pair of 
geniculate layers, some cells 
have center-surround fields, 
but others seem to be 
orientation-specific, with 
simple receptive fields. Farther 
downstream, in the layers 
above and below 4C, the great 
majority of cells are complex. 
End-stopping occurs in about 20 
percent of cells in layers 2 and 
3 but seldom occurs elsewhere. 
On the whole, then, we find a 
loose correlation between 
complexity and distance along 
the visual path, measured in 
numbers of synapses.

    Stating that most cells above 
and below layer 4 are complex 
glosses over major layer-to-
layer differences, because 
complex cells are far from all 
alike. They all have in common 
the defining characteristic of 
complex cells--they respond 
throughout their receptive field 
to a properly oriented moving 
line regardless of its exact 
position--but they differ in 
other ways. We can distinguish 
four subtypes that tend to be 
housed in different layers. In 
layers 2 and 3, most complex 
cells respond progressively 
better the longer the slit (they 
show length summation), and 
the response becomes weaker 
when the line exceeds a 
critical length only if a cell is 
end stopped. For cells in layer 
5, short slits, covering only a 
small part of the length of a 
receptive field, work about as 
well as long ones; the receptive 
fields are much larger than the 
fields of cells in layers 2 and 3. 
For cells in layer 6, in contrast, 
the longer an optimally 
oriented line is, the stronger 
are the responses, until the 
line spans the entire length of 
the field, which is several 
times greater than the width 
(the distance over which a 
moving line evokes responses). 
The field is thus long and 
narrow. We can conclude that 
axons running from layers 5, 6, 
and 2 and 3 to different targets 
in the brain (the superior 
culliculus, geniculate, the 
other visual cortical areas) 
must carry somewhat different 
kinds of visual information.

    In summary, from layer to 
layer we find differences in the 
way cells behave that seem 
more fundamental than 
differences, say, in optimal 
orientation or in ocular 
dominance. The most obvious of 
these layer-to-layer 
differences is in response 
complexity, which reflects the 
simple anatomical fact that 
some layers are closer than 
others to the input.