Although we have only 
recently come to realize how 
numerous these visual areas 
are, we are already building up 
knowledge about the 
connections and single-cell 
physiology of some of them. Just 
as area 17 is a mosaic of two sets 
of regions, blob and nonblob, so 
the next visual area, area 18 or 
visual area 2, is a mosaic of 
three sets. Unlike the blobs and 
interblobs, which formed 
islands in an ocean, the mosaic 
in area 18 takes the form of 
parallel stripes. In these 
subdivisions we find a striking 
segregation of function. In the 
set of thick stripes, most of the 
cells are highly sensitive to the 
relative horizontal positions of 
the stimuli in the two eyes, as 
described in Chapter 7; we 
therefore conclude that this 
thick-stripe subdivision is 
concerned at least in part with 
stereopsis. In the second set, 
the thin stripes, cells lack 
orientation selectivity and 
often show specific color 
responses. In the third set, the 
pale stripes, cells are 
orientation selective and most 
are end stopped. Thus the three 
sets of subdivisions that make 
up area 18 seem to be concerned 
with stereopsis, color, and 
form.

    A similar division of labor 
occurs in the areas beyond area 
18, but now entire areas seem to 
be given over to one or perhaps 
two visual functions. An area 
called MT (for middle temporal 
gyrus) is devoted to movement 
and stereopsis; one called V 4 (V 
for visual) seems to be 
concerned mainly with color. 
We can thus discern two 
processes that go hand in hand. 
The first is hierarchical. To 
solve the various problems in 
vision outlined in previous 
chapters--color, stereopsis, 
movement, form--information 
is operated upon in one area 
after the next, with progressive 
abstraction and increasing 
complexity of representation. 
The second process consists of a 
divergence of pathways. 
Apparently the problems 
require such different 
strategies and hardware that it 
becomes more efficient to 
handle them in entirely 
separate channels.

    This surprising tendency for 
attributes such as form, color, 
and movement to be handled by 
separate structures in the brain 
immediately raises the question 
of how all the information is 
finally assembled, say for 
perceiving a bouncing red ball. 
It obviously must be assembled 
somewhere, if only at the motor 
nerves that subserve the action 
of catching. Where it's 
assembled, and how, we have 
no idea.