Our main goal in this text 
will be to understand why all 
these chains of neuronal 
structures exist, how they 
work, and what they do. We 
want to know what kind of 
visual information travels 
along a trunk of fibers, and how 
the information is modified in 
each region--retina, lateral 
geniculate body, and the 
various levels of cortex. We 
attack the problem by using the 
microelectrode, the single most 
important tool in the modern 
era of neurophysiology. We 
insert the microelectrode 
(usually a fine insulated wire) 
into whatever structure we 
wish to study--for example, the 
lateral geniculate body--so that 
its tip comes close enough to a 
cell to pick up its electrical 
signals. We attempt to influence 
those signals by shining spots 
or patterns of light on the 
animal's retina.

An experimental plan for 
recording from the visual 
pathway. The animal, usually a 
macaque monkey, faces a 
screen onto which we project a 
stimulus. We record by 
inserting a microelectrode into 
some part of the pathway, in 
this case, the primary visual 
cortex. (The brain in this 
diagram is from a human, but a 
monkey brain is very similar.)

    Because the lateral 
geniculate body receives its 
main input from the retina, 
each cell in the geniculate will 
receive connections from rods 
and cones--not directly but by 
way of intermediate retinal 
cells. As you will see in Chapter 
3, the population of rods and 
cones that feed into a given cell 
in the visual pathway are not 
scattered about all over the 
retina but are clustered into a 
small area. This area of the 
retina is called the receptive 
field of the cell. So our first 
step, in shining the light here 
and there on the retina, is to 
find the cell's receptive field. 
Once we have defined the 
receptive field's boundaries, we 
can begin to vary the shape, 
size, color, and rate of 
movement of the stimulus--to 
learn what kinds of visual 
stimuli cause the cell to 
respond best.