EXPLORATION OF THE CORTEX
    Microelectrodes are one-
dimensional tools. To explore a 
three-dimensional structure in 
the brain, we push an electrode 
slowly forward, stop at 
intervals to record from and 
examine a cell, or perhaps two 
or three cells, note the depth 
reading of the advancer, and 
then go on. Sooner or later the 
electrode tip penetrates all the 
way through the cortex. We can 
then pull the electrode out and 
reinsert it somewhere else. 
After the experiment, we slice, 
stain, and examine the tissue 
to determine the position of 
every cell that was recorded. In 
a single experiment, lasting 
about 24 hours, it is usual to 
make two or three electrode 
penetrations through the 
cortex, each about 4 to 5 
millimeters long, and from 
each of which some 200 cells 
can be observed.

    The electrodes are slender, 
and we do well if we can even 
find their tracks under a 
microscope; we consequently 
have no reason to think that in 
a long penetration enough cells 
are injured to impair 
measurably the responses of 
nearby cells. Originally it was 
hard to find the electrode track 
histologically, to say nothing of 
estimating the final position of 
the electrode tip, and it was 
consequently hard to estimate 
the positions of the cells that 
had been recorded. The problem 
was solved when it was 
discovered that by passing a 
tiny current through the 
electrode we could destroy cells 
in a small sphere centered on 
the electrode tip and could 
easily see this region of 
destruction histologically. 
Luckily, passing the current did 
no damage to the electrode, so 
that by making three or four 
such lesions along a single 
penetration and noting their 
depth readings and the depth 
readings of the recorded cells, 
we could estimate the position 
of each cell. The lesions, of 
course, kill a few cells near the 
electrode tip, but not enough to 
impair responses of cells a 
short distance away. For cells 
beyond the electrode tip, we 
can avoid losing information by 
going ahead a bit and recording 
before pulling back to make the 
lesion.