This was the situation in 
1958, when Torsten Wiesel and I 
made one of our first 
technically successful 
recordings from the cortex of a 
cat. The position of 
microelectrode tip, relative to 
the cortex, was unusually 
stable, so much so that we were 
able to listen in on one cell for 
a period of about nine hours. We 
tried everything short of 
standing on our heads to get it 
to fire. (It did fire 
spontaneously from time to 
time, as most cortical cells do, 
but we had a hard time 
convincing ourselves that our 
stimuli had caused any of that 
activity.) After some hours we 
began to have a vague feeling 
that shining light in one 
particular part of the retina was 
evoking some response, so we 
tried concentrating our efforts 
there. To stimulate, we were 
using mostly white circular 
spots and black spots. For black 
spots, we would take a 1-by-2-
inch glass microscope slide, 
onto which we had glued an 
opaque black dot, and shove it 
into a slot in the optical 
instrument Samuel Talbot had 
designed to project images on 
the retina. For white spots, we 
used a slide of the same size 
made of brass with a small hole 
drilled through it. (Research 
was cheaper in those days.) 
After about five hours of 
struggle, we suddenly had the 
impression that the glass with 
the dot was occasionally 
producing a response, but the 
response seemed to have little 
to do with the dot. Eventually 
we caught on: it was the sharp 
but faint shadow cast by the 
edge of the glass as we slid it 
into the slot that was doing the 
trick. We soon convinced 
ourselves that the edge worked 
only when its shadow was swept 
across one small part of the 
retina and that the sweeping 
had to be done with the edge in 
one particular orientation.