The most dramatic 
demonstration of orientation 
columns comes from the use of 
voltage-sensitive dyes, 
developed over many years by 
Larry Cohen at Yale and applied 
to the cerebral cortex by Gary 
Blasdel at the University of 
Pittsburgh. In this technique, a 
voltage-sensitive dye that 
stains cell membranes is poured 
onto the cortex of an 
anesthetized animal and is 
taken up by the nerve cells. 
When an animal is stimulated, 
any responding cells show 
slight changes in color, and if 
enough cells are affected in a 
region close enough to the 
surface, we can record these 
changes with modern TV 
imaging techniques and 
computer-aided noise 
filtration. Blasdel stimulated 
with stripes in some particular 
orientation, made a photograph 
of the pattern of activity in a 
region of cortical surface a few 
centimeters in area, and 
repeated the procedure for 
many orientations. He then 
assigned a color to each 
orientation--red for vertical, 
orange for one o'clock, and so 
on--and superimposed the 
pictures. Because an iso-
orientation line should be 
progressively displaced 
sideways as orientation 
changes, the result in any one 
small region should be a 
rainbowlike pattern. This is 
exactly what Blasdell found. It 
is too early, and the number of 
examples are still too few, to 
allow an interpretation of the 
patterns in terms of fractures 
and reversals, but the method is 
promising.